Journal of Colloid and Interface Science 424 (2014) 56–61
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Nanoscale adhesive forces between silica surfaces in aqueous solutions Paula Troncoso a, Jorge H. Saavedra a, Sergio M. Acuña b, Ricardo Jeldres a, Fernando Concha c, Pedro G. Toledo a,⇑ a
Chemical Engineering Department and Surface Analysis Laboratory (ASIF), University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile Department of Food Engineering, University of Bio-Bio, PO Box 447, Chillán, Chile c Metallurgical Engineering Department, University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile b
a r t i c l e
i n f o
Article history: Received 2 May 2013 Accepted 6 March 2014 Available online 15 March 2014 Keywords: Nanoscale forces Adhesion Silica AFM Cavitation Contact surfaces Dehydroxylation
a b s t r a c t Nanoscale adhesive forces between a colloidal silica probe and a flat silica substrate were measured with an atomic force microscope (AFM) in a range of aqueous NaCl, CaCl2, and AlCl3 solutions, with concentrations ranging from 106 to 102 M at pH 5.1. Notably, the measured force curves reveal large pull-off forces in water which increase in electrolyte solutions, with jump-off-contact occurring as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between mineral surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities or nanobubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured force data. A simple model based on several cavities bridging the two surfaces we show that is able to capture all the features in the measured force curves. The silica surfaces used are clean but not intentionally hydroxylated, as contact angle measurements show, and as such may be responsible for the cavities. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction It is well known that silica suspensions exhibit remarkable stability at their isoelectric point [1] against coagulation and sedimentation [2,3] and low viscosity [1,4]. The short-range repulsion not predicted by DLVO theory [5,6] but usually observed in direct force measurements between silica surfaces arises from a surface-induced water effect, from the creation of a hydrogenbonding network at the surface level [4,7–24]. This short-range repulsion as well as the small attractive van der Waals forces for silica in water and in aqueous electrolyte solutions are likely responsible for the unusual stability of silica suspensions particularly at their isoelectric point. In the past this issue has been of interest to a wide range of processes involving transport and processing of silica slurries and pulps. Interest today is even greater considering that industry is moving fast toward higher solids loadings, and thus an appropriate control of rheological behavior and physicochemical stability of the suspensions is crucial to obtain fluidity and stability as desired. A great variety of
⇑ Corresponding author. Fax: +56 41 2204691. E-mail address: [email protected] (P.G. Toledo). http://dx.doi.org/10.1016/j.jcis.2014.03.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
experimental techniques have been used to characterize silica– silica interactions, however for assessing interaction forces between silica surfaces mediated by aqueous solutions the surface force apparatus (SFA) and the atomic force microscope (AFM) are the favorites. Here we are interested in the forces that arise in the separation of two silica surfaces after reaching direct contact. Given the repulsive character of the interaction in the approaching of the surfaces, one might expect a separation virtually free of hysteresis. This is true for the interaction in water [20,22,25,26] but not in aqueous electrolyte solutions [18,27–31]. Although the silica–silica system has been reinvestigated several times, interest has focused on approach force curves rather than on separation force curves, therefore adhesive forces have not always been observed [14,19,25–27,32–34]. It has been reported that adhesion between silica surfaces in water occurs only when the surfaces remain in contact for a long time [19] although according to Yaminsky et al. [25,26], after several days in water the strength of the adhesion decreases. Similar behavior is reported by Chapel [18]; silica adhesion in 0.1 M NaCl at pH 5.5 disappears after few minutes. Less common in the literature is to find trends followed by silica–silica adhesion with electrolyte concentration in the measurement of forces. Yaminsky et al. [26] and Meagher [28]
57
P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
