Thin Liquid Films from Dilute Sodium Dodecyl Sulfate Solutions

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Thin Liquid Films from Dilute Sodium Dodecyl Sulfate Solutions Formed on a Mercury Electrode M. Kaisheva,*,† M. Krasteva,† and S. Usui‡,§ Faculty of Chemistry, University of Sofia, No. 1 “James Bouchier”, Sofia 1126, Bulgaria, and Institute for Advanced Materials Processing, Tohoku University, Katahira, Sendai 980-8577, Japan Received May 18, 1999. In Final Form: September 23, 1999 The stability and rupture of thin aqueous films formed between a hydrogen bubble and a mercury droplet were studied in aqueous solutions of 5 × 10-6 or 10-3 M sodium dodecyl sulfate in the presence of 0.05 M Na2SO4. The mercury droplet was positively polarized at E ) 0.2 V in the rational potential scale. The critical thickness of film rupture, or the equilibrium thickness of stable films, was measured using the microinterferometric method. Results were discussed in terms of DLVO and heterocoagulation theory by referring to the differential capacity previously reported.

Introduction The influence of sodium dodecyl sulfate (SDS) on thin liquid films formed on a mercury electrode has been studied in refs 1-5. The authors of refs 2-5 have revealed the influence of the structure of adsorbed layers of SDS on the interaction between a hydrogen bubble and a charged mercury surface. It has been shown that film stability is strongly dependent on electrode potential E (in the rational potential scale, i.e., versus the potential of zero charge in supporting electrolyte solutions) and on surfactant concentration. At low SDS concentrations and positive potentials, films are not stable and film rupture occurs, leaving a bare mercury surface. In the region of the potential of zero charge (PZC) and at negative E films are stable and no rupture is observed. This phenomenon has been explained by comparing results from film stability with measurements of differential capacitance at the mercury/solution interface.2-8 The authors of refs 2-8 have reached the conclusion that at PZC and at negative E dodecyl sulfate anions are adsorbed as a monolayer, with their negatively charged “head” groups turned toward the solution and hydrocarbon chains oriented toward mercury. At positive electrode potentials the orientation of specifically adsorbed organic anions is the opposite. Thus, the capacity minimum at E ) 0.4 V is connected with the maximum adsorption of dodecyl sulfate (DS) anions with an orientation of the hydrophobic chains toward the aqueous solution, hydrophobizing mercury surface. The capacity minimum at E ) -0.2 V is connected with the opposite orientation of DS-, with hydrophilic groups turned toward the aqueous solution making the mercury * Author to whom correspondence should be addressed. † University of Sofia. ‡ Tohoku University. § Present address: 43-23 Hagurodai, Taihaku-ku, Sendai 9820817, Japan. (1) Mushiake, K.; Imaizumi, T.; Inoue, T. Proc. Eleventh Int. Mineral Process. Congr., Cagliary, Italy 1975, 405. (2) Kaisheva, M.; Usui, S.; Dai, Q. Colloids Surf. 1988, 29, 147. (3) Dai, Q.; Sasaki, H.; Usui, S.; Kaisheva, M. J. Colloid Interface Sci. 1990, 139, 30. (4) Kaisheva, M.; Usui, S.; Girkov, T. Ann. Sofia Univ., Fac. Chem. 1984, 78, 153. (5) Kaisheva, M. Adv. Colloid Interface Sci. 1992, 38, 319. (6) Eda, K. J. Chem. Soc. Jpn. 1959, 80, 343, 347, 461, 708. (7) Eda, K. J. Chem. Soc. Jpn. 1960, 81, 689, 875, 879; 1964, 85, 828. (8) Damaskin, B. B.; Nikolaeva-Fedorovich, N. V.; Ivanova, R. V. Zh. Phyz. Khim. 1960, 34, 894.

