Electrosorption of amphiphilic surfactants at the mercury-solution interface and its influence on the stability of thin liquid films

Advances in Colloid and Interface Science, 38 (1992) 3194352 Elsevier Science Publishers B.V., Amsterdam

319

ELECTROSORPTION OF AMPHIPHILIC SURFACTANTS AT THE MERCURY-SOLUTION INTERFACE AND ITS INFLUENCE ON THE SI’ARILITY OF THIN LIQUID FILMS MILLIANA

K. KAISHEVA

Department ofphysical Chemistry, UniversityofSofia, No. 1 A. Ivanov Au., Sofia 1126, Bulgaria CONTENTS Abstract ......................................... 1. Introduction .................................... 2. Adsorption of Amphiphilic Surfactants on a Mercury Electrode ........ A. Non-Ionic Surfactants .............................. B. Anionic Surfactants ............................... C. Cationic Surfactants .............................. 3. Stability of Thin Films Formed From Aqueous Solutions of Amphiphilic Surfactants on Polarized Mercury ........................ A. Non-Ionic Surfactants .............................. B. Anionic Surfactants ............................... C. Cationic Surfactants .............................. References ........................................

319 319 322 322 328 332 335 336 338 345 350

ABSTRACT The adsorption of non-ionic, anionic and eationic surfactants on a stationary mercury electrode from aqueous solutions of a supporting electrolyte is discussed with respect to phase transformations in the adsorption monolayer, the determination of adsorption parameters and outer Helmholtz potentials. Investigations of the stability of thin liquid films, formed from solutions of these surfactants between a mercury electrode and a hydrogen bubble reveal certain peculiarities. The latter are explained by the assumption of the existence, in some cases, of a hydrophobic component of the of the disjoining pressure in the films, withthe estimation of the corresponding hydrophobic force constant. 1. INTRODUCTION

~phiphiIic surface active agents (SAA) are very widely used in industry and agriculture and investigation of their surface and bulk properties is an important area of physical chemistry. The terms amphiphilic, colloid surfactants or association colloids are accepted by IUPAC [13 and characterize these substances as having not only high ~l-~ff9~15.~

0 1992 - Elsevier Science Publishers B.V. All rightsreserved.

320

surface activity, but also stabilizing ability, a tendency for micellization and solubilization. The characteristic properties of the amphiphilic SAA, which are responsible for their practical applications, are closely related to the adsorption of these substances at interfaces and to the influence of the electrical double layer on this process. The electrical double layer has been most profoundly studied at the mercury-aqueous solution interface. The results of these investigations have been excellently summarized in a number of reviews by Grahame [21, Parsons [3,41, Lyklema and Parsons [51, Damaskin and co-workers [6,71, Delahay 181,Mohilner 191,Trasatti [lOI and Nikitas [ill . R. de Levie [121and C. Buess-Herman [131have recently reviewed the two-dimensional condensation phenomena at the mercury-water interface, occurring with substances like pyridines, quinolines, camphor, borneol, adamantanol, coumarin etc., which have the ability to form solid-like adsorption layers at the interface, depending not only on molecular interactions but also on considerations of molecular packing. Such a solid-like condensed monolayer is expected to have relatively few structural defects [121and almost no water molecules incorporated, with the exception of the water of crystallization. In the case of adsorption of amphiphilic surfactants at the free-water surface another kind of a phase transformation has been observed, namely a first order surface phase transition from a gaseous to a liquidexpanded state [14,151. It occurs at a bulk concentration of the colloid surfactant equal to a tenth of the critical micelle concentration (CMC) 114-161 and results in the formation of a condensed monolayer of the liquid-expanded type. The adsorption of colloid surfactants at the mercury-solution (M/S) interface has been studied by a number of authors using a dropping mercury electrode i17-431. Some of this work has been reviewed by Schinoda 1441and Ottewill [451. The dropping mercury electrode is a convenient method of studying the kinetics of adsorption of colloid surfactants [46,471. Specific and interesting results have been obtained, which will shortly be discussed further. However, because ofthe comparatively large dimensions of the molecules of the colloid SAA, in cases where low bulk concentrations should be used the adsorption equilibrium is rarely attained on a dropping electrode within the lifetime of a mercury drop as a result of the slow diffusion and slow surface rearrangements. Electrosorption investigations of the electrical double layer of a stationary mercury electrode have indicated that the molecules of the amphiphilic surfactants have an analogous behaviour to that at the free-water surface [48-531, i.e. they undergo analogous two-dimensional

321

phase transformations. We will summarize the most significant of these findings in the discussion that follows. In 1959 Scheludko and Platikanov [541studied the stability of thin films formed from benzene, from some other organic liquids and from a 0.1 mol dm3 aqueous solution of KNO, on the surface of a non-polarized mercury. They have investigated the rate of thinning of the benzene films by the microinterference method and have shown that, for certain very small film thicknesses h, the disjoining pressure of the benzene films was negative with a minimum of about h = 33 nm. For higher film thicknesses the authors predicted the existence of a positive disjoining pressure of the films on mercury. The first investigations of thin films formed on the surface of polarized mercury from pure aqueous solutions of Na,SO, without a surfactant were performed by Frumkin and co-workers K&5-591.The authors have measured the lifetime of the films by observing, through an inclined microscope at an angle to the mercury surface, a bubble of hydrogen, brought in contact with the mercury electrode. Thinning and rupture of liquid films, formed from aqueous saline solutions without SAA between a mercury electrode and a gas bubble 160,611, between a mercury electrode and glass 1621and between two mercury electrodes [63,641 have also been investigated by other authors. The role of the dissimilar electrical double layers at the mutual approach of two surfaces has been discussed in Refs [65,661. The coalescence of mercury droplets in aqueous solutions in the presence of amphiphilic surfactants has been studied by Sonntag [67] and Watanabe et al. [681. Contact angles in the system composed of polarized mercury/aqueous electrolyte solution with 169,701 and without [711 SAA/gas have been obtained in connection with the process of wetting and de-wetting of mercury. A direct practical application of the mercury/surfactant solution/gas system has been found by Pomianowski 1721who developed an apparatus for model flotation of mercury drops, which has since been very widely used 173-761. The formation of thin liquid films on the surface of a polarized mercury substrate from aqueous solutions of colloid surfactants is of significant interest in connection with many practical applications. These include the separation of gas bubbles from electrodes and the understanding of the basic phenomena underlying the flotation process, since the latter is connected with thinning and rupture of liquid films formed between the foam bubble and the mineral particle.