2. Experimental Glass microslides (B&C) typically 1 1 cm2 were used. The flat silica substrates were glued to AFM stubs before use. Glass colloidal probes were prepared [39] by gluing a 20 lm in diameter sphere (Duke Scientific Corporation) to the end of a tipless Vshaped, 100 lm long, 0.6 lm thick, Si3N4 cantilever (Veeco) with Norland Optical Adhesive 61 (Norland Products, USA). Spring constants of individual cantilevers were determined by the method of standards (standards provided by Park Scientific, USA) with the Dimension 3100 AFM microscope and were typically 0.14 N/m. SEM and AFM images verified the quality of the modified cantilevers. Interaction forces were measured in bi-distilled water, pH was 5.1, and also in electrolyte solutions. NaCl, CaCl2, and AlCl3 (analytical chemical grade, Merck, Germany) were used in concentrations ranging from 104 to 102 M. Experiments were carried out without any buffering, pH was 5.1. All glassware used in the preparation of solutions was detergent and alkali washed with final thorough rinsing in bi-distilled water. Prior to force measurement the mineral surfaces, substrate and sphere, were thoroughly rinsed in high purity water (18.6 MO/cm), then with ethanol, and then again with bidistilled water. No efforts were made to improve hydroxylation of the glass surface groups into silanol groups, thus perfect wetting should not be expected. Contact angles were measured through the liquid phase for the various electrolyte solutions used here with the Ramé-Hart contact angle goniometer. Surface roughness, assessed by AFM imaging with the Dimension 3100 and measured as the root-mean-square roughness, was small for both substrate, 2 nm, and microsphere, 1 nm. Force measurements between AFM probes and substrates as function of separation were conducted using an SPM-3 (Thermo, USA) multimode atomic force microscope equipped with a Nanoscope IIIa SPM control station, fluid cell (0.1 cm3), silicone pad for
vibration isolation, and acoustic enclosure. AFM allows continuous measurement of cantilever deflection vs. position as probe and substrate approach, commonly named extension, or separate, commonly named retraction. Approaching force curves can be found in Acuña and Toledo [23], here we report on retraction force curves. Measurement of a typical force curve took less than 20 min; during this time the AFM roughness of the substrate remained unaltered. The force measurement protocol is well established [14,16] and the sample manipulation procedure is available [23]. Extension and retraction driving speeds were low, 280 nm/s, in order to minimize hydrodynamic contribution to the measured force. Force curves were first verified to be independent of position on the substrate; measurements were always highly reproducible. Forces are reported normalized by the microsphere probe radius, that is, as interaction energy between silica flat surfaces by virtue of Derjaguin´s approximation, F(D) = 2pRE(D), where F is force, D distance, R probe radius, and E energy per unit area. 3. Results and discussion Here we are interested in the forces that arise in the separation of two perfectly clean silica surfaces but not hydroxylated after reaching direct contact. Fig. 1 shows glass contact angles measured through the liquid phase for the various electrolyte solutions of interest here. Contact angle in pure water is 27° and in electrolyte solutions increases with concentration, very rapidly in the range R2. Force curves in Figs. 2 and 3, and data in Table 1, show this behavior even though the effect of electrolyte concentration and valence is small, although distinguishable, over surface tension and contact angle. Formation of cavities or persistent air bubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured
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P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
Table 1 Adhesion force between silica surfaces measured in electrolyte solutions at 20 °C and pH 5.1. Electrolytes are NaCl, CaCl2, and AlCl3. Concentrations range from 104 to 102 M. F = kDD, where k = 0.14 N/m is the elastic constant of the cantilever and DD is the surface-to-surface distance from the jump-off-contact distance to the equilibrium position of the cantilever (the zero line), which we read directly from the curves. The work of adhesion for identical solid surfaces is Wadhesion = 2c, the surface energy of the solid c is obtained from the DMT model through F/Rsphere = 4pc, here Rsphere = 10 lm. For example, for a suspension of silica particles with 0.1 lm in radius, according to DMT model the contact radius between the particles under zero external load is a0 = (4pR2c/K)1/3, where K is reduced elastic modulus of contacting particles. For silica K = 4.94 1010 N/m2. Total adhesive energy W accumulated in the contact area between the particles is W ¼ W adhesion pa20 which is in units of kBT.