surface hydrophilic. Comparison of results from capacity measurements and the time of life of thin liquid films containing SDS has shown2-5 that there is a direct correlation between reorientation of surfactant anions on the mercury surface and film stability. At higher concentrations of SDS, e.g., 5 × 10-4 M, films are stable at all electrode potentials. At this concentration the surfactant is not adsorbed as a monolayer, and bi- and multilayer groups of adsorbed molecules on the electrode surface are formed. This is confirmed also by studies of the interface air/aqueous solution of SDS.9 Thus, the bulk SDS concentration, at which a surface phase separation is observed, is much lower than the critical micelle concentration (cmc). The latter is 1.36 × 10-3 M SDS in 0.1 M NaCl at 22 °C.10 Results from double-layer capacity measurements have been used in refs 2-5 and 11 for calculating the electrostatic component of disjoining pressure Πel in thin films as a function of the distance d between outer Helmholtz planes for mercury and bubble. A very strong adsorption of SDS at positive potentials of mercury has been revealed in refs 1-5 indicating the existence of a superequivalent negative charge of the electrode and, as a result, a high electrostatic barrier to film rupture at both negative and positive potentials. The Hamaker coefficient A necessary for calculations of the van der Waals component of disjoining pressure Πvw has been estimated for the system mercury/aqueous film/gas bubble as -6.3 × 10-20 J in ref 4 and as -7.22 × 10-20 J in ref 11. The negative value of A indicates that Πvw facilitates film stability. It was shown in refs 2-5 that a film formed on a positively charged electrode in the presence of SDS was unstable and ruptured, whereas there was a high enough force barrier to ensure film stability. To explain this contradiction, the authors of refs 2-5 supposed the existence of a hydrophobic force in the film when surfactant orientation was with the hydrophobic tails turned toward the aqueous solution. Hydrophobic force was first revealed by Israelachvili and Pashley,12,13 (9) Nikolov, A.; Martynov, G.; Exerova, D.; Kaishev, V. Kolloidn. Zh. 1980, 42, 672. (10) Betts, J. J.; Pethica, B. A. Proc. Second Int. Congr. Surf. Act. London 1957, 393. (11) Usui, S.; Sasaki, H.; Hasegawa, F. Colloids Surf. 1986, 18, 53. (12) Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98, 500.

10.1021/la9905968 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/16/1999

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who proposed the following equation for its description:

Πh ) -(Ch/d0) exp(-d/d0)

(1)

