Langmuir 1991, 7, 2253-2260
2263
Diffuse Layer Electrostatic Potential and Stability of Thin Aqueous Films Containing a Nonionic Surfactant+ E.D.Mane4 and R. J. Pugh' Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden Received October 9, 1990. In Final Form: April 22, 1991 The stability and equilibrium thicknesses of free thin films produced from aqueous solutionsof C12(EO)a surfactant were studied in a wide range of concentrations and pH values. The films were found to be stabilized by repulsive electrostatic disjoining pressure, and the % potential of the diffuse layer of the film was estimated from DLVO theory. At the natural pH, ca. 5.7, it was found to have a value of ca. 42 mV at the lower surfactant concentrations and decrease at concentrations above 106 M. The decrease is noticeable already below the critical micelle concentration. Also the equilibrium film thickness was found to be pH dependent; the results showed a pHi,, = 3 and negative surface charge at pH > 3. It was concluded that the dependence of * O potential on surfactant concentration was due to the displacement of ions (H+ and OH-) from the nonionic surfactant adsorption layer at high surface coverage, where structural changes are expected to occur.
Introduction DLVO theory considers the electrostatic interactions as a major factor in the stability of the dispersed systems. There are several indications in the literature that poly(ethylene oxide) type surfactants adsorbed at an interface can alter the electrostatic potential which in turn can have a pronounced influence on the stability. For example, Schick1 used an air-electrode technique, which consisted of a direct reading electrometer and an ionized conducting path produced by a-radiation, to measure the surface potential at the air/solution interface as a function of concentration and chain length of adsorbed poly(ethy1ene oxide) 1-dodecanols and 1-octadecanols. A change in surface potential of some 90 mV was reported for 1-dodecanol + 14 EO as the surfactant concentrations varied between 10" and lo-" M, up to its critical micelle concentration (cmc). The surface potential was found to remain constant between pH 3 and 13.5, but below pH 3 a rapid increase was reported. These effects have been discussed more recently by Lange and Jeschke.2 There are also several other studies where changes in the electrokinetic (I;) potential, estimated from electrokinetic mobility measurements, have been reported to occur due to the adsorption of nonionicsurfactant at the colloid/ aqueous solution interface. In some cases, even reversal in f potential is reported as, e.g., by Hough and Thompson3 for positively charged silver iodide particles on adsorption of C12(EO)6 or by Glazman and Kabysh' for the same charged colloid on addition of C12(E0)18. With negatively charged silver iodide sols Mathai and Ottewills observed only a reduction in I; potential for the adsorption of C,(EO)e, where n = 8,10,12, and 16at PI 4, but no charged reversal effects were observed. The results were discussed in terms of a shift in the shear plane. Changes in I; potentials were observed to be caused by the adsorption of nonionics with specific adsorption of lanthanum ions on negatively charged silver iodide.s t Dedicated to Profeesor A. Scheludko on his 70th birthday. t On leave from the Department of Physical chemistry, Sofia
Univemitv. Bulnaria. _ . .~_.. ~ ~,Sofia. _ ~ .~~ _, (1) Schick, M. J. J. Colloid Sci. 1963, 18, 378. (2) h g e , H.;Jeschke, P. In Nonionic Surfactants;Schick,M. J., Ed.; Marcel Dgkker: New York, 1987; Chapter 1, p 31. (3) Hough, D. B.; Thompson, L. In Nonionic Surfactants; Schick, M. J., Ed.;Marcel Dekker: New York, 1987; Chapter 11, p 616. (4) G h a n , Yu. M.; Kabyeh, G.M. Kolloidn. Zh. 1969,31, 27. 1966,62,759. (5) Mathai, K. G.;Ottewill, R. H. Trans. Faraday SOC.
