Journal of Colloid and Interface Science 440 (2015) 16–22
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Moderate the adsorption of cationic surfactant on gold surface by mixing with sparingly soluble anionic surfactant Wei Wang a,b,⇑, Wensheng Lu c,⇑, Haifei Wang c, Hongtao Xie a, Jide Wang a a
College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China Centre for Pharmacy and Department of Chemistry, University of Bergen, Bergen 5007, Norway c Beijing National Laboratory for Molecular Science, Key Laboratory of Colloid and Interface, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b
a r t i c l e
i n f o
Article history: Received 29 July 2014 Accepted 28 October 2014 Available online 7 November 2014 Keywords: C18N3 Arachidic acid Cationic Adsorption Gold surface Morphology Aggregation
a b s t r a c t Surfactants with amine groups are often used in the nanoparticle synthesis due to the high affinity with Au atoms. The match of charges of a capping reagent with Au has significant influence on structures in nanoparticle synthesis. Thus we studied the adsorption of a catanionic surfactant system on Au surface. The surfactants used in the study are bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3) and arachidic acid. Three combinations of the surfactants were studied with regard to the protonation state of the amine groups and the match of charges of the surfactant headgroup. The morphology of the surfactant mixtures changes from high-curvature aggregates to low-curvature with increasing the molar ratio of arachidic acid in the mixtures or the pH of the surfactant solutions. The adsorption of the mixed surfactant systems was studied by means of scanning electron microscopy (SEM), quartz crystal microbalance (QCM) and cyclic voltammetry (CV). The results revealed that the homogeneity and the compactness of the adsorbed layer on a gold surface were increased with the molar ratio of arachidic acid in the complexes. Furthermore, we may obtain the construction of the film of the mixed surfactant on gold surface using the result obtained by QCM. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Surfactant adsorption at an air–liquid or liquid–solid interface, along with the resulting lowering of the surface tension, plays a central role in controlling the desired behavior in many practical applications involving surfactants. In many practical applications, better properties can be attained by using mixed surfactant systems comparing to using an individual surfactant [1]. Mixtures of cationic and anionic surfactant represent interesting models for understanding structure formation in two-dimensional (2D) and three-dimensional (3D) structures. One of their interesting features is the competitive interaction between the cationic part and the anionic part leading to a richness of phases and special aggregation structures. This calls for systematic variations of these interactions and one of the conventional methods is to vary the charge of the hydrophilic headgroups. For the mixed systems of fatty acid and fatty amine as the most frequently studied system, this is possible via variation of the hydrophilic part.
⇑ Corresponding authors at: Postbox 7803, Bergen N-5007, Norway (W. Wang). E-mail addresses: [email protected] (W. Wang), [email protected] (W. Lu). http://dx.doi.org/10.1016/j.jcis.2014.10.063 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Multihead surfactants have drawn the attention of researchers due to its unique properties and applications in nanotechnology [2]. With regard to the increase of steric repulsion and the decrease of hydrophobicity, multihead surfactants exhibit a higher critical micellar concentration (CMC) comparing to conventional surfactants with the alkyl chain of an equal length. The aggregates formed by multihead surfactants in an aqueous solution are generally micelles with a low aggregation number, although it is possible to form other types of aggregates by balancing the hydrophilic part with a longer alkyl chain [3]. This may be understood by the molecular packing parameter (p). The molecular packing parameter is defined as p = v/a⁄l where a is the area of the surfactant headgroup, l equals to the length of the alkyl chain at a stretched configuration, and v is the volume of the surfactant molecule [4]. Due to the large area of the headgroup for a multihead surfactant, the packing parameter is commonly smaller than 1/3, which indicates a large spontaneous curvature of the aggregates [5]. In the previous studies from our group, we have synthesized a multihead surfactant with a dendritic headgroup, bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (abbreviated as C18N3), which exhibited many interesting features as a capping reagent in nanocrystal synthesis and nanoparticle self-assembling [6–16]. C18N3
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is a surfactant with a eighteen-carbon chain and a dendritic headgroup analogy to the dendritic unit of poly(amidoamine) (PAMAM) [17]. The dendritic headgroup is composed of two primary amines and a tertiary one. The three-amine groups are protonated at low pH, and maintain uncharged at high pH. Thus the area of the headgroup changes corresponding to the pH of the solution. It also resulted different aggregation morphology at different pHs, such as, micelles at pH 2 and lamellar structures at pH 10.5. Due to the multiple-amine groups, the surfactant shows a strong interaction with metal elements, such as aurum, silver and copper [8,9,12]. For example, the surfactant interacted with silver ions and the aggregates of the surfactant transferred from micelles to vesicles at low pH [12]. The vesicles were used as a template to assemble AgCl nanoparticles on the vesicle surface and synthesize hollow nanostructures. The diameter of the hollow nanostructure increases with a controlled growth rate due to the competitive interaction with silver ions between the surfactant and the chloride ions [12]. Many interesting self-assembling structures have been achieved by applying the surfactant as a capping reagent in synthesizing gold nanoparticles. Recently C18N3 was used to self-assemble highaspect-ratio gold nanorods and resulted in a highly ordered 2D structure on a micrometer scale, which is by far the largest 2D self-assembling structure of nanoparticles by using bottom-up methodology [13]. These results suggest that the increase of hydrophobic interaction by elongating the surfactant chain may benefit the self-assembling; while the liquid property of the alkyl chain has to be retained. Therefore in this study, an anionic surfactant, arachidic acid (AA) was introduced to the C18N3 system. Arachidic acid is an amphiphilic molecule, however sparingly soluble in water due to a long alkyl chain. Mixing with arachidic acid will maintain the strong hydrophobic interaction, and in the meantime moderate the charge density of the aggregates. The adsorption of the aggregates on gold surface was studied by means of scanning electron microscope (SEM), quartz crystal microbalance (QCM), and electrochemistry. Since C18N3 mostly has three protonated sites, three molar combinations of arachidic acid and C18N3 were thus studied concerning the match of the charges in the headgroups of the surfactants.
H 2N
O N H
O N
N H
NH 2
Fig. 1. The chemical structure of bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3).