322

The influence of the amphiphilic surfactants on the properties of the solution films between a mercury electrode and a gas bubble has been investigated recently [77-851. The results of these investigations and their correlation with electrosorptionstudies will be the focus ofthe present review. 2. ADSORPTION ELECTRODE

OF AMPHIPHILIC

SURFACTANTS

ON A MERCURY

A. Non-ionic surfixtants The first scientist to notice certain peculiarities in the adsorption of the non-ionic surfactant n-octanol was Grahame 1171.He measured the differential capacity C of the electrical double layer in a 1 mol dm3 aqueous solution of KNO, with the addition of n-octanol, as a function of a.c. frequency and electrode potential E in the rational scale (i.e. versus the potential of zero charge in the pure supporting electrolyte). Grahame found that the two maxima of adsorptiondesorption were very narrow and infinitely high. Later Melik-Gaikazyan [191 revealed a tendency for the capacity curve, in the case of n-octanol solutions, to exhibit a vertical step at potentials Q vs. the saturated calomel electrode (SCE) of about -0.6 to -0.7 V, again for a high concentration of the supporting electrolyte (1 to 3 mol dm3 KCl). These findings were indications of a high attraction between n-octanol molecules in the adsorption monolayer and for the occurrence of a surface phase transformation in this layer in the potential region of the vertical capacity step. Both authors worked with a dropping mercury electrode. This fact did not give them the opportunity of studying adsorption phenomena for longer periods of time. When a stationary mercury electrode and a lower concentration of the supporting electrolyte (0.1 mol dm3 Na,SOJ were applied [511 for the investigation of n-octanol adsorption, it was found (Fig. 11, that in the region of (p = -0.6 to 4.7 V SCE, which corresponds to an electrode potential E = -0.2 V in the rational scale, a capacity maximum arises (denoted by d in Fig. 1). This maximum is not an equilibrium one. It increases with time and the values illustrated in Fig. 1 are measured after the second hour of the life of the mercury drop. The capacity values in the minimum, a, in Fig. 1 do not change over a period of more than two hours. Over a long period of time, changes in the differential capacity have been observed [861during the growth of compact layers of some bipyridine complexes at the stationary mercury electrode/solution interface. It is interesting to note, that in this case the supporting electrolyte concentration has also been low (0.1 mol dm3 NaC104).

323

The time dependence of the differential capacity C(t) can be used as a diagnostic test for surface phase-transformation processes in monolayers of adsorbed SAA. The time dependence of the degree of coverage 6 of the electrode by SAA molecules, observed in the papers mentioned above, which can be found by measuring C, is totally different from the direct proportionality to t‘I’, characteristic for fast adsorption governed by slow diffusion of SAA to the electrode. Some investigations of the kinetics of adsorption of amphiphilic surfactants on a dropping mercury electrode [46,471 show that when a vertical step is observed on the capacity vs. electrode potential curve, very often the time dependence of capacity at the potential of the step exhibits a maximum. Such is the case with the adsorption of dodecylhexaoxyethylene glycol monoether (C&E& at the stationary mercury electrode from 0.05 mol dm3 aqueous NagSO solutions as a supporting electrolyte 147,491. The vertical step in the CO curve (Fig. 2), which is not very well defined, indicates an irreversible,

\ i

3c

20

10

I

I

0.5

0.0

I

-05

I

-1.0

LV

Fig. 1. Curves of differential capacity vs. potential for a stationary mercury electrode in 0.05 A4 Na2S04 (1) and with lo9 M n-octanol added (2).

324

non-equilibrium surface-phase transition. It is observed at E about -0.6 V for concentration of C izE, higher than 5.1O-c mol dm3. The dependence of the capacity in the region of the step (at E = -6.6 W, first increases with time and then diminishes, as can be seen in Fig. 3. This is a clear picture of the process of slow association of the surfactant molecules in the interfacial layer, which is observed only for concentrations of Cn,E6 higher than 5.104 mol dm3. It should be noted that the

90 -

ao70 6Q?$ *LL:500 I

40 30 2010 cl I

I

OS3 04

I

0 4

I

- 04

I

-0s

I

I

-1.2 -1.6 E/V

Fig. 2. Curves of differential capacity vs. potential for a stationary mercury electrode in contact with aqueous 0.05 M NazS04 solutions containing C12Ee in concentrations: (a) 0 M; (b) lo-‘M; (c) 5.104 M, (d) l.083~104 M.

325

. t/min Fig. 3. Dependence of differential capacity on time after formation of a new mercury surface at an electrode potentialE = -0.6 V for a 1.1~104MC~~Es solution. The supporting electrolyte is a 0.05 M aqueous solution of Na2S04.

CMC obtained for 0.05 mol dm3 Na$O, solutions of CIzEe at 20°C is 7.103 mol dm3 [491. The time dependence of the capacity in the region of E = 0 V is entirely different. These values have been measured after the differential capacity has reached a constant value, as shown in Fig. 4. A characteristic feature of the CO curves (see Fig. 2) is that for C12E6 concentrations > 5.10” mol dm3 and for potentials E more negative than a.7 V, the capacity is higher than that measured in lower SAA concentrations. An analogous phenomenon has also been observed on a dropping mercury electrode 125,331; however because equilibrium cannot be attained within the lifetime of the mercury drop for low SAA concentrations, the authors have observed this phenomenon only in the case of high surfactant concentrations. Obviously, the presence of a larger amount of Na’ counterions in the electrical double layer at values of E more negative than -0.6 V leads to a two-dimensional micellization effect. So for C,,Ee concentrations higher than 5-10a M in the potential region between 0 and -0.6 V the adsorption layer consists of closely packed surfactant molecules and very little water, while for potentials more negative than 4.6 V the surface solution contains surface micelles of C12E6and a larger amount of water. These considerations have been confirmed by the statistical estimation of the adsorption parameters of C rzE, in the region from 0 to a.6 V [WI. A method has been recently proposed [871for the estimation of adsorption

326

parameters from differential capacity data, according to which the analytical expression for the differential capacity is treated not deterministically, but as a stochastic one. This allows the adequacy of the model to be checked using a sample of experimental data and to find such estimates of the parameters, that the regression description should be closest to the sample of experimental capacity data. Analytical models of differential capacity, based on the isotherms of Frumkin [63,Hill-de Boer [871,Flory-Huggins El ,521etc. can be applied,

2422 N 20 c -‘-‘-.‘E

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i

‘1

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.\ “\ ..\c d \.

