Langmuir 2007, 23, 12457-12460
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Comments Comment on “Hydrophobic Forces in the Foam Films Stabilized by Sodium Dodecyl Sulfate: Effect of Electrolyte” and Subsequent Criticism
The paper cited in the title1 is concerned with the disjoining pressure Π acting between the two surfactant monolayers of foam films stabilized by 10-4 M sodium dodecyl sulfate (SDS) in the absence and the presence of the electrolyte sodium chloride (NaCl). It is argued that the resulting disjoining pressure versus thickness curves cannot be described with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Thus, an attractive long-range hydrophobic force is added to the repulsive electrostatic and attractive van der Waals forces. Although the physicochemical origin of this force is still unknown (the lack of knowledge is convincingly described by the authors themselves6,7), all deviations are attributed to this force. However, the experimental curves the authors are referring to can be described with the DLVO theory if the surface potentials deduced from disjoining pressure measurements are regarded as apparent values. Arguments for this approach will be given below, and thus, an additional force is not required in this case. If the existence of a long-range hydrophobic force in foam films was claimed only in the cited paper, we would not feel forced to comment on it. However, not only have an increasing number of papers appeared over the past few years2-5 but also a whole book chapter is devoted to the “Hydrophobic Forces in Foam Films”.6 It is primarily because of this increase in “publicity” that it is necessary not only to comment on this approach but also to show that in foam films the existence of such a force has not been proven and that alternative explanations can be given. The main goals of this comment are the following: (i) To initiate a detailed scientific discussion on the question about the possible role of hydrophobic forces in differen types of systems, starting with a particular emphasis on the foam films. (ii) To turn the attention of both the experimentalists and the theorists on the fundamental flaws of the strategy to reveal and substantiate new types of forces by simply fixing the difference between the experimentally measured data and the data calculated with one or another version of an extended DLVO theory. We do not express any doubts on the quality or reliability of the * To whom correspondence should be addressed. Fax +353-17161177. Telephone: +353-1-7161923. E-mail: cosima.stubenrauch@ ucd.ie. † University College Dublin. ‡ Universite ´ Paris Sud. § Bulgarian Academy of Sciences. | University of Sofia. ⊥ Royal Institute of Technology. O Institute for Surface Chemistry. # A. N. Frumkin Institute of Physical Chemistry and Electrochemistry. ∇ Technische Universita ¨ t Berlin. (1) Wang, L.; Yoon, R.-H. Langmuir 2004, 20, 11457. (2) Yoon, R.-H.; Aksoy, B. S. J. Colloid Interface Sci. 1999, 211, 1. (3) Angarska, J. K.; Dimitrova, B. S.; Danov, K. D.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2004, 20, 1799. (4) Wang, L.; Yoon, R.-H. Colloids Surf., A 2005, 263, 267. (5) Wang, L.; Yoon, R.-H. Colloids Surf., A 2006, 84, 282-283. (6) Yoon, R.-H.; Wang, L. In Colloid Stability: The Role of Surface Forces, Part 1; Tadros, T., Ed.; Colloid and Interface Science Series; Wiley-VCH: Weinheim, 2006; Chapter 7.
experimental data,1-5 but we would like to challenge the interpretation of this data. We first will comment on the paper cited in the title which is about ionic surfactants (sections 1-3) and then extend the discussion to nonionic surfactants (section 4) because the same authors also claim to have experimental evidence for the existence of long-range hydrophobic forces in foam films stabilized by nonionic surfactants.5 Finally, the assertion that a long-range hydrophobic force is necessary to describe the thinning of foam films will be questioned (section 5).