2.3. Transmitted electron microscope (TEM) C18N3 and arachidic acid were dissolved in chloroform with the concentration of 1 mmol/L. The solution of C18N3 and arachidic acid at different molar ratio was mixed in a flask and the solvent was evaporated using rotary evaporator. The film of the mixtures was dried in a nitrogen atmosphere, and then rehydrated with pure water. In each sample, the final concentration of the total mixtures was 1 mmol/L. The pH of solution was then adjusted by 1 mol/L HCl or NaOH solution, and measured by a HI 8314 pH-meter (HANNA instrument) with a HI 1200B glass electrode. The solutions were placed in a thermal room of 25 °C to reach equilibrium. One drop of the solution was placed on a carbon film supported by copper grids and maintained for 5 min in a sealed environment. Then the residuals were removed using a filter paper. TEM analysis was carried out with a negative-staining method on JEOL (JEM2011).
2. Materials and methods
2.4. Scanning electron microscope (SEM)
2.1. Materials
A silica substrate covered with a layer of gold film was immersed in a pre-mixed surfactant solution. After the adsorption of the sample on the substrate for half an hour, the substrate was removed from the solution, and rinsed with pure water. The substrate was then dried in a nitrogen environment. SEM images were taking on a HITACHI S4300.
Arachidic acid (99.5%), hydrochloride acid (37%), and sodium hydroxide (99.5%) were purchased from Aldrich–Sigma, and they were used without further purification. Chloroauric acid, methylene blue (MB) and trisodium citrate were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Milli-Q water with a resistivity of 18 M ohm cm was used in all experiments. Bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3) was synthesized according to the method described in a previously published paper, and the structure is shown in Fig. 1 [17].
2.2. Au nanoparticles preparation 13 nm Au nanoparticles were prepared by reduction of chloroauric acid in an aqueous trisodium citrate solution. 2% chloroauric acid (0.3 ml) and 2% trisodium citrate (0.9 mL) were mixed with 60 mL water. The solution was then heated to the boiling temperature under rigorous agitation for 30 min. After cooling to room temperature, the Au nanoparticles solution was stored at 4 °C. The TEM image of the nanoparticles is shown in the supporting information (Fig. S2).
2.5. Quartz crystal microbalance (QCM) The gravimetric measurements were performed on a QCM. An AT-cut quartz crystal with a fundamental frequency of 9 MHz coated with gold layers on both sides, was purchased from Seiko EG&G (Tokyo, Japan). The effective adsorbing area on the quartz was 0.196 cm2. The gold surface of the quartz resonator was precleaned by detergents. Then the piranha solution (H2SO4:30%H2O2 = 3:1) was dropped on the gold surface and the surface was headed up to 50 °C for 2 min. The surface of the quartz was then rinsed by pure water and dried with a stream of nitrogen. However the glue between the gold layer and the quartz could not stand in an acidic solution, so the experiments were only performed for the samples without the addition of HCl and NaOH solution. The resonators were immersed in the solution of the mixed surfactants for 30 min. Then the quartz was removed from the
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solution and rinsed by pure water. The measurements were performed in a dried state; therefore the precondition of Sauerbrey equation was satisfied. The change of frequency was transferred into the adsorbed mass based on the equivalence of 1 Hz frequency corresponding to 1.07 ng mass with regard to our experimental setup. Hence the mass of the adsorbed layer was directly obtained. The resonator with the dried surfactant layer was again immured into the gold nanoparticle solution for 30 min. Then the quartz was removed from the nanoparticle solution and dried in a nitrogen stream. The mass of the adsorbed gold nanoparticles was again measured. Based on the adsorbed amount of the gold nanoparticles, we can obtain the charge density of the outer layer. 2.6. Cyclic voltammetry (CV) A polycrystalline gold electrode (1 cm2) was polished with alumina powder of 0.3 lm and 0.05 lm subsequently. The polished gold electrode was rinsed in water and ultrasonicated to remove the residuals of the alumina powder. The electrode was pretested by potential cycling in H2SO4 solution, and then it was rinsed thoroughly in water. The freshly prepared gold electrode was immersed in the mixed surfactant solution with the concentration of 1 mmol/L for 30 min. The modified electrode was removed from the solution and rinsed by water and then dried by a nitrogen stream. Electrochemical experiments were carried out with a CHI625B Electrochemical Analyzer (CH instrument Co. Shanghai). A singlecompartment cell with a three-electrode system was used for the cyclic voltammetry measurements. An Ag/AgCl electrode was used as a reference electrode, and a platinum wire as a counter electrode. The cyclic voltammograms were collected in a 50 lM MB solution between 0.4 and 0.15 V with the scan rate of 10 mV/s. 3. Results 3.1. TEM results The aggregation morphology of the mixture of C18N3 and arachidic acid (AA) was captured by TEM. C18N3(AA)n (n = 1, 2 and 3) will be used to denote the molar ratio of C18N3 and AA in the samples. The images of C18N3(AA)1 and C18N3(AA)2 are shown in Fig. 2 at pH = 4, 7 and 10. Arachidic acid is insoluble in
water at room temperature, and it usually exists at a crystalline state. However, C18N3 has shown strong ability in dissolving other substance in an acidic solution, but not in a base solution. In the present experiment, the same scenario has been observed. On the first row of Fig. 2, we observe the change of the aggregate corresponding to the pH value of the solution for C18N3(AA)2. In the acidic solution, small aggregates are observed in Fig. 2a. The aggregates are irregular, which may be micelles with small vesicles coexisting in the solution. At neutral pH, ill-shaped vesicles are observed, with sharp edges and corners. The vesicles are polydispersed and the size of the vesicles is about 2 lm. The large vesicles in Fig. 2b have hexagonal shape with six corners in each vesicle. The non-spherical vesicles have observed in other systems, which usually indicate a segregation of one component, and will then consequently result a one-component rich domain in the corner. In this case, it is possible that there are two types of complexes: one is C18N3(AA)1 and the other C18N3(AA)2 since at the neutral condition, the primary amine is not totally protonated. Some of the non-protonated amine groups did not form complexes with the acid group of AA. At pH 10, the lamellar structure is observed in Fig. 2c. The aggregate shown in Fig. 2c has a multilayer structure and the size is about 3 lm. The second row in Fig. 2 is the TEM images for C18N3(AA)1 samples at different pHs. At pH 4, small vesicles are observed in Fig. 2d. The vesicles are mono-dispersed with the size of 100 nm in diameter. The shape of the vesicles is nearly spherical. In Fig. 2e and f, spherical vesicles are observed with a larger size comparing with them at pH 4. The results shown in Fig. 2e and f indicate the morphology of the aggregates C18N3(AA)1 is not sensitive to pH when it is above 7. The mixture of C18N3(AA)3 exhibits a phase separation in the aqueous solution. There was precipitation coexisting with a transparent solution (see Fig. S1) during the rehydration of the film of the mixed surfactant. This may indicate the amount of AA in the mixture is over the limit of the solubility of C18N3, even at pH 4. The charged group of C18N3 may completely associate with the headgroup of AA, which lead to a phase separation. However, there were still mixed surfactants in the solution, and the morphology of the aggregates was captured by means of TEM. The TEM images for C18N3(AA)3 at different pHs are shown in Fig. 3. The morphology of the aggregates has been dramatically changed as shown in Fig. 3. At pH 4, the mixed surfactants tend
Fig. 2. TEM images of the mixed surfactant solutions at different pH conditions: (a), (b) and (c) are C18N3(AA)2 at pH 4, 7, and 10 respectively; (d), (e), and (f) are C18N3(AA)1 at pH 4, 7 and 10 respectively.