Values of the diffuse electric layer potential (%I) at the air/aqueous solution interface in the presence of adsorbed nonionics have also been determined by Exerowa et aleH from equilibrium film thickness studies. Experimental measurements6 were carried out at constant ionic strength at pH -5.5 at a range of surfactant concentrations (from 10sto le2 M solutions). Three nonionic Surfactants (nonylphenol20-glycol ether, dodecyl 11-ethylene oxide, and decylmethyl sulfoxide) and an anionic surfactant (sodium dodecylsulfonate) were used in the experiments. The equilibrium film thicknesses reached a plateau at surfactant concentrations before cmc and the potential of the diffuse electric layer corresponding to this region was estimated. For the nonionics, values of 37, 39, and 44 mV were obtained for the nonylphenol2O-glycolether, DMS, and dodecyl 11-ethylene oxide, respectively. In the work of Exerowa and Zacharieva8 the equilibrium thickness of films containing nonionics of the C l r (EO), type was found to be sensitive to the pH and electrolyte concentration and an isoelectric point at pH = 3.7 was reported. Huddleston and Smiths have determined the electrokinetic potential and associated charge at the air/aqueous solution interface with three different surfactants, one of which was nonionic (decyl methyl sulfoxide). For the latter, they estimated a I; potential of about 45 mV and pHiep = 2; at higher pH values, a buildup of negative charge was found to occur. In the present study we have considered electrostatic effects in aqueous foam films containing a nonionic surfactant and have discussed some of these early results. We have determined the equilibrium thickness and stability of films stabilized by pentaethylene glycol monododecyl ether of high purity. The influence of pH and ionic strength on the values of the diffuse-layer electrostatic potential (\k~),charge (uo),the sign of the charge, and the value of pHiepat the air/surfactant interface wae determined from DLVO theory. Experimental Section The equilibrium thickneeses of free films from aqueous solution of pentaethylene glycol monododecyl ether, CdEO)s, were
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(6) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1090. (7) Exerowa, D. Kolloid-Z. 1969,232, 7093. (8)Exerowa, D.; Zacharieva, M. Research in SurfaceForces;Plenum: New York, 1972; p 234. (9) Huddleston, R. W.; Smith, A. L. In nothe and Foam; Academic Press: London, 1975; p 163.
0 1991 American Chemical Society
2254
Langmuir, Vol. 7, No. 10, 1991
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Figure 1. Schematic of the experimental setup for studying microscopic thin liquid films. established over a wide range of surfactant concentrations (from lo-' to 4.5 X 10-2 M). High purity grade surfactant (>99.9%) was supplied by Nikko Chemicals, Japan. All solutions were prepared with doubly distilled water. The capillary pressure was determined by the method of capillaryrise in a glass capillary. Surface tension was determined by the Du Nooy ring method. Microscopic films of radius from 0.1 to 0.25 mm were studied by the microinteferometric method as previously described by Scheludko and Exerowa.loJ1 We shall consider here, some necessary details of the equipment. A schematic of the experimental setup is presented in Figure 1. A film is formed between the tips of the menisci of a biconcave drop held in a cylindrical glass tube (of 2.2 mm radius) by sucking the liquids out of the tube. The amount of liquid in the biconcave drop is controlled by a Teflon microsyringe by which the radius of the film can be varied. The measuring cell (containing the glass tube holder of the film) is enclosed in a water jacket which is maintained at a constant temperature (hO.1 OC). The cell is placed on the stage of a metallographic microscope (Carl Zeiss IM 35). Light from a quartz-halogen source (100 W) passing through a monochromatic interference filter (X = 546 nm) is incident on the film surface. The light signal reflected from a small portion of the film (radius ca. 0.02 mm) is directed onto a photodiodide, converted into an electric signal, amplified, and finally recorded onto a stripchart recorder as photocurrent versus time. The film is observed in reflected light through a calibrated eyepiece and the radius of the film is determined visually with an accuracy of a0.005 mm. Precautions are taken to eliminate the effect of external disturbances, such as vibrations, on the film thinning process as well as the critical thickness of rupture and black film formation. The film thickness (h)at any instant of time is estimated from the intensity of the photocurrent through the relationship
where G = [(no- l)/(no + 1)12,no is the refractive index of the studied solution, X is the wavelength of the monochromatic light, and A = (Z - Zmin)/Z- Zmh),where Z, Z-, and Zmin are the instantaneous, maximum, and minimum values of the photocurrent. The error of film thickness determination did not exceed 0.5 nm. corrections at small film thicknesses due to optical nonhomogeneity (according to the model, taking into account the (10) Scheludko,A.; Exerowa, D. Commun. Chem. Inst., Bulg. Acad. Sci. 1959, 7, 123. (11) Scheludko,A. ThinLiquidFilms,Adu. ColloidlnterfaceSci. 1967, 1, 391.