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! ‘\ 10 + ‘: \ i \ $3;

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_. -..-..-

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f I

I

10

20

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30 t/min

Fig. 4. Dependence of differential capacity on time. The potential of the stationary mercury electrode is E = -0.03 V and the supporting electrolyte is 0.05 M NazS04. Concentrations of C12E6are as follows: (a) 0 M; (b) 10e7 M; (c) 510W7M, (d) lOa M, (e) 10J M, 0 10” M.

327

and the statistical analysis described in Ref. 1871gives a new possibility for distinguishing between these models and finding the most adequate one. For example, the model based on the isotherm of Flory-Huggins consists of the following three equations [X21: (i) the Flory-Huggins type isotherm Bc =

0

n(1 - e)”

exp(-Znae)

(ii) the potential dependence of the equilibrium constant of adsorption B E + c%$

~qodE--0.5C’E2 0

! 1 /RT~,

(2)

which is obtained by applying the model of the two parallel condensers [61and (iii) the expression for the differential capacity

(3)

’ =‘0 W3 +“f3+m

r,[i -

zn~e(i

-e) + (n

- i)e] +

e

Here c is the bulk concentration of SAA, n is the project area ratio of the solute and solvent molecule on the electrode surface, q. and Co are the charge density and the differential capacity respectively of the mercury electrode in pure supporting electrolyte without a surfactant, C’ is the double layer capacity when the surface is entirely covered by SAA (0 = 11, E, is the potential difference arising as a result ofthe oriented adsorption of organic dipoles, Tm is the maximum number of moles of adsorbed substance per unit area of the electrode, e is a normally distributed random variable corresponding to the experiment error with zero mean and a certain variance, B. = co,5;on(O.!V-’ exp(na) [

-1

1

(4)

where co.5;ois the bulk SAA concentration for which 8 = 0.5 when E = 0, and a is a constant, reflecting the interaction between adsorbed SAA molecules. When a > 0 attraction is observed and when a < 0 repulsion prevails. B. is connected to the standard Gibbs energy of adsorption

328

AG): = -RT In (55.5 Bo)

(5)

When Eqns (1) and (2) are combined, the dependence of 8 on E and c can be calculated. The insertion of 8 into Eqn (3) allows the differential capacity to be described as a function of SAA concentration and potential E. Estimates of the adsorption parameters co.5;o, c’, EN, Tm, a and n, necessary for that purpose are found by applying a non-linear regression analysis. Using the least squares criterion to determine the closeness of the model to the experimental data, the vector of the parameters was found 1871,for which the analytical capacity model is closest to the capacity values from the experimental sample for C12Ee.The sample was taken in the interval of electrode potentials E from 0.3 to -0.6 V, discussed earlier. The statistical estimate of T,.,,obtained in Ref. [871 was 8.510-lo * 0.5~10-‘0mol dmm2.This result confirms the idea of the existence of an adsorption monolayer, containing almost no water and consisting of mainly C r2E6molecules packed very closely and oriented vertically to the M/S interface. The positive sign of the estimate obtained for EN indicates that the adsorbed molecules are oriented with the positive pole of the dipole toward the mercury surface. The estimate for co,5;oillustrates the great surface activity of C12E6. The standard Gibbs energy of its adsorption at the mercury electrode is AGj = -52.3 kJ mol-‘. For the adsorption of C12E, at the solution/gas (S/G) interface, Lange [881 found that AGl = ~47.7 kJ mol-‘. Consequently the main reason for the activity of this SAA is the difference in the solvation energies of the organic molecules in the bulk solution and at the S/G or M/G interface. B. Anionic surfmtants The study of anionic surfactant adsorption on a mercury electrode was started almost simultaneously by two groups of scientists [Zl-24,481. Substances like alkyl sulphates with different paraffin chainlengths (C = 6,8,9,10, 12,14,16 and 18) were investigated. Both groups of authors worked with high surfactant concentrations (close to the CMC) and varied the concentration and type of the supporting electrolyte. Eda and coworkers [21-241 used a dropping mercury electrode and Damaskin et al. 1481 used a stationary one. Both groups observed peculiar curves of differential capacity which at high electrolyte concentrations had four capacity maxima. They gave similar explanations for the observed phenomena. The authors supposed that in the region of the potential of zero

329

charge (PZC) the adsorbed surfactants formed a monolayer, the polar head groups being located in the aqueous phase and the hydrocarbon chains turned to the mercury surface. It was supposed that at certain, limited concentrations of the supporting electrolyte a two-dimensional phase separation occurred, caused by the adsorption of counter-ionsto the monolayer of anionic surfactants, leading to the appearance of another cathodic peak on the capacity curves, besides the one connected with the adsorption-desorption process. At the positively polarized surface, however, the small peak at less positive potential was named the “rotation peak” and was ascribed to the reorientation of the surfactants to a position in which the negatively charged polar groups turned closer to the positively charged mercury surface and the hydrocarbon “tails” had to be oriented to the solution. It was assumed that the appearance of an anodic maximum at even higher positive potentials was due to the formation of a bilayer of adsorbed surfactant anions. Some other studies of alkyl sulphate adsorption on mercury were made later [32,34,50,78,89,901. They reached the same conclusions about the complexity of the adsorption behaviour of anionic surfactants. Trujillo and Bennes 1341and Schuhmann et al. 1891investigated the similarities between the phenomena: bulk micellization of SAA and their adsorption at electrodes. In the case of sodium dodecyl sulphate (SDS) adsorption on mercury, when the concentration of the surfactant was 5103 mol dm3, the area per adsorbed surfactant anion DS- was found to be about 0.5 nm2 [341. The dependence of the charge of the adsorbed anions on the electrode charge, obtained by the drop time method was used for that purpose. Electrocapillary curves for SDS solutions with concentrations varying from 6~10~ to 6.103 mol dm3 were obtained by Mushiake et al. 1781. The temperature dependence of the differential capacity of a stationary mercury electrode in SDS solutions showed that the decrease of the standard Gibbs energy of the system on SDS adsorption was due mainly to an entropy factor [501.The increase in entropy on the adsorption of the surfactant is explained by a breakdown of the structured layer of water at the M/S interface. The capacity curves obtained [501 are illustrated in Fig. 5 at 16°C in the case of 0.05 M Na,SO, as a supporting electrolyte. It follows from the measurement that for SDS concentrations lower than 5~10~ mol dm3 there are two capacity minima. The one in the potential region from 0 to -0.5 V corresponds to the adsorption of the DS- ions with their hydrocarbon chains oriented towards mercury, and the other at E = 0.4 V corre-