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Fig. 3. TEM images of C18N3(AA)3 at pH (a) 4; (b) 4; (c)7 and (d)10.
to form big clusters, and the size of the clusters is on the scale of micrometer, as shown in Fig. 3a. In Fig. 3b, the aggregates are shown in a smaller scale where the detailed structure is observable. The big clusters are composed of a disordered aggregation of small worm-like micelles. Fig. 3c is the TEM image of the complex aggregates at pH 7, which are flat lamellar structure. At pH 10, the aggregates transformed to strings. The length of the strings is on the scale of 10 lm. 3.2. SEM results The adsorption of the aggregates on a gold surface was first examined by means of SEM on a large scale. A strong interaction between the headgroup of C18N3 and Au has been generally observed in the previous studies; however the interaction between carboxylic acid and Au has not been reported. From the SEM images, we are able to observe the homogeneity of the film formed of adsorption. In Fig. 4, the images were captured under the same scale where it is easy to compare the adsorbed film from different samples. Fig. 4a shows a film formed by pure C18N3 where the dark part in the image is the holes on the film. On a large scale the film is rather homogeneous however with large amount of holes, and the substrate may expose to the outer medium through the imperfection of the film. The mixed surfactants show a better coverage of the Au substrate. Fig. 4b shows the SEM image of the C18N3(AA)1 on the Au surface. It shows much less holes on the film comparing to the film formed by pure C18N3. With further increasing ratio of AA in the composition of the complex, the fine film shows fewer holes as shown in Fig. 4c and d.
3.3. QCM results QCM was used to measure the mass adsorbed on the gold surface. The results are shown in Fig. 5. For the mixed surfactant aggregates, it shows a general increase of adsorbed amount with the composition of AA in the mixtures. The film on the gold surface is supposed to be a double layer since the adsorption was performed in the aqueous solution. The hydrophilic part of the surfactant should be exposed to the solution where the hydrophilic headgroups are also the part interacting with the gold surface, and as shown above the aggregates have generally a bilayer structure. Therefore, it is most possible to form a double layer on the gold surface; however the double layer may not be symmetrical due to the different chemical property of the substrate and the solution. For pure C18N3, 38 ng of adsorbed amount is obtained, and the value is 50 ng for C18N3(AA)1. This small increase may be attributed to reducing the charge density on the headgroup. For C18N3(AA)2, the adsorbed amount is about 110 ng, which is slightly double of what is obtained for C18N3(AA)1. This may suggest that another bilayer structure built on top of the first layer. This second layer was stable since rinsing with water could not remove it. For C18N3(AA)3, the adsorbed amount is 372 ng, which indicates a six-bilayer construction. The neutralization of the charges on the headgroups is the main reason for the multilayer construction, which corresponds to the coacervation in the bulk phase. The repulsive force becomes weak due to the reduced charge density, which may also lead to the interaction between C18N3 and AA in different layers.
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Fig. 4. SEM images of the film formed by the adsorption of the mixed surfactant solution: (a) C18N3; (b) C18N3(AA)1; (c) C18N3(AA)2; (d) C18N3(AA)3.
Fig. 5. The adsorbed amounts of the mixed surfactants and negatively charged Au nanoparticles were measured by means of QCM.
The quartz with pre-adsorbed surfactant layer was immersed in a solution containing Au nanoparticles in order to study the features of the outer layer. The size of the Au nanoparticles is about 13 nm. The results are shown in Fig. 5. A clear decrease of adsorption of nanoparticles is observed with increasing the molar ratio of AA in the composition. The trend is presumable regarding the decrease of the available amine groups on the outer layer. For C18N3, the adsorbed amount of the nanoparticles is about 160 ng, and it decreases to 78, 55, and 46 ng for C18N3(AA)1, C18N3(AA)2 and C18N3(AA)3, respectively.
pactness of the layer and the electroactive species in the solution [18]. When the electrode is electrochemically cleaned, the oxidation potential (Epa) and reduction potential (Epc) are 0.22 and 0.27 V vs a saturated calomel electrode (SCE), respectively [19,20]. The results for C18N3(AA)n-modified gold electrodes are shown in Fig. 6. For C18N3 modified electrode, the baseline of the scanned curve is rather flat due to the blocking effect of all redox reactions, which includes electroactives dissolved in the solution other than MB, such as O2. The Epa, Epc and DE are 0.28, 0.31 and 0.03. A significant trailing effect can be observed in the reversible electron transfer reaction, which indicates the diffusion of electron has been retarded on the modified electrode. With increasing the ratio of AA in the mixed surfactant, the oxidation peak of MB could not be observed, and the potentials of the reduction peaks are 0.31, 0.32 and 0.34 V for C18N3(AA)1, C18N3(AA)2 and C18N3(AA)3, respectively. This result may suggest that the adsorbed layer on the electrode becomes more compact with the ratio of AA in the mixed surfactant system. The CV curves were also measured at different concentrations for each system, and the results are presented in the supporting information (Fig. S3). We found that the concentration of the
3.4. CV results Cyclic voltammetry is a commonly used method in evaluating self-assembling layers on gold surface as the modified layer acts as a barrier to block the access of the redox species to the gold surface. As a blocker, the self-assembling layer reduces the redox peak in a cyclic voltammograph. It depends on the thickness and com-
Fig. 6. Cyclic voltammograms (CVs) of 50 lm MB at the modified gold electrodes.