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Figure 2. Equivalent equilibrium film thickness as a function of C12(EO)bbulk concentration: open circles, +lW M KC1; closed circles, +1W9 M KCl; y indicates the surface tension isotherm at 20 OC. pH = 5.7 f 0.1. differences in the optical densities of the adsorption layers and the aqueous core) were made in the manner described in refs 12 and 13. By use of the refractive index of water for no in eq 1 the 'equivalent" water thickness (h,) could be derived. The "realn film thickness (h)can then be obtained from h, when corrections are made for the surfactant adsorption layers, which differ in refractive index from water. The real thickness (h) can be derived from the equation
where dl is thickness of the layer formed by the hydrocarbon tails of the surfactant molecules adsorbed on the film surfaces, nl is refractive index of this layer, n, is refractive index of the aqueous solution, n2 is refractive index of the layer formed by the surfactant polar heads, and dz is ita thickness. We have assumed nl = 1.42 (i.e. equal to that of dodecane), n, = 1.333 and n2 = 1.47 (ref 13). The value of dl will obviously depend on the density of the adsorption layers. It can be calculated from the volume per hydrocarbon chain (assumed to be 0.38 nm3, as, e.g., for pure dodecane) and the area per adsorbed molecule at the respective surfactant concentration. The distance between the planes of potential \ko in the film can be estimated, as in ref 14, by subtracting from the real thickness value (h)the correction Ah Ah = 2d1
+ 26
(3)
where 6 = d2/2,6 is the "radius" of the hydrophilic head, and the value of dz is obtained from the area per molecule data (Figure 6), and assuming volume of 2.5 X nm3 per methylene group or an oxygen atom (cf. ref 13). The thickness correction with respect to h, increases upon surfactant concentration and reaches -3.2 nm at the high adsorption levels.
Experimental Results Figure 2 shows the equivalent equilibrium film thicknesses of the aqueous foam films as a function of C12(E0)6 bulk concentration at natural pH (pH = 5.7 f 0.1) at two ionic strength values and 10-3 M KC1 added, respectively) and temperature 20 f 0.1 "C. The surface tension of t h e C d E 0 ) 6 aqueous solutions determined in the concentration range from lo-' to M at 20 O C are (12) van den Boomsgaard, A. Ph.D. Thesis, Wageningen Agricultural University, Wageningen, 1985; Chapter 5. (13) Barneveld,P.;Scheutjene, J. M. H. M.; Lyklema,J. Colloids Surf. 1991,52, 107. (14) Kolarov, T.; Cohen, R.; Exerowa, D. Colloids and Surf. 1989,42, 49.
Langmuir, Vol. 7, No. 10, 1991 2255
Stability of Thin Aqueous Films
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also shown in this figure. The surface tension isotherm thus obtained indicates the high quality of the surfactant used; the value of the critical micelle concentration obtained from these measurements confirms the manufacturers claim. There are no indications of a minimum or any other irregularities in the surface tension isotherm which would suggest the presence of impurities in the surfactant sample used for the present experimental study. The film thicknesses determined experimentally as a function fo nonionic surfactant concentration in the M exhibit concentration range from to 4.5 X several features of interest. Equilibrium films of varying thickness form throughout this concentration range. Exceptions are the unstable (rupturing) films which occur in all cases at the lowest studied surfactant concentration M) and the occasional ruptures at the other concentrations below the cmc. At a higher C12(E0)5 concentration M) critical phenomena are also observed. In this case, however, they do not lead to the destruction of the films. Instead stable, so-called “black films” of smaller thickness (