330

I .\

1\

35

30

25

20

15

10

d 5

-0-d

Fig. 5. Capacity dependence on E in SDS solutions with concentrations: (1) lOa M, (2) 10m5M; (3) 5.10” M, (4) 1Od M, (5) 510A M, (6) in pure supporting electrolyte (0.05 M NazS04).

sponds to the adsorption of DS- with the negative polar group towards mercury. So the maximum “c” is connected with the reorientation of the adsorbed surfactant ions, while maxima “a” and 71” are connected with adsorption-desorption processes. When the SDS concentration becomes high enough, i.e. 5.10” mol dm3, a vertical step appears in the capacity curve at about -0.5 V,

331

connected with a non-equilibrium, two-dimensional phase transition. Obviously in the case of this concentration the adsorption monolayer in the potential region from 0 to -0.5 V has reached saturation (this is proved in Ref. [901by a statistical estimation of 6) and the approach of a greater number of sodium counterions to the electrode at more negative potentials leads to a surface phase separation in the adsorption monolayer. Thus the bulk SDS concentration, at which a surface phase separation is observed, is lower by about one order of magnitude than the CMC. The latter is 1.36.103 mol dm3 SDS in 0.1 M NaCl at 22°C [911. For potentials E more positive than 0.5 V and for 510A M SDS, desorption is not observed. The sharp decrease of differential capacity to values much lower than those for the supporting electrolyte solution indicates the formation of an adsorption layer consisting of two or more monolayers. On the basis of experimental data illustrated in Fig. 5 and applying the approach for the statistical estimation of adsorption parameters described earlier [87], I,.,, for SDS was found in Ref. [901. It was shown that I,.,, = 2.8.10-‘” mol cmM2,which is of the same order as for the S/G interface [921. The Gibbs free energy of adsorption of SDS at the M/S interface was AGj = -52.1 kJ mol-’ [901. This value is very close to the one obtained for Ci2E, [871 and is obviously determined by the length of the hydrocarbon chain. Differential capacity results 1501and the statistical estimates of the adsorption parameters of SDS on mercury [901were used in Refs [83,931 for calculating the electrostatic component of the disjoining pressure II,, in the mercury/thin solution film/gas system. The electrostatic component of the disjoining pressure II,, arising when two surfaces are approaching each other, is determined by the potentials $. of the outer Helmholtz planes of the two interfaces (often called Stern potentials) 1641. On the basis of capacity data, +. was calculated 1831for the electrode potentials E = -0.2 V and E = 0.4 V of the capacity minima, corresponding to the two different orientations of the surfactant ions at the interface. For this purpose the charge density qi of the specifically adsorbed DSions was found by the equation q,=-l?F=-er,F

(6)

applying the estimated values for 8 and Tm [83,90,931. For 5~10~ M SDS, Qi = -22.1 ClCcme2 in the case of E = 0.4 V and had almost the same value for E = -0.2 V. Because of the high surface activity of DS- ions, the charge density of the inner Helmholtz plane qi was much greater in absolute

332

value than the charge density Q of the mercury surface for both negative and positive polarizations (e.g. in 5.lo3 1K SDS q was less than -1 pC cme2 for E = -0.2 V and q r 6yC cme2 for E = 0.4 VI. A superequivalent adsorption of SDS at the interface was observed. According to the theory of Grahame and GouyChapman L&94,951

where Ciois the concentration of the ith ion in bulk of solution, Zi is the valence of the ion and F is the Faraday number. Applying Eqn (7), the potential I/J~of the outer Helmholtz plane was determined for the M/S interface. For the SDS concentration 510” Mand E = 0.4 V, $. = -0.108 V and for E = -0.2 V, I/J~= -0.129 V [831. C. Cationic surfmtants The adsorption of cationic SAA at the dropping mercury electrode was studied by Eda [231 who investigated the solutions of dodecylpyridinium chloride and dodecyltrimethylammonium chloride by the impedance bridge method. Eda firstly showed that these cationic surfactants were desorbed from the mercury surface at high positive charge densities and, secondly, that the differential capacity of zero or negatively charged mercury had its minimum value at an intermediate bulk surfactant concentration, i.e. at the higher bulk SAA concentrations the capacity increased with increase in concentration. The adsorption of n-alkyltrimethyl- and n-alkyltriethylammonium chalogenides with an alkyl radical chainlength no longer than 16 carbon atoms was studied by measuring the differential capacity of the mercury electrode [29-31,96,971. A general result was the appearance of a vertical step in the capacity curves for a certain bulk concentration of the cationic surfactant. Most of these studies, however, were carried out on a dropping mercury electrode which restricts the significance of results only to a qualitative level. Recently the adsorption of n-hexadecyltributylphosphonium bromide (CTBPB) was investigated at the stationary mercury electrode 1531.The capacity curves obtained in lo5 and 510m5M solutions of CTBPB are illustrated in Fig. 6. Two regions of adsorption with respect to concentration were distinguished in Ref. [531. On one hand, for surfactant concentrations lower

333

c-4

'E

U

Li

z 4c

3c

2c

I

05

I

0

I

- 0.5

I

E/ v

Fig. 6. Differential capacity dependence on electrode potential E in the case of pure supporting electrolyte (1) 0.1 M NaF and with added CTBPB (2) lo4 M and (3) 5*104 M.

than 10” mol dm3 the capacity values in the interval of the minimum were equilibrium values and did not depend on the direction of the potential scan. They were obtained after waiting for a constant capacity value in a way analogous to that illustrated in Fig. 4 for C12Ep These values were used in Ref. [531 for the statistical estimation of the adsorption parameters of CTBPB. For CTBPB concentrations lower than 10d M adsorption-desorption was observed at high cathodic and anodic polarizations.

