Langnuir 1989, 5 , 1111-1113 useful discussions. Dr. Sven Engstrom is thanked for helpful advice and Dr. Thomas Arnebrant for help with the ellipsometry work. Prof. KAre Larsson and co-workers at the Division of Food Technology, Lund University, are thanked for putting the ellipsometer a t our disposal and
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for helpful advice. Registry No. TP, 555-44-2; NaLa, 629-25-4;NaOBS, 2867511-8;NFE~,,,9016-45-9;CTAC, 11242-7; SDS,151-21-3;c 1 2 ~ * , 5274-68-0; CI2E6,3055-95-6; C12Es, 3055-96-7; CH3(CH2)oH, 112-30-1;CH3(CH2)&H,,124-18-5; NaCl, 7647-14-5.
Letters Entropic Orientational Forces between Surfaces in Anisotropic Liquids Jacob N. Israelachvili,* Stephen J. Kott, Michelle L. Gee, and Thomas A. Wittent Department of Chemical & Nuclear Engineering and Materials Department, University of California, Santa Barbara, California 93106, and Corporate Research Laboratories, E x x o n Research and Engineering Company, Annandale, N e w Jersey 08801 Received January 19, 1989. I n Final Form: February 23, 1989 The forces between both hydrophilic mica surfaces and between surfactant-coated (hydrophobic) mica surfaces immersed in 2-methyloctadecane have been measured. The force laws exhibit a monotonically attractive regime and, at short range, a monotonically repulsive steric force. The strengths of the attractive adhesion forces are about 1 order of magnitude greater than the predicted van der Waals forces. Similar effects are observed with other isoparaffin liquids and with mixtures. We tentatively conclude that such attractive forces may be a general feature of the interactions across certain anisotropic liquids (e.g., water) and that they arise from molecular orientational ordering effects at surfaces. There exist four well-established forces between like particles or surfaces in liquids. These are the attractive van der Waals and repulsive electrostatic “double-layer” forces, oscillatory “solvation” or “structural” forces, and steric forces between polymer-covered surfaces.’ The origin and properties of these four interactions are now fairly well understood, at least for simple liquids. However, when the solvent is water, there arise two important additional forces: the monotonically repulsive “hydration”24 and attractive “hydrophobic”” forces. It is widely believed that the hydrophobic interaction is mainly entropic in origin, arising from the perturbation of the orientationdependent hydrogen-bonding network of the water molecules between two inert surfaces or two solute molecules as they approach each other.“l0 More generally, such attractive forces may be present whenever asymmetric molecules are forced into an unfavorable alignment (ordering) near an inert surface since, on approach of two such surfaces, the entropically unfavorable configurations are relieved as the molecules are sent back into the more random bulk liquid. In an attempt to obtain some insight into whether orientation-dependent forces exist in some system other than water, a series of force measurements have been conducted for surfaces immersed in a model anisotropic liquid (an isotropic liquid would have no orientational entropy associated with it). Previous force measurements with hydrocarbons have only looked at the unbranched n-alkanes” which, because of their ability to pack into discrete layers, exhibit short-range forces which are dominated by oscillatory f ~ r c e s . ’ ~ JHence, ~ one cannot easily ascertain Exxon Research a n d Engineering Co.