1. Evaluation of Π-h Curves It is not surprising that the route described by Wang and Yoon to evaluate the experimentally determined Π-h curves leads to the conclusion that the classical DLVO theory does not describe the experimental data. (The expression “classical DLVO theory” refers to the sum of double-layer forces and van der Waals forces. However, the evaluation of these forces not necessarily has to be “classical”. It is especially the Poisson-Boltzmann approximation for describing the double-layer forces which has its limits (for example, it does not consider ion-ion correlations and confinement effects).) However, we believe that the discrepancy found by Wang and Yoon is not due to the presence of a long-range hydrophobic force but due to the route via which the authors evaluate their data, which will briefly be summarized in the following. It was already in 1999 that the authors calculated the surface potentials in two different ways similar to the approach described in ref 7. First, surface tension isotherms were used to calculate the surface concentration of the ionic species and corrections taking into account counterion condensation were made. The authors call this approach the “counterion binding model”. Second, the surface potentials were calculated from the equilibrium film thicknesses of the foam films applying the classical DLVO theory. Comparing the resulting values, one finds that the surface potentials obtained via the counterion binding model are larger than those calculated with the DLVO theory. Up to this point, the arguments and calculations are straightforward and consistent. However, it is the interpretation of this difference and the consequent postulation of a new force which are questionable. The authors themselves concede that “its (they refer to the hydrophobic force constant) accuracy depends critically on the validity of the double-layer potential” (see p 166 in ref 6). In fact, Attard et al.8 showed already in 1988 that surface potentials (and thus surface charge densities) calculated within the Poisson-Boltzmann (PB) model are lower than the real values. This is due to the fact that ion-ion correlation effects are neglected in the mean field PB model. Let us look at one particular example to illustrate the problem, namely, at a foam film stabilized by 10-4 M SDS in the absence of NaCl. In ref 1, a surface potential of ψ ) -114.3 mV (q ) 5.4 mC m-2) was calculated from the counterion binding model, while a value of ψ ) -74.4 mV (q ) 2.4 mC m-2) was obtained (7) Tchaliovska, S.; Manev, E.; Radoev, B.; Eriksson, J. C.; Claesson, P. M. J. Colloid Interface Sci. 1994, 168, 190. (8) Attard, P.; Mitchell, D. J.; Ninham, B. W. J. Chem. Phys. 1988, 89, 4358.
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from the classical DLVO theory. The corresponding surface charge densities are given in parentheses. Note that the slope of the Π-h curve is fully consistent with DLVO theory and no long-range interaction needs to be invoked if a lower surface potential is used. We argue that the neglect of ion-ion correlation effects in the PB model is one reason why the effective doublelayer potential is lower than the surface potential calculated for a single interface. However, there are also other reasons to expect a difference between the potentials calculated by the two methods. It is well-known that confining systems between two walls changes various properties of the system. For example, one could argue that the dissociation degree of a surfactant in a foam film and at a single surface is not the same. This argument is fully supported by results published only recently.9 In an extensive experimental and theoretical study, the classical DLVO theory was tested over a wide range of ionic strengths by comparing surface charge densities obtained from DLVO fits of Π-h curves with those estimated independently from film conductance and surface tension data. To reconcile the different values of charge densities calculated from surface tension and film conductance with those from disjoining pressure, a simple ion-binding electrostatic model was suggested, which predicts a decrease of the dissociation degree with increasing ionic strength. This model is in agreement with surface tension and film conductance data but does not describe the trends of the measured Π-h curves for which the surface charge density increases with increasing ionic strength. It was concluded that the ion-binding extension of the classical DLVO theory does not permit agreement between theory and independent experimental data from surface tension, disjoining pressure, and film conductance and that thus further theoretical and experimental work is needed. In other words, obtaining different surface potentials for single surfaces and for surfaces of a foam film can tell us something about the dissociation degree in a confined geometry10-12 and does not imply the presence of a long-range hydrophobic force. In addition to different dissociation degrees, a difference in the surface concentration excess Γ could also play a role. A confinementinduced adsorption regulation was not only described theoretically13 but has also been found experimentally for thin liquid films confined between two solid14 and two liquid surfaces15. For foam films stabilized by SDS, it was found that Γ is slightly lower than that of the corresponding single surface.15 The difference was found to be in the range of 10-7-10-8 mol m-2, and it is thus relevant in the present case: at 10-4 M SDS, the surface concentration excess is e10-7 mol m-2.1 Note that a lower Γ is a very reasonable explanation for the different surface potentials calculated by Yoon and Wang. Our main conclusion is that the properties of thin foam films are not simply the sum of two single surfaces. It is the influence of ion-ion correlation effects and effects due to confinement that we need to understand. In our opinion, these effects can be accounted for within the framework of the DLVO theory and thus a long-range hydrophobic force is not needed to explain the results reported by Wang and Yoon. (9) Yaros, H. D.; Newman, J.; Radke, C. J. J. Colloid Interface Sci. 2003, 262, 442. (10) Bergeron, V. Langmuir 1997, 13, 3474. (11) Schulze-Schlarmann, J.; Buchavzov, N.; Stubenrauch, C. Soft Matter 2006, 2, 584. (12) Buchavzov, N.; Stubenrauch, C. Langmuir 2007, 23, 5315. (13) Boinovich, L. B.; Emelyanenko, A. M. AdV. Colloid Interface Sci. 2003, 104, 93. (14) Lokar, W. J.; Koopal, L. K.; Leermakers, F. A. M.; Ducker, W. A. J. Phys. Chem. B 2004, 108, 15033. (15) Mishra, N. C.; Muruganathan, R. M.; Mu¨ller, H.-J.; Krustev, R. Colloids Surf., A 2005, 256, 77.