334

On the other hand, for surfactant concentrations 10” M and higher, hysteresis was detected in the whole region of zero and negative potentials measured, and the capacity increased in comparison to that found in the case of the lower SAA concentrations. As seen in Fig. 6, desorption was not observed at the high cathodic polarizations. Kaisheva et al. [531 concluded that a surface phase transition takes place at the M/S interface for bulk CTBPB of concentration 10” mol dm3 and higher, in which the inter-facial monolayer is separated into two phases. This process should be accompanied by an increase in differential capacity, and this has indeed been observed. The CMC value for CTBPB (1.6*104 mol dm3) should be mentioned for comparison 1981. The broad and low maximum observed in Fig. 6, curves 2 and 3, was connected with a reorientation of the surface active cations 1531.It was supposed that at potentials near the PZC, corresponding to the minimum in curve 2 (E = 0.23 V), the surfactant cations are adsorbed with the more hydrophobic cetyl chain oriented to the electrode surface and the hydrophilic phosphonium group oriented to the aqueous solution. With the increase of the negative electrode charge the energetically more favourable orientation becomes that with the positive phosphonium group of CTBPB nearer to the metal surface, thus causing a reorientation of the surfactant in the monolayer. Applying the analytical model of differential capacity described by Eqns (l-3) and the non-linear regression analysis, it was shown b31,that the model based on the isotherm of Flory-Huggins is adequate to describe the experimental sample, including the capacity results for CTBPB concentrations lower than lo4 mol dm 3. Thus the statistical estimate for Im was 1.8~10-10f 0.2*10-‘” mol cm-‘. The standard Gibbs energy of adsorption was high, AG: = -88 kJ mol-‘, showing the very high surface activity of CTBPB. As a result of the quantitative investigation of CTBPB adsorption on mercury, applying Eqns (6,7), it became possible to calculate the potential of the outer Helmholtz plane $. for this interface. In the case ofE = -0.57 V calculations showed that I)~ = 0.077 V and in the case of E = 0.23 V, Q. = 0.095 V 1531. It became clear, that for both negative and positive electrode potentials E, $. was always positive and was almost independent of E. This result was due to the very high surface activity of the positively charged CTBP’ ion leading to its superequivalent adsorption at the M/S interface. So it can be concluded that the surface phase-transitions in monolayers of colloid surfactants is accompanied by the appearance of a vertical step in the capacity vs. potential curves. A characteristic feature of this step

335

is the fact that the capacity in one of the potential regions next to the step, corresponding to a greater electrode charge density, is usually higher for the higher SAA concentrations. This is explained by the existence of a greater quantity of counterions of the supporting electrolyte in the double layer, which is the cause for the rearrangement of the densely packed surfactant monolayer and for the uncovering of a part of the electrode surface. The bulk surfactant concentration, corresponding to the occurrence of a two-dimensional phase separation at the mercury/colloid SAA solution interface is of the order of a tenth of the CMC. The role of the concentration of the supporting electrolyte is important for the kinetics of the surface phase-transformation and for the structure of the adsorption layer. Because of their very high surface activity, ionic surfactants are characterized by a superequivalent adsorption, resulting in an outer Helmholtz plane potential $o which is almost constant with E. $. has always the same sign in the whole interval of positive and negative electrode charge densities and potentials E. It is always positive in the case of cationic surfactants, and negative in the case of the anionic SAA. 3. STABILITY OF THIN FILMS FORMED FROM AQUEOUS SOLUTIONS OF AMPHIPHILIC

SURFACTANTS

ON POLARIZED

MERCURY

The interaction between particles and bubbles in surfactant solutions is the basis of flotation and is a fundamental problem in colloid and interface science. This interaction is a heterocoagulation process [65,66, 991 in which surface forces play a decisive role. According to DLVO theory surface forces include van der Waals and electrical double layer forces. Recently Israelachvili and Pashley [lo01 revealed the essential role of another type of attractive force between surfaces-the hydrophobic force - which seems to be caused by the structural rearrangement of water molecules around hydrophobic surfaces [loll. For the investigation of surface forces precise control of the inter-facial potentials is important, as well as the application for quantitative analysis of the potentials 21)of the outer Helmholtz plane (Stern potentials) instead of the zeta potentials often used. These requirements make the mercury electrode especially convenient for studying film stability because besides having a molecularly smooth surface, mercury gives the possibility for obtaining a highly purified, ideally polarizable electrode with well defined parameters of the electrode/solution interface. A. Non-ionic surfactants In order to study more completely the properties of some SAA of

practical importance, the aim of the work of Kaisheva 179,821 was to investigate the correlation between the influence of the electrical double layer on the adsorption of the non-ionic surfactant Ci2E6 at the M/S interface on one hand, and the stability of thin films formed from Ci2E6 solutions on the same electrode, on the other. For this purpose the results from differential capacity investigations of SAA adsorption were compared with the lifetime t of the thin solution films formed in the system: polarized mercury substrate/thin film/hydrogen, the films consisting of 0.05 mol dm3 aqueous solutions of Na#O, with the addition of Ci2E6 in different concentrations. The experimental set-up for direct observation of thin liquid films between a mercury electrode and a hydrogen bubble is shown schematically in Fig. 7 and has been more precisely described elsewhere 179,821. A cell of similar construction has been used in Refs Ml, 1021.As seen from Fig. 7, the cell is isolated from the air and gives the possibility of working in a hydrogen atmosphere. The necessity for an inert gas atmosphere is an important feature of experimental work with mercury surfaces because of their strong tendency for oxidation. In the studies quoted above hydrogen was bubbled through the solution for at least one hour immediately before the experiment. Mercury was squeezed out of glass tube 1 and hydrogen out of glass tube 2 by the use of microscrew devices. Tubes 1 and 2 had the same inner diameter of 1.72.103 m. They were immersed in the solution contained in the glass cell. A potential was applied between the auxiliary platinum cylinder electrode, 3, and mercury by means of platinum wires sealed in glass. A saturated calomel electrode was used as a reference electrode

Fig. 7. Schematic diagram of the experimental cell. (1) Glass tube holding the mercury electrode; (2) glass tube holding the hydrogen bubble; (3) auxiliary platinum electrode; (4) salt bridge to the reference calomel electrode; (5) vertical microscope.

Fig. 8. A picture in reflected light of a thin film, formed on the surface of polarized mercury from a 0.05 M solution of NazS04 with the addition of C12Es.

and was connected to the cell by a salt bridge, 4. An all-glass apparatus was used with the purpose ofavoiding possible contamination. Films were illuminated through the objective, 5, and observed in reflected light with a microscope. For this purpose the upper glass surface of the hydrogenholding tube 2 was flat-parallel. The moment of film formation was distinctly marked by the appearance of Newton interference fringes, illustrated in Fig. 8, which served as a start of time measurement performed by means of an electronic counter. In the cases of unstable films the rupture of the film was accompanied by a clear observation of a solution-free mercury surface, a sharp increase of the area of contact and the disappearance of the Newton fringes, this moment being the end of the lifetime t (induction time) measurement. Thin liquid films were formed by the slow approach of an electrolytically-generated hydrogen bubble to a stationary mercury surface. In the process of film formation the pressure in the glass tube 2 was controlled. The radii of the films were of the order of 5103 m. The results of the t vs. E measurements are ihustrated in Fig. 9. In pure supporting electrolyte solutions (dashed curve 0 thin films ruptured immediately in the region of the PZC, or lived for several seconds at higher positive or negative potentials. For 5*1O-e M and lad M solutions of C,sEe (curves “a”

