Langmuir 1990,6, 1083-1087 with changing composition. The values for cmci (the cmc of the single surfactants in water) are independent of composition only if the ionic strength is constant, as would be the case for solutions with an excess of an added (swamping) electrolyte. This condition has not been considered in the present study, because it would invalidate the conductance method used for determining the mixture cmc*. Therefore, the values of cmci to be used in eq 18 will strictly be defined as the cmc of unicomponent surfactants at the same conditions (T, p, ionic strength) as the mixture at the mixture cmc*.21 The effect of counterion concentration on the cmc of a pure ionic surfactant is well established12and can be expressed quantitatively cmci = (19) where XO represents total concentration of counterion and (cmco)i is the cmc for the pure surfactant with no excess of added electrolyte. For the exponent, bi, the following values can be found in the literature for the surfactants used: 0.67 (CTAB),220.65 (TTAB),220.62 (DTAB),23and 0.35 (DeTAB).24 The combined relation that results from eqs 18 and 19, using the mixture cmc* as the concentration of total added electrolyte, XO,in eq 19 leads to an acceptable description of the experimen(21) We acknowledee a reviewer for this sueeestion. (22) Barry, B. W.; korrison, J. C.; Russell, F. J. J . Colloid Interface Sci. 1970, 33, 554. (23)Emerson, M. F.; Holtzer, A. J . Phys. Chem. 1967, 71, 1898. (24) Corrin, M.L., Harkins, W. D. J. Am. Chem. SOC.1947, 69,683.
1083
tal cmc* for all binary mixtures studied, as can be seen in Figure 3. The obtained results may confirm not only the expected ideal behavior of the considered binary mixtures but also the usefulness of the newly developed conductivity method in a more precise determination of the cmc* in mixtures of ionic surfactants. Whether its application would also be extensible to other kind of mixtures, such as those formed by ionic/nonionic surfactants, would depend on the assignment of the correct meaning of the Gaussian maxima concentrations for the conductivity/concentration second derivative in these systems, as well as on its development for physical quantities other than conductivity.
C X
D M K
x P cmc* Xi
Symbols total concentration of surfactants concentration of free counterion total surfactant monomer concentration total micellized surfactant concentration conductivity ionic conductance micelle counterion binding parameter cmc of mixed system mole fraction of surfactant in total mixed solution
E.
Registry No. DeTAB, 2082-84-0;DTAB, 1119-94-4;TTAB, 1119-97-7; CTAB, 57-09-0.
Effect of Nonionic Surfactant on Ion Adsorption and Hydration Forces Mark W. Rutland and Hugo K. Christenson* Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, GPO Box 4, Canberra ACT 2601, Australia Received September 8, 1989 Measurements have been carried out of the force between mica surfaces immersed in aqueous micellar solutions of the nonionic surfactant penta(oxyethy1ene) n-dodecyl ether with sodium sulfate as a background electrolyte. At surfactant concentrations below about 5 x 10-3 M (;=lo0 X cmc), adsorption to the mica surface is weak and can only be indirectly inferred from a reduction in the surface potential in dilute electrolyte solution. Further evidence for adsorption is provided by a decrease in the measured adhesion between the surfaces and an enhanced viscous drag at small (lo0 x cmc), extensive aggregation occurs between the surfaces, and this gives rise to very long-range,repulsive forces. We speculate that the reduction in surface potential may be a more general feature of nonionic adsorbates and rationalize some earlier results on the adsorption of dextran.
