Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb

Bile salts at the air–water interface: Adsorption and desorption J. Maldonado-Valderrama a,∗ , J.L. Muros-Cobos b , J.A. Holgado-Terriza b , M.A. Cabrerizo-Vílchez a a b

Department of Applied Physics, Campus de Fuentenueva, sn. University of Granada, 18071 Granada, Spain Department of Software Engineering, C/Periodista Daniel Saucedo Aranda, sn. University of Granada, 18071 Granada, Spain

a r t i c l e

i n f o

Article history: Received 10 December 2013 Received in revised form 7 May 2014 Accepted 9 May 2014 Available online 22 May 2014 Keywords: Surface tension Surface rheology Desorption Surface conformation Bile salts Digestion

a b s t r a c t Bile salts (BS) are bio-surfactants which constitute a vital component in the process of fat digestion. Despite the importance of the interfacial properties in their biological role, these have been scarcely studied in the literature. In this work, we present the adsorption–desorption profiles of two BS (NaTC and NaGDC) including dilatational rheology. Findings from this study reveal very different surface properties of NaTC and NaGDC which originate from different complexation properties relevant to the digestion process. Dynamic adsorption curves show higher adsorption rates for NaTC and suggest the existence of various conformational regimes in contrast to NaGDC which presents only one conformational regime. This is corroborated by analysis of the adsorption isotherms and more in detail by the rheological behaviour. Accordingly, the dilatational response at 1 Hz displays two maxima of the dilatational modulus for NaTC as a function of bulk concentration, in contrast to NaGDC which displays only one maximum. The desorption profiles reveal that NaTC adopts an irreversibly adsorbed form at high surface coverage whereas NaGDC fully desorbs from the surface within the whole range of concentrations used. Analysis of the adsorption–desorption profiles provides new insight into the surface properties of BS, suggesting a surface complexation of NaTC. This knowledge can be useful since through interfacial engineering we might control the extent of lipolysis providing the basis for the rational design of food products with tailored digestibility. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Bile salts (BS) are bio-surfactants present in the gastrointestinal tract that play a crucial role in digestion and absorption of nutrients [1]. The importance of BS for controlled release and transport of lipid soluble nutrients/drugs has recently stimulated the scientific interest in these physiological compounds. BS are a peculiar type of surfactants with unusual properties. In contrast to classical surfactants, BS do not have a well defined tail and head group but they exhibit instead a planar polarity. Chemically, BS are rigid, almost flat molecules with weakly separated hydrophilic and hydrophobic faces [2]. This peculiar molecular structure facilitates the formation of dynamic aggregates able to solubilise and transport lipid soluble compounds, hence, constituting a vital component in the process of fat digestion. Most lipid digestion in humans occurs in the duodenum. The reason for this is principally due to BS, which adsorb onto

∗ Corresponding authors at: Campus de Fuentenueva n, Facultad de Ciencias Departamento, Granada, Spain. Tel.: +0034 958241000x20387; fax: +003 4958243214. E-mail address: [email protected] (J. Maldonado-Valderrama). http://dx.doi.org/10.1016/j.colsurfb.2014.05.014 0927-7765/© 2014 Elsevier B.V. All rights reserved.

and remove other materials such as proteins and emulsifiers from the lipid surface [3,4]. This allows lipase and its cofactor co-lipase to adsorb onto the lipid surface and instigate lipolysis. The unusual surface behaviour of BS is directly related to their intriguing molecular structure and further knowledge could provide an improved understanding and rational control of lipid digestion. The detergent nature of BS has been studied in the literature, mostly concentrating on the self-assembly behaviour of BS in solution. Many literature reviews provide interesting information about the current knowledge of self-assembly of BS in solution emphasizing the peculiar properties of BS aggregates as compared to classical surfactants [2,5–7]. Also, recent studies with computer simulations have provided new insight into the formation mechanisms of micelles [8,9]. As a result, the self-assembly and aggregation mechanisms of BS in solution are rather well understood nowadays. Interaction of BS with proteins and lipids has also been a subject of intensive research. In particular, there are some recent works dealing with interactions between interfacial material on oil droplets and BS such as proteins [4,10,11], aminoacids [12] or lipids [13]. In contrast, studies dealing directly with the interfacial behaviour of BS are very limited in the literature [1,14]. Although investigation of the adsorption behaviour of classical surfactants