338

Fig. 9. Lifetime versus potential curves for films formed on mercury from solutions of 0.05 M Na2S04 (0 and with the addition of ClzE6 in concentrations: (a) 5*104 M and (b) lo3 M.

and “b” in Fig. 9) the region of potentials around -0.3 V was characterized by higher film stability. Solutions with bulk concentrations of C12E6higher than lo4 M formed very stable films living longer than 20 min in the whole interval of electrode potentials, where adsorption of the SAA was taking place, namely from E = 0.5 V to E = -1.0 V. Comparison of Fig. 9 with Fig. 2 indicated that the film stability in lad M C12E6solutions was directly connected to the surface phase transformation, observed with the help of differential capacity measurements for surfactent concentrations lower than the CMC 179,821.Obviously the formation of the thin liquid films became possible when densely packed adsorption monolayers of C,,Ee were formed at both interfaces M/S and S/G. B. Anionic surfmtants

Smolders [701 measured the contact angles in the system polarized mercury/aqueous sodium decane sulfonate solution/hydrogen. He observed that there was a difference, depending on the electrode potential, in the time necessary to reach the equilibrium values of the contact angles. The author noticed that at positively charged surfaces, solution films on mercury were not stabilized and a rapid film-break occurred. At

339

zero and negative rational scale potentials up to -0.5 V, films were stable. It was not the purpose of this work, however, to give a quantitative description of this phenomenon. The thinning and rupture of films from aqueous SDS solutions between polarized mercury and air was studied in Ref. 1781.The thinning process was microphotometrically investigated with an experimental technique and apparatus similar to that constructed by Scheludko and Platikanov [541. Disjoining pressure isotherms were obtained in relation to the polarizing potential of the mercury. The critical film thicknesses were found to be of the order of 200 nm. When the mercury surface was positively charged the film was observed to thin very rapidly and collapsed forming a definite contact angle. As the mercury surface was given higher negative charges the thinning became slower and, according to Mushiake et al. [781,there sometimes remained a stable wetting film free from collapse. This phenomenon was explained in terms of the adsorption of the highly hydrated sodium ions to the surface. It is a pity, however, that the inert gas atmosphere was not used in these experiments since the thinning process was of reduced reproducibility [781. However, very good electrocapillary and capacity results were obtained in this work for the interface mercury electrode/SDS solutions in 0.1 mol dm3 Na$O, and an attempt was made to correlate them with the film thinning behaviour. In Refs [83,85,931 the influence ofthe structure of SDS layers adsorbed on mercury on the interaction between a hydrogen bubble and a positively or negatively charged mercury electrode immersed in SDS solution was revealed. The results from the measurements of the lifetime t of the thin liquid films formed in the system M/S/hydrogen were compared with calculations of the electrostatic H,, and the van der Waals Dvw components of the disjoining pressure. The lifetime t (induction time) measurements were made by the same apparatus and cell, illustrated in Fig. 7. The results from these measurements in the case of liquid films formed from a 510p5 M aqueous SDS solution with 0.05 M Na2S04 as a supporting electrolyte are shown in Fig. 10. Vertical lines indicate the standard deviation oft. As seen in Fig. 10, the stability of the thin films depends strongly on the electrode potential E. At higher positive E the films were not stable and ruptured within one minute, leaving a solutionfree mercury surface. In the region of zero and negative E, films were stable and no rupture could be observed. These results confirm the observations made earlier in Refs 170,781.An explanation ofthis phenomenon was sought in Refs [83,85,931 by comparing it to the results from the independent differential capacity measurements at the M/S interface

340

[21,48,50,78,85,901. For comparison, in Fig. 11, curve 2, differential capacity vs. electrode potential is shown for a stationary mercury electrode in 0.05 mol dm3 Na2S04 with the addition of 5-10q mol dm3 SDS. As it was already pointed out, scientists [21,48,50,78,85,901 reached the conclusion that at the PZC, as well as at negative E, DS- ions were adsorbed in a monolayer, with their negatively charged headgroups oriented towards solution and the hydrocarbon chains oriented towards mercury. At positive electrode potentials the orientation of the specifically adsorbed organic anions is reversed. Thus the minimum at E = 0.4 V in Fig. 11 was connected with the maximum adsorption of DS-with the hydrocarbon chains turned towards the solution and the negatively charged headgroups towards positively charged mercury, making the surface hydrophobic. The minimum at E = -0.2 V was connected with the maximum adsorption of specifically adsorbed DS- ions in the opposite orientation. Comparison of Figs. 10 and 11 shows that there is a direct

._ i +

FILM UUoTURE

OBSERVED

2

Fig. 10. Lifetime (T) of the thin films formed between a mercury electrode and a hydrogen bubble in 5.10” M SDS aqueous solution, as a function of electrode potential. Supporting electrolyte: 0.05 M NazS04.

341

correlation between the reorientation of DS- ions at the M/S interface, marked by the small capacity peak at E = 0.26 V in Fig. 11, and the stability of the thin liquid films as reflected by the lifetime measurements in Fig. 10. In order to visualize these assumptions, the surfactant orientation is schematically represented in Fig. 12B. The results from double layer capacity measurements have been applied to calculate the electrostatic component of the disjoining pressure in the thin films as a function of the separation distance d between the outer Helmholtz planes for mercury and bubble [83,85,931. The potential $s,o of the outer Helmholtz plane at the S/G interface ?

F

u

3 u

50 \

40

30 -

2cI-

1C1 -

I

05

I

0.0

I

- 0.5

E/V

Fig. 11. Differential capacity vs. electrode potential for the stationary mercury electrode in (1) 0.05 mol dm” NazS04 aqueous solution and (2) in the presence of 5104M SDS.