Ch in this equation is a constant, d0 is a decay constant of hydrophobic interaction and is on the order of molecular dimensions. For the attraction between two mica surfaces hydrophobized by the adsorption of a hexadecyltrimethylammonium cation, it was found12 that Ch ) 22.3 mN/m and d0 ) 10-9 m. For the system mercury/0.05 M aqueous Na2SO4 with 5 × 10-5 M SDS/gas bubble, the minimum value of Ch was determined as a first approximation3,5 on the basis of films’ lifetime measurements, by setting d0 equal to 1.4 nm and supposing that the sum of hydrophobic and van der Waals interactions should be at least as great as the barrier of the electrostatic component of disjoining pressure. Thus, it was estimated3,5 that Ch was 3.98 mN/m. A similar estimation of Ch for the same system, however for a lower concentration of SDS (5 × 10-6 M) and for another supporting electrolyte (0.01 M aqueous NaClO4) was performed in ref 3, where minimum Ch was found to be 3.0 mN/m for E ) 0.06 V. In the previous studies on films formed on mercury electrodes, the stability or rupture of films was observed by a microscope from a horizontal direction3,11 or a vertical direction.1,2,4,5 It should be noted that in either case no direct information was available on the thickness of thin aqueous films, which varies with time in the course of approaching of mercury and bubble surfaces. For these reasons it was thought to be worth applying a new method for direct measurement of the thickness of aqueous films between bubbles and polarized mercury surfaces as a function of time of their approaching. The purpose of the present work is to study the thickness of thin films from aqueous SDS saline solutions formed on positively polarized mercury by applying the microinterferometric method14-17 and to determine the disjoining pressure involving hydrophobic interaction which played an important role in the presence of surfactants. Experimental Section Double-distilled mercury and triple-distilled water were used for all measurements. SDS, chromatographically pure, produced by Merck was used in concentrations of 5 × 10-6 and 10-3 M. Supporting electrolyte was 0.05 M Na2SO4, p.a. produced by Baker, Phillipsburg, NJ. The experimental setup used in the present work for direct observation of thin liquid films between a mercury electrode and a hydrogen bubble is similar to the one precisely described earlier.2,5 Monochromatic light of a wavelength λ ) 675 nm was used to illuminate the films. Interfering light reflected from both film surfaces was observed by a vertical metallographic microscope at a point in the center of the film. Its intensity I proportional to the photocurrent was detected by a photocell (Hamamatsu, Japan), and curves I vs time t (microinterferograms) were recorded by a computer. Thin films’ radii were on the order of 2.5 × 10-5 m. Mercury electrode potential was E ) +0.2 V in the rational scale, which is equal to -0.309 V vs saturated calomel electrode (SCE). (13) Israelachvili, J. N. Intermolecular and Surface Forces, end ed., 4th printing; Academic Press: London, 1994. (14) Scheludko, A.; Platikanov, D. Kolloid-Z. 1961, 175, 150. (15) Scheludko, A. Kolloid Z. 1957, 155, 39. (16) Scheludko, A.; Exerowa, D. Kolloid Z. 1959, 165, 148; 1960, 168, 27. (17) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391.

Figure 1. Dependence on time of the light intensity interfering from both surfaces of a film, containing 5 × 10-6 M NaLS and 0.05 M Na2SO4. The potential of the mercury electrode is E ) 0.2 V in the rational potential scale.

The gas phase is optically transparent, and its refractive index n1 ) 1. We assume that the index of refraction of the aqueous solution films is n2 ) 1.331, approximately coinciding with the index of refraction of water. The refractive index of mercury is a complex number n3 ) n3 + ik, where n3 is the real part of the refractive index, k is its imaginary part, and i is the imaginary constant. The optical constants of mercury for λ ) 675 nm were n3 ) 1.75 and k ) 2.98.18 For the determination of the capillary pressure in the system, some electrocapillary measurements of the interfacial tension of mercury were performed by a capillary electrometer and the surface tension at the solution/gas interface was measured by the method of Wilhelmy. Experiments were carried out at 20 °C. Results and Discussion Microinterferograms obtained in the case of thin films from 0.05 M aqueous Na2SO4 solutions with the addition of 5 × 10-6 M SDS formed on a mercury electrode polarized at E ) 0.2 V are illustrated in Figure 1. As seen in the figure, interference minima and maxima were detected as a function of time at a central point of the film, reflecting the process of the film’s thinning. The appearance (at t ) 0) of extremes in the microinterferograms marks the thin liquid film’s formation. The region before that corresponds to the approach of the gas bubble toward the mercury surface. As can be observed in Figure 1, films formed on mercury polarized at 0.2 V and immersed in a 0.05 M Na2SO4 solution with 5 × 10-6 M SDS added rupture after about 1.2 s of their life. Rupture appears as a vertical line at 1.2 s in Figure 1, followed by a horizontal line corresponding to the strong reflection from a bare mercury surface. Results shown in Figure 1 have a good repeatability and confirm the fact that mercury at 0.2 V is not wetted by aqueous saline solutions containing 5 × 10-6 M SDS. Microinterferograms were used in the present work to study the kinetics of films’ thinning, i.e., to calculate the dependence of film thickness on time. For that purpose the reflectivity R of the three-phase system was calculated (18) Mellor, J. W. Inorganic and Theoretical Chemistry; Longmans, Green and Co.: London, 1957; Vol. IV, p 725.