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2. Film Rupture The authors argue in ref 1 (p 11462, first column, last two sentences) that the films rupture at larger thicknesses and lower pressures than predicted by the DLVO theory. First of all, the DLVO fit they are referring to is the one calculated with ψ ) -114.3 mV. However, we argue that a more correct reference would have been the film thickness and the pressure calculated for ψ ) -74.4 mV (see explanation given above). Second, it is well-known that film rupture does not occur at the pressure maximum calculated by the DLVO theory due to the following two reasons. (1) While the DLVO theory and thus the interaction forces are static values, film rupture is a dynamic process. In the case of common black films (CBFs), film rupture occurs due to fluctuations of both film thickness16,17 and surface concentration.11 The latter fluctuations are dampened if the surface has a sufficiently high elasticity,that is, the higher the surface elasticity the more stable the film should be. Indeed, the latest attempts to correlate film stabilities and surface elasticities turned out to be quite promising.18,19 (2) In the case of Newton black films (NBFs), fluctuations of the surface concentration lead to the nucleation of holes that cause film rupture.20 In addition, it was shown only recently for foam films stabilized by low molecular weight nonionic surfactants that film rupture of CBFs can also occur via nucleation (in this case of NBF spots and not of holes). The amplification of fluctuations and the formation of nuclei (holes in the case of an NBF and NBF spots in the case of a CBF) are purely stochastic events;21that is, the film rupture is not expected to take place at one specific pressure (distance) but in a pressure (distance) range.22,23 We note that the DLVO theory describes van der Waals and double-layer forces between two solid surfaces and between two fluid interfaces equally well. However, the DLVO theory does not consider peculiarities of fluid interfaces such as shape fluctuations or nuclei formation which cause film rupture. Thus, it is misleading to infer a longrange hydrophobic interaction from the pressure (distance) at which film rupture occurs. Moreover, short-range repulsive forces that build up when the critical micelle concentration (cmc) of the surfactant is approached do also play a role. These forces are so large that without surface concentration fluctuations no film rupture would ever occur.
3. Origin of the Long-Range Hydrophobic Force A whole book chapter is devoted to the origin of an attractive long-range hydrophobic force between solid surfaces.24 It is argued that the hydrophobic force is a structural response of water molecules that are in contact with a hydrophobic surface. However, the authors themselves mention computer simulations which indicate that a significant surface effect is noticeable in liquid water only for short distances of ∼1 nm (see p 103 in refs 24 and 25). To extend this range to 50-80 nm (the range of the hydrophobic force), the “formation of extensive long-range structures in a cooperative manner” is claimed,24 although no (16) Scheludko, A. Colloid Chemistry; Elsevier: Amsterdam, 1966. (17) Vrij, A. Faraday Discuss. Chem. Soc. 1966, 42, 23. (18) Stubenrauch, C.; Miller, R. J. Phys. Chem. B 2004, 108, 6412. (19) Santini, E.; Ravera, F.; Ferrari, M.; Stubenrauch, C.; Makievski, A.; Kra¨gel, J. Colloids Surf., A 2007, 298, 12. (20) Kashchiev, D.; Exerowa, D. J. Colloid Interface Sci. 1980, 77, 501. (21) Tsekov, R.; Radoev, B. J. Chem. Soc., Faraday Trans. 1992, 88, 576. (22) Stubenrauch, C. ChemPhysChem 2005, 6, 35. (23) Stubenrauch, C.; Strey, R. J. Phys. Chem. B 2005, 109, 19798. (24) Eriksson, J. C.; Yoon, R.-H. In Colloid Stability: The Role of Surface Forces, Part 1; Tadros, T., Ed.; Colloid and Interface Science Series; WileyVCH: Weinheim, 2006; Chapter 5. (25) Chandler, D. Nature 2005, 437, 640.