a
c (mN/m)
Wadhesion (mJ/m2)
W (kBT)
4.05 ± 0.15
0.32
0.64
0.44
370.8 ± 78.7 568.3 ± 67.6 649.2 ± 72.0
5.19 ± 0.55 7.96 ± 0.48 9.09 ± 0.51
0.41 0.63 0.72
0.83 1.27 1.45
0.66 1.35 1.69
1 104 1 103 1 102
485.0 ± 96.9 536.7 ± 89.1 666.7 ± 20.4
6.79 ± 0.68 7.51 ± 0.63 9.33 ± 0.15
0.54 0.60 0.74
1.08 1.20 1.49
1.04 1.23 1.76
1 104 1 103 1 102
732.5 ± 18.9 741.7 ± 132.9 764.2 ± 96.3
10.26 ± 0.13 10.38 ± 0.93 10.70 ± 0.68
0.82 0.83 0.85
1.63 1.65 1.70
2.06 2.11 2.21
Medium
Concentration (M)
DD (nm)
Water
1 106a
289.2 ± 21.5
NaCl
1 104 1 103 1 102
CaCl2
AlCl3
F/Rsphere (mN/m)
Water used is bi-distilled with conductivity of 18.6 MO/cm and concentration estimated in 106 M.
ϕ R D
θ1 θ2
R2 R1
Fig. 4. Axisymmetric cavity/bubble between a sphere and a plane in a liquid. R is the sphere radius, u the semi-angle subtended at the centre of the sphere by the cavity, h1 and h2 are the solid–liquid contact angles at the two triple lines, h1 = h2 = h when sphere and plane are made of the same material, R1 is the radius of curvature descriptive of the rotation about the vertical axis in the plane of the paper, R2 is the radius of curvature of the vapor–liquid surface in the plane perpendicular to the paper, D is the length of the cavity.
force data. In a related work we have demonstrated by means of a simple macroscopic force balance (Eq. (1)) and molecular dynamics simulation of a Lennard-Jones liquid that capillary bridges between surfaces of different nature are responsible for attractive forces not just between solvophilic surfaces but also for moderately solvophophic surfaces; and thus for cavities or bubbles surrounded by the same liquid the force is attractive even when the substrates are moderately solvophilic [49]. These results although for a LJ fluid support the idea that for moderately hydrophilic surfaces it is also possible to have attractive forces due to cavities or persistent bubbles bridging the silica surfaces. We propose a simple model to explain our force data based on several cavities and or nanobubbles bridging the two surfaces. Such cavities have a distribution of sizes and stability upon separation. The least stable cavity fails first at small separation leading to a small step in aqueous solutions with low electrolyte concentration and valence and to a jump in the presence of high electrolyte concentration and valence; the second less stable cavity fails next and so on. Fig. 5 shows a schematic representation of several vapor cavities trapped between two solid surfaces, some of the cavities are larger and more stable than others. Three stages of the separation are illustrated. Frame (a) illustrates the array when contact angle is intermediate, proper of interaction in aqueous solutions with high electrolyte concentration and valence, so the initial shape of the cavities is prolate spheroidal but changes to almost a circular cylinder as separation of the surfaces increases. Pressure inside the cavities is low and decreases as separation of the surfaces increases. With increasing separation, R1 increases very rapidly and R2 decreases slowly, the result according to Eq. (1) is an increasing net adhesive force between the surfaces. The third cavity from the left in Frame (a) is the weakest. The third stage shows the point at which this cavity fails (the cavity right before
(a) F R
D
iii
(b)
i
ii
Fig. 5. Schematic representation of several cavities/bubbles trapped between two solid surfaces, some of the cavities are larger and more stable than others. Three stages of the separation are illustrated. (a) Contact angle is intermediate and remains the same in the three stages; the shape of the cavities is prolate spheroidal but changes to almost a circular cylinder as separation of the surfaces increases. Cavities at the verge of failing are drawn with segmented lines. (b) Schematics of the corresponding forces in (a).