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Figure 2. Calculated dependence on film thickness of the intensity of light interfered from both surfaces of a thin aqueous solution film formed between mercury and gas.

according to the optical equations given by Hansen,19 which are analogous to those applied in ref 14. In the latter work it was shown that

h)

[

δ 2m + 1 λ π- ( 2πn2 2 2

arcsin

x

∆

1+

]

4xR12R23(1 - ∆) (1 - xR12R23 )2

d(1/h2)/dt ) 4∆P/3ηr2 (2)

2kn2 2

2

n2 - n3 - k

2

(3)

∆ ) (I - Imin)/(Imax - Imin) ) (R - Rmin)/(Rmax - Rmin) (4) R12 ) R23 )

|

(n1 - n2)2

(5)

(n1 + n2)2

| |

|

n2 - (n3 + ik) 2 n2 - n3 - ik 2 ) ) n2 + n3 + ik n2 + n3 + ik (n2 - n3)2 + k2 (n2 + n3)2 + k2

(6)

The dependence of ∆ on the thickness h of aqueous solution films formed on mercury is shown in Figure 2, calculated on the basis of equations derived earlier14,19 (e.g., eq 2). As seen in Figure 2, this dependence is periodic with respect to film thickness with a period of 253.56 nm. On the other hand, experimental microinterferograms reflect the process of film thickness h changing in time, leading to the respective change in I (or ∆) on time and shedding light on the kinetics of film thinning. Film thickness was calculated as a function of time replacing ∆ in eq 2 with experimental ∆(t) from the microinterferograms. Because the amplitude of the initial part of the I(t) curves (respectively ∆(t) curves) changed in time, we have used for calculations by eq 2 only the final part of the microinterferograms, around the last maximum and/or minimum. There the film radius was already constant in time. It was large enough so that the observed light spot was only in the film center, the amplitude of the microinterferograms remained constant in time, and the film thickness was in the range of the first period in Figure (19) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380.

(7)

Here η is the viscosity of film’s liquid, r is the film’s radius, and ∆P is the pressure difference between the thin film and the bulk phase. In the general case

∆P ) Pσ - Π(h)

where m ) 0, 1, 2, 3, ... is the interference order

tan δ ) -

2. For earlier stages of film thinning only the time of the maxima and minima of microinterferograms was taken into consideration, to which h was ascribed equal to the values of the maxima and minima in Figure 2. Here ∆ was 1 or 0, and no error could be inserted from the amplitude changing in time. When experimental microinterferograms of the type shown in Figure 1 was compared with calculations according to eq 2 illustrated in Figure 2, the thickness of films containing 5 × 10-6 M SDS was obtained as depicted in Figure 3. The critical thickness of film rupture was 20 nm, as seen in this figure. The microinterferometric method allows film thickness to be determined with a precision of about (3 nm. Reproducibility, however, depends on the nature of the process. Because film rupture is a stochastic process, the average critical thickness of the rupture of films from 0.05 M aqueous Na2SO4 with 5 × 10-6 M SDS added was 35 ( 15 nm. Disjoining pressure in the studied films was determined supposing that film surfaces’ flow is stopped by the adsorbed surfactant, using the theory developed in14,20

(8)

where Pσ is the capillary pressure. In the case of asymmetric films between a spherical liquid/gas surface with a radius of curvature R1 and interfacial tension σ1, and a spherical liquid/mercury surface with a radius of curvature R2 and interfacial tension σ2, the following expression holds:14

( ) ( )

R1 R2 Pσ ) σ1 R1 1 + σ2 2σ1 1 +

(9)

