1684
Langmuir 2007, 23, 1684-1693
A pH-Study of n-Dodecyl-β-D-maltoside Foam Films Cosima Stubenrauch,*,† Rashel Cohen,‡ and Dotchi Exerowa‡ School of Chemical and Bioprocess Engineering, UniVersity College Dublin, Belfield, Dublin 4, Ireland, and Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. BoncheV Str. 11, Sofia 1113, Bulgaria ReceiVed August 4, 2006. In Final Form: NoVember 7, 2006 The influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-β-Dmaltoside (β-C12G2) was studied. Foam film measurements were carried out with the thin film pressure balance (TFPB) technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure Pc and, with the latter, complete disjoining pressure versus thickness curves (Π-h curves) were measured. Most of the results were obtained for 10-4 and 10-5 M β-C12G2 solutions that contained 10-3 M electrolyte. Measurements were carried out in a pH range from 3 to 9. The major results are the following: (1) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs ∼ cmc. This holds true for both low and high pH values. (2) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (3) The pH of the isoelectric point increases with increasing surfactant concentration. (4) The q0-pH curve of β-C12G2 shows two pH ranges (3-5.5 and 7-10) in which the surface charge density q0 is pH-insensitive, while a significant change of q0 was observed between pH ) 5.5 and 7.0. A possible explanation is given.
1. Introduction In well-drained quasi-static foam films (thickness 6 have to be performed under N2 atmosphere. In a CO2-containing atmosphere, CO2 is solubilized in the aqueous solution, which leads to a decrease of the pH down to pH ) 5.5-6.0, the so-called “natural” pH. Up to now, only one single paper has been published dealing with measurements at high pH, which were performed under N2 atmosphere.23 Consequently, results obtained at pH > 6 would have to be interpreted carefully if they were performed in air. In addition, the error bars of some published data are quite large. Although the trend toward thicker films and higher potentials with increasing pH cannot be denied in most of the studies, the thicknesses and the corresponding potentials can often be averaged, leading to a pH-independent constant value with a large error bar. Another indication of pH-independent behavior is the Π-h curves published in ref 24. The Π-h curves of foam films stabilized by the nonionic sugar surfactant β-dodecyl maltoside (β-C12G2) were indistinguishable in a pH range from 4 to 8. It is argued that the investigated pH range is likely to be from 4 to 5.5 only as the measurements were not carried out under N2. The authors expect to find a pH influence on β-C12G2 films for pH < 4 and pH > 5.5, namely an isoelectric point at pH < 4 and an increase of the film thickness at pH > 5.5. In summary, one can say that there are not enough experimental (16) Exerowa, D. Kolloid-Z. 1969, 232, 703. (17) Manev, E. D.; Pugh, R. J. Langmuir 1991, 7, 2253. (18) Cohen, R.; Exerowa, D. Colloids Surf., A 1994, 85, 271. (19) Waltermo, Å.; Manev, E.; Pugh, R.; Claesson, P. J. Dispersion Sci. Technol. 1994, 15, 273. (20) Bergeron, V.; Waltermo, A.; Claesson, P. M. Langmuir 1996, 12, 1336. (21) Khristov, K.; Exerowa, D.; Yankov, R. Colloids Surf., A 1997, 129-130, 257. (22) Karraker, K. A. Ph.D. Thesis, University of California, Berkeley, 1999. (23) Karraker, K. A.; Radke, C. J. AdV. Colloid Interface Sci. 2002, 96, 231. (24) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem. Phys. 2002, 4, 4504 and Phys. Chem. Chem. Phys. 2003, 5, 2736 (erratum).
Langmuir, Vol. 23, No. 4, 2007 1685
data to make generalizations about the influence of the pH on the properties of nonionic foam films. The present paper addresses the question of how the pH influences the properties of foam films stabilized by nonionic surfactants in general and of foam films stabilized by β-C12G2 in particular. In fact, we studied the influence the pH has on the equilibrium film thickness h, on Π-h curves, on the film stability, and on the CBF-NBF transition of β-C12G2 foam films. Equilibrium film thicknesses were measured in a pH range from 3 to 6.5, while Π-h curves were measured from pH ) 3-9. Most of the results were obtained for 10-4 and 10-5 M β-C12G2 solutions that contained 10-3 M electrolyte. For the sake of clarity and comparison, selected data for different surfactant and electrolyte concentrations are also presented. The results are compared with those obtained for other nonionic surfactants, and general conclusions are drawn wherever possible and reasonable. 2. Experimental Section 2.1. Materials and Cleaning Procedure. The nonionic surfactant n-dodecyl-β-D-maltoside (β-C12G2) was used as received from Sigma (>98% GC). The purity was checked by measuring the surface tension as a function of the concentration at 22 °C by the DuNou¨y ring method using a Kru¨ss K10ST tensiometer. The results are published in ref 25. Sodium chloride was obtained from Merck (Germany) and roasted at 500 °C before use to remove organic impurities. Sodium hydroxide (NaOH) was purchased from Merck, hydrochloric acid (HCl) from Acros, and acetone (pro analysis) from Merck. All solutions were prepared with Milli-Q water. All glassware (except the film holders) was cleaned with Deconex from Borer Chemie (as replacement for chromic sulfuric acid) and rinsed thoroughly with Milli-Q water before use. The porous plates were boiled twice in acetone and six times in water, and at least 0.5 L of hot water was sucked through each disk afterward. The ScheludkoExerowa cell was cleaned with hot chromic sulfuric acid and rinsed first with hot water and then with cold triple-distilled water. Surfactant solutions at different concentrations were prepared in 1.0 × 10-3 M and 1.0 × 10-4 M background electrolyte. All concentrations are below the critical micelle concentration (cmc), which is 1.5 × 10-4 M. The pH was set with NaOH or HCl and measured with a portable pH electrode from Hanna Instruments before and after the experiment. 2.2. Techniques. 2.2.1. Determination of Π-h CurVes. (a) General. With the thin film pressure balance technique, free-standing horizontal liquid foam films can be investigated by subjecting the film to a defined gas pressure and measuring its equilibrium thickness h interferrometrically. Experimental details have been published elsewhere.24 Briefly, films are formed in the hole of a porous plate which is attached to a glass tube and placed in a gas-tight measuring cell in such a way that the film is exposed to the gas pressure Pg and the free end of the glass tube is exposed to the reference pressure Pr. The pressure difference ∆P ) Pg - Pr can be adjusted via two syringes and measured by a difference pressure transducer. Knowing ∆P, one can easily calculate the corresponding disjoining pressure Π (see eq 2 in ref 24). Π-h curves are generated by interferrometrically measuring the equivalent film thickness heq after applying a fixed pressure in the cell. The “true film thickness” h can be obtained according to the three-layer model where the film is considered as a water core of refractive index ns surrounded by two surfactant layers of different refractive indexes (see ref 24 for details). The error bars given for Π-h curves are (50 Pa for the pressure, which results from the uncertainty in the hydrostatic pressure in the glass tube of the film holder. The error of the film thickness is (5% and results from both the scattering of the intensity of the reflected light and the inaccuracy of the three-layer model. (b) Experimental Procedure. The film holder is kept in the surfactant solution overnight to establish equilibrium. To make sure (25) Santini, E.; Ravera, F.; Ferrari, M.; Stubenrauch, C.; Makievski, A.; Kra¨gel, J. Colloids Surf., A, published online Dec 15, http://dx.doi.org/10.1016/ j.colsurfa.2006.12.004.