342

E = -0,6V

E= 0,2V

E=-02V

E=04V

Fig. 12. A schematic representation of the structure of adsorption monolayers at the two interfaces of the films in the case of (A) cationic and (B) anionic SAA at positive (1) and negative (2) electrode potentials.

was estimated tion c of SDS

by measuring

the surface

tension

OS/Gvs. bulk concentra-

1851

is the Gibbs adsorption of SDS at the S/G interface. Supposing that DS- ions were potential determining, it was found that I)~,~was of the order of -50 mV for bulk SDS concentration up to 4~10~ M. The determination of II,, dependence on the distance d was achieved [83,931 by an exact integration using the theory of heterocoagulation as proposed by Derjaguin 1991. As shown earlier in this review, the values obtained for the outer Hehnholtz plane potentials at the mercury/510J M SDS solution interface were & = -108 mV for E = 0.4 V and $o = -129 mV for E = -9.7 V [831. The D,,(h) dependences for the system M/S/G in the case of 5.10” M SDS solution with 0.05 M Na,S04 as a supporting electrolyte, obtained where

rsIG

343

Fig. 13. Electrostatic (curves 1,2) and van der Waals (curve 3) components of disjoining pressure in thin films, formed on mercury from 5.10” M solutions of SDS with 0.05 M Na2S04 as a supporting electrolyte. Curve 1 corresponds to an electrode potential E = -0.2 V and curve 2 corresponds to E = 0.4 V.

by using the theory of heterocoagulation 1991are shown in Fig. 13. In the figure h = d + 26 is the surface-to-surface separation distance and 6 is the distance from an interface to the corresponding outer Helmholtz plane, assumed here as a first approximation to be 6 = 1.4 nm [931. Curve 1 in Fig. 13 corresponds to negative electrode polarization E = -0.2 V, where $c = -129 mV and curve 2 corresponds to positive E = 0.4 V, where Q. = -108 mV. In both cases the value -50 mV was used for $s,G. As seen in Fig. 13, a high Il,, barrier to coagulation exists for both E = 0.4 V and E = -9.2 v. In order to obtain the total disjoining pressure in the thin liquid films, according to the heterocoagulation theory 166,991 the van der Waals

344

component Il,, was calculated in [83,85,931. In all calculations the planeparallel model was used. In some of these works 185,931lIvwwas determined by the equation l-l VW

A

=-6Jch3

where A is the Hamaker constant. It has been proved [611 that the Hamaker constant for the M/S/G system is -7.22*10-20 J. Assuming as a first approximation that 26 = 2.8 nm is twice the length of a vertically oriented DS- ion, a lIvwdependence on h was obtained, illustrated in Fig. 13, curve 3. In Ref. [831 the van der Waals force was calculated by taking into account the effect of adsorbed layers on the basis of Han-raker11031and Vold [lo41 theories, in which the Hamaker constant of hydrocarbon was used as the constant of the hydrocarbon adsorption layer by assuming 8 = 1 on both mercury and bubble surface. The results have shown, however, that the correction for the effect of adsorption layers was negligible. Moreover, lIvwin the investigated system was much lower than Il,, and D, = D,, + I&,., was determined by D,,. As seen in Fig. 13, for the two opposite in sign electrode potentials (respectively charge densities) the dependences of Il,, on h (correspondingly of Il, on h) almost coincided and no explanation could be given on this basis for the different film stability at E = -0.2 V (stable films) and at E = 0.4 V (rupture) of films from 5.10” M SDS solutions. The estimation of the sucking capillary pressure P, in the system was made in Refs [83,85,931 by using the formula derived by Scheludko and Platikanov 1541

(10) where om is the interfacial tension at the M/S interface and r = 8.6*10A m is the radius of curvature, which is the same for the bubble and mercury. The value for P, at E = 0.4 V was shown in Ref. 1831to be 238 N rne2.As is obvious from the comparison of P, with curve 2 in Fig. 13, the barrier value for Il,, is high enough to prevent film rupture not only at negative but also at positive polarizations of the electrode. Curves 1 and 2 in Fig. 13 were obtained by using a constant potential heterocoagulation theory [991. If interaction at constant charge were assumed, or a mixed model, i.e. constant surface potential-constant surface charge model, Il,, would give an even higher force barrier. In order to account for the film instability at E = 0.4 V and c = 5~10~

345

M SDS, the authors of Refs [83,85,931 incorporated an additional attractive force - the hydrophobic force Fh. The latter was recently found by Israelachvili and Pashley to be a strong attractive force acting between two hydrophobized mica surfaces 11001,and was later observed by some other authors [105,1061. So Il, becomes I-I, = l-I,, + l-Ivw + l-lb

(11)

where, according to Ref. [lOOI Dh = -@,/do)

exp (d/do)

(12)

Equation (12) was derived for the interaction of hydrophobic hexadecyltimethylammonium bromide (CTAB) monolayers on mica [loo]. For the latter system C, = 0.14/2x = 2.23~10-~N m-l = 22.3 dyn cm-’ and the decay length do = lo-’ m [lOOI. In order to estimate the contribution of lIh to Il, in the system mercury/5105 M SDS solution/hydrogen, C,.,was regarded as a variable and do was set to be 1.4 nm. Assuming that in order to make the solution film at E = 0.4 V rupture, an attraction force Fh is necessary at least as high in absolute value as the barrier illustrated in Fig. 13, i.e. Fh = -1.5106 N rnv2,the force constant C, is determined to be 3.98*103 N m-i, or 3.98 dyn cm-‘. Such estimations of Ch were made in Ref [851 for lower bulk concentrations of SDS as well, and for another supporting electrolyte andE. The values obtained for C, were of the same order. For example in the case of 5-104 M SDS in 0.01 M aqueous NaC104 solution, Ch was found to be 3.0 dyn cm-‘. On the basis of the investigations described above, a conclusion could be made, that the rupture or stability of the thin liquid films from solutions of the anionic surfactant SDS, formed between mercury and bubble, depends on the balance between the attractive hydrophobic force and the capillary pressure, on one hand, and the repulsive double-layer and van der Waals forces, on the other. C. Cationic surfactants The hydrophilic or hydrophobic nature of the mercury electrode/ndecyltriethylammonium bromide solution interface was examined by static (contact angle) and dynamic (film rupture) methods [771. In the dynamic experiment the reaction of an air bubble in contact with a dropping mercury electrode to the falling of the mercury drop from a