In the present work we measured the interfacial tension between the mercury electrode at 0.2 V and the 0.05 M aqueous Na2SO4 solution with 5 × 10-6 M SDS added to be σ2 ) 410 mN/m. The surface tension of 5 × 10-6 M SDS aqueous saline solution was 63 mN/m, which coincided with the results of ref 9. Thus, capillary pressure in the films from 5 × 10-6 M SDS aqueous saline solutions was 75 N/m2. On the basis of curves of the type illustrated in Figure 3, the derivative d(1/h2)/dt was calculated as a function of time. When eqs 10 and 11 are applied, an experimental isotherm of disjoining pressure was found, as shown by the points in Figure 4. To reveal the types of forces acting in the studied films, an attempt was made in the present work to apply the theory of heterocoagulation of DLVO.21,22 To calculate the electrostatic component of disjoining pressure Πel, it was necessary to have estimates of the outer Helmholtz planes potentials ψ1 and ψ2 for the two film surfaces solution/gas and metal/solution, respectively. (20) Scheludko, A.; Dessimirov, G.; Nikolov, K. Ann. Sofia Univ., Fac. Chem. 1954/1955, 49, 126. (21) Derjaguin, B. V. Discuss. Faraday Soc. 1954, 18, 85. (22) Usui, S. In Electrical Phenomena at Interfaces; Kitahara, A., Watanabe, A., Eds.; Dekker: New York, 1984; p 285.

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Figure 3. Thickness of an aqueous 0.05 M Na2SO4 film containing 5 × 10-6 M NaLS, as a function of time. The potential of the mercury electrode is E ) 0.2 V in the rational potential scale.

Figure 4. Disjoining pressure isotherm for a thin film from aqueous 0.05 M Na2SO4 containing 5 × 10-6 M NaLS formed on mercury at E ) 0.2V. Points represent experimental results. The line is calculated on the basis of the DLVO theory, taking into account hydrophobic interaction with Ch ) 3.98 mN/m and d0 ) 3.5 nm.

The potential ψ2 of the mercury/solution interface was determined on the basis of the results of works.4,23 In ref 4 the differential capacity of the mercury electrode immersed in 0.05 M aqueous Na2SO4 was measured without and with SDS added in different concentrations. When capacity curves are integrated, it was found that charge density q of the electrode at 0.2 V was 1.6 µC/cm2 for the case of 5 × 10-6 M SDS + 0.05M aqueous Na2SO4. From adsorption measurements for the same system the charge density q′ of the outer Helmholtz plane was determined to be q′ ) -17 µC/cm2. Thus, the charge density of the diffuse part of the electrical double layer on mercury was qd ) q - q′ ) 15.4 µC/cm2. On the basis of GouyChapmen theory, we calculated ψ2 ) -0.107 V. The potential ψ1 of the solution/gas interface was estimated on the basis of results on foam films from aqueous SDS + 4 × 10-4 M NaCl.24 The potential of 5 × (23) Kaisheva, M. Ann. Sofia Univ., Fac. Chem. 1985, 79, 466. (24) Exerowa, D.; Zacharieva, M.; Platikanov, D.; Cohen, R. Colloid Polym. Sci. 1979, 257, 1089.

Kaisheva et al.

Figure 5. Electrostatic and van der Waals components of disjoining pressure for a thin film from aqueous 0.05 M Na2SO4 containing 5 × 10-6 M NaLS formed on mercury at E ) 0.2 V estimated on the basis of the DLVO theory at constant potentials of the two surfaces ψ1 ) -0.005 V and ψ2 ) -0.107 V and Hamaker coefficient A ) -6.3 × 10-20 J.