W. Wang et al. / Journal of Colloid and Interface Science 440 (2015) 16–22
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Fig. 7. An illustration of the structures of the mixed-surfactant systems on a gold substrate.
mixed surfactant has little effect on the peak potential for all four systems, which indicates that the structure of the adsorbed layer does not change accordingly with the concentration of the mixed surfactants. All the results of electrochemical behavior of MB indicate that the ratio of AA in the mixed surfactant system has significant effects on the compactness of the adsorbed layer on the gold electrode. 4. Discussions 4.1. Transition of the aggregation morphology The transition of the aggregation morphology corresponding to pH can be understood in terms of the packing parameter. In the mixed surfactant system, the packing parameter (p) is defined as
p¼
v al
¼
xa pa þ xc pc xa þ xc
where xa and xc are the molar ratio of the anionic surfactant and the cationic surfactant respectively, pa and pc are the packing parameter of anionic surfactant and the cationic surfactant [4]. The presumable shape of the surfactant associates in the mixture can be possibly predicted. First it is examined for the systems with respect to the variation of the composition. By using the values of p in the previous studies, we are able to calculate the p value for the mixed surfactant system [11,21]. At pH 7, the calculated values of the p for C18N3(AA)1, C18N3(AA)2, and C18N3(AA)3 are 0.79, 0.88, and 0.92, respectively, which all fall in the range of 1/2–1, and close to 1. This indicates that the morphology of the aggregates is presumable lamellar structure including vesicle. As it was shown in Figs. 2b, e and 3c, the actual structures are vesicles and lamellar structure. With the increase of p, the curvature of the aggregates becomes smaller, which may attribute to the rigid molecule of arachidic acid. Two factors may cause the transition from high-curvature aggregates to low-curvature aggregates with respect to the variation of pH. One is the protonation state of C18N3; the other is the match of the charges in the complex with regard to the protonation state. The two factors simply compete with each other. When C18N3 is fully protonated at pH 4, a larger area of the headgroup is expected with comparison to the area at pH 7 and 10, meanwhile three amine groups are available to interact with arachidic acid. As a consequence, the morphology of the aggregates changes from small vesicles to larger ones as shown in Fig. 2d–f. Please note that the composition of the samples shown in these figures is C18N3(AA)1, and all three amine groups are protonated; while only one of them interact with arachidic acid. At pH = 7, the tertiary amine is protonated and forms complexes with arachidic acid. The same trends were found for the change of the morphology of the other two combinations, although it was not as identical as it is for C18N3(AA)1.
4.2. The adsorbed amount of mixed surfactant on Au surface The other important information obtained in the study is that the adsorbed film on Au surface became more compact and homogenous when increasing the ratio of arachidic acid in the complexes. The morphology of the film was observed by SEM images. With the combination of the results from QCM and CV, the physicochemical properties of the film on Au surfaces can be reckoned. As shown in Fig. 5, the results from QCM could be used for deducting the construction of the adsorbed film of the complexes. The effective adsorbing area on the quartz was 0.196 cm2. For pure C18N3, the occupied area per molecule is 0.84 nm2 based on bilayer model. The bilayer structure on the gold substrate has been illustrated in Fig. 7 (please note the illustration of the film morphology is more as an assumption than a proved structure). The value is slightly larger than the area of the headgroup of the surfactant in an aqueous solution. However, considering the configuration of the surfactant on the dried solid surface the value is in an acceptable range. The calculated value of the area per molecule for the C18N3(AA)1 film on Au surface is 0.52 nm2. For C18N3(AA)2 and C18N3(AA)3 films, the calculations are based on double-bilayer and six-bilayer structure, respectively (see Fig. 7). The values of the area per molecule for these two films are 0.44 and 0.42 nm2, respectively. The area of per molecule for these two systems reveals that further increasing the ratio of arachidic acid in the complexes will have minor contribution to the compactness of the film. The results of the molecular area on Au surface show a great consistency with the CV results where the results also manifest the increasing compactness of the adsorbed film with the ratio of arachidic acid in the complexes. 5. Conclusion We studied the aggregation morphology of the catanionic system, C18N3(AA)n and the adsorption of the surfactant complexes on Au surface by SEM, QCM and CV. The aggregation morphology was studied for three combinations of the two surfactants and at three different pH values. We observe that the aggregation morphology changes from high-curvature aggregates to low-curvature aggregates by increasing pH or the amount of arachidic acid in the surfactant mixtures. The adsorption of the catanionic surfactant solution shows an increasing homogeneity and compactness with increasing the molar ratio of arachidic acid in the surfactant mixtures. Acknowledgments WW acknowledges the financial support by TianShan and Qianren projects at Xinjiang University. WL acknowledges the National Natural Science Foundation of China (Grant Nos. 20903106 and 21321063).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.10.063. References [1] M. Mulqueen, D. Blankschtein, Langmuir 16 (20) (2000) 7640–7654. [2] S. Bhattacharya, S.K. Samanta, J. Phys. Chem. Lett. 2 (8) (2011) 914–920. [3] T. Kunitake, Y. Okahata, M. Shimomura, S. Yasunami, K. Takarabe, J. Am. Chem. Soc. 103 (18) (1981) 5401–5413. [4] S. Segota, Du-i Tezak, Adv. Colloid Interface Sci. 121 (1–3) (2006) 51–75. [5] S. Bhattacharya, J. Haldar, Langmuir 20 (19) (2004) 7940–7947. [6] W. Jia, J. Li, L. Jiang, Synthesis of highly branched gold nanodendrites with a narrow size distribution and tunable NIR and SERS using a multiamine surfactant, ACS Appl. Mater. Interfaces, 2013. [7] W. Jia, J. Li, G. Lin, L. Jiang, Cryst. Growth Des. 11 (9) (2011) 3822–3827.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Moderate the adsorption of cationic surfactant on gold surface by mixing with sparingly soluble anionic surfactant Wei Wang a,b,⇑, Wensheng Lu c,⇑, Haifei Wang c, Hongtao Xie a, Jide Wang a a
College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China Centre for Pharmacy and Department of Chemistry, University of Bergen, Bergen 5007, Norway c Beijing National Laboratory for Molecular Science, Key Laboratory of Colloid and Interface, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b