Introduction The adsorption of ionic surfactants to solid surfaces from aqueous solution depends, in general, upon electrostatic attachment of the headgroups to surface sites, sup0743-7463/90/2406-1083$02.50/0
ported by a favorable reduction in hydrophobic tail-water contact. In systems of nonionic surfactants, the adsorption mechanism is less well defined. For surfactants of the poly(oxyethy1ene) alkyl ether type (and related species), the adsorption process seems quite well under0 1990 American Chemical Society
1084 Langmuir, Vol. 6, No. 6, 1990 stood for substrates with the capacity for hydrogen bonding such as The ether groups hydrogen bond to silanol groups presenting the hydrophobic tail toward the solution, with which, at higher concentration, a second layer intercalates. The adsorption of nonionics to hydrophobic surfaces is also well characterized. The hydrophobic tail points toward the surface with the headgroup in the solution. This has been studied both by direct surface force measurements6 and by monitoring the decrease in bulk concentration in a dispersion of the hydrophobic ~ u b s t r a t e . ~On - ~ surfaces which are neither hydrophobic nor hydrogen bonding, it is not obvious whether or not adsorption will occur and, if so, under what conditions and in what orientation the surfactant will adsorb. Nonionics are of interest in both mineral flotationlo and for oil recovery,11J2although they are susceptible to salting out effects in solution and their phase behavior is extremely temperature dependent. A characteristic feature is the occurrence of a closed miscibility gap in the micellar pha~e.13.1~ The temperature (cloud point) a t which a two-phase region first appears is dependent on the relative sizes of the headgroup and tail. It has been shown15that a very large, polydisperse surfactant (octyl phenyl poly(-4O)(oxyethylene)) can be made to adsorb to mica from micellar solution in high salt (0.1 M KN03). The experiments presented below were performed below the cloud point of a poly(oxyethy1ene)alkyl surfactant with the intention of shedding more light on the adsorption of a monodisperse nonionic surfactant from micellar solutions at low electrolyte concentrations.
Materials and Methods Penta(oxyethy1ene) mono-n-dodecyl ether (C12E5) was used as supplied by Nikko Chemicals (critical micelle concentration (crnc) 4 x 10-5 M).16 The water was treated with activated charcoal, ion exchanged, and doubly distilled and typically had a conductivity of 1-2 pS cm-1. Sodium sulfate was supplied by BDH and was roasted overnight a t about 500 "C. Sodium sulfate was used as the electrolyte in this study as it was discovered that halide ions, especially in the presence of nonionic surfactant, attack the silver layer on the back side of the mica. Surface force rneasurementsl7 were performed between molecularly smooth mica sheets (with silver backing) mounted on curved silica disks in a crossed-cylinder configuration (radius = 2 cm). The separation of the surfaces, D , is determined interferometrically to within 0 . 2 nm, and the force is measured by the deflection of a double cantilever spring on which one of the surfaces is mounted. The force, F , is normalized by R, the mean radius (1) Rupprecht, H. B o g . Colloid Polym. Sci. 1978, 65, 29. (2) Rupprecht, H.; Liebl, H. Kolloid-2. 2.Polym. 1970,239, 685. (3) Seng, H. P.; Sell, P. J. Tenside Deterg. 1977, 14, 4. (4) Furlong, D. N.; Aston, J. R. Colloids Surf. 1982, 4, 121. (5) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf. 1984, 12, 255. (6) Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J . Chem. SOC., Faraday Trans. I 1986,82, 2735. (7) Abe, R.; Kuno, H. Kolloid-2. 2. Polym. 1986, 70,181. (8) Wolf, F.; Wurster, S. Tenside Deterg. 1970, 7, 140. (9) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1959, 62, 1241. (IO) Doren, A.; Vargas, D.; Goldfarb, J. Inst. Mining Metall. Trans. C 1975, 84, 34. (11) Rouquerol, J.; Partyka, S.; Rouquerol, F. Stud. Surf. Sci. Catal. 1982. 10., 69. ~. ._ , ~.
(12) Akstinat, M. H. Tenside Deterg. 1985,22, 77. (13)Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (14) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.;MacDonald, M. P.J. Chem. Soc., Faraday Trans 1 1983, 79, 975. (15) Luckham, P. F.; Klein, J. J. Colloid Interface Sci. 1987, 117, 149. (16) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueow Surfactant Systems; US National Bureau of Standards: Washington, DC, 1970. (17) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 975.