10-6 M SDS saline solution/gas interface was -0.068 V, and the potential of the same interface when the bulk SDS concentration exceeded 10-4 M, i.e., at saturation of the interface with dodecyl sulfate anions, was -0.082 V. The authors24 have estimated the absolute value of the charge density for the latter case to be 0.57 µC/cm2, obtained by recalculations from the respective potential and application of the Gouy-Chapmen theory. The charge density of 5 × 10-6 M SDS saline solution/gas interface was qd ) 0.42 µC/cm2. When the electrolyte concentration is increased to 0.05 M Na2SO4, the diffuse part of the electrical double layer becomes thinner, and by using an analogous recalculation according to the Gouy-Chapmen theory, we obtained ψ1 ) -0.005 V for the interface gas/ 0.05 M aqueous Na2SO4with 5 × 10-6 M SDS added. Thus, the absolute value estimated for ψ1 is the lowest possible, i.e., if there were uncertainty in ψ1 estimation, the correction might only be sought in the direction of a more negative ψ1. Results from the determination of the electrostatic component of disjoining pressure on the basis of the DLVO theory at constant potentials of the two surfaces ψ1 ) -0.005 V and ψ2 ) -0.107 V are shown in Figure 5. In this figure the van der Waals component of disjoining pressure is illustrated as well, calculated by using the equation

Πvw ) - A/6πh3

(10)

The value of the constant A used by us was -6.3 × 10-20 J for the system mercury/aqueous solution/gas.4 From Figure 5 it follows that the barrier of Πel is very high even when the value of ψ1 is only -5 mV. A possible increase in the absolute value of ψ1 would lead to an even greater barrier of Πel. As seen in Figure 5, both components of disjoining pressure have high positive values, which might only cause film stabilization, contrary to experimental results. The capillary pressure is acting in a direction opposite to Πel and Πvw; however, it is much smaller in the studied system (75 N/m2) and cannot be the reason for the observed instability of the studied films, i.e., for the contradiction between experimental results and calculations based on the DLVO theory. This contradiction will not be remedied if Πel is calculated on the basis of a constant surface charge model in double-layer interactions, because the constant surface charge model gives more repulsive Πel.

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Figure 6. Dependence of the intensity of interfered light with time for films from a 10-3 M solution of SDS + 0.05 M Na2SO4 on a mercury electrode at E ) 0.2 V.

An explanation of the obtained results in the present work has been sought in the existence of a hydrophobic interaction between film surfaces. As mentioned in the Introduction, a similar contradiction found by measuring film’s time of life has been explained earlier2-4 by the hydrophobic force Πh. Israelachvili et al.12,13 proved the existence of a hydrophobic interaction between mica surfaces treated with surfactants and demonstrated that it was much stronger and with greater radius of action than van der Waals interaction. In the present work the equation for hydrophobic interaction25 derived for two parallel flat surfaces was used for the calculation of Πh

Πh ) -(Ch/d0) exp[-(h - δh - δp)/d0]

(11)

Both film surfaces in this investigation could be accepted as being approximately plane parallel, because the radii of curvature of mercury and the bubble were much greater than the film thickness. In eq 11 δh is the length of the hydrocarbon chain of the surfactant, and δp is the diameter of the hydrophilic “head”. The sum of δh and δp gives the total length of SDS, which has been estimated as 1.4 nm. Results from calculations of the total disjoining pressure

Πt ) Πel + Πvw + Πh

Figure 7. Dependence of thickness with time for films from a 10-3 M solution of SDS + 0.05 M Na2SO4 on a mercury electrode at E ) 0.2 V.

(12)

are represented by the line in Figure 4. For the estimation of Πh it was accepted that Ch ) 3.98 mN/m3,5 and that d0 ) 3.5 nm. This means that hydrophobic interaction decays rather slowly with distance, and the decay length is on the order of 2.5 monolayers of SDS. Figure 4 indicates that the attractive force becomes significant from h lower than 30-40 nm. This result is in good agreement with the result that the average critical thickness for film rupture in that system was found to be 35 nm. Furthermore, comparison of Figures 4 and 5 indicates that hydrophobic interaction plays an important role in film rupturing: for film thickness higher than 20 nm, the basic component of disjoining pressure is the hydrophobic interaction. When the concentration of SDS was increased to 10-3 M with the same supporting electrolyte 0.05 M Na2SO4, thin films on positively polarized mercury remained stable and did not rupture for more than 3 min. The respective microinterferograms are illustrated in Figure 6. As seen in Figure 6, films in this case reach an equilibrium thickness for about 6 s, which is obvious from the plateau at the right-hand side of the figure. The film thickness calculated by using eq 2 or eq 7 is shown in Figure 7 as a function of time for films from 10-3 M SDS with 0.05 M aqueous Na2SO4. In the case of this high surfactant concentration, an equilibrium film thickness is reached with an average h0 ) 10 nm. (25) Nishimura, S.; Chen, Z.; Sasaki, H.; Usui, S. J. Colloid Interface Sci. 1990, 139, 238.