J. Maldonado-Valderrama et al. / Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

onto air–water and oil–water interfaces has proven to be extremely useful in improved understanding of their diffusion and aggregation behaviour, the interfacial tension profiles of BS are often not reported in the literature and even less studies deal with surface rheological characterisation. Another important innovation of the present work are the desorption profiles. In order to proceed with the transport and absorption of lipid nutrients, the ability to desorb from the lipid surface plays a crucial role. The lipolysis reaction which occurs at the interface involves complex equilibria between adsorption–desorption processes and conformational changes at the interface. Accordingly, studying the reversibility of adsorption of BS onto hydrophobic surfaces definitely provides interesting information applicable to the role of BS in lipid digestion as already spotted in some preliminary measurements in previous works [1]. The present work combines novel surface characterisation methods to study the surface properties of two bile salts: NaTC and NaGDC. We present a systematic study comprising dynamic adsorption curves, surface tension isotherms, dilatational rheology and desorption profiles. Such combination has not been reported to date on bile salts and reveals interesting differences between adsorbed layers formed by NaTC and NaGDC, hence importantly improving our understanding of adsorbed layers of BS. The efficacy of lipid digestion depends closely on the physical-chemistry of the interface and on the unconventional behaviour of BS. Hence, a better understanding of surface activity of BS can facilitate manipulation of physico-chemical and interfacial properties to modulate lipid digestion, improve bioavailability of lipid soluble nutrients and reduce absorption of saturated fats, cholesterol and trans fats.

2. Materials and methods 2.1. Materials Sodium glycodeoxycholate (NaGDC, >97% TLC, cat n. G9910, lot no. 039K0308), and sodium taurocholate (NaTC, >97% TLC, cat n. 86339, lot no. BCBG6336V) were obtained from Sigma-Aldrich® and used as received. Both bile salts are negatively charged, and their molecular weights are 537.68 Da (NaTC) and 471.6 Da (NaGDC). BS are a family of soluble amphiphilic molecules with an unusual molecular structure exhibiting planar polarity. Bile acids comprise two connecting units, a rigid steroid backbone with a hydrophobic and a hydrophilic face to which a flexible aliphatic tail is attached [15]. Fig. 1 shows a schematic representation of a bile salt molecule and the molecular formula of NaTC and NaGDC. The hydrophobic surface lies on the convex side of the rigid steroid ring system. The concave side of the molecule contains one two or three hydroxyl groups and an amino group that can be conjugated with taurine, glycine or other aminoacids. Different BS differ in the number, position and stereochemistry of the hydroxyl group as well as on the conjugated amino acid; glycine (75%) or taurine (25%). The buffer used in all solutions was 2 × 10−3 M Bis Tris (SigmaAldrich® , ≥99.0%, cat n. 148779) 0.15 M NaCl, 0.01 M CaCl2 , adjusted to pH 7 with HCl. This buffer mimics the physiological conditions in the duodenum. Bile salts solutions were prepared daily by dilution from a stock solution of 0.1 M. The stock solution was prepared by dissolving the bile salt directly in the duodenal buffer for 1 h under mild agitation. Dilutions were kept under mild agitation for 30 min before use. Ultrapure water, cleaned using a Milli-Q water purification system (0.054 ␮S), was used for the preparation of buffer solutions. All glassware was washed with 10% Micro-90 cleaning solution and exhaustively rinsed with tap water, isopropanol, deionised water, and ultrapure water in this sequence. All other chemicals used were of analytical grades and used as received. The temperature was

177

adjusted to 20 ◦ C with an external temperature control and surface tension of the clean air–water interface was measured before every experiment, in order to confirm the absence of surface-active contaminants, yielding values of 72.8 ± 0.2 mN/m at 20 ◦ C. 2.2. The OCTOPUS All the measurements were made in The OCTOPUS; a Pendant Drop Surface Film Balance equipped with a subphase multiexchange device which has been fully designed and assembled at the University of Granada (UGR) (patent submitted P201001588) and is described in detail in [16]. The pendant drop is placed on a three axis micro-positioner and is immersed in a glass cuvette (Hellma) which is kept in a thermostatically-controlled cell. Drop images are captured by a CCD camera (Pixelink® ) connected to an optical microscope (Edmund Optics® ). The computer program DINATEN© fits experimental drop profiles, extracted from digital drop micrographs, to the Young–Laplace equation of capillarity by using ADSA (Axisymmetric Drop Shape Analysis), and provides as outputs the volume (V), the surface tension (), and the interfacial area (A) of the pendant drop. The OCTOPUS allows also measuring the dilatational rheology of the surface layers at different oscillating frequencies by recording the response of the surface tension to a triangular area deformation [17]. In a general case, the dilatational modulus (E) is a complex quantity that contains a real and an imaginary part: E = E  + iE  = ε + iv