1686 Langmuir, Vol. 23, No. 4, 2007
Stubenrauch et al.
that the film holder did not pollute the solution, its surface tension was measured over a period of 2 h before each experiment. Eventually, part of the cell was filled with surfactant solution, the lid of the cell was treated with a commercial antifogging agent, and the experimental setup was assembled and left for at least 3 h to ensure vapor-liquid equilibrium. All Π-h curves were reproduced at least once. Measurements were carried out at room temperature (23 ( 1 °C). With regard to the pH-dependent measurements, we first set the pH with either NaOH or HCl and measured it before starting the measurement. In the case of pH ) 5.5, we made sure that the solutions reached their “natural” pH of 5.5 ( 0.3 (due to the dissolution of CO2 in the solution) by mixing them for several minutes in an open flask before loading the film holder. As suggested by Karraker and Radke,23 for measurements at pH > 5.5, the whole instrument (including the syringes, all tubes, and the measuring cell) was purged with N2 gas for several minutes to avoid pH changes due to the dissolution of CO2. To check the efficiency of this procedure, we measured the pH of the surfactant solution that was placed in the measuring cell once again after the measurement, which was about 10-12 h later. Unfortunately, we still observed a drift of 1-2 pH units during the course of the day, the reason for which is unclear. 2.2.2. Determination of the Equilibrium Thickness. The measurements of the film thickness at constant capillary pressure were carried out by employing the microinterferrometric technique that has been described in detail in previous works (e.g., ref 8 and references therein). Horizontal microscopic films with a constant radius of 100 µm were formed in the Scheludko-Exerowa cell, and their thicknesses h were registered. Note that h was measured at least 1 h after the tightly closed measuring cell was mounted on the microscope, so that the vapor pressure inside the cell and the surfactant adsorption at the film interfaces could have reached equilibrium. We assumed that the film thickness reached its equilibrium value when it did not change for 5 min. The experimental points were determined from 10-15 measurements of the thickness of films formed from two to three different solutions. The measurements were carried out at constant temperature T ) 22 °C. The capillary pressures Pc were determined by the Laplace equation Pc ) 2σ/R, where R ) 2.65 mm is the radius of the cell capillary in which the film is formed and σ is the surface tension for the different surfactant concentrations which are listed in Table 1. 2.3. Calculations. The measured Π-h curves were compared with interaction curves calculated within the framework of the DLVO theory. The van der Waals component of the disjoining pressure was calculated with the Hamaker constant A ) 3.7 × 10-20 J for the air-water-air system.1 To obtain the electrostatic component of the disjoining pressure, the nonlinear Poisson-Boltzmann equation was solved. The calculations were done with the algorithm of Chan et al.26 using constant charge boundary conditions and the theoretical Debye length κ-1.1 The parameter extracted from these calculations is the apparent surface potential ψ0, with which the corresponding surface charge density q0 can be calculated using the Grahame equation:1 q0 ) x80RTc sinh
( ) Fψ0 2RT
(1)
To calculate the surface potentials and surface charges from the measured equilibrium thicknesses, we used the same equations. The parameters necessary for these calculations are the film thickness h at the corresponding capillary pressure Pc, which equals the disjoining pressure Π.
3. Results The effect of the pH on equilibrium thicknesses and disjoining pressure versus thickness curves (Π-h curves) of foam films stabilized by the nonionic sugar surfactant n-dodecyl-β-Dmaltoside (β-C12G2) has been examined. For that purpose, the (26) Chan, D. Y.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283.
Figure 1. Π-h of β-C12G2 foam films measured for different pHs (3.0-9.2) at a constant ionic strength of 10-3 M (NaOH + HCl) and a constant surfactant concentration of 6.8 × 10-5 M (cmc ) 1.5 × 10-4 M24). Data for pH ) 4 and 5.5 (the “natural” pH) are taken from ref 24. The lines are calculated according to the DLVO theory, from which the surface charge density is calculated to be 3.2 and 2.1 mC m-2.
pH of surfactant solutions containing different amounts of surfactant and electrolyte was varied. Film thicknesses were found to range from nearly 100 nm down to 6 nm, depending on the composition of the sample and the applied pressure, which ranges from 30 to 9000 Pa. Model DLVO calculations using constant charge boundary conditions and the theoretical Debye length κ-1 were performed and provide unequivocal evidence for the electrostatic nature of the long-range repulsive force stabilizing the common black films (CBFs). 3.1. Foam Films Stabilized with 6.8 × 10-5 M n-Dodecylβ-D-maltoside. The starting point of (and the motivation for) the present work was a previous study in which we observed that Π-h curves of foam films stabilized by the nonionic sugar surfactant β-dodecyl maltoside (β-C12G2) are indistinguishable in a pH range from 4 to 8,24 although a pH dependence of Π-h curves is regarded as a general feature of nonionic surfactants. As previously mentioned in the Introduction, it is very likely that the effective pH range studied in ref 24 was only from 4 to 5.5 as the measurements were not carried out under N2. To complement the results obtained in ref 24, we, first of all, extended the pH range, which is now 3-9. Second, all measurements above the natural pH of 5.5 were carried out under N2 to prevent a decrease of the pH due to the solubilization of CO2 in the aqueous solution. The results obtained at a surfactant concentration of 6.8 × 10-5 M and a constant electrolyte concentration of 10-3 M are seen in Figure 1. (It is important to mention that the concentrations given in ref 24 are not correct. All concentrations have to be multiplied by 0.68 to get the real concentration, which is explained in the erratum of ref 24. Thus, other than those stated in ref 24, the pH-dependent measurements shown in Figure 6 were not carried out at 1.0 × 10-4 M but at 6.8 × 10-5 M β-C12G2, which is the reason why our complementary measurements were carried out at 6.8 × 10-5 M as well.) According to Figure 1, the film thickness and, thus, the electrostatic part of the disjoining pressure increase with increasing pH, which is in agreement with observations made for other nonionic surfactants (see Introduction). More precisely, the surface charge density q0 increases from 2.1 to 3.2 mC m-2 if the pH is increased from 3 to 9. A closer look reveals that the Π-h