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whether some additional unexpected monotonic component is also present. However, recent experiments with polymer meltsI4 have shown that the presence of branched groups can prevent the molecules from ordering into discrete layers, thereby eliminating the oscillatory force a t small separations. It is for this reason that P-methyloctadecane, CH3(CH2)&H(%H&H3, was chosen for these studies. Additionally, this liquid does not have a viscosity as high as polymer melts, which greatly reduces the accuracy of the measurements (ref 14-16 and Horn, R. G.;
(1)Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, New York, 1985. (2) LeNeveu, D. M.; Rand, R. P.; Parsegian, V. A. Nature 1976,259, 601. (3)Israelachvili, J. N. Chem. Scr. 1985, 25, 7. (4)Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153; 1981, 83, 531-546. (5)Israelachvili, J. N.; Pashley, R. M. Nature 1982,300,341;J.Colloid Interface Scz. 1984, 98,500. (6) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (7)Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986, 114, 234. (8)Tanford, C. The Hydrophobic Effect; Wiley: New York, 1980. (9)Marcelja, S.;Mitchell, D. J.; Ninham, B. W.; Sculley, M. J. J. Chem. SOC., Faraday Trans 2 1977, 73, 630. (10) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (11)Christenson, H. K.;Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987,87, 1834. (12)Christenson, H. K.;Horn, R. G. Chem. Scr. 1985, 25, 37. (13)Israelachvili, J. N. Acc. Chem. Res. 1987,20, 415. (14)Israelachvili, J. N.; Kott, S. J. J. Chem. Phys. 1988, 88, 7162. (15)Montfort, J. P.; Hadziioannou, G. J. Chem. Phys. 1988,88, 7187. (16)Horn, R.G.; Israelachvili, J. N. Macromolecules 1988,21, 2836.
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1112 Langnuir, Vol. 5, No. 4 , 1989
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Letters
ATTRACTION
VDW
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Distance, D (nm) Figure 1. Results of the force F between two curved surfaces of radius R immersed in isooctadecane as a function of the surface separation D at 22 O C . Curve A: Forces between untreated mica surfaces. The shaded band gives the force law obtained by superimposing the results of three independent experiments; for each particular experiment the individual force curves had much less scatter. Curve B: Forces between surfaces coated with a surfactant monolayer. Full circles, double-chained surfactant monolayer of (C&DAB (from two independent experiments); open circles, single-chained surfactant monolayer of CTAB (from two independent experiments). Due to the small quantity of purified isooctadecane available to us,all experiments were carried out with a macroscopic droplet between the surfaces, in an atmosphere of nitrogen dried with P2Ob For curve B, D = 0 refers to molecular contact of the coated surfaces in dry nitrogen prior to injecting the isooctadecane between them. For each system, the theoretically expected van der Waals force is shown by the dashed curve.
Hadziioannou, G. personal communication). More importantly, the molecules are highly anisotropic flexible chains that might be expected to display a significant degree of orientational ordering at surfaces, thereby leading to an entropic orientational force. It should be noted that previous work has shown that for unbranched hydrocarbons and a polymer melt the molecules orient preferentially parallel to a smooth surfacel1J6 (which is entropically unfavorablez6). In the present study, the forces between both mica and surfactant monolayer coated mica surfaces were measured. The monolayer-coated surfaces were especially chosen because (1)the surfaces are quite inert, (Le., there are no specific solid-liquid interactions) and (2) the van der Waals forces between the hydrocarbon surfaces across the hydrocarbon liquid are extremely weak, therby enabling us to more easily ascertain whether or not some additional force is present. A "surface forces apparatus" was used for these studies. This apparatus is capable of measuring the forces between surfaces in liquids with a distance resolution of about 1 8, and has been much described in the literature.l7Js The mica surfaces were installed into the apparatus either in their virgin state or after deposition of a monolayer of either the single-chained surfactant CTAB (hexadecyltrimethylammonium bromide; CH3(CH2)15N(CH3)3+Br-)or the double-chained surfactant (C1&DAB (didodecyldimethylammonium bromide; (CH3(CHz)11)zN(CH3)2+Br-)onto each surface. The depositions were carried out by retraction from solution, as previously de~ c r i b e d . ~ In ~ ~all * ' cases, ~ the surfactants adsorb with the cationic head groups attached to the mica, thereby exposing a purely hydrocarbon surface.