Comments
experimental evidence has yet been found. This theory and numerous others that have been put forward to explain the origin of a long-range hydrophobic force need to be recognized as yet unproven suggestions. The questions to be asked in relation to these theories are the following: What do the theories really predict? Which experiments need to be carried out to critically test the theories? Note that it was found experimentally for alcohols that the structural response of a liquid to confinement can indeed cover a range of up to 50 nm film thickness.26 We do not question the general idea of having a “long-range” structural response. What we do criticize is the claim of the existence of a long-range attractive force in aqueous foam films for which there is neither theoretical nor experimental evidence. The only long-range interaction (several tens of nanometers) related to hydrophobicity found so far is due to an excess of negatively charged ions at hydrophobic interfaces, which was found experimentally27-29 and theoretically.30 However, since a foam film is a symmetric film, this force should be repulsive. Last but not least, it is described convincingly in ref 24 that the reproducibility of those experiments in which a long-range attractive surface force “was observed” is not very high. Having these uncertainties and inconsistencies in mind, care should be taken before establishing the presence of a long-range hydrophobic force in foam films. Let us go one step further and assume that there is indeed a long-range hydrophobic force. In this case, we cannot see any justification why this force is described with a “van der Waals-like” equation (see eq 12 in ref 1). While the van der Waals equation has a well-established theoretical background, the corresponding equation for the long-range hydrophobic force has none, and it was introduced by the authors themselves as being purely empiric.
4. Long-Range Hydrophobic Force in Nonionic Foam Films The authors extended their long-range hydrophobic force concept to nonionic surfactants and claimed to have experimental evidence for the existence of an additional long-range attractive surface force in foam films stabilized by nonionic surfactants.5 This time, the conclusions are only based on the kinetics of film thinning, while no equilibrium film thicknesses or Π-h curves were measured by the authors. However, these experiments have been carried out by others who, in nearly all cases, have found good agreement between measured Π-h curVes and interactions calculated within the DLVO framework (reviewed and summarized in refs 31-34). In nearly all cases, the classical DLVO theory provides a perfect explanation for the measured Π-h dependence using the surface potential as a fitting parameter. In the other cases, the reason for the deviation is well-known, namely, the repulsive steric interactions of nonionic surfactants with large (26) Boinovich, L. B.; Emelyanenko, A. M. AdV. Colloid Interface Sci. 2002, 96, 37. (27) Kolaric, B.; Jaeger, W.; Hedicke, G.; Klitzing, R. v. J. Phys. Chem. B 2003, 107, 8152. (28) Ciunel, K.; Arme´lin, M.; Findenegg, G.; Klitzing, R. v. Langmuir 2005, 21, 4790. (29) Qu, D.; Ha¨nni-Ciunel, K.; Rapoport, D.; Klitzing, R. v. In Colloid Stability: The Role of Surface Forces; Tadros, T., Ed.; Colloid and Interface Science Series; Wiley-VCH: Weinheim, 2007; Vol. 3, Chapter 10. (30) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361. (31) Exerowa, D.; Kruglyakov, P. Foam and Foam FilmssTheory, Experiment, Application; Elsevier: Amsterdam, 1998. (32) Stubenrauch, C.; Klitzing, R. v. J. Phys.: Condens. Matter 2003, 15, R1197. (33) Stubenrauch, C.; Rippner-Blomqvist, B. In Colloid Stability: The Role of Surface Forces, Part 1; Tadros, T., Ed.; Colloid and Interface Science Series; Wiley-VCH: Weinheim, 2006, Chapter 11. (34) Kolarov, T.; Cohen, R.; Exerowa, D. Colloids Surf. 1989, 42, 49.
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head groups that extend to distances that allow overcoming of the attractive van der Waals force. This long-range steric repulsive interaction and the compressibility of the large head groups explain the deviations from the classical DLVO theory in these cases. The authors restrict the influence of the long-range hydrophobic force to low surfactant concentrations and argue correctly that at these low surfactant concentrations foam films are very unstable. However, we do have data for concentrations as low as 1/20 cmc,35 and all of the Π-h curves measured at low concentrations follow the DLVO theory. Moreover, we even have data for a “surfactant-free” foam film36 that follows the DLVO theory, which is the most convincing argument against the existence of a long-range hydrophobic force. It is above all in the surfactant-free system (highest possible hydrophobicity of the surfaces) that a long-range hydrophobic force should act. However, this is clearly not the case. In ref 36, the pH-dependence of the equilibrium film thickness was studied systematically at constant low ionic strength. An isoelectric point is reached at a certain pH at which the potential of the diffuse electric layer inclines to zero and film rupture occurs. This experiment is direct proof that the thicker equilibrium films are stabilized by electrostatic forces and thus does not support the existence of a long-range hydrophobic force in foam films.