pH Study of n-Dodecyl-β-D-maltoside Foam Films
curves and, thus, q0 do not change linearly with increasing pH but that two pH ranges (3-5.5 and 7-10) can be identified in which q0 is pH-insensitive while a significant change is observed between pH ) 5.5 and 7.0. We will come back to this point in section 4.3. Another experimental observation that needs to be mentioned is the fact that the pH changes during the day, although the measurements were carried out under N2 atmosphere. The drifts that occurred in the course of ∼12 h are indicated in the legend of Figure 1. Fortunately, these drifts do not influence the quality of the DLVO fits and, thus, of the derived surface charge densities, as will be discussed in section 4.2. Coming back to Figure 1, one sees that, even at a pH as low as 3, the foam film is a stable CBF and indistinguishable from the films formed at pH ) 4 and pH ) 5.5. Neither a higher tendency toward film rupture nor the formation of an NBF was observed at pH ) 3. Thus, one can conclude that, for the given system (6.8 × 10-5 M β-C12G2 and 10-3 M electrolyte), the isoelectric point must be below 3, i.e., pH* or pHcr < 3. To determine the isoelectric point, we could have reduced the pH further. In this case, however, we would not have been able to carry out measurements at an electrolyte concentration of 10-3 M as pH < 3 requires a H+ concentration of >10-3 M. For the sake of consistency, we decided to change the surfactant and not the electrolyte concentration to learn more about the isoelectric points of this particular surfactant. Moreover, as the focus was on the isoelectric points, we mainly measured equilibrium thicknesses at one capillary pressure and restricted the timeconsuming Π-h measurements to a few illustrative examples. Based on the knowledge we gained so far, two different surfactant concentrations were chosen, namely, 10-5 and 10-4 M. In the first case, i.e., at c < 6.8 10-5 M, the probability of film rupture is expected to be higher and, thus, the influence of the pH on the rupturing of films can be studied. In the second case, i.e., at c > 6.8 × 10-5 M, the probability of an NBF formation is expected to be higher and, thus, the influence of the pH on the NBF formation can be studied. The aim was to determine a pH* and a pHcr value for this particular surfactant. (One always has to keep in mind that there is not a characteristic pH* or pHcr value for a given surfactant, as these values depend on the surfactant concentration, which makes it very difficult to generalize experimental data. To do so, one has to agree on a reference state, which could be, e.g., 2/3(cmc) of the respective surfactant, as suggested in ref 23.) 3.2. Foam Films Stabilized with 10-5 M n-Dodecyl-β-Dmaltoside. The first series of equilibrium measurements was carried out at a surfactant concentration of 1.0 × 10-5 M and a constant electrolyte concentration of 10-3 M. The equilibrium film thicknesses h were measured in a pH range of 3.6-6.0, and the respective surface potentials ψ0 and surface charge densities q0 were calculated using the DLVO theory. The results are seen in Figure 2. As seen in Figure 2a, a pH-insensitive equilibrium film thickness of h ) 53 ( 5 nm is observed from pH ) 6 down to pH ∼ 4.2. At pH < 4.2, h decreases steeply until it reaches 33 nm at pH ) 3.63. At even lower pH values, the film is no longer stable but ruptures immediately. Describing this trend in terms of surface interactions, one sees that ψ0 drops from 27 ( 5 mV down to 13 mV and q0 drops from 2.1 ( 0.5 mC m-2 down to 0.9 mC m-2. Based on these results, one obtains an isoelectric point of pH* ) 3.6. At this point, ψ0 and q0 are zero and an electrostatic film stabilization is no longer possible. As the surfactant concentration is far below the cmc and, thus, the surface concentration of β-C12G2 is not high enough to sterically stabilize the film, an immediate rupture is observed once pH < 3.63.
Langmuir, Vol. 23, No. 4, 2007 1687
Figure 2. (a) Equilibrium thickness h, (b) diffuse electric layer potential φ0, and (c) surface charge density q0 of β-C12G2 foam films as a function of the pH at a constant ionic strength of 10-3 M (NaCl + HCl), a constant surfactant concentration of 10-5 M (cmc ) 1.5 × 10-4 M24), and a constant capillary pressure of 45.7 Pa. All lines are to guide the eye.
The results described above are in perfect agreement with corresponding Π-h studies. First, it is reported in ref 24 that films formed by 6.8 × 10-6 M and 3.4 × 10-5 M β-C12G2 are stable at 10-4 M NaCl and pH ) 5.5. Thus, it is reasonable to claim that a 10-5 M β-C12G2 solution at 10-3 M NaCl and pH ) 5.5 equally forms stable foam films. Second, we carried out additional Π-h measurements at 1.0 × 10-5 M β-C12G2 and pH ) 4. At this pH, neither films containing 10-4 M nor those containing 10-3 M electrolyte were stable: an NBF nucleus was formed in all cases followed by rapid film rupturing; that is, pH* ∼ 4. The slight difference between the pH* values obtained from the equilibrium thickness measurements (pH* ) 3.6) and the Π-h measurements (pH* ∼ 4) is most likely due to the higher pressure that needs to be applied in the latter case. (It was already mentioned in the Introduction that, in a Scheludko-Exerowa cell, Pc is usually 30-100 Pa, while it can be 200-100.000 Pa using the porous plate as film holder. The limits are given by the porosity of the porous plate. The minimum equals the pressure that needs to be applied to form a film at all, while the maximum equals the capillary entry pressure.) In other words, by measuring Π-h curves, one observes rapid film rupturing at surface charge
1688 Langmuir, Vol. 23, No. 4, 2007
Stubenrauch et al.
Figure 4. Π-h of β-C12G2 foam films measured for different pHs (4.0-9.4) at a constant ionic strength of 10-3 M (NaOH + HCl) and a constant surfactant concentration of 10-4 M (cmc ) 1.5 × 10-4 M24). Data for cs ) 6.8 × 10-5 M at pH ) 4.0 are the same as those shown in Figure 1. The lines are calculated according to the DLVO theory, from which the surface charge density is calculated to be 2.1 and 1.5 mC m-2.