the failure is drawn with segmented lines). After the failure, small bubbles remain attached to the surfaces obeying the contact angle at the triple line. Frame (b) depicts a typical force curve for the system in (a). At point (i), the net adhesive force is given by Eq. (1) for R1 larger than R2. At point (ii) the net adhesive force increases according to Eq. (1) because R1 increases more rapidly than R2 decreases, the third cavity in Frame (a) is about to fail. Contribution of this cavity to the net adhesive force is so significant that its failure drives a force jump to a much smaller adhesive force at point (iii). This event is repeated as many times as there are steps
P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
in the force curve. The enhanced wetting of the cavities in this case increases their attachment to the surfaces and so does the pull-off force at the failure. This very simple model captures all the features in the force curves in Figs. 2 and 3. 4. Conclusion Measured long range adhesive forces clearly increase with electrolyte concentration and valence. Contact forces may be too short range to be invoked to explain them. Likewise the attractive force of van der Waals. In our experiments the time of contact of the surfaces is sufficiently short to discard any sintering mechanism by the action of water. Notably, our force curves reveal large pull-off forces in water and in electrolyte solutions, the jump-off-contact occurs as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between silica surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities and or nanobubbles cannot be observed directly in our experiments; however, we cannot discard them as responsible for the discontinuities in the measured force data. We offer a simple model based on several cavities and/or nanobubbles bridging the two surfaces that is able to capture all the features in the measured force curves. In the model the least stable cavity fails first at small separation leading to a small step in aqueous solutions with low electrolyte concentration and valence and to a jump in the presence of high electrolyte concentration and valence, the second less stable cavity fails next and so on. Dehydroxylation of our silica surfaces may be responsible for the cavities/bubbles; however, cavities/bubbles need to be considered in the analysis of measured force curves between silica surfaces because they might never be totally absent. Acknowledgments We thank Project Conicyt/Fondecyt 1101023, Centro CRHIAMProject Conicyt/Fondap-15130015 and Red Doctoral REDOC.CTA, MINEDUC Grant #UCO1202 for financial support. J.H.S. and R.J. thank CONICYT-Chile for graduate student fellowships. References [1] R.K. Iler, The Chemistry of Silica, Wiley Interscience, NY, 1979.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
61
L.H. Allen, E. Matijevíc, J. Colloid Interface Sci. 31 (1969) 287. J. Depasse, A. Watillon, J. Colloid Interface Sci. 33 (1970) 430. M. Colic, M.L. Fisher, G.V. Franks, Langmuir 14 (1998) 6107. B.V. Derjaguin, L. Landau, Acta Physicochim. URSS 14 (1941). E.J.W. Verwey, J.Th.G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. D.M. LeNeveu, R.P. Rand, V.A. Parsegian, Nature 259 (1976) 601. D.M. LeNeveu, R.P. Rand, V.A. Parsegian, D. Gingell, Biophys. J. 18 (1977) 209. J.N. Israelachvili, G.E. Adams, J. Chem. Soc. Faraday Trans. I 74 (1978) 975. R.M. Pashley, J. Colloid Interface Sci. 80 (1981) 153. Ya.I. Rabinovich, B.V. Derjaguin, N.V. Churaev, Adv. Colloid Interface Sci. 16 (1982) 63. J.N. Israelachvili, R.M. Pashley, Nature 306 (1983) 249. R.G. Horn, D.T. Smith, W. Haller, Chem. Phys. Lett. 162 (1989) 404. W.A. Ducker, T.J. Senden, R.M. Pashley, Nature 353 (1991) 239. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, Second Edition, London, 1992. W.A. Ducker, T.J. Senden, R.M. Pashley, Langmuir 8 (1992) 1831. A. Grabbe, R.G. Horn, J. Colloid Interface Sci. 157 (1993) 375. J.-P. Chapel, Langmuir 10 (1994) 4237. G. Vigil, Z. Xu, S. Steinberg, J.N. Israelachvili, J. Colloid Interface Sci. 165 (1994) 367. J.N. Israelachvili, H. Wennerstrom, Nature 379 (1996) 219. J.C. Fröberg, O.J. Rojas, P.M. Claesson, Int. J. Miner. Process. 56 (1999) 1. H.