(1)

where E is the storage modulus that accounts for the elasticity of the interfacial layer (ε), E is the loss modulus that accounts for the viscosity of the interfacial layer () and  is the angular frequency of the applied oscillation. The applied oscillations in interfacial area are maintained at amplitude values of less than 5%, in order to avoid excessive perturbation of the interfacial layer. The measurement frequency () can be changed between 0.01 and 2 Hz depending on the requirements of the system. Computer software DINATEN© records the images of the dilatational experiments in real time and these are then analysed and processed by computer software CONTACTO© , which provides as outputs the mean surface tension (), the surface elasticity (ε), the surface viscosity (), the storage modulus (E ), the loss modulus (E ) and the dilatational modulus (E) of the adsorbed layer. Automatic adsorption and dilatational rheology measurements can be made of up to 12 solutions adjusting the surface area, the total adsorption time and the oscillating frequency. In this work, we have measured the adsorption process at 30 mm2 , for 1 h and the dilatational rheology at fixed periods of 10 s, 5 s and 1 s. The concentrations used range between 10−6 and 0.1 M. The OCTOPUS software automatically discards the solution once the measurement is over, injects the new solution, creates a new droplet and starts the new measurement. This can be done automatically up to 12 times. The desorption experiments are carried out by means of the double capillary [18] which allows an automatic, non-invasive and complete exchange of the subphase of the drop preserving the pendant drop volume and surface area during the subphase exchange. In the case of soluble surfactants, exchanging the bulk subphase by pure buffer enables the study of the desorption profile of the adsorbed surfactants [19]. In this case, the evolution of the surface tension of the surface layer increases as the bulk solution is depleted of bile salts, which is done after the surface layer has equilibrated at constant surface area (Fig. 2). In order to accurately obtain a desorption profile, the conditions of the subphase exchange need to be optimised prior to the experiment, establishing a complete subphase exchange when the changes in interfacial tension are negligible upon further exchanges of the subphase of

178

J. Maldonado-Valderrama et al. / Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

Fig. 1. (a) Schematic representation of the facial amphiphilic structure of bile salts. The convex face is hydrophobic while the hydroxyl groups are oriented to the concave side (b) chemical formula of sodium taurocholate (NaTC) and sodium glycodeoxycholate (NaGDC).

the drop [16]. This was found to occur for bile salts, by exchanging at least 50 times the volume of the drop. Fig. 2 shows the evolution of the surface tension while the subphase exchange proceeds. It can be seen that, once the subphase exchange is complete, the surface tension is not affected by further subphase exchanges and the new steady state is adequately reached. Only then, we stop the subphase exchange and monitor the surface tension at constant surface area for a selected period of time. As in the adsorption experiments, The OCTOPUS software automatically discards the solution once the measurement is over, injects the new solution, creates a new droplet and starts a new adsorption–desorption profile. 3. Results and discussion BS are extremely surface active despite only reaching low surface pressures [1]. This intriguing behaviour is ascribed to their facial amphiphilic structure (Fig. 1), which contrast with the conventional head/tail structure of surfactants. However, in order to fully understand the surface behaviour of these facial amphiphiles we still lack of a detailed study that looks into different surface phenomena. Surface tension profiles of BS are often not reported in the literature, especially, regarding dynamic adsorption or surface rheology. Thus, in this section, we present a fundamental characterisation of two representative BS (NaTC and NaGDC) at the air–water

Fig. 2. Evolution of the surface tension during the subphase exchange in a desorption experiment were the bulk subphase it substituted by buffer and the bulk depleted of surfactants. The arrow indicates the time point when the subphase exchange starts and it continues until reaching a steady state of the surface tension.