Rutland a n d Christenson
i
2 0
20
40
60
Distance (nm)
Figure 1. Measured force (normalized by the radius of curvature, R ) between mica surfaces immersed in M Na2S04 with and without added C12E5. The solid lines are fits obtained by using a nonretarded van der Waals interaction and nonlinear Poisson-Boltzmann equation according to the algorithm of Chan et a1.20 (Hamaker constant for mica across water of 2.2 X 10-19 J).17 The open symbols are the results of two force measurements made in the absence of C12E5 and are fitted with the following parameters: the surface potential at infinite separation = -122 mV, the Debye length ~ - 1= 17.3 nm (nominal concentration 1.0 X 10-4 M Na2S04), giving a surface charge u = -9.4 X 10-3 C m-2. The closed symbols are two force measurements with 6 X 10-4 M C12E5 (15 X cmc) with the parameters @ = -105 mV, K - ~ = 17.3 nm, and u = 6.7 X 10-3 C m-2.
*
of curvature of the surfaces and is plotted as a function of D. The zero of separation is taken as the position into which the surfaces jump in pure water (or dilute electrolyte) and is assumed to be mica-mica, or primary, contact. From the Derjaguin approximation,'* the quantity FIR is equal to 2rE, where E is the interaction free energy per unit area between flat surfaces. Measurements were carried out a t 22.0 f 0.2 "C, which is well below the cloud point for this surfactant (27 OC).l9 Unless otherwise stated, the time between points in each force measurement was about 30 s. The curve fitting was done using a nonretarded van der Waals interaction and the nonlinear PoissonBoltzmann equation according to the algorithm of Chan et a1.20 (with a Hamaker constant for mica across water of 2.2 X 10-20 J)."
Results Figures 1-4 show force measurements in various solutions of C12E5 and Na2S04. Figure 1 is a comparison of forces measured in M NaZS04 with and without surfactant. The open symbols represent two runs performed consecutively in salt alone. In this case, classic DLVO behavior (a long-range double-layer repulsion and a short-range van der Waals attraction) is observed; the surfaces jump into adhesive contact from about 2.8 nm. After surfactant was injected to a concentration of 6 X M (15 X cmc), the results represented by the closed symbols were obtained. The interaction of the surfaces still seems to follow DLVO behavior, but the magnitude of the interaction is reduced as the surface potential drops from -122 to -105 mV. Note that even a t 15X the cmc the surfaces jump into primary contact, suggesting that any surfactant adsorbed to the surfaces is easily pushed out. A t higher concentrations (Figure 2,500 X cmc, open symbols), the surface potential is further reduced to -84 mV. If the force run is done a t high speed without allowing the system time to equilibrate between measure(18) Derjaguin, B. V. Kolloid-2. 1934, 69, 155. (19) Clunie, J. S.; Corkhill, J. M.; Goodman, J. R.; Symons, P. C.; Tate, J . R. J. Chem. Soc., Faraday Trans. I 1967,63, 2839. Clunie, J. S.; Goodman, J. R.; Symons, P. C. J. Chem. Soc., Faraday Trans. 1 1969, 65, 287. (20) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77.
Langmuir, Vol. 6, No. 6, 1990 1085
Effect of Nonionic Surfactant on Zon Adsorption
-
c
.
'E
5. k
23-
J \
2
0
5
10
20
15
0
25
Distance (nm)
I
I
'E
i
g 4 E0
t I - 3P
.
M
9
0
50
100
Distance (nm)
Figure 2. Forces in M Na2S04 and 0.02 M C12& (500 x cmc). At this stage, equilibrium had not been reached at the surfaces. The closed symbols are a force run performed too fast for the system to relax or equilibrate between measurements ( e 5 s between points). In this case, a large additional repulsion due to viscous drag is seen, compared with the open symbols in which run the system was allowed time to equilibrate between points (approximately 30 s between points). The fit to the open symbols yields a further reduction in surface potential compared to the lower curve in Figure 1: 'k = -84 mV, K-1 = 17.3 nm, and u = -4.3 X 10-3 C m-2. 5
...,..
a 0 I
I
5
10
\.i
..