-J. Butt, B. Cappella, M. Kappl, Surf. Sci. Rep. 59 (2005) 1. S.M. Acuña, P.G. Toledo, Langmuir 24 (2008) 4881. S.M. Acuña, P.G. Toledo, J. Colloid Interface Sci. 361 (2011) 397. V.V. Yaminsky, R.K. Yusupov, E.A. Amelina, V.A. Pchelin, E.D. Shchukin, Colloid J. USSR 37 (1975) 918. V.V. Yaminsky, B.W. Ninham, R.M. Pashley, Langmuir 14 (1998) 3223. W.R. Bowen, N. Hilal, R.W. Lovitt, C.J. Wright, Colloid Surf. A 157 (1999) 117. L. Meagher, J. Colloid Interface Sci. 152 (1992) 293. D.T. Atkins, P. Kékicheff, O. Spalla, J. Colloid Interface Sci. 188 (1997) 234. J.J. Adler, Ya.I. Rabinovich, B.M. Moudgil, J. Colloid Interface Sci. 237 (2001) 249. H. Guleryuz, A.K. Royset, I. Kaus, C. Filiàtre, M.-A. Einarsrud, J. Sol-Gel Sci. Technol. 62 (2012) 460. Ya I. Rabinovich, Colloid J. USSR 39 (1977) 1094. G. Peschel, P. Belouschek, M.M. Müller, R.M. Müller, R. König, Colloid Polym. Sci. 260 (1982) 444. R.G. Horn, D.T. Smith, J. Non-Cryst. Solids 120 (1990) 72. A.M. Freitas, M.M. Sharma, J. Colloid Interface Sci. 233 (2001) 73. I.U. Vakarelski, K. Ishimura, K. Higashitani, J. Colloid Interface Sci. 227 (2001) 111. I.U. Vakarelski, K. Higashitani, J. Colloid Interface Sci. 242 (2001) 110. M.L. Fielden, R.A. Hayes, J. Ralston, Phys. Chem. Chem Phys. 2 (2000) 2623. S. Huntington, S. Nespolo, Microscopy Today 1–3 (2001) 32. B.V. Derjaguin, V.M. Muller, Y.P. Toporov, J. Colloid Interface Sci. 53 (1975) 314. D. Tabor, J. Colloid Interface Sci. 58 (1977) 2. Y.-P. Zhao, L.S. Wang, T.X. Yu, J. Adhes. Sci. Technol. 17 (2003) 519. J.L. Parker, P.M. Claesson, J. Phys. Chem. 98 (1994) 8468. Ch. Li, P. Somasundaran, J. Colloid Interface Sci. 148 (1992) 587. V.S. Yushchenko, V.V. Yaminsky, E.D. Shchukin, J Colloid Interface Sci. 96 (1983) 307. H.K. Christenson, P.M. Claesson, R.M. Pashley, Proc. Indian Acad. Sci. (Chem. Sci.) 98 (1987) 379. H.K. Christenson, P.M. Claesson, Science 239 (1988) 390. V.V. Yaminsky, V.S. Yushchenko, E.A. Amelina, E.D. Shchukin, J. Colloid Interface Sci. 96 (1983) 301. J.H. Saavedra, R.E. Rozas, P.G. Toledo, J. Colloid Interface Sci. 13 (2014) 2686.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Nanoscale adhesive forces between silica surfaces in aqueous solutions Paula Troncoso a, Jorge H. Saavedra a, Sergio M. Acuña b, Ricardo Jeldres a, Fernando Concha c, Pedro G. Toledo a,⇑ a
Chemical Engineering Department and Surface Analysis Laboratory (ASIF), University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile Department of Food Engineering, University of Bio-Bio, PO Box 447, Chillán, Chile c Metallurgical Engineering Department, University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile b
a r t i c l e
i n f o
Article history: Received 2 May 2013 Accepted 6 March 2014 Available online 15 March 2014 Keywords: Nanoscale forces Adhesion Silica AFM Cavitation Contact surfaces Dehydroxylation
a b s t r a c t Nanoscale adhesive forces between a colloidal silica probe and a flat silica substrate were measured with an atomic force microscope (AFM) in a range of aqueous NaCl, CaCl2, and AlCl3 solutions, with concentrations ranging from 106 to 102 M at pH 5.1. Notably, the measured force curves reveal large pull-off forces in water which increase in electrolyte solutions, with jump-off-contact occurring as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between mineral surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities or nanobubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured force data. A simple model based on several cavities bridging the two surfaces we show that is able to capture all the features in the measured force curves. The silica surfaces used are clean but not intentionally hydroxylated, as contact angle measurements show, and as such may be responsible for the cavities. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction It is well known that silica suspensions exhibit remarkable stability at their isoelectric point [1] against coagulation and sedimentation [2,3] and low viscosity [1,4]. The short-range repulsion not predicted by DLVO theory [5,6] but usually observed in direct force measurements between silica surfaces arises from a surface-induced water effect, from the creation of a hydrogenbonding network at the surface level [4,7–24]. This short-range repulsion as well as the small attractive van der Waals forces for silica in water and in aqueous electrolyte solutions are likely responsible for the unusual stability of silica suspensions particularly at their isoelectric point. In the past this issue has been of interest to a wide range of processes involving transport and processing of silica slurries and pulps. Interest today is even greater considering that industry is moving fast toward higher solids loadings, and thus an appropriate control of rheological behavior and physicochemical stability of the suspensions is crucial to obtain fluidity and stability as desired. A great variety of
⇑ Corresponding author. Fax: +56 41 2204691. E-mail address: [email protected] (P.G. Toledo). http://dx.doi.org/10.1016/j.jcis.2014.03.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
experimental techniques have been used to characterize silica– silica interactions, however for assessing interaction forces between silica surfaces mediated by aqueous solutions the surface force apparatus (SFA) and the atomic force microscope (AFM) are the favorites. Here we are interested in the forces that arise in the separation of two silica surfaces after reaching direct contact. Given the repulsive character of the interaction in the approaching of the surfaces, one might expect a separation virtually free of hysteresis. This is true for the interaction in water [20,22,25,26] but not in aqueous electrolyte solutions [18,27–31]. Although the silica–silica system has been reinvestigated several times, interest has focused on approach force curves rather than on separation force curves, therefore adhesive forces have not always been observed [14,19,25–27,32–34]. It has been reported that adhesion between silica surfaces in water occurs only when the surfaces remain in contact for a long time [19] although according to Yaminsky et al. [25,26], after several days in water the strength of the adhesion decreases. Similar behavior is reported by Chapel [18]; silica adhesion in 0.1 M NaCl at pH 5.5 disappears after few minutes. Less common in the literature is to find trends followed by silica–silica adhesion with electrolyte concentration in the measurement of forces. Yaminsky et al. [26] and Meagher [28]
57
P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
2. Experimental Glass microslides (B&C) typically 1 1 cm2 were used. The flat silica substrates were glued to AFM stubs before use. Glass colloidal probes were prepared [39] by gluing a 20 lm in diameter sphere (Duke Scientific Corporation) to the end of a tipless Vshaped, 100 lm long, 0.6 lm thick, Si3N4 cantilever (Veeco) with Norland Optical Adhesive 61 (Norland Products, USA). Spring constants of individual cantilevers were determined by the method of standards (standards provided by Park Scientific, USA) with the Dimension 3100 AFM microscope and were typically 0.14 N/m. SEM and AFM images verified the quality of the modified cantilevers. Interaction forces were measured in bi-distilled water, pH was 5.1, and also in electrolyte solutions. NaCl, CaCl2, and AlCl3 (analytical chemical grade, Merck, Germany) were used in concentrations ranging from 104 to 102 M. Experiments were carried out without any buffering, pH was 5.1. All glassware used in the preparation of solutions was detergent and alkali washed with final thorough rinsing in bi-distilled water. Prior to force measurement the mineral surfaces, substrate and sphere, were thoroughly rinsed in high purity water (18.6 MO/cm), then with ethanol, and then again with bidistilled water. No efforts were made to improve hydroxylation of the glass surface groups into silanol groups, thus perfect wetting should not be expected. Contact angles were measured through the liquid phase for the various electrolyte solutions used here with the Ramé-Hart contact angle goniometer. Surface roughness, assessed by AFM imaging with the Dimension 3100 and measured as the root-mean-square roughness, was small for both substrate, 2 nm, and microsphere, 1 nm. Force measurements between AFM probes and substrates as function of separation were conducted using an SPM-3 (Thermo, USA) multimode atomic force microscope equipped with a Nanoscope IIIa SPM control station, fluid cell (0.1 cm3), silicone pad for