interface as a model interface. Firstly, the dynamic adsorption curves are presented followed by the adsorption isotherms. Second, the dilatational rheology profiles are evaluated. And finally, the desorption profiles of these BS from the air–water interface is discussed in detail. 3.1. Dynamic adsorption Figs. 3 and 4 show the dynamic adsorption profiles recorded automatically by the OCTOPUS for NaTC and NAGDC, respectively. These figures provide the adsorption profile process for 12 bulk concentrations within the range of 10−6 M and 10−1 M under the physiological conditions of the duodenum. The OCTOPUS software automatically discards the solution once the measurement is over, injects the new solution, creates a new droplet and starts the new measurement. This can be done automatically up to 12 times. For each concentration, we measure the surface tension for 1 h at constant surface area, followed by three successive dilatational cycles in which we measure the dilatational rheology of the adsorbed layer by subjecting the drop to 10 oscillations periods of 10 s, 5 s and 1 s which will be discussed in Section 3.3. Each different colour in Figs. 3 and 4 corresponds hence to a new droplet corresponding to a new (higher) concentration. Fig. 3 shows how the surface tension of each solution diminishes progressively as the concentration of NaTC increases in the bulk.

Fig. 3. Adsorption dynamics of NaTC at the air–water interface in 2 mM Bis Tris, 150 mM NaCl, 10 mM CaCl2 , pH 7, T = 20 ◦ C. Each curve corresponds to a new drop with the new concentration.

J. Maldonado-Valderrama et al. / Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

179

Fig. 5. Surface tension-bulk concentration isotherms of NaTC (closed triangles) and NaGDC (open squares) in 2 mM Bis Tris, 150 mM NaCl, 10 mM CaCl2 , pH 7, T = 20 ◦ C. Error bars represent standard deviations from three independent measurements. Fig. 4. Adsorption dynamics of NaGDC at the air–water interface in 2 mM Bis Tris, 150 mM NaCl, 10 mM CaCl2 , pH 7, T = 20 ◦ C. Each curve corresponds to a new drop with the new concentration.

Within the whole range of concentration, the adsorption process proceeds extremely fast and the surface tension rapidly reaches a low value for each concentration (Fig. 3). This agrees with literature works and is attributed to the area per BS molecule, which is much higher than those corresponding to conventional surfactants, implying that the adsorption is very efficient [12]. However, we can still distinguish three different regions of adsorption behaviour by looking on more detail in to the kinetics recorded for the different solutions. The lowest concentrations (10−6 M, 10−5 M and 10−4 M NaTC), attain a value of surface tension almost immediately after the drop formation, which then remains practically unchanged. This value of surface tension decreases as the concentration of NaTC increases, but remains always above 65 mN/m. Hence, within this concentration regime, NaTC adsorbs very efficiently onto the air–water interface attaining instantaneously a stable conformation at the surface. second region of bulk concentration values A (2 × 10−4 –7 × 10−3 M) shows a somehow different dynamic behaviour. Although the surface tension starts off with an already low value of surface tension, this value continues to decrease along the 1 h of adsorption recorded. This decrease is not very steep, but there is a certain diminishing tendency in the adsorption profile (Fig. 3). Also, the starting surface tension decreases as the concentration of NaTC increases in the bulk and the surface tension ranges in this region between 65 mN/m and 50 mN/m. This adsorption profile could indicate that, within this concentration range, the adsorption proceeds as follows. First, the NaTC molecules adsorb very efficiently onto the surface accounting for the low initial value of the surface tension immediately after drop formation. Subsequently, the diminishing tendency could be indication of a conformational changes occurring at the surface, such as an aggregation, complexation or molecular reorientation phenomena. We will look into this aspect later on. Finally, the highest concentrations recorded (10−2 –10−1 M) show again a very rapid adsorption profile in which the final surface tension is attained almost immediately after drop formation. The difference from the first regime is that now the surface tension attained is considerably lower and also, this value does not seem to decrease further with increasing bulk concentration nor with time. This behaviour implies that the surface is rapidly saturated of molecules forming a stable adsorbed layer characterized by a value of surface tension of 48–45 mN/m. Analysis of the adsorption profile, as recorded in Fig. 4, provides information about the surface activity of NaGDC at the air–water interface already providing differences in the adsorption behaviour with respect to the NaTC (Fig. 3). Fig. 4 shows how the surface tension diminishes progressively as the concentration of NaGDC