a r t i c l e
i n f o
Article history: Received 29 July 2014 Accepted 28 October 2014 Available online 7 November 2014 Keywords: C18N3 Arachidic acid Cationic Adsorption Gold surface Morphology Aggregation
a b s t r a c t Surfactants with amine groups are often used in the nanoparticle synthesis due to the high affinity with Au atoms. The match of charges of a capping reagent with Au has significant influence on structures in nanoparticle synthesis. Thus we studied the adsorption of a catanionic surfactant system on Au surface. The surfactants used in the study are bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3) and arachidic acid. Three combinations of the surfactants were studied with regard to the protonation state of the amine groups and the match of charges of the surfactant headgroup. The morphology of the surfactant mixtures changes from high-curvature aggregates to low-curvature with increasing the molar ratio of arachidic acid in the mixtures or the pH of the surfactant solutions. The adsorption of the mixed surfactant systems was studied by means of scanning electron microscopy (SEM), quartz crystal microbalance (QCM) and cyclic voltammetry (CV). The results revealed that the homogeneity and the compactness of the adsorbed layer on a gold surface were increased with the molar ratio of arachidic acid in the complexes. Furthermore, we may obtain the construction of the film of the mixed surfactant on gold surface using the result obtained by QCM. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Surfactant adsorption at an air–liquid or liquid–solid interface, along with the resulting lowering of the surface tension, plays a central role in controlling the desired behavior in many practical applications involving surfactants. In many practical applications, better properties can be attained by using mixed surfactant systems comparing to using an individual surfactant [1]. Mixtures of cationic and anionic surfactant represent interesting models for understanding structure formation in two-dimensional (2D) and three-dimensional (3D) structures. One of their interesting features is the competitive interaction between the cationic part and the anionic part leading to a richness of phases and special aggregation structures. This calls for systematic variations of these interactions and one of the conventional methods is to vary the charge of the hydrophilic headgroups. For the mixed systems of fatty acid and fatty amine as the most frequently studied system, this is possible via variation of the hydrophilic part.
⇑ Corresponding authors at: Postbox 7803, Bergen N-5007, Norway (W. Wang). E-mail addresses: [email protected] (W. Wang), [email protected] (W. Lu). http://dx.doi.org/10.1016/j.jcis.2014.10.063 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
Multihead surfactants have drawn the attention of researchers due to its unique properties and applications in nanotechnology [2]. With regard to the increase of steric repulsion and the decrease of hydrophobicity, multihead surfactants exhibit a higher critical micellar concentration (CMC) comparing to conventional surfactants with the alkyl chain of an equal length. The aggregates formed by multihead surfactants in an aqueous solution are generally micelles with a low aggregation number, although it is possible to form other types of aggregates by balancing the hydrophilic part with a longer alkyl chain [3]. This may be understood by the molecular packing parameter (p). The molecular packing parameter is defined as p = v/a⁄l where a is the area of the surfactant headgroup, l equals to the length of the alkyl chain at a stretched configuration, and v is the volume of the surfactant molecule [4]. Due to the large area of the headgroup for a multihead surfactant, the packing parameter is commonly smaller than 1/3, which indicates a large spontaneous curvature of the aggregates [5]. In the previous studies from our group, we have synthesized a multihead surfactant with a dendritic headgroup, bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (abbreviated as C18N3), which exhibited many interesting features as a capping reagent in nanocrystal synthesis and nanoparticle self-assembling [6–16]. C18N3
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is a surfactant with a eighteen-carbon chain and a dendritic headgroup analogy to the dendritic unit of poly(amidoamine) (PAMAM) [17]. The dendritic headgroup is composed of two primary amines and a tertiary one. The three-amine groups are protonated at low pH, and maintain uncharged at high pH. Thus the area of the headgroup changes corresponding to the pH of the solution. It also resulted different aggregation morphology at different pHs, such as, micelles at pH 2 and lamellar structures at pH 10.5. Due to the multiple-amine groups, the surfactant shows a strong interaction with metal elements, such as aurum, silver and copper [8,9,12]. For example, the surfactant interacted with silver ions and the aggregates of the surfactant transferred from micelles to vesicles at low pH [12]. The vesicles were used as a template to assemble AgCl nanoparticles on the vesicle surface and synthesize hollow nanostructures. The diameter of the hollow nanostructure increases with a controlled growth rate due to the competitive interaction with silver ions between the surfactant and the chloride ions [12]. Many interesting self-assembling structures have been achieved by applying the surfactant as a capping reagent in synthesizing gold nanoparticles. Recently C18N3 was used to self-assemble highaspect-ratio gold nanorods and resulted in a highly ordered 2D structure on a micrometer scale, which is by far the largest 2D self-assembling structure of nanoparticles by using bottom-up methodology [13]. These results suggest that the increase of hydrophobic interaction by elongating the surfactant chain may benefit the self-assembling; while the liquid property of the alkyl chain has to be retained. Therefore in this study, an anionic surfactant, arachidic acid (AA) was introduced to the C18N3 system. Arachidic acid is an amphiphilic molecule, however sparingly soluble in water due to a long alkyl chain. Mixing with arachidic acid will maintain the strong hydrophobic interaction, and in the meantime moderate the charge density of the aggregates. The adsorption of the aggregates on gold surface was studied by means of scanning electron microscope (SEM), quartz crystal microbalance (QCM), and electrochemistry. Since C18N3 mostly has three protonated sites, three molar combinations of arachidic acid and C18N3 were thus studied concerning the match of the charges in the headgroups of the surfactants.