346

capillary was observed. Both methods showed a definite dependence of surface hydrophobicity on electrode potential. However, perhaps because of air coming in contact with mercury, leading to possible oxidation of the surface, results were not quantitatively discussed. Stable unsymmetric liquid films were observed in the mercury/ aqueous dodecyltriethylammonium perchlorate (DTEAClOd solutions/ air system [801. Under similar experimental conditions in the case of the chloride salt (DTEACI), films ruptured within 20 s. Electrochemical investigations of the mercury electrode in DTEAC104 and DTEACl solutions suggested that strong lateral interactions between adsorbed ions occurred for DTEAClO,. Film stability studies in this work were again mainly qualitative. Thin liquid films from solutions of the cationic surfactant cetyltributylphosphonium bromide (CTBPB), formed between a mercury electrode and a hydrogen bubble, were investigated 1841.The supporting electrolyte was aqueous 0.1 M NaF. The purpose of this work was to compare the results from the measurements of the lifetime z of the films with differential capacity data for one of the film’s surfaces -the M/S interface. The film induction time was measured with the same experimental set-up, shown schematically in Fig. 7 and in the same way as was done for CIzEG and for SDS solutions [79,82,85,931. The obtained potential dependence of t is illustrated in Fig. 14 for 0.1 M NaF solution (curve 1) and with the addition of CTBPB in concentrations lo3 M (curve 2) and 5~10~ M (curve 3). Results shown in Fig. 14 were compared with the differential capacity measurements illustrated in Fig. 6 for the same solutions of CTBPB: 10” M (curve 2 in Fig. 6) and 5.10” M (curve 3 in Fig. 6). As seen in Fig. 14, stable CTBPB solution films exist in the potential region around the PZC and at positive E for a surf’actant concentration of 5.10” M. For potentials more negative than -0.2 Vliquid films from CTBPB solutions with c I 5+10-5M spontaneously ruptured, leaving a bare mercury surface. Film rupture was observed within seconds in the whole interval of potentials studied for pure supporting electrolyte solutions (curve 1 in Fig. 14), as well as for solutions of CTBPB with c ( 10” M. From the comparison of Fig. 14 with Fig. 6 it becomes obvious that a correlation exists between the results of both experiments. As shown in Ref. [531 and discussed earlier, for positive E around 0.23 V the cation CTBP’ is adsorbed with its long hydrophobic cetyl radical oriented towards the mercury surface, and with its hydrophilic phosphonium headgroup towards the solution. When a more negative electrode potential is set (E = -0.57 Vj, an orientation with the positive headgroup nearer

347

STABLE

FlLM

EQUILIBRIUM

RUPTURE

OBSERVED

FILMS

3

Fig. 14. Lifetime of the thin films formed between a mercury electrode and a hydrogen bubble in a 0.1 M aqueous solution of NaF (1) and with the addition of CTBPB in concentrations: lo4 M (2) and 5*103 M (3).

to the negatively charged surface becomes energetically more favourable. In this latter orientation the cetyl radical is turned to the solution, hydrophobizing the electrode. The adsorption layer structure in the case of these two possible orientations is shown schematically in Fig. 12A. Together with this reorientation process connected with the appearance of the broad and low maxima in curves 2 and 3 of Fig. 6 at electrode potentials E around -0.3 V, a two-dimensional surface-phase micellization was supposed to occur 1531. It can be concluded that films on a polarized mercury support are stable when the orientation of the adsorbed surfactant cations in the densely packed adsorption layer is such that the hydrophilic group is towards the solution. The opposite orientation of SAA causes hydrophobization of the mercury surface and rupture of the films, in some cases within tenths of a second. The results discussed here are in good agreement with those obtained for the anionic SDS (Fig. 12B). When the surfactant anion is oriented with its hydrophobic “tail” to the solution at positive electrode potentials, the

films are not stable. The same is observed for the cationic surfactant at negative E. An attempt was made in Ref. 1841to explain the experimental results by calculating the electrostatic component II,, of the disjoining pressure using Derjaguin’s theory [991, as discussed above. For this purpose the estimates for the outer Helmholtz plane potentials $. for the M/S interface were used. Because of the very high surface activity of CTBPB, a superequivalent adsorption of this substance on mercury was observed, as was the case with SDS adsorption. As a result, $. was positive in the whole interval of potentials E from -0.5 to 0.5 V. Two electrode potentials were chosen for the calculation of II,,(d) dependences, corresponding to the two different orientations of CTBP’ ions [841. In the case of the electrode potential E = 0.23 V the value +o = 0.095 V was used in the calculations and in the case of E = -0.57 V the value of 0.077 V was obtained for $o and applied further. It was accepted that the potential $s/~ of the outer Helmholtz plane at the S/G interface was the same, as found by Exerova et al. I1071for the aqueous solutions of cetyltrimethylammonium cations: $s/~ = 0.070 V. The heterocoagulation theory of Derjaguin 1991was used, as in the case of SDS [83,85,931. The van der Waals component of the disjoining pressure was calculated 1841in the same way as in the case of SDS [931, but using Eqn (9). The distance 6 was estimated on the basis of the length of CTBPB to be 2.05 nm. Il,_, was with two orders of magnitude lower than II,,. By summation of I& and II,,, the dependence of II, on h was calculated, and is illustrated in Fig. 15. As seen in the figure, for the two opposite in sign electrode potentials E = 0.23 V (curve 1) and E = 0.57 V (curve 2), the II,(h) dependences almost coincide and no explanation could be given on this basis of the difference in the film stability. In order to estimate the sucking capillary pressure P, in the investigated system, Eqn (10) was applied 1841.The interfacial tension as/G = 47 mN m-l 1981and ows was taken to be 415 mN m-l in the case ofE = 0.23 V and 350 mN m-l at E = -0.57 V 1841.Calculations on the basis of Eqn (10) showed that P, = 196 N rnT2at E = 0.23 V and P, = 193 N m-’ at E = -0.57 V. Consequently the capillary pressure in both cases was very low, several orders of magnitude lower than the height of the barriers, illustrated in Fig. 15. In order to explain the film rupture in the case of the negatively charged mercury surface, the existence of an additional hydrophobic force l& was assumed in Ref. 1841.At a distance d = 0.2 nm it was supposed to be at least equal in value and opposite in sign to the maximum of the barrier in Fig. 15 ll, = 3.34~10~N rne2.Equation (12) was used for the estimation of the hydrophobic force constant C,. For this

349

Fig. 15. The sum Ilt of the electrostatic and van der Waals components of the disjoining pressure, as a function of film thickness h for (1) I)O= 0.095 V at E = 0.23 V and (2) $0 = 0.077 V at E = -0.57 V. it was accepted that d, = 2.05 nm. It was found that Ch = 7.5103 N m-i = 7.5 dyn cm- ‘. As expected, this value is slightly higher than that obtained for SDS. However, it is lower than Ch found by Israelachvili and Pashley [loo]. Obviously the strength of the hydrophobic force, reflected by the force constant C, in this heterointeraction system, in which only one of the surfaces is hydrophobic, should be smaller than that for the purpose

350

two hydrophobic CTAB monolayers investigated by Israelachvili and Pashley [lOOI. REFERENCES 1 2 3 4 5

6

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