Figure 3. (a) Equilibrium thickness h, (b) diffuse electric layer potential φ0, and (c) surface charge density q0 of β-C12G2 foam films as a function of the pH at a constant ionic strength of 10-3 M (NaCl + HCl), a constant surfactant concentration of 10-4 M (cmc ) 1.5 × 10-4 M24), and a constant capillary pressure of 29.2 Pa. All lines are to guide the eye.
densities that are slightly higher than zero. Thus, the determination of isoelectric points is more accurate with equilibrium thickness than with Π-h measurements. 3.3. Foam Films Stabilized with 10-4 M n-Dodecyl-β-Dmaltoside. The second series of equilibrium measurements was carried out at a surfactant concentration of 1.0 × 10-4 M and a constant electrolyte concentration of 10-3 M. The equilibrium film thicknesses h were measured in a pH range of 3.8-6.2, and the respective surface potentials ψ0 and surface charge densities q0 were calculated using the DLVO theory. The results are seen in Figure 3.As seen in Figure 3a, a pH-insensitive equilibrium film thickness of h ) 43 ( 4 nm is observed from pH ) 6.2 down to pH ) 4.0. At pH < 4.0, only NBFs of 6 nm thickness are formed. Describing this trend in terms of surface interactions, one sees that ψ0 ) 17 ( 2 mV and q0 ) 1.2 ( 0.2 mC m-2 from pH ) 6.2 down to pH ) 4.0, while no measurable ψ0 and q0 values were found for pH < 4.0. Based on these results, one obtains an isoelectric point of pHcr ) 3.8; that is, at this pH, ψ0 and q0 are zero and an electrostatic film stabilization is no longer possible. However, in contrast to the observations made for the 10-5 M β-C12G2 solution, the surface concentration of β-C12G2
is now high enough to sterically stabilize an NBF. Comparing the isoelectric point of the 10-5 M β-C12G2 solution (pH* ) 3.6) with that of the 10-4 M β-C12G2 solution (pHcr ) 3.8), one sees that the pH of the isoelectric point increases slightly with increasing surfactant concentration. This observation is in agreement with previous findings17,18 and will be discussed in section 4.2. A closer look at Figure 3a reveals that, at most pH values, two film thicknesses are given. The reason for this is that, in the studied pH range, both CBFs and NBFs are formed. This pH range belongs to the so-called metastable zone as described earlier by Cohen and Exerowa.18 In this zone, a CBF may or may not transfer to an NBF. The probability of the CBF-NBF transformation increases with decreasing pH until NBFs are immediately formed at pH < pHcr. (Metastable zones were also observed at low surface concentrations at which an NBF is not stable.18,21 In this case, the CBF may or may not rupture and the probability of the CBF rupture increases with decreasing pH until the film ruptures immediately at pH < pH*.) This observation can be understood very easily if we consider a Π-h data pair as a point in a Π-h phase diagram, as was suggested only recently.27 In this case, the CBF-NBF transition is a nucleation process as long as it takes place in the metastable region between the binodal and the spinodal regions, while it is a spinodal decomposition in the two-phase region of the phase diagram. In the present case, the spinodal decomposition can obviously be induced by changing the pH. Again, complementary Π-h measurements were carried out to underpin the results obtained by measuring the equilibrium thicknesses. For that purpose, we studied films stabilized by a 1.0 × 10-4 M β-C12G2 solution containing 10-3 and 10-4 M electrolyte at two different pHs, namely, pH ) 9.4 and 4.0. The results are seen in Figures 4 and 5. We need to mention two general experimental observations before describing the results in more detail. First, as was the case for the 6.8 × 10-5 M β-C12G2 solution, the pH changed during the day although the measurements were carried out under N2 atmosphere. The drifts are slightly lower than those reported in Figure 1 and do not affect the quality of the DLVO fit in the case (27) Stubenrauch, C.; Strey, R. J. Phys. Chem. B 2005, 109, 19798.
pH Study of n-Dodecyl-β-D-maltoside Foam Films
Langmuir, Vol. 23, No. 4, 2007 1689
at pH ) 4.0, which we assume to be approximately the pHcr. As was already the case for the pH* value, we observe a slight difference between the pHcr obtained from the equilibrium thickness (pHcr ) 3.8) and that obtained from the Π-h measurements (pHcr ∼ 4), which is, again, most likely due to the higher pressures that are applied in the latter case. A pHcr ) 3.8 is also supported by the results shown in Figure 5, from which the same general conclusion can be drawn as that from Figure 4, because the isoelectric point does not depend on the electrolyte concentration celec, as was shown by Li and Somasundaran for the bare water/air interface.29 Last but not least, we plotted in Figures 4 and 5 one additional Π-h curve that was measured under slightly different conditions. The results will be compared and discussed in section 4.2.
4. Discussion Figure 5. Π-h of β-C12G2 foam films measured for different pHs (4.0-9.7) at a constant ionic strength of 10-4 M (NaOH + HCl) and a constant surfactant concentration of 10-4 M (cmc ) 1.5 × 10-4 M24). Data for cs ) 6.8 × 10-5 M refer to the “natural” pH of 5.5 and are taken from ref 24. The solid line is calculated according to the DLVO theory, from which the surface charge density is calculated to be 1.2 mC m-2.
of the solution containing 10-3 M electrolyte (Figure 4). However, a jumplike reduction of the film thickness by ∼10 nm is seen around 700 Pa for the Π-h curve measured at 10-4 M electrolyte (Figure 5). Speculative as it might be, we believe that the pH drift might be the reason for this observation although one would expect a continuous rather than a jumplike change if the pH and, thus, the surface charge density changed continuously. Note that, at the same surfactant concentration and pH, the total surface charge density q0 at 10-4 M electrolyte is much lower than that at 10-3 M electrolyte and, thus, much more sensitive to slight changes. Leaving this question unanswered, we would like to point out a second observation, namely that some of the Π-h curves cannot be fitted at low thicknesses (see also Figure 1). As can be seen in Figures 1, 4, and 5, as well as in refs 15, 24, and 28, this is a general observation and, thus, not related to any pH drifts. We face here the same phenomenon as the one discussed in connection with Figure 3a, namely a metastable zone in the phase diagram of a foam film.27 The difference between Figure 3a and the Π-h curves is the fact that, in the latter case, the CBF-NBF transition is not induced by pH changes but by the applied pressure. We attribute the “nonfittable” thicknesses to the fact that no equilibrium was reached at these particular pressures and that an NBF formation would have finally taken place if we had waited long enough. As we measured these Π-h curves at a time when we were not aware that phase diagrams can be used to describe Π-h curves of foam films, it is very likely that we simply “overshot” the pressure limit at which the transition to the NBF occurs. Let us come back to the question of whether the measured Π-h curves support the results obtained by measuring the equilibrium thicknesses. First, by comparing Figures 3c and 4, one sees that the surface charge densities at pH ) 4 are the same for the foam films studied with the Scheludko-Exerowa cell (q0 ) 1.2 ( 0.2 mC m-2) and for those studied with the porous plate (q0 ) 1.5 mC m-2). Second, in Figure 4, further experimental evidence for the finding that the pH of the isoelectric point is pHcr ) 3.8 is given. While an electrostatically stabilized CBF is formed at pH ) 9.4-8.2, it is an NBF that is directly formed (28) Schlarmann, J.; Stubenrauch, C. Tenside, Surfactants, Deterg. 2003, 40, 190.