Distance (nrn)
Figure 3. Forces in 4.5 X 10-3 M Na2S04. Closed symbols correspond to inward runs (surfacesbrought toward one another), and the open symbols are outward runs (measured as surfaces are taken apart). The squares show measurements in 6 X 10-5 M Cl2E5 (1.5 X cmc), and these measurements are within error equal to forces measured in the absence of surfactant.The circles show force measurements performed in 6 X 10-3 M C&5 (e150 X cmc). The higher surfactant concentration causes a significant reduction in the hydration force. Both curves can be fitted beyond the range of the hydration force with 'k = -110 mV, K-1= 2.6 nm (nominal concentration of 4.5 X 10-3 M), and u = -4.8 X 10-2 C m-2. ments (e.g., 5 s between points, Figure 2, closed symbols), then a large additional repulsion is observed starting about 5.0 nm from contact. This effect was not observed a t lower concentrations. Figure 3 shows the results of two force runs in 4.5 X 10-3 M Na2S04. In both curves, the closed symbols are inward runs (measured as the surfaces are brought toward one another) and the open symbols are outward runs (measured as the surfaces are taken apart). The squares show measurements in 6 X M C12E5 (just above the cmc) and are almost identical with measurements (not shown) in the absence of surfactant. At separations above about 2.5 nm, the interaction is again well described by DLVO theory. However, a t separations of less than 2.5 nm, an additional short-range repulsive force is observed; this repulsion is caused by the adsorption of hydrated sodium ions to the surface and is commonly called a hydration f0rce.'~,2~The hydration force has been shown to be an
Figure 4. Long-range forces observed at high surfactant concentrations (>lo0 X cmc) after long (>24 h) equilibration time. The squares show a force measurement in 10-4 M Na2S04 and 6 X 10-3 M C12E5. The circles show forces measured in the same surfactant concentration but with 4.5 X 10-3 M NaZS04. Closed circles are inward runs; open circles outward.) The dotted lines approximate the force curves shown in Figures 1 and 3.
oscillatory function (with a period equivalent to the diameter of a water molecule) superimposed on a concentration-dependent monotonic repulsion, with the amplitude of the oscillations increasing with increasing Na+ concentration.z2 The maxima and minima were not accurately determined in this experiment. The circles in Figure 3 show measurements performed when the Cl2E5 concentration had been increased to 6 X 10-3 M (roughly 150X the crnc). The longer range part of the curve shows identical decay, but the hydration force is reduced by approximately a factor of 2. Figure 4 shows the effect of a high surfactant concentration ( > l o 0 cmc) in different salt regimes. An additional, usually hysteretic, very long range repulsive force appears. For comparison the dotted lines approximate the forces shown in Figures 1 and 3. At the higher salt concentration (4.5 X M Na2S04, 6 X M C12E5, closed circles inward, open circles outward), the longrange force is of a smaller magnitude than that observed in lower salt concentrations (squares, M Na2S04, 6 X 10-3 M C12E5). The concentration of C12E5 a t which this long-range repulsion first appears for different electrolyte concentrations was found to vary somewhat from experiment to experiment but was always about 100 X cmc. In some cases, the surfaces could be forced into primary adhesive contact even though a hydration force had prevented this a t lower surfactant concentrations. For clarity, only one outward run is shown, but it illustrates the typical hysteresis seen in most of the longrange, repulsive force runs. In M Na2S04,the adhesion of the mica surfaces was indistinguishable from that in water, giving the surface energy in the solution y = 3.7 and 4.9 mJ m-2. (A range of values is quoted, as there is controversy as to whether y = F/4?rR or F/3irR, where F is the pull-off force.23 On addition of surfactant to 1.5 and 15 X cmc, the adhesion became progressively smaller, and at 150 X cmc the adhesion was reduced to 1.8-2.4 mJ m-2. Discussion It is apparent from the results presented above that, even with a concentration of nonionic surfactant of many times the cmc, the effect on the interaction of the mica (21) Pashley, R. M.J. Colloid Interface Sci. 1981,83, 531. Israelachvili, J. N. J.Colloid Interface Sci. 1984, (22) Pashley, R.M.; 101, 511. (23) Christenson, H.K.J. Colloid Interface Sci. 1988, 121, 170.