vibration isolation, and acoustic enclosure. AFM allows continuous measurement of cantilever deflection vs. position as probe and substrate approach, commonly named extension, or separate, commonly named retraction. Approaching force curves can be found in Acuña and Toledo [23], here we report on retraction force curves. Measurement of a typical force curve took less than 20 min; during this time the AFM roughness of the substrate remained unaltered. The force measurement protocol is well established [14,16] and the sample manipulation procedure is available [23]. Extension and retraction driving speeds were low, 280 nm/s, in order to minimize hydrodynamic contribution to the measured force. Force curves were first verified to be independent of position on the substrate; measurements were always highly reproducible. Forces are reported normalized by the microsphere probe radius, that is, as interaction energy between silica flat surfaces by virtue of Derjaguin´s approximation, F(D) = 2pRE(D), where F is force, D distance, R probe radius, and E energy per unit area. 3. Results and discussion Here we are interested in the forces that arise in the separation of two perfectly clean silica surfaces but not hydroxylated after reaching direct contact. Fig. 1 shows glass contact angles measured through the liquid phase for the various electrolyte solutions of interest here. Contact angle in pure water is 27° and in electrolyte solutions increases with concentration, very rapidly in the range R2. Force curves in Figs. 2 and 3, and data in Table 1, show this behavior even though the effect of electrolyte concentration and valence is small, although distinguishable, over surface tension and contact angle. Formation of cavities or persistent air bubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured
60
P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
Table 1 Adhesion force between silica surfaces measured in electrolyte solutions at 20 °C and pH 5.1. Electrolytes are NaCl, CaCl2, and AlCl3. Concentrations range from 104 to 102 M. F = kDD, where k = 0.14 N/m is the elastic constant of the cantilever and DD is the surface-to-surface distance from the jump-off-contact distance to the equilibrium position of the cantilever (the zero line), which we read directly from the curves. The work of adhesion for identical solid surfaces is Wadhesion = 2c, the surface energy of the solid c is obtained from the DMT model through F/Rsphere = 4pc, here Rsphere = 10 lm. For example, for a suspension of silica particles with 0.1 lm in radius, according to DMT model the contact radius between the particles under zero external load is a0 = (4pR2c/K)1/3, where K is reduced elastic modulus of contacting particles. For silica K = 4.94 1010 N/m2. Total adhesive energy W accumulated in the contact area between the particles is W ¼ W adhesion pa20 which is in units of kBT.
a
c (mN/m)
Wadhesion (mJ/m2)
W (kBT)
4.05 ± 0.15
0.32
0.64
0.44
370.8 ± 78.7 568.3 ± 67.6 649.2 ± 72.0
5.19 ± 0.55 7.96 ± 0.48 9.09 ± 0.51
0.41 0.63 0.72
0.83 1.27 1.45
0.66 1.35 1.69
1 104 1 103 1 102
485.0 ± 96.9 536.7 ± 89.1 666.7 ± 20.4
6.79 ± 0.68 7.51 ± 0.63 9.33 ± 0.15
0.54 0.60 0.74
1.08 1.20 1.49
1.04 1.23 1.76
1 104 1 103 1 102
732.5 ± 18.9 741.7 ± 132.9 764.2 ± 96.3
10.26 ± 0.13 10.38 ± 0.93 10.70 ± 0.68
0.82 0.83 0.85
1.63 1.65 1.70
2.06 2.11 2.21
Medium
Concentration (M)
DD (nm)
Water
1 106a
289.2 ± 21.5
NaCl
1 104 1 103 1 102
CaCl2
AlCl3
F/Rsphere (mN/m)
Water used is bi-distilled with conductivity of 18.6 MO/cm and concentration estimated in 106 M.
ϕ R D
θ1 θ2
R2 R1
Fig. 4. Axisymmetric cavity/bubble between a sphere and a plane in a liquid. R is the sphere radius, u the semi-angle subtended at the centre of the sphere by the cavity, h1 and h2 are the solid–liquid contact angles at the two triple lines, h1 = h2 = h when sphere and plane are made of the same material, R1 is the radius of curvature descriptive of the rotation about the vertical axis in the plane of the paper, R2 is the radius of curvature of the vapor–liquid surface in the plane perpendicular to the paper, D is the length of the cavity.