H 2N
O N H
O N
N H
NH 2
Fig. 1. The chemical structure of bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3).
2.3. Transmitted electron microscope (TEM) C18N3 and arachidic acid were dissolved in chloroform with the concentration of 1 mmol/L. The solution of C18N3 and arachidic acid at different molar ratio was mixed in a flask and the solvent was evaporated using rotary evaporator. The film of the mixtures was dried in a nitrogen atmosphere, and then rehydrated with pure water. In each sample, the final concentration of the total mixtures was 1 mmol/L. The pH of solution was then adjusted by 1 mol/L HCl or NaOH solution, and measured by a HI 8314 pH-meter (HANNA instrument) with a HI 1200B glass electrode. The solutions were placed in a thermal room of 25 °C to reach equilibrium. One drop of the solution was placed on a carbon film supported by copper grids and maintained for 5 min in a sealed environment. Then the residuals were removed using a filter paper. TEM analysis was carried out with a negative-staining method on JEOL (JEM2011).
2. Materials and methods
2.4. Scanning electron microscope (SEM)
2.1. Materials
A silica substrate covered with a layer of gold film was immersed in a pre-mixed surfactant solution. After the adsorption of the sample on the substrate for half an hour, the substrate was removed from the solution, and rinsed with pure water. The substrate was then dried in a nitrogen environment. SEM images were taking on a HITACHI S4300.
Arachidic acid (99.5%), hydrochloride acid (37%), and sodium hydroxide (99.5%) were purchased from Aldrich–Sigma, and they were used without further purification. Chloroauric acid, methylene blue (MB) and trisodium citrate were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Milli-Q water with a resistivity of 18 M ohm cm was used in all experiments. Bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3) was synthesized according to the method described in a previously published paper, and the structure is shown in Fig. 1 [17].
2.2. Au nanoparticles preparation 13 nm Au nanoparticles were prepared by reduction of chloroauric acid in an aqueous trisodium citrate solution. 2% chloroauric acid (0.3 ml) and 2% trisodium citrate (0.9 mL) were mixed with 60 mL water. The solution was then heated to the boiling temperature under rigorous agitation for 30 min. After cooling to room temperature, the Au nanoparticles solution was stored at 4 °C. The TEM image of the nanoparticles is shown in the supporting information (Fig. S2).
2.5. Quartz crystal microbalance (QCM) The gravimetric measurements were performed on a QCM. An AT-cut quartz crystal with a fundamental frequency of 9 MHz coated with gold layers on both sides, was purchased from Seiko EG&G (Tokyo, Japan). The effective adsorbing area on the quartz was 0.196 cm2. The gold surface of the quartz resonator was precleaned by detergents. Then the piranha solution (H2SO4:30%H2O2 = 3:1) was dropped on the gold surface and the surface was headed up to 50 °C for 2 min. The surface of the quartz was then rinsed by pure water and dried with a stream of nitrogen. However the glue between the gold layer and the quartz could not stand in an acidic solution, so the experiments were only performed for the samples without the addition of HCl and NaOH solution. The resonators were immersed in the solution of the mixed surfactants for 30 min. Then the quartz was removed from the
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solution and rinsed by pure water. The measurements were performed in a dried state; therefore the precondition of Sauerbrey equation was satisfied. The change of frequency was transferred into the adsorbed mass based on the equivalence of 1 Hz frequency corresponding to 1.07 ng mass with regard to our experimental setup. Hence the mass of the adsorbed layer was directly obtained. The resonator with the dried surfactant layer was again immured into the gold nanoparticle solution for 30 min. Then the quartz was removed from the nanoparticle solution and dried in a nitrogen stream. The mass of the adsorbed gold nanoparticles was again measured. Based on the adsorbed amount of the gold nanoparticles, we can obtain the charge density of the outer layer. 2.6. Cyclic voltammetry (CV) A polycrystalline gold electrode (1 cm2) was polished with alumina powder of 0.3 lm and 0.05 lm subsequently. The polished gold electrode was rinsed in water and ultrasonicated to remove the residuals of the alumina powder. The electrode was pretested by potential cycling in H2SO4 solution, and then it was rinsed thoroughly in water. The freshly prepared gold electrode was immersed in the mixed surfactant solution with the concentration of 1 mmol/L for 30 min. The modified electrode was removed from the solution and rinsed by water and then dried by a nitrogen stream. Electrochemical experiments were carried out with a CHI625B Electrochemical Analyzer (CH instrument Co. Shanghai). A singlecompartment cell with a three-electrode system was used for the cyclic voltammetry measurements. An Ag/AgCl electrode was used as a reference electrode, and a platinum wire as a counter electrode. The cyclic voltammograms were collected in a 50 lM MB solution between 0.4 and 0.15 V with the scan rate of 10 mV/s. 3. Results 3.1. TEM results The aggregation morphology of the mixture of C18N3 and arachidic acid (AA) was captured by TEM. C18N3(AA)n (n = 1, 2 and 3) will be used to denote the molar ratio of C18N3 and AA in the samples. The images of C18N3(AA)1 and C18N3(AA)2 are shown in Fig. 2 at pH = 4, 7 and 10. Arachidic acid is insoluble in