increases and reveal also three adsorption regimes as indicated by the dynamics recorded in Fig. 4. The lowest concentrations (10−6 M, 10−5 M), hardly display surface activity and the surface tension remains during the whole adsorption process above 70 mN/m indicating that there is hardly any material adsorbed at the surface within this range of concentrations. A second region of bulk concentration values (3 × 10−5 –3 × 10−4 M) shows more variation and also the steep drop in surface tension seen at the start of each data set appears more pronounced in the NaGDC data (Fig. 4) than in the NaTC data (Fig. 3) set. Interestingly, all the curves within this concentration regime show this fast initial dynamics in which the surface tension decreases approximately 5 × mN/m in less than 5 min. Increasing the bulk concentration decreases the initial value of the surface tension. This regime proceeds between surface tension values of 65 mN/m and 50 mN/m. The rapid dynamics could correspond to the diffusion of molecules which rapidly fill the surface rather than to a possible conformational change at the surface. The rapid kinetics recorded for NaTC in Fig. 3 suggests that the adsorption process proceeds faster than for NaGDC. This agrees with previous findings with other experimental techniques [1]. Finally, the highest concentrations recorded (5 × 10−4 –10−2 M) show again a very rapid adsorption profile in which the final surface tension is attained almost immediately after drop formation, implying that the surface is rapidly saturated of molecules forming a stable adsorbed layer and characterized by a value of surface tension of 50–45 mN/m. The flat conformation of BS at the surface appears responsible for the efficient decrease in surface tension recorded by occupation of large surface area per molecule [12]. 3.2. Adsorption isotherms Fig. 5 shows the surface tension-bulk concentration isotherms recorded for NaTC and NaGDC at the air–water interface. The surface tension values plotted in Fig. 5 are the final values recorded after 1 h of adsorption at constant surface area as shown in Figs. 3 and 4. NaTC and NaGDC display similar values of surface tension ranging between that of pure buffer (72 mN/m and 45 mN/m). Bile salts seem to arrange loosely at the surface instead of forming a compact layer, providing relatively high values of the surface tension even at maximum surface coverage (Fig. 5) [20,21]. NaGDC appears more surface active than NaTC as indicated by the displacement of the surface tension isotherm to lower bulk concentrations (Fig. 5). This is consistent with findings of O’Connor et al. and can be attributed to a higher hydrophobicity of the NaGDC as compared to NaTC [21]. Also, the amphiphilicity of the bile salts is strongly influenced by subtle variations in molecular structure as stated by Armstrong and Carey [22]. These authors report that the molecular hydrophobicity of BS depends on the cholate conjugation group. Between Deoxycholate (DC), Cholate (C),

180

J. Maldonado-Valderrama et al. / Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

Fig. 6. Dilatational modulus versus bulk concentration. NaTC (closed symbols) and NaGDC (open symbols) in 2 mM Bis Tris, 150 mM NaCl, 10 mM CaCl2 , pH 7, T = 20 ◦ C. 0.1 Hz (squares), 0.2 Hz (triangles), 1 Hz (rhomboids). Error bars represent standard deviations from three independent measurements.

ChenoDeoxycholate (CDC) and Ursodeoxycholate (UDC) it decreases in the order DC > CDC > C > UDC with free bile salts > glycine-conjugates > Tauro-conjugates. Accordingly, NaGDC is more hydrophobic than NaTC, as affected by both constitutive groups; the DC and the glycine conjugate. Moreover, glycine is a non chiral aminoacid with a minimal side chain of only one hydrogen atom which can fit into hydrophilic or hydrophobic environments. This last feature also accounts for the increased affinity of NaGDC for the surface as quantified by the surface tension isotherm in Fig. 5. The shape of the surface tension isotherms recorded for NaTC and NaGDC is also different. The former seems to show discontinuities in contrast to the linear shape of the latter. According to O’Connor, this suggests that NaTC aggregation occurs in a stepwise fashion forming dimers and oligomers in the early stages of aggregation [21]. This fact would importantly agree with the findings from the dynamic curves in Figs. 3 and 4 and might have implications in the structure of the surface layer as we will see in following sections. The concentration that provides a saturated interface, i.e. constant interfacial tension, is a good indication of the critical micelle concentration (cmc). This value can be inferred from Fig. 5 obtaining (3.0 ± 0.5) × 10−3 M and (9.0 ± 0.5) × 10−3 M for NaGDC and NaTC, respectively. These values are in the range of cmc values reported for BS in the literature [23,24]. The lower cmc obtained for NaGDC proves the higher surface activity of this bile salt as compared to NaTC. Moreover, the higher bulk concentration of NaTC required for saturation of the surface could be indication of an increased packing achieved by NaTC at the surface. 3.3. Surface rheology of BS The surface dilatational rheological behaviour of adsorbed layers is a constitutive characteristic of the system, independent of the type of excitation. Due to this fact, the surface dilational viscoelasticity appears to be a most suitable characteristic of the surface to compare the results of different dynamic experiments [25]. Fig. 6 shows the dilatational elastic values of NaTC and NaGDC as a function of bulk concentration at the air–water interface. The values plotted in Fig. 6 were obtained, for each of the concentrations considered, once a stable value of the surface tension had been reached; after 1 h of adsorption at constant interfacial area (Figs. 3 and 4). Only then, we applied three successive oscillating cycles at 0.1 Hz, 0.2 Hz and 1 Hz. At these frequencies, the value of the loss modulus (E ) was negligible in all cases and the dilatational elastic modulus (E) practically coincides with the storage modulus (E ) or elasticity of the surface layer (ε) as given by Eq. (1) (results not shown). Hence, both BS layers exhibit mostly elastic properties at this frequency range and hence, the dilatational viscosity will be ignored [19]. The dependence of the dilatational modulus on the oscillation frequency is caused by relaxation processes at the interface [25].