4.1. Theoretical Background. It is no longer in doubt that the charge-giving ions in foam films stabilized by nonionic surfactants are OH- ions at the water/air interface, as was originally proposed by Exerowa.16 Zeta potential29-31 as well as thin film studies on polyelectrolyte-coated substrates32 provided independent evidence for a negative net charge at the water/air interface both in the absence and in the presence of a nonionic surfactant. (As demonstrated and discussed in refs 10, 35, and 37 and references therein, the water/air and the water/oil interfaces behave in a similar manner. Thus, all given explanations and proposed models refer to both interfaces. In the present work, however, we will only talk about the water/air interface.) Other sources such as the adsorption of HCO3- ions (from the dissolution of CO2) or negatively charged electrolyte ions (reviewed in ref 10) as well as the adsorption of surface active charged impurities33,34 were ruled out experimentally. However, neither the source of the OH- ions nor the adsorption mechanism is clear. It is widely accepted that the source of the OH- ions is the dissociation of water. This source, however, requires a specific adsorption of OH- at the water/air interface, as the amount of OH- ions needed for the measured surface charge densities is at least 3 orders of magnitude higher than the corresponding OH- bulk concentration (see ref 10 for details). Thus, a new mechanism explaining the source of the OH- ions at the water/ air interface has been proposed33 which is not based on adsorption. However, as this mechanism does not explain all experimental observations, we will use the traditional picture of specific OHadsorption in the following. The current discussion is based on the fact that the water molecules at the water/air interface are ordered.1,36 It is suggested that strong hydrogen bonds between the interfacial water and OH- ions lead to the specific adsorption of OH- ions. Although these bonds also exist in the bulk water, it is proposed by Marinova et al.37 that fractions of the H-bonds in the bulk are broken due to the Brownian motion. In other words, specific adsorption could result from restricted movement of the water molecules in the interfacial layer leading to more pronounced H-bonds between OH- and water molecules compared to the bulk. Having (29) Li, C.; Somasundaran, P. J. Colloid Interface Sci. 1991, 146, 215. (30) Usui, S.; Sasaki, H. J. Colloid Interface Sci. 1978, 65, 36. (31) Yoon, R. H.; Yordan J. L. J. Colloid Interface Sci. 1986, 113, 430. (32) Ciunel, K.; Armelin, M.; Findenegg, G. H.; v. Klitzing, R. Langmuir 2005, 21, 4790. (33) Stubenrauch, C.; Rojas, O. J.; Schlarmann, J.; Claesson, P. M. Langmuir 2004, 20, 4977. (34) Rojas, O. J.; Stubenrauch, C.; Schulze-Schlarmann, J.; Claesson, P. M. Langmuir 2005, 21, 11836. (35) Beattie, J. K.; Djerdjev, A. M. Angew. Chem. 2004, 43, 3568. (36) Pratt, L. R.; Pohorille, A. Chem. ReV. 2002, 102, 2671. (37) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045.
1690 Langmuir, Vol. 23, No. 4, 2007
said this, we can ask ourselves why the surface charge density q0 (a) decreases with increasing concentration of nonionic surfactant cs and (b) increases with increasing pH. (a) The decrease of q0 with increasing cs is a well-known phenomenon, an example of which is given in the Appendix. The explanation commonly given is a decrease of the area available for the adsorption of OH- ions. The ions are expelled from the interface while a nonionic surfactant adsorption layer is built up. However, there is ample space for OH- ions, even at very high surfactant adsorption levels (i.e., next to the cmc), so that the expulsion of the ions is not for want of geometrical space. Remembering the fact that the water molecules at the pristine water/air interface are highly ordered, one can argue that, in the presence of surfactant, the interfacial water consists mainly of hydration water attached to the polar head groups. The more surfactant is adsorbed, the less “oriented” water molecules are present, which was the condition for specific OH- adsorption. In this case, a decrease of the interfacial charge with increasing surfactant concentration is expected. However, it was found that the charge is nearly constant at low surfactant concentrations and does not decrease significantly until the concentration approaches and exceeds the cmc (see Figure 7 as well as refs 17, 19, 20, 22, 24, and 44). This behavior contrasts sharply with the adsorption of the surfactant, which changes significantly at low concentrations and already stays constant far below the cmc. Thus, a typical competitive adsorption between the surfactant and OH- was excluded and a two-site mechanism for the adsorption was proposed.23 This model is based on adsorption sites on the interface at which only surfactant can adsorb (S1), whereas at other sites either surfactant or OH- can adsorb (S2). As the surfactant adsorption (0.5-0.4 nm2/surfactant) is 2 orders of magnitude larger than the OH- adsorption (∼100-500 nm2/ charge), a “permanent surfactant background” (S1) is present. It is only for the S2 sites that competition occurs. Note that, if such sites do exist, they should be considered as highly dynamic and being related to the specific structure of the interfacial water, as discussed above. Unfortunately, this model does not describe the experimental results satisfactorily and can, thus, be considered as a qualitative description only. We will come back to this model in section 4.3. (b) The assumption that the electrostatic repulsion in nonionic foam films is due to the specific adsorption of OH- ions at the water/air interface is underpinned by pH-dependent measurements. As already mentioned in the Introduction, a decrease of the pH of the solution at constant ionic strength leads to a decrease of the surface charge density until q0 ) 0 at the isoelectric point.16-18,20,21,23 In other words, q0 increases with increasing pH as the overall amount of OH- ions increases. Note that it is not only for low but also for high pH values that a limit is reached. It is argued in ref 23 that a maximum coverage of OH- ions is already reached at very low q0 values (∼2.5 mC m-2 in a 3 × 10-3 M electrolyte solution, which corresponds to 65 nm2 per charge), underlining the argument that the adsorption is not limited by finite size but by the availability of adsorption sites. In the following, we will discuss our results in the light of the specific adsorption of OH- ions at the water/air interface. 4.2. Π-h Curves and Equilibrium Thicknesses of β-C12G2. It was concluded from Figure 1 that the Π-h curves and, thus, q0 do not change linearly with increasing pH as two pH ranges (3-5.5 and 7-10) were identified in which q0 is pH-insensitive, while a significant change was observed between pH ) 5.5 and 7.0. It has to be pointed out that this observation is in agreement with the results obtained for β-C8G1 foam films (see Figure 7 in ref 20) for which pH-dependent measurements were also carried
Stubenrauch et al.