Figure 8. Theoretically estimated electrostatic and van der Waals components of disjoining pressure, as well as their sum, for films from a 10-3 M aqueous solution of SDS + 0.05 M Na2SO4 on a mercury electrode at E ) 0.2 V.

To explain this experimental result, an estimation was made by using the heterocoagulation theory of the forces acting in the films containing 10-3 M SDS. From former investigations5-8 of the differential capacity of a mercury electrode immersed in SDS solutions, it follows that at such a high concentration of SDS surfactant adsorption does not take place in a monolayer but rather in a bilayer and that no hydrophobic interaction can be expected, because the second adsorption layer is oriented with its hydrophilic “heads” toward the aqueous solution. The electrostatic component of disjoining pressure was estimated by using the theory developed in ref 21. For that purpose the outer Helmholtz plane potential of the mercury/10-3 M SDS solution interface in the presence of 0.05 M Na2SO4 was calculated on the basis of the results in refs 3 and 23 as being ψ2 ) -0.130 V. It was shown4 that the outer Helmholtz plane potential at the solution/air interface for SDS concentrations higher than 10-5 M found on the basis of results from ref 24 was ψ1 ) -0.006 V. van der Waals disjoining pressure was calculated using eq 13 with A ) -6.3 × 10-20 J.4 Results from these calculations are shown in Figure 8. The use of A ) -7.22 × 10-20 J11 leads to insignificant changes in Πvw. As seen in Figure 8, there is a very high barrier of the total disjoining pressure which cannot be overcome under the influence of the low capillary pressure in the system, Pσ ) 54 N/m2, and thus the film stability at the high surfactant concentration can be explained. For the determination of Pσ the surface tension at the air/10-3 M SDS solution interface in the presence of 0.05 M Na2SO4 was measured by us as 35 mN/m, and the interfacial tension of a mercury/solution was 380 mN/m. It follows from the DLVO theory that thin films should be stable at the thickness at which capillary pressure

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equals disjoining pressure. For the experimentally observed equilibrium thickness of 10 nm, the electrostatic component of disjoining pressure is on the same order as Pσ; however, the van der Waals component is higher in magnitude. Thus, the total disjoining pressure accounts for the stability of the film in the presence of 10-3 M SDS, but no quantitative explanation is available about the equilibrium thickness of the film. This will be a subject of further study. Conclusion The stability and rupture of thin aqueous films formed between positively polarized (E ) 0.2 V, where E refers to the rational potential scale) mercury droplets and hydrogen bubbles were studied by measuring the thickness of the films by using a microinterferometric method in

Kaisheva et al.

the presence of SDS and 0.05 M Na2SO4. The results are summarized as follows: (1) Films formed from 5 × 10-6 M SDS were unstable and ruptured with a lifetime of 1.2 s. A mean value of critical thickness of the film was found to be 35 nm. This result was reasonably interpreted in terms of the heterocoagulation theory in which the Hamaker constant was negative, A ) -6.3 × 10-20 J, and hydrophobic interaction was taken into consideration. (2) Thin films formed from 1 × 10-3 M SDS were stable with an equilibrium thickness of about 10 nm. Repulsive forces due to double-layer interactions and a negative Hamaker constant in the absence of hydrophobic interaction are responsible for the film stability. LA9905968