water at room temperature, and it usually exists at a crystalline state. However, C18N3 has shown strong ability in dissolving other substance in an acidic solution, but not in a base solution. In the present experiment, the same scenario has been observed. On the first row of Fig. 2, we observe the change of the aggregate corresponding to the pH value of the solution for C18N3(AA)2. In the acidic solution, small aggregates are observed in Fig. 2a. The aggregates are irregular, which may be micelles with small vesicles coexisting in the solution. At neutral pH, ill-shaped vesicles are observed, with sharp edges and corners. The vesicles are polydispersed and the size of the vesicles is about 2 lm. The large vesicles in Fig. 2b have hexagonal shape with six corners in each vesicle. The non-spherical vesicles have observed in other systems, which usually indicate a segregation of one component, and will then consequently result a one-component rich domain in the corner. In this case, it is possible that there are two types of complexes: one is C18N3(AA)1 and the other C18N3(AA)2 since at the neutral condition, the primary amine is not totally protonated. Some of the non-protonated amine groups did not form complexes with the acid group of AA. At pH 10, the lamellar structure is observed in Fig. 2c. The aggregate shown in Fig. 2c has a multilayer structure and the size is about 3 lm. The second row in Fig. 2 is the TEM images for C18N3(AA)1 samples at different pHs. At pH 4, small vesicles are observed in Fig. 2d. The vesicles are mono-dispersed with the size of 100 nm in diameter. The shape of the vesicles is nearly spherical. In Fig. 2e and f, spherical vesicles are observed with a larger size comparing with them at pH 4. The results shown in Fig. 2e and f indicate the morphology of the aggregates C18N3(AA)1 is not sensitive to pH when it is above 7. The mixture of C18N3(AA)3 exhibits a phase separation in the aqueous solution. There was precipitation coexisting with a transparent solution (see Fig. S1) during the rehydration of the film of the mixed surfactant. This may indicate the amount of AA in the mixture is over the limit of the solubility of C18N3, even at pH 4. The charged group of C18N3 may completely associate with the headgroup of AA, which lead to a phase separation. However, there were still mixed surfactants in the solution, and the morphology of the aggregates was captured by means of TEM. The TEM images for C18N3(AA)3 at different pHs are shown in Fig. 3. The morphology of the aggregates has been dramatically changed as shown in Fig. 3. At pH 4, the mixed surfactants tend
Fig. 2. TEM images of the mixed surfactant solutions at different pH conditions: (a), (b) and (c) are C18N3(AA)2 at pH 4, 7, and 10 respectively; (d), (e), and (f) are C18N3(AA)1 at pH 4, 7 and 10 respectively.
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Fig. 3. TEM images of C18N3(AA)3 at pH (a) 4; (b) 4; (c)7 and (d)10.
to form big clusters, and the size of the clusters is on the scale of micrometer, as shown in Fig. 3a. In Fig. 3b, the aggregates are shown in a smaller scale where the detailed structure is observable. The big clusters are composed of a disordered aggregation of small worm-like micelles. Fig. 3c is the TEM image of the complex aggregates at pH 7, which are flat lamellar structure. At pH 10, the aggregates transformed to strings. The length of the strings is on the scale of 10 lm. 3.2. SEM results The adsorption of the aggregates on a gold surface was first examined by means of SEM on a large scale. A strong interaction between the headgroup of C18N3 and Au has been generally observed in the previous studies; however the interaction between carboxylic acid and Au has not been reported. From the SEM images, we are able to observe the homogeneity of the film formed of adsorption. In Fig. 4, the images were captured under the same scale where it is easy to compare the adsorbed film from different samples. Fig. 4a shows a film formed by pure C18N3 where the dark part in the image is the holes on the film. On a large scale the film is rather homogeneous however with large amount of holes, and the substrate may expose to the outer medium through the imperfection of the film. The mixed surfactants show a better coverage of the Au substrate. Fig. 4b shows the SEM image of the C18N3(AA)1 on the Au surface. It shows much less holes on the film comparing to the film formed by pure C18N3. With further increasing ratio of AA in the composition of the complex, the fine film shows fewer holes as shown in Fig. 4c and d.
3.3. QCM results QCM was used to measure the mass adsorbed on the gold surface. The results are shown in Fig. 5. For the mixed surfactant aggregates, it shows a general increase of adsorbed amount with the composition of AA in the mixtures. The film on the gold surface is supposed to be a double layer since the adsorption was performed in the aqueous solution. The hydrophilic part of the surfactant should be exposed to the solution where the hydrophilic headgroups are also the part interacting with the gold surface, and as shown above the aggregates have generally a bilayer structure. Therefore, it is most possible to form a double layer on the gold surface; however the double layer may not be symmetrical due to the different chemical property of the substrate and the solution. For pure C18N3, 38 ng of adsorbed amount is obtained, and the value is 50 ng for C18N3(AA)1. This small increase may be attributed to reducing the charge density on the headgroup. For C18N3(AA)2, the adsorbed amount is about 110 ng, which is slightly double of what is obtained for C18N3(AA)1. This may suggest that another bilayer structure built on top of the first layer. This second layer was stable since rinsing with water could not remove it. For C18N3(AA)3, the adsorbed amount is 372 ng, which indicates a six-bilayer construction. The neutralization of the charges on the headgroups is the main reason for the multilayer construction, which corresponds to the coacervation in the bulk phase. The repulsive force becomes weak due to the reduced charge density, which may also lead to the interaction between C18N3 and AA in different layers.
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Fig. 4. SEM images of the film formed by the adsorption of the mixed surfactant solution: (a) C18N3; (b) C18N3(AA)1; (c) C18N3(AA)2; (d) C18N3(AA)3.
Fig. 5. The adsorbed amounts of the mixed surfactants and negatively charged Au nanoparticles were measured by means of QCM.