There are two types of relaxation processes: exchange of molecules between the bulk solution and the interface and conformational change of molecules in the interfacial layer. Upon compression, some molecules can dissolve into the underlying water, to respond to the gradient in surface area, i.e. surface concentration. At low frequency, this process occurs fully and there is no resistance to compression–expansion so that the dilatational modulus is very small. At high frequency, the exchange rate is much lower than the frequency of the disturbance so that the layer can behave as an insoluble monolayer and produce a higher dilatational response [26]. As a result, the dilatational modulus increases with increasing oscillation frequencies. Fig. 6 shows how the dilatational response of NaTC at 0.1 Hz is significantly lower than that obtained for NaGDC and again increases as the frequency of the oscillation increases. The hydrophobic face of NaTC is mainly composed of rigid methylene groups and arranges loosely at the surface providing hardly any resistance to deformation. As we increase the frequency of oscillation, the dilatational response increases and at 1 Hz we get two small maxima (Fig. 6). For NaGDC, the dilatational response is again very low at 0.1 Hz but still shows a small maximum located at 2 × 10−5 M. He et al. also found a small maximum in the dilatational response of NaDC at the air–water interface at this frequency [12]. However, this maximum practically doubles its height for a frequency of oscillation of 1 Hz. Very few literature works show rheological properties of BS and all of them use a frequency of oscillation of 0.1 Hz, hence showing a very low dilatational response [12]. Dilatational elasticity is caused by the energy change due to departure from equilibrium state after perturbation which is related to molecular interaction. Hence, the stronger dilatational response recorded for NaGDC reflects the formation of a more compact layer as compared to NaTC. Accordingly, the results presented here constitute a major improvement with respect to previous works highlighting the dilatational properties of BS. Concerning the differences encountered between the dilatational response of NaTC and NaGDC, these are very notable in Fig. 6. Firstly, the dilatational moduli of NaTC is much lower than that recorded for NaGDC within the whole range of concentrations [1]. This is consistent with the more rapid adsorption profile recorded for NaTC (Fig. 3) as compared to NaGDC (Fig. 4). It is also indicative of a more deformable surface layer caused by lower lateral interactions between NaTC molecules. This argument is consistent with Matuyabasi et al. who show that the interactions occurring between NaTC molecules in a monolayer are very weak [27]. Second, NaGDC shows a clear maximum appearing at a bulk concentration of 2 × 10−5 M. Fig. 4 also displays lower adsorption rates for NaGDC as compared to NaTC and this could originate a higher dilatational response of the layer, which behaves as an insoluble monolayer for NaGDC at this relatively high frequency. Once the concentration reaches the cmc (3 × 10−3 M), the dilatational response is negligible again. The height of the maximum

J. Maldonado-Valderrama et al. / Colloids and Surfaces B: Biointerfaces 120 (2014) 176–183

181

Fig. 7. Desorption profile of NaTC in 2 mM Bis Tris, 150 mM NaCl, 10 mM CaCl2 , pH 7, T = 20 ◦ C. Adsorption (closed symbols), desorption (open symbols). (a) 1E − 4 M (squares), 1E − 3 M (triangles), 1E − 2 (rhomboids). (b) 3E − 3 M (squares), 5E − 3 M (triangles), 7E − 3 (rhomboids). Curves are mean of six independent measurements with standard deviation

Bile salts at the air-water interface: adsorption and desorption.

Bile salts (BS) are bio-surfactants which constitute a vital component in the process of fat digestion. Despite the importance of the interfacial prop...
1MB Sizes 2 Downloads 3 Views