1086 Langmuir, Vol. 6, No. 6, 1990 surfaces is very slight. Previous work on the adsorption of cationic s u r f a c t a n t ~ ~has * ? ~shown ~ a marked change in surface interaction well below the cmc. With increasing surfactant concentration, a drop in surface potential (even a charge reversal) is observed, followed by a hydrophobic attraction as a monolayer forms and then a repulsion as a charged, hydrophilic bilayer forms at concentrations below the cmc. With the formation of each layer, the contact position (or position of closest approach) changes. None of these effects is seen here as the absence of any electrostatic component precludes this type of adsorption. Although there is no measurable steric contribution to the force, the reduction in both the surface potential and the magnitude of the hydration force suggests that some slight adsorption of the surfactant takes place. PashleyZ1has described the surface potential of mica in aqueous solution in terms of competing adsorption of metal and hydrogen ions. With this model, he successfully explained both the nonmonotonic behavior of the surface potential with metal ion concentration and the nature of the hydration force. The model has been extended to explain the interesting charging properties of silica in solutions of symmetrical quaternary ammonium ions.26 The model incorporates the size of adsorbing cations and the area of negative surface sites. Sodium ions, which have an adsorbed area of 0.55 nm2,21thus obscure more than one negative site (area 0.48 nm2).27 Both the reduction in the surface potential and the reduction in the hydration force can be explained if the presence of adsorbed surfactant shifts the adsorption equilibrium for sodium ions without affecting the adsorption of hydronium ions. This is a reasonable proposition if one accepts that the adsorption of hydrogen ions is to negative sites in the crystal lattice and they are thus within the oxide layer forming the surface, in contrast to that of metal ions which, due to their larger size, are constrained to reside exterior to the mica surface. The result is that the potential drops as the free energy of adsorption of Na+ (but not H+) decreases with increasing surface concentration of surfactant. (This is because for each Na+ ion prevented from adsorbing, more than one H+ may adsorb.) Although the magnitude of the change in free energy (AG,d,(Na+)) is unknown, a good fit to the measured drop in potential in Figure 1is obtained by assuming that AG,d,(Na+) drops from -19.8 to -16.9 k J mol-'. The effect on the adsorbed Na+ density of decreasing AGad,(Na+) is shown in Figure 5. The effect of the change in Na+ ion density on the surface potential (at m separation) is plotted in Figure 6 . It can be seen that in M Na2S04 (Le., 2 X M Na+) the effect on the potential (q)is large, but in 4.5 X M NazS04 (Le., 9 X M Na+) the effect on the potential is small, as observed in Figure 3. The reduction in Na+ ion density though would easily be enough to appreciablyreduce the hydration force, again as observed in Figure 3. The reductions in potential and in hydration force can, of course, be equally well fitted by assuming that AG,dH+) increases without a corresponding increase in AGada(Na+). It seems unlikely, though, that a change in the solution properties and adsorption layer should give rise to an effect which would change the adsorption of ions within (24) KBkicheff, P.; Christenson, H. K.; Ninham, B. W . Colloids Surf. 1989, 40, 31. (25) Herder, P. C. J . Colloid Interface Sci., in press. (26) Rutland, M. W.; Pashley, R. M . J . Colloid Interface Sci. 1989, 130, 448. (27) Gaines, G. L. Nature (London) 1956,178,1304.