Figure 6. Surface charge densities q0 of foam films as a function of the pH at a constant surfactant (cs) and constant electrolyte (celec) concentration. (a) β-C12G2 with cs ) 6.8 × 10-5 M (0.5 cmc), celec ) 10-3 M and β-C8G1 with cs ) 1.0 × 10-2 M (0.5 cmc), celec ) 10-3 M.20 The q0 values are calculated from the respective Π-h curves. The dashed lines are to guide the eye. (b) C12E5 with cs ) 10-5 M [1/7(cmc)], celec ) 10-3 M17 and C10E8 with cs ) 8 × 10-4 M [2/3(cmc)], celec ) 3 × 10-4 M.23 Equilibrium film thicknesses (in ref 17) and Π-h curves (in ref 23) were measured to obtain q0. The data for the bare water/air interface were obtained by measuring the electrophoretic mobility of gas bubbles in an electrolyte solution of celec ) 10-2 M.29 The solid lines are linear regressions to guide the eye.
out at c ) 0.5(cmc) and can, thus, be compared directly. For β-C8G1, a pH insensitivity was observed from pH ) 10 to 6, followed by a steep decrease of q0 from 4.1 to 1.5 mC m-2 as the pH was decreased from 6 to 5. A further decrease to pH ) 4 led to an immediate film rupture, which suggests that q0 ∼ 0 and, thus, pH* ∼ 4. One could argue that the pH insensitivity of β-C8G1 between 6 and 10 is simply due to the fact that there was no pH difference at all as these measurements were carried out under normal, i.e., CO2 containing, atmosphere. However, for some reason, the pH of the solutions studied under N2 also drifted by up to 2 units. This drift, however, turned out to be very helpful as it demonstrates the pH insensitivity very convincingly. Although measured at “changing pH”, the Π-h curves could be fitted reasonably well with the DLVO theory. In other words, if the drift in pH caused a change of the surface charge density, we would not be able to fit the Π-h curves with a constant surface charge density, namely 2.1 or 3.2 mC m-2. As a rule of thumb, one can say that Π-h curves, and thus q0, of foam films stabilized by sugar surfactants at c ∼ 0.5(cmc) are insensitive to pH changes for pH > 6-7. A further decrease of the pH leads to a steep decrease of q0 around pH ) 5.5 (natural pH), which, in turn, leads to the formation of a thinner CBF in the case of β-C12G2 and to film rupture in the case of β-C8G1. For β-C12G2, it is only at even lower pH values that q0 is neutralized, which would most likely lead to an NBF formation at the concentration in question. According to Figures 2c and 3c, this latter change of q0 is expected to be as steep as the one around pH ) 5.5. Based on the knowledge gained so far, one can construct q0-pH curves for β-C12G2 and β-C8G1 (see Figure 6a). We will discuss these
pH Study of n-Dodecyl-β-D-maltoside Foam Films
curves in the light of OH- ion adsorption and compare them with q0-pH curves obtained for other nonionic surfactants in section 4.3. From Π-h curves measured at slightly different conditions, we can learn a lot about the influence that changes in the surfactant concentration cs and the pH can have. As is clearly seen in Figure 4, slight changes of the surfactant concentration may have an enormous effect on the resulting surface interactions. At pH ) 4, the system forms a stable CBF at 6.8 × 10-5 M β-C12G2, while it immediately forms an NBF at 10-4 M β-C12G2. Comparing the results obtained at pH ∼ 9 at these two concentrations (see Figures 1 and 4), the same enormous influence of cs is observed, namely a decrease of q0 from 3.2 mC m-2 (Figure 1) at 6.8 × 10-5 M β-C12G2 down to 1.5 mC m-2 at 1.0 × 10-4 M β-C12G2 (Figure 4), which is huge compared to the effect a pH change has. The strong influence of the surfactant concentration is supported by the results obtained for the equilibrium thickness h, which also changes significantly between 6.8 × 10-5 M and 1.0 × 10-4 M (see Figure 7 where the values of h, ψ0, and q0 are given as a function of the β-C12G2 concentration at pH ) 5.5 and 10-3 M NaCl). These findings are in line with all observations made so far, namely that q0 is nearly constant at low surfactant concentrations and does not decrease significantly until the concentration approaches the cmc, as was already mentioned. For the present case, it holds cs g 0.5 cmc with cmc ) 1.5 × 10-4 M. In other words, at cs , cmc, we would not expect significant changes in q0 with changing cs, which is indeed the case and is illustrated in Figure 7. An interesting example of “compensating effects” is seen in Figure 5, where Π-h curves measured at 10-4 M electrolyte are shown. Comparing the curve measured at pH ) 5.5 and 6.8 × 10-5 M β-C12G2 with that measured at pH ) 9.7-8.4 and 1.0 × 10-4 M β-C12G2, one can argue that the effects of the concentration and the pH compensate each other: at the lower surfactant concentration, thicker films are expected, which is obviously counterbalanced by the lower pH at which thinner films should be formed. Note that both changes are expected to significantly affect q0 as the surfactant concentrations are around the cmc and the change in pH covers the range in which a steep change is expected (see Figure 6a). Note that a quantitative comparison of the influence the surfactant concentration and the pH have at 10-4 M electrolyte would only be possible if we had Π-h curves at pH ) 5.5 and pH ) 9 for both 6.8 × 10-5 M β-C12G2 and 1.0 × 10-4 M β-C12G2. In any case, none of the effects dominate, which results in the same Π-h curves for 6.8 × 10-5 M and 1.0 × 10-4 M β-C12G2, ignoring the thickness jump of the latter at 700 Pa. As discussed above, a lot of information can be derived from Π-h curves. However, measuring them is very time-consuming, which considerably limits the number of concentrations and pH values that can be studied. On the other hand, equilibrium film thicknesses can be measured much more easily, thus providing a chance to systematically vary concentrations and pH values. We made use of this and determined two isoelectric points of the pseudobinary system (water/electrolyte/β-C12G2) by reducing the pH in small steps. The results are seen in Figures 2 and 3 and are described in sections 3.2 and 3.3. Comparing Figures 2 and 3, one sees that the pH of the isoelectric point slightly increases with increasing surfactant concentration, namely from pH* ) 3.6 at 10-5 M β-C12G2 up to pHcr ) 3.8 at 10-4 M β-C12G2. Although the absolute increase is very small, the tendency is noticeable and in line with previous studies.17,18 This trend is what one expects, as the surface charge density generally decreases with increasing surfactant concentration at a fixed pH. Hence, the total number of positive charges needed to neutralize the
Langmuir, Vol. 23, No. 4, 2007 1691