5. Role of Long-Range Hydrophobic Force in the Thinning of Foam Films The long-range hydrophobic force concept was also used to describe film thinning, that is, drainage. In refs 3-5, it was found that the Reynolds equation (in which the driving force for the drainage is given by the disjoining pressure) does not describe the drainage: the foam films drain faster than theoretically predicted. Attributing the faster drainage to an attractive longrange hydrophobic force and extending the DLVO expression with the respective term for the long-range hydrophobic force indeed leads to a good fit of the experimental data. However, the Reynolds equation was derived for film thinning between flat solid surfaces. Thus, the assumption of tangentially immobile surfaces is incorrect in the case of many foam films as was shown in ref 37. The lower the surfactant concentration and the thicker the film, the more pronounced is this effect.37,38 As the foam films studied by the authors are indeed stabilized by low surfactant concentrations and as they are relatively thick, the foam film surfaces could be tangentially mobile and the accelerated film thinning could be partly due to the convective surface motion of the surfactant (Marangoni effect). A discussion of this effect is missing in the papers of Yoon and Wang. In addition, the film radius Rf might change during drainage, which is not documented either in their papers. Last but not least, the effect of thickness non-homogeneities on film thinning has not been considered. It was found that thickness non-homogeneities accelerate drainage and that this effect strongly increases with increasing film size.39-41 In any case, we are convinced that the assumption of a long-range hydrophobic force to describe the (35) Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem. Phys. 2002, 4, 4504; Stubenrauch, C.; Schlarmann, J.; Strey, R. Phys. Chem. Chem. Phys. 2003, 5, 2736 (erratum). (36) Exerowa, D. Kolloid-Z. 1969, 232, 703. (37) Sonin, A.; Bonfillon, A.; Langevin, D. Phys. ReV. Lett. 1993, 71, 2342. (38) Radoev, B.; Manev, E.; Ivanov, I. Kolloid-Z. 1969, 234, 1037. (39) Manev, E. D.; Sazdanova, S. V.; Wasan, D. T. J. Colloid Interface Sci. 1984, 97, 591. (40) Manev, E.; Tsekov, R.; Radoev, B. J. Dispersion Sci. Technol. 1997, 18, 769. (41) Manev, E. D.; Nguyen, A. V. Int. J. Miner. Process. 2005, 77, 1.
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measured thinning of foam films is quite an unsatisfactory and a highly improbable explanation. There may well be plausible explanations other than those suggested by us, and the authors are kindly asked to check if our explanations explain their data and/or think of alternative ones. We really do hope that this comment will stimulate further discussions and work in this important area. Final Remark. We would like to finish this comment with a last quote. On p 182 in ref 6, it is written: “The strongest evidence that the hydrophobic forces present in foam films are long-range has been provided by the disjoining pressure isotherms.” As was explained in this comment, we think that this is definitely not the case. We suggest that limitations in the PB model and differences in the ionization properties of single surfaces and surfaces confined in foam films is a more likely explanation. Support for this suggestion comes from the fact that the data by Yoon and Wang and others can be explained with the DLVO theory when the surface potential used in the PB model is used as a fitting parameter. The scientific problem is to correctly calculate the double-layer repulsion. Although the use of the surface potential as a fitting parameter in the PB approximation is a convenient way, it is not a scientifically fully satisfactory way. The alternative approach advocated by Yoon and Wang (i.e., treating the PB model as exact and assuming that the ionization is independent of the confinement) indeed leads to the conclusion that an extra attractive force is present. We question this conclusion and do not find any “strong evidence”
Comments
for the existence of a long-range hydrophobic force in foam films. It is our hope that this comment will stimulate further discussion on surface interactions in general and on interactions in foam films in particular. Cosima Stubenrauch,*,† Dominique Langevin,‡ Dotchi Exerowa,§ Emil Manev,| Per M. Claesson,⊥,O Ludmila B. Boinovich,# and Regine v. Klitzing∇
School of Chemical and Bioprocess Engineering, UniVersity College Dublin, Belfield, Dublin 4, Ireland, Laboratoire de Physique des Solides, Baˆ t. 510, UniVersite´ Paris Sud, 91405 Orsay, France, Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. BoncheV Str. 11, Sofia 1113, Bulgaria, Department of Physical Chemistry, UniVersity of Sofia, 1164 Sofia, Bulgaria, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, Stockholm SE-100 44, Sweden, Institute for Surface Chemistry, Box 5607, Stockholm SE-114 86, Sweden, A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, 31 Leninsky prospect, 119991 Moscow, Russia, and Stranski-Laboratorium, Institut fu¨r Chemie, Technische UniVersita¨t Berlin, Straβe des 17. Juni 124, 10623 Berlin, Germany ReceiVed April 25, 2007 In Final Form: September 14, 2007 LA701208G