Rutland and Christenson
150
2 .-,x 100 yi
8 +
2
50 0
1
3 PNa
2
5
4
Figure 5. Adsorbed sodium ion density at the mica surface, calculated from the model of metal ion adsorption in ref 21. The solid line is the adsorbed Na+ density as a fundion of sodium ion concentration in the bulk obtained by using a free energy of adsorption (AG,b(Na+)) of -19.8 kJ mol-l. The broken line shows the same thing for a reduced adsorption energy (AGd(Na+) = -16.9 kJ mol-'). The bold arrow shows the approximate minimum concentration which will ordinarily give rise to a hydration force ((6 f 1) X 10-3 M Na+), and the other arrows show the Na+ concentrations at which the results of Figures 1-3 were obtained. I
40
'
1
I
I
2
,
I
3
4
5
PNa
Figure 6. Surface potential *om (at infinite separation) of mica as a function of sodium ion concentration. The solid line shows the behavior of 90" with Na+ concentration obtained by using AC.d.(Na+) = -19.8 kJ mol-1.21 The dotted line shows how the potential would behave if AG,&(Na+) = -16.9 kJ mol-1. The arrows show the Na+ concentrationsat which the results of Figures 1-3 were obtained. the surface (H+) without equally changing the adsorption energy of ions adsorbed exterior to the surface. It is possible that the change in the ion-binding equilibrium could be caused by a change in activity of the adsorbing ions in bulk rather than in the adsorbed layer. This seems less likely, especially in view of the fact that it is usually anions that interact strongly with nonionic surfactants (as seen in the effect on cloud points at salt concentrations 1 order of magnitude higher than used here).28 The reduction in adhesion (by about a factor of 2) provides further evidence for weak adsorption. Since the state of the surfaces in contact is the same with and without surfactant, the decrease must be due to a reduction in the surface energy at infinite separation in surfactant solution. It is reasonable to assume that surfactant adsorption reduces the surface energy. The additional repulsion in Figure 2 gives further indi(28) Meguro, K.; Ueno, M.; Eaumi, K. In Nonionic Surfactants: Physical Chemistry;Schick, M., Ed.; Marcel Dekker: New York. (29) Dorfler, H.-D.; Bergk, K.-H.; Miiller, K.; Miiller, E. Tenside Deterg. 1984,21, 5. (30) Ueno, M.; Takasawa, Y.; Miyashiga, H.; Tabata, Y.; Meguro, K. Colloid Polym. Sci. 1981,259, 761. (31) Doren, A.; Goldfarb, J. J . Colloid Interface Sci. 1970,32, 67.
Langmuir, Vol. 6, No. 6, 1990 1087
Effect of Nonionic Surfactant on Ion Adsorption cation of adsorption to the surface. This repulsion is entirely due to a viscous effect, as quantified by Horn et al.32 As this force was not observed in lower surfactant concentrations, it means that the viscosity at the surface is now much higher-indicative of a high surface concentration of surfactant. Thus it seems reasonable to suppose that, for concentrations of many times the cmc, there is some weak adsorption of C12E5 which is easily displaced as the surfaces approach. It seems likely that adsorption of the surfactant to mica is either parallel to the surface or has the nature of a monolayer with the hydrophobic moiety toward the mica surface. The thickness of an adsorbed monolayer on a hydrophobic surface at 22 OC is approximately 2.0 f 0.4 nm,6 and it should be noted that the extra repulsion in Figure 2 starts at 5.0 nm-perhaps equivalent to two monolayer thicknesses. The generally hysteretic, long-range force in Figure 4 may be due to something in the nature of lamellar ordering between the surfaces or to adsorption of micellar aggregates. The difference in the magnitude of the longrange repulsive force at different salt concentrations is not easily explained. The bulk solution remains micellar, and it should be noted that the concentration of surfactant used here is considerably less than that required to form a bulk lamellar phase.14 At 22 "C, the micellar/ lamellar phase boundary is at about 55% by weight C12E5. The maximum concentration of surfactant used here is about 0.8 % A substance with a similar surface to that of mica is magadiite which is a lamellar type silicate with a (nonhydrogen-bonding) oxide surface. Dorfler et al.29 studied the adsorption isotherms of ClzEs to synthetic Magadiite by monitoring the bulk concentration of a (centrifuged) dispersion. It was shown that maximum adsorption took place at roughly 40 X the cmc (cmc = 7.1 X M,30adsorption only to external surfaces, as determined by X-ray diffraction). These authors also observed an increase in adsorption with 7 X M Na2S04 which was ascribed to the solution structure-breaking properties of S042- i0ns.3~ All this is markedly different to observations of adsorption of a nonionic surfactant on a silica surface in a similar concentration of Na2S04.10 In the absence of electrolyte, the adsorption plateaued (7.5 X lo-' mol g-l) at the cmce5 The coverage at this plateau is of the same order as a monolayer at the air-water interface. In the presence of electrolyte, it was initially observed that increased adsorption took place (compared to the adsorption in the absence of electrolyte at the same pH) at concentrations up to 0.6 X cmc, where it plateaued at a coverage of only 40% of that in the absence of electrolyte. This different behavior (vastly lower concentrations of surfactants are required for full adsorption to silica, and different behavior in sodium sulfate), implies a different type of adsorption. On silica, the surfactant adsorbs due to hydrogen bonding between undissociated silanol groups and the poly(oxyethy1ene) headgroup.l+ Initially, a hydrophobic monolayer forms which is then rapidly intercalated by a second layer, with overlapping of hydrophobic tails, presenting a more hydrophilic surface. Presumably the inhibition by sodium ionslO is caused by substitution of hydrated sodium ions on the silanol groups, thus precluding the possibility of H-bonding. The model used to explain the reduction in surface potential and the hydration force can be used to ratio-
.