surface decreases, which is reflected in a higher pH* or pHcr value. The fact that the pH increases only by 0.2, although the surfactant concentration is increased by a factor of 10, is easily understandable if we look at Figure 7. In the concentration range under discussion, there is no significant change in the surface charge density at pH ) 5.5. Anticipating that this also holds true for pH ∼ 4, we expect only a slight difference in q0 and, thus, in the isoelectric point. Our results support previous findings for other nonionic surfactants and can, thus, be considered as a general phenomenon. For the sake of completeness, we ought to mention that the opposite behavior, namely a decrease of pH* and pHcr, with increasing surfactant concentration was also observed.8,38 This behavior can only be explained if the surfactant carries a charge or is contaminated with negatively charged surface-active impurities, the surface concentration of which increases with increasing total surfactant concentration. In conclusion, one can say that the magnitude and nature of surface interactions in foam films stabilized by nonionic surfactants clearly depend on the surfactant concentration, the pH, and the electrolyte concentration. The challenge is to pinpoint the conditions under which changes of these parameters lead to either significant or negligible effects. This knowledge will enable us to tune properties of single foam films and, in the long term, of foams. Three important findings can be extracted from the pH dependence of Π-h curves and equilibrium thicknesses. First, for a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs ∼ cmc. This holds true for both low and high pH values. Second, for a given cs, a change in the pH leads to at least one pronounced effect, namely at the isoelectric point (i.e., at pH ) pH* and pH ) pHcr), where the surface charge density q0 is zero. We will come back to this point in section 4.3. Third, the pH of the isoelectric point increases with increasing surfactant concentration. 4.3. Adsorption of OH- Ions at the Water/Air Interface. Let us come back to the q0-pH curves constructed for β-C12G2 and β-C8G1 and compare them with the q0-pH curves obtained for the nonionic surfactants C12E517 and C10E823 . It has to be kept in mind that only the measurements for β-C12G2 and C10E8 were carried out under N2 atmosphere, so the results obtained for β-C8G1 and C12E5 have to be interpreted carefully. The results for these four nonionic surfactants are seen in Figure 6. For comparison, the q0-pH curves for the bare water/air interface measured via the electrophoretic mobility of gas bubbles in an electrolyte solution are also shown.29 As is clearly illustrated in Figure 6, the surface charge density q0 drops steeply in a relatively small pH range in the case of the sugar surfactants. The main difference between β-C8G1 and β-C12G2 is the fact that only one steep change was observed for the former, whereas two such changes were observed for the latter. (We expect the q0-pH curves seen in Figures 2c and 3c to look like Figure 6 once measurements at higher pH values have been carried out. Note that equilibrium thicknesses were only measured up to pH ∼6.) The situation is completely different for the CiEj surfactants for which a continuous change of q0 was found in agreement with the observations made for the bare water/air interface. A continuous change would be expected if there were a linear relation between pH and the number of surface charges. The examples shown in Figure 6 are those for which the most experimental data are available. Because q0-pH curves of other nonionic surfactants16,18,21 were measured at pH < 6 only and, thus, cover no more than the first part of the q0-pH (38) Exerowa, D.; Zacharieva, M. In Research in Surface Forces; Derjaguin, B. V., Ed.; Plenum Press: London, 1972; Vol. 2, p 234.
1692 Langmuir, Vol. 23, No. 4, 2007
curve (see, e.g., Figures 2c and 3c), they do not lend themselves to comparison. To explain the different q0-pH curves, we have to take a close look at the adsorption mechanism. In ref 23, a Langmuir-like adsorption of OH- ions is proposed for foam films stabilized by C10E8 and C12E4. What is strange, however, is the fact that the experimental data cannot be fitted at all with the proposed Langmuir-type monolayer formation (see Figure 8 in ref 23), and we are puzzled by the author’s comment that “the hydroxideion specific adsorption process seems well described by a Langmuir expression” (p 245). However, these data can be connected by a straight line as seen in Figure 6b and are, thus, in agreement with the results obtained for C12E5. The differences in the absolute q0 values are due to the different electrolyte concentrations used in these measurements. Unfortunately, the adsorption mechanism is not known yet. However, what can be said at this stage is that the adsorption mechanism is expected to be the same as the one taking place at the bare water/air interface. As discussed in section 4.1, there is no lack of space for OH- adsorption, which means that the surfactant can indeed be considered to be a “permanent background”, as was suggested in ref 23. A variation of the pH simply changes the number of OH- ions at the “available adsorption sites” and, thus, the surface charge density, as seen in Figure 6b. Again, neither the mechanism nor the driving force for such an adsorption is understood. Let us now have a look at Figure 6a. The q0-pH curve of β-C8G1 indeed follows a Langmuir-type monolayer adsorption of OHions. Assuming that it is indeed a Langmuir-type adsorption (which still has to be proven), one could argue that, in the case of β-C12G2, the adsorption takes place via a multilayer formation (i.e., bilayer in the present case) and not via a monolayer formation. Speculative as it might be, a bilayer formation would explain the experimental results described above, and we hope that our proposition will trigger further discussion and theoretical considerations. Unfortunately, as previously mentioned, there are no other data available for comparison. To prove or disprove the two steps observed in the q0-pH curve, additional h-pH curves have to be measured over a much broader concentration and electrolyte range. Let us conclude this section with suggestions for the different behaviors seen for the two classes of nonionic surfactants. It is known that sugar- and ethylene oxide-based surfactants behave quite differently, although they are both nonionic. One example of how different their properties can be is the adsorption of C12E6 and β-C12G2 on hydrophilic silica. At first sight, one would not expect a big difference as both surfactants are uncharged; that is, the interactions between the silica surface and the nonionic surfactants cannot differ very much. In fact, quite the opposite was observed. While C12E6 adsorbs strongly on silica, β-C12G2 does not adsorb at all. This is surprising and still not understood.39,40 The most prominent difference, however, lies in their temperature sensitivity: while the physicochemical properties of aqueous solutions of sugar surfactants are not very temperature sensitive, those of the corresponding ethylene oxide solutions are. The temperature insensitivity of sugar surfactants in aqueous solution results from the strength of the hydrogen bonds between the hydroxy groups of the sugar unit and water, which prevents any significant dehydration of the head group in the experimentally relevant temperature range. In contrast to the strong hydrogen bonds between the water and sugar units, the hydration water of the corresponding ethylene oxide units is attached only via weak dipole-dipole interactions, which leads to an easy dehydration (39) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2004, 20, 4051. (40) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2005, 21, 2766.