force data. In a related work we have demonstrated by means of a simple macroscopic force balance (Eq. (1)) and molecular dynamics simulation of a Lennard-Jones liquid that capillary bridges between surfaces of different nature are responsible for attractive forces not just between solvophilic surfaces but also for moderately solvophophic surfaces; and thus for cavities or bubbles surrounded by the same liquid the force is attractive even when the substrates are moderately solvophilic [49]. These results although for a LJ fluid support the idea that for moderately hydrophilic surfaces it is also possible to have attractive forces due to cavities or persistent bubbles bridging the silica surfaces. We propose a simple model to explain our force data based on several cavities and or nanobubbles bridging the two surfaces. Such cavities have a distribution of sizes and stability upon separation. The least stable cavity fails first at small separation leading to a small step in aqueous solutions with low electrolyte concentration and valence and to a jump in the presence of high electrolyte concentration and valence; the second less stable cavity fails next and so on. Fig. 5 shows a schematic representation of several vapor cavities trapped between two solid surfaces, some of the cavities are larger and more stable than others. Three stages of the separation are illustrated. Frame (a) illustrates the array when contact angle is intermediate, proper of interaction in aqueous solutions with high electrolyte concentration and valence, so the initial shape of the cavities is prolate spheroidal but changes to almost a circular cylinder as separation of the surfaces increases. Pressure inside the cavities is low and decreases as separation of the surfaces increases. With increasing separation, R1 increases very rapidly and R2 decreases slowly, the result according to Eq. (1) is an increasing net adhesive force between the surfaces. The third cavity from the left in Frame (a) is the weakest. The third stage shows the point at which this cavity fails (the cavity right before
(a) F R
D
iii
(b)
i
ii
Fig. 5. Schematic representation of several cavities/bubbles trapped between two solid surfaces, some of the cavities are larger and more stable than others. Three stages of the separation are illustrated. (a) Contact angle is intermediate and remains the same in the three stages; the shape of the cavities is prolate spheroidal but changes to almost a circular cylinder as separation of the surfaces increases. Cavities at the verge of failing are drawn with segmented lines. (b) Schematics of the corresponding forces in (a).
the failure is drawn with segmented lines). After the failure, small bubbles remain attached to the surfaces obeying the contact angle at the triple line. Frame (b) depicts a typical force curve for the system in (a). At point (i), the net adhesive force is given by Eq. (1) for R1 larger than R2. At point (ii) the net adhesive force increases according to Eq. (1) because R1 increases more rapidly than R2 decreases, the third cavity in Frame (a) is about to fail. Contribution of this cavity to the net adhesive force is so significant that its failure drives a force jump to a much smaller adhesive force at point (iii). This event is repeated as many times as there are steps
P. Troncoso et al. / Journal of Colloid and Interface Science 424 (2014) 56–61
in the force curve. The enhanced wetting of the cavities in this case increases their attachment to the surfaces and so does the pull-off force at the failure. This very simple model captures all the features in the force curves in Figs. 2 and 3. 4. Conclusion Measured long range adhesive forces clearly increase with electrolyte concentration and valence. Contact forces may be too short range to be invoked to explain them. Likewise the attractive force of van der Waals. In our experiments the time of contact of the surfaces is sufficiently short to discard any sintering mechanism by the action of water. Notably, our force curves reveal large pull-off forces in water and in electrolyte solutions, the jump-off-contact occurs as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between silica surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities and or nanobubbles cannot be observed directly in our experiments; however, we cannot discard them as responsible for the discontinuities in the measured force data. We offer a simple model based on several cavities and/or nanobubbles bridging the two surfaces that is able to capture all the features in the measured force curves. In the model the least stable cavity fails first at small separation leading to a small step in aqueous solutions with low electrolyte concentration and valence and to a jump in the presence of high electrolyte concentration and valence, the second less stable cavity fails next and so on. Dehydroxylation of our silica surfaces may be responsible for the cavities/bubbles; however, cavities/bubbles need to be considered in the analysis of measured force curves between silica surfaces because they might never be totally absent. Acknowledgments We thank Project Conicyt/Fondecyt 1101023, Centro CRHIAMProject Conicyt/Fondap-15130015 and Red Doctoral REDOC.CTA, MINEDUC Grant #UCO1202 for financial support. J.H.S. and R.J. thank CONICYT-Chile for graduate student fellowships. References [1] R.K. Iler, The Chemistry of Silica, Wiley Interscience, NY, 1979.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
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