The quartz with pre-adsorbed surfactant layer was immersed in a solution containing Au nanoparticles in order to study the features of the outer layer. The size of the Au nanoparticles is about 13 nm. The results are shown in Fig. 5. A clear decrease of adsorption of nanoparticles is observed with increasing the molar ratio of AA in the composition. The trend is presumable regarding the decrease of the available amine groups on the outer layer. For C18N3, the adsorbed amount of the nanoparticles is about 160 ng, and it decreases to 78, 55, and 46 ng for C18N3(AA)1, C18N3(AA)2 and C18N3(AA)3, respectively.
pactness of the layer and the electroactive species in the solution [18]. When the electrode is electrochemically cleaned, the oxidation potential (Epa) and reduction potential (Epc) are 0.22 and 0.27 V vs a saturated calomel electrode (SCE), respectively [19,20]. The results for C18N3(AA)n-modified gold electrodes are shown in Fig. 6. For C18N3 modified electrode, the baseline of the scanned curve is rather flat due to the blocking effect of all redox reactions, which includes electroactives dissolved in the solution other than MB, such as O2. The Epa, Epc and DE are 0.28, 0.31 and 0.03. A significant trailing effect can be observed in the reversible electron transfer reaction, which indicates the diffusion of electron has been retarded on the modified electrode. With increasing the ratio of AA in the mixed surfactant, the oxidation peak of MB could not be observed, and the potentials of the reduction peaks are 0.31, 0.32 and 0.34 V for C18N3(AA)1, C18N3(AA)2 and C18N3(AA)3, respectively. This result may suggest that the adsorbed layer on the electrode becomes more compact with the ratio of AA in the mixed surfactant system. The CV curves were also measured at different concentrations for each system, and the results are presented in the supporting information (Fig. S3). We found that the concentration of the
3.4. CV results Cyclic voltammetry is a commonly used method in evaluating self-assembling layers on gold surface as the modified layer acts as a barrier to block the access of the redox species to the gold surface. As a blocker, the self-assembling layer reduces the redox peak in a cyclic voltammograph. It depends on the thickness and com-
Fig. 6. Cyclic voltammograms (CVs) of 50 lm MB at the modified gold electrodes.
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Fig. 7. An illustration of the structures of the mixed-surfactant systems on a gold substrate.
mixed surfactant has little effect on the peak potential for all four systems, which indicates that the structure of the adsorbed layer does not change accordingly with the concentration of the mixed surfactants. All the results of electrochemical behavior of MB indicate that the ratio of AA in the mixed surfactant system has significant effects on the compactness of the adsorbed layer on the gold electrode. 4. Discussions 4.1. Transition of the aggregation morphology The transition of the aggregation morphology corresponding to pH can be understood in terms of the packing parameter. In the mixed surfactant system, the packing parameter (p) is defined as
p¼
v al
¼
xa pa þ xc pc xa þ xc
where xa and xc are the molar ratio of the anionic surfactant and the cationic surfactant respectively, pa and pc are the packing parameter of anionic surfactant and the cationic surfactant [4]. The presumable shape of the surfactant associates in the mixture can be possibly predicted. First it is examined for the systems with respect to the variation of the composition. By using the values of p in the previous studies, we are able to calculate the p value for the mixed surfactant system [11,21]. At pH 7, the calculated values of the p for C18N3(AA)1, C18N3(AA)2, and C18N3(AA)3 are 0.79, 0.88, and 0.92, respectively, which all fall in the range of 1/2–1, and close to 1. This indicates that the morphology of the aggregates is presumable lamellar structure including vesicle. As it was shown in Figs. 2b, e and 3c, the actual structures are vesicles and lamellar structure. With the increase of p, the curvature of the aggregates becomes smaller, which may attribute to the rigid molecule of arachidic acid. Two factors may cause the transition from high-curvature aggregates to low-curvature aggregates with respect to the variation of pH. One is the protonation state of C18N3; the other is the match of the charges in the complex with regard to the protonation state. The two factors simply compete with each other. When C18N3 is fully protonated at pH 4, a larger area of the headgroup is expected with comparison to the area at pH 7 and 10, meanwhile three amine groups are available to interact with arachidic acid. As a consequence, the morphology of the aggregates changes from small vesicles to larger ones as shown in Fig. 2d–f. Please note that the composition of the samples shown in these figures is C18N3(AA)1, and all three amine groups are protonated; while only one of them interact with arachidic acid. At pH = 7, the tertiary amine is protonated and forms complexes with arachidic acid. The same trends were found for the change of the morphology of the other two combinations, although it was not as identical as it is for C18N3(AA)1.
4.2. The adsorbed amount of mixed surfactant on Au surface The other important information obtained in the study is that the adsorbed film on Au surface became more compact and homogenous when increasing the ratio of arachidic acid in the complexes. The morphology of the film was observed by SEM images. With the combination of the results from QCM and CV, the physicochemical properties of the film on Au surfaces can be reckoned. As shown in Fig. 5, the results from QCM could be used for deducting the construction of the adsorbed film of the complexes. The effective adsorbing area on the quartz was 0.196 cm2. For pure C18N3, the occupied area per molecule is 0.84 nm2 based on bilayer model. The bilayer structure on the gold substrate has been illustrated in Fig. 7 (please note the illustration of the film morphology is more as an assumption than a proved structure). The value is slightly larger than the area of the headgroup of the surfactant in an aqueous solution. However, considering the configuration of the surfactant on the dried solid surface the value is in an acceptable range. The calculated value of the area per molecule for the C18N3(AA)1 film on Au surface is 0.52 nm2. For C18N3(AA)2 and C18N3(AA)3 films, the calculations are based on double-bilayer and six-bilayer structure, respectively (see Fig. 7). The values of the area per molecule for these two films are 0.44 and 0.42 nm2, respectively. The area of per molecule for these two systems reveals that further increasing the ratio of arachidic acid in the complexes will have minor contribution to the compactness of the film. The results of the molecular area on Au surface show a great consistency with the CV results where the results also manifest the increasing compactness of the adsorbed film with the ratio of arachidic acid in the complexes. 5. Conclusion We studied the aggregation morphology of the catanionic system, C18N3(AA)n and the adsorption of the surfactant complexes on Au surface by SEM, QCM and CV. The aggregation morphology was studied for three combinations of the two surfactants and at three different pH values. We observe that the aggregation morphology changes from high-curvature aggregates to low-curvature aggregates by increasing pH or the amount of arachidic acid in the surfactant mixtures. The adsorption of the catanionic surfactant solution shows an increasing homogeneity and compactness with increasing the molar ratio of arachidic acid in the surfactant mixtures. Acknowledgments WW acknowledges the financial support by TianShan and Qianren projects at Xinjiang University. WL acknowledges the National Natural Science Foundation of China (Grant Nos. 20903106 and 21321063).
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