Stubenrauch et al.
of the head group (see, e.g., ref 41, where the temperature sensitivity of microemulsions formulated with these different surfactants is discussed). It is, however, not only the strength of hydration but also the hydration number that is completely different. It was found that, under similar conditions and for similar head group sizes (a glucose unit is comparable with four ethylene oxide units), the hydration of ethylene oxide-based surfactants is 1 order of magnitude higher than that of sugarbased surfactants.42 Last but not least, the flexibility of the head groups is different. While a maltoside unit behaves like a hard disk, the ethylene oxide units behave more like short polymer chains, which, in turn, means that they are much more flexible.43 Having these differences in mind, we can come back to our problem of different q0-pH curves. We have to recall that, in the presence of surfactant, the interfacial water consists mainly of hydration water attached to the polar head groups. The more surfactant is adsorbed, the less “highly ordered” water molecules are present at the water/air interface, which was the condition for specific OH- adsorption. In other words, for a continuous increase of q0 with increasing pH, we need a continuous increase of “highly ordered” interfacial water molecules although the total composition of the system does not change. In the case of ethylene oxide units, these molecules could be easily provided by releasing some of the hydration water, a process that would be much more difficult for the strongly bonded hydration water of the sugar units. In other words, an ethylene oxide unit is able to “react” to pH changes by changing either its hydration degree (easy uptake and release of water) or its conformation (high flexibility), thus being able to provide enough interfacial water molecules at which OH- ions could adsorb. As both processes are very unlikely in the case of a sugar unit, the uptake of OHions is given by the total surfactant concentration rather than by the pH. As neither the number of “highly ordered” interfacial water molecules nor the conformation can change significantly, there is no process via which more sites for the adsorption of OH- ions could be created, which explains the pH insensitivity. At the isoelectric point, the system seems to collapse, an observation that we cannot explain yet. In addition, the “two step” behavior of β-C12G2 is also far from being understood. Leaving these questions unanswered, we want to conclude with the remark that any adsorption mechanism that would explain the results seen in Figure 6 needs to consider the differences between the two types of surfactants.
5. Conclusion We studied the influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-β-Dmaltoside, β-C12G2. Foam film measurements were carried out with the thin film pressure balance technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure and, with the latter, complete disjoining pressure versus thickness curves (Π-h curves) were measured. Equilibrium film thicknesses were measured in a pH range from 3 to 6.5, while Π-h curves were measured from pH ) 3 to 9. Most of the results were obtained for 10-4 and 10-5 M β-C12G2 solutions that contained 10-3 M electrolyte. For the sake of clarity and comparison, selected data for different surfactant and electrolyte concentrations were also studied. (41) Kluge, K.; Sottmann, T.; Stubenrauch, C.; Strey, R. Tenside, Surfactants, Deterg. 2001, 38, 30. (42) Claesson, P. M.; Kjellin, M.; Rojas, O.; Stubenrauch, C. Phys. Chem. Chem. Phys., submitted (43) Persson, C. M.; Kjellin, U. R. M.; Eriksson, J. C. Langmuir 2003, 19, 8152.
pH Study of n-Dodecyl-β-D-maltoside Foam Films
Measuring Π-h curves and equilibrium thicknesses as a function of the pH yielded three results which we believe to be very helpful. (a) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs ∼ cmc. This holds true for both low and high pH values. (b) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (c) The pH of the isoelectric point increases with increasing surfactant concentration. Previous findings were contradictory, and we believe that this independent study clarified this issue. These findings are an important step toward the final goal, i.e., to specify the conditions under which changes of the surfactant concentration, the pH, or the electrolyte concentration, or any kind of additive lead to either significant or negligible effects. The most important result of the present study, however, is the q0-pH curves that we constructed on the basis of both the present study and previous works. The q0-pH curve of β-C12G2 shows two pH ranges (3-5.5 and 7-10) in which q0 is pH insensitive, while a significant change of q0 was observed between pH ) 5.5 and 7.0. A similar pH insensitivity was found for foam films stabilized by β-C8G1, while for C10E8 and C12E5 q0 increases linearly with the pH. The first results could be explained with a Langmuir-type adsorption of OH- ions, which leads to the formation of a bilayer in the case of β-C12G2 and of a monolayer in the case of β-C8G1. To prove or disprove this adsorption model, we need (a) to extend the existing pH-dependent measurements toward lower and higher pH values to cover a broader pH range (at least pH ) 2-11) and (b) to develop a theoretical description of the underlying adsorption process. Another open question is the difference in the q0-pH curves of the two types of surfactant. Speculative as it may be, we would like to give a qualitative explanation for this difference. We argue that an ethylene oxide unit is able to “react” to pH changes by changing either its hydration degree (easy uptake and release of water) or its conformation (high flexibility), thus being able to provide enough interfacial water molecules at which OH- ions could adsorb. As both processes are very unlikely in the case of a sugar unit, the uptake of OH- ions is given by the total surfactant concentration rather than by the pH, which would explain the observed pH insensitivity. Acknowledgment. C.S. is indebted to the DFG, the Fond der Chemischen Industrie, and the European Commission (Marie Curie RTN SOCON, Contract No. MRTN-CT-2004-512331) for financial support. The authors wish to thank Dr. Judith SchulzeSchlarmann for measuring the Π-h curves.
Appendix For the sake of completeness, we added results that were obtained by measuring the equilibrium thickness h of β-C12G2 stabilized foam films as a function of the surfactant concentration at pH ) 5.5 and at 10-3 M NaCl. The experimental results as well as the calculated surface potentials ψ0 and surface charge densities q0 are seen in Figure 7. The capillary pressures Pc and surface tensions σ used for calculating the latter are listed in Table 1. As is seen in Figure 7, the film thickness and, thus, the surface interactions are nearly constant at low surfactant concentrations and do not change significantly until the concentration approaches the cmc, which is in perfect agreement with observations made
Langmuir, Vol. 23, No. 4, 2007 1693
Figure 7. (a) Equilibrium thickness h, (b) diffuse electric layer potential φ0, and (c) surface charge density q0 of β-C12G2 foam films as a function of the surfactant concentration cs (cmc ) 1.5 × 10-4 M24) at a constant ionic strength of 10-3 M (NaCl + HCl) and the “natural” pH of 5.5. The capillary pressure for each concentration is listed in Table 1. All lines are to guide the eye. Table 1. Capillary Pressures Pc and Surface Tensions σa for the Surfactant Concentrations cs Studied in the Present Paper cs/M σ/(mN m-1) Pc/Pa a
1 × 10-6 71.0 54.6
5 × 10-6 65.0 50.7
1 × 10-5 60.5 44.7
3 × 10-5 51.0 38.4
1 × 10-4 38.7 29.2
Data are taken from ref 25.
for other nonionic surfactants.17,19,20,22,24,44 At 10-4 M β-C12G2 [2/3(cmc)] the thickness changes abruptly and an NBF is formed. Note that Muruganathan et al.45 also studied how the equilibrium thickness of foam films stabilized by β-C12G2 depends on the surfactant concentration. The focus of this work, however, was on the properties of NBFs, and the measurements were carried out at a high salt concentration (2 × 10-1 M) to ensure NBF formation. LA062310M (44) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1089. (45) Muruganathan, R. M.; Krustev, R.; Mu¨ller, H.-J.; Mo¨hwald, H.; Kolaric, B.; v. Klitzing, R. Langmuir 2004, 20, 6352.