(32) Horn, R. G.; Hirz, S. J.; Hadziioannou, G.; Frank, C. W.; Catala, J. M. J. Chern. Phys. 1989,90,6767.
nalize results obtained in recent surface force measurements by Perez and P r o u ~ t .They ~ ~ studied the effect of the helical polysaccharidedextran on the forces between mica sheets in solutions of sodium chloride. The result of adding dextran to a concentration of about 0.1 g L-l (the amount of dextran varied between solutions) was a reduction in the measured force. It was suggested that the reduction was due to a depletion force. Although these curves were smaller in magnitude than those measured both here and by Pashley,21 we have fitted them with DLVO theory. We have found that the reduction in force observed on the addition of dextran is entirely consistent with a decrease in surface potential similar to the one found here. For example, the force measured in 10-3 M NaCl has a P, of -70 mV, and the reduced force in the presence of dextran gives \k, of -53 mV. There is no need to resort to depletion forces to explain the results. An effect on the surface potential might well be a common feature in the weak adsorption of nonionic compounds. Although the future of nonionics in the field of tertiary oil recovery looks bleak (due to salting out effects, low cloud point, and the fact that the source rocks are in all probability rendered hydrophobic by the adsorption of ashphaltenes), they would appear to have at least one advantage. It is apparent from this work that little surfactant would be lost by adsorption to rocks with nonhydrogen-bondingoxide surfaces, as adsorption is not only very weak but also appears to occur at concentrations well above the cmc. The effect here observed on the hydration force may well have implications for clay swelling-a soil problem caused by hydration of cations such as sodium between the clay platelets. It is known that clay swelling can be temporarily inhibited by the replacement of cations in the surfaces of the platelets by other cationic species with different hydration properties. Our observations suggest that nonionic species might also have applications in this field. It is known that nonionic surfactants intercalate in a parallel orientation between clay plateletsF4 and if they simultaneouslychange the ion adsorption properties of the surface, they will also alter the swelling behavior of the clay.
Conclusion We have identified four separate effects of nonionic surfactants on the surface forces measured in electrolyte solutions: (i) a reduction in surface potential, (ii) a reduction in the hydration force, (iii) an additional repulsion in more concentrated surfactant solutions due to viscous effects at high-speed approach, and (iv) a reduction in the measured adhesion between the surfaces. Taken together, these observations are strong evidence for the weak adsorption of nonionic surfactants to mica surfaces at concentrations where there is no steric effect on the equilibrium interaction. All these effects were only observed at concentrations greatly above the cmc. At very high (=lo0 X cmc and above) concentrations, adsorption is clearly evident from the presence of a long-range, hysteretic repulsion. A similar reduction in the surface potential and the hydration force may explain some earlier work in dextran solution and presumably can be extended to other systems, for example, polymers or alcohols. Registry No.
C12E5,3055-95-6; NaZS04,
7757-82-6.
(33) Perez, E.; Proust, J. E. J . Phys., Lett. 1985,46, 79. (34) Schott, H. J. Colloid Interface Sci. 1968,23, 133.