Figure 1 shows the results obtained for the forces between mica (Figure 1A) and surfactant-coated mica surfaces (Figure 1B) in 2-methyloctadecane. Each force curve is monotonic and has an attractive regime and a shortrange repulsive "wall". The Ionger range attractive regime is far stronger than can be explained by the Lifshitz theory of van der Waals forces' for all systems investigated in the present study. The theoretically calculated Hamaker constant for mica interacting across 2-methyloctadecane is A = 7.5 X J. This leads to the theoretical force law' F I R = -ARID2, given by the dashed curve in Figure 1A. It is clear from comparison of this with the experimental curve that the strength of the adhesive minimum is about 1 order of magnitude greater than the theoretical value. It should be emphasized that this increased adhesion is not a result of any long-range layering of the liquid molecules, as has been observed with unbranched alkanes, where the force is an oscillatory function of surface separation." Oscillatory forces in unbranched alkane systems are characterized by the symmetry of the oscillations about the Lifshitz prediction.20 This is definitely not the case for the 2-methyloctadecane system studied here; the form of the adhesive minimum and the hard-wall repulsion do not satisy the above criterion for oscillations or even an oscillatory force where the oscillations have been simply smeared out. In none of the experiments were oscillations or quantized adhesive minima observed even after the surfaces had been pushed close together under a large force and kept there for periods up to 20 min. Unfortunately, accurate measurements of the forces further out were made difficult since the high viscosity of 2-methyloctadecane imparted a sluggishness to the system. However, from inward jump (Le., instablity) measurements5J7it was ascertained that by D = 6 nm the attractive forces were within 10% of the unretarded van der Waals force expected from the Lifshitz theory. The range of the extra attractive force is therefore less than this distance. The extra attraction is even more evident in the case of the surfactant monolayer coated surfaces (Figure IB). Each monolayer has a thickness of about 1.8 nm,5 so that for separations up to D = 2 nm one expects the van der Waals force to be practically nonexistent. The strongest possible van der Waals force curve is shown by the dashed line in Figure l B , based on the Lifshitz equation for the interaction of layered surfaces.21s2zConsequently, while these forces are weaker and closer than in the bare mica/ hydrocarbon case, they are still much greater than theoretically predicted. Before discussing these findings, it should be mentioned that the experimental curves plotted in Figure 1 are the equilibrium forces. The force curves were totally reproducible and reversible when measured slowly, i.e., when equilibration times between I and 20 min were allowed before data points were recorded. The reversibility of the force curves is further proof that oscillations are not present in the 2-methyloctadecane systems. However, it was observed that, for equilibration times progressively shorter than 1 min, the adhesion force often fell significantly; i.e., the adhesive minima were not as deep when the surfaces were maintained in the minima for periods less than about 60 s down to 20 s. It is entirely possible that if one could measure these forces even more rapidly, the attractive force would approach the theoretically expected van der Waals force. The observation that a certain equilibration time was required to attain a maximum value of the adhesion is also
(17) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC., Faraday Trans 1 1978, 74, 975.
(18)Israelachvili, J. N.; McGuiggan, P. M. Science 1988, 241, 795. (19) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169.
(20) Israelachvili, J. N. J . Colloid Interface Sci. 1986, 110, 263. (21) Ninham, B. W.; Parsegian, V. A. J . Chem. Phys. 1970,52,4578. ( 2 2 ) Israelachvili, J. N.; Tabor, D. Prog. Surf. Membr. Sci. 1973, 7, 1.
Langmuir 1989, 5, 1113-1115 consistent with the idea that molecular orientational effects are involved in this interaction. A question arises concerning the "hard-wall" type repulsion measured a t very small separations, especially between the untreated mica surfaces. Such short-range repulsive walls have also been recently measured in polymer melts.14J5 They are most likely due to the strong binding of the liquid molecules to the surfaces (the contact angle of 2-methyloctadecane on mica is zero or very small), which prevents the last few layers from being easily squeezed out. That the positions of these hard walls were closer with the surfactant-coated surfaces may therefore be due to the weaker adhesion of the liquid to these more inert surfaces. The progressive reduction in the strength of the adhesion forces as we go from untreated mica to surfactantcoated surfaces and-just within experimental error-from double-chained (Clz),DAB-coated surfaces to the singlechained CTAB-coated surfaces is likely to be related to the progressively increasing surface roughness of these surfaces.z3 Again, this observation is consistent with the notion that the attractive force is related to molecular orientation effects; viz., the ability of surfaces to align anisotropic molecules parallel to them must diminish as the surface roughness increases. Similar experiments have also been performed with other isoparaffins and mixtures of isoparaffins, resulting in force laws similar to those obtained here. Thus, the results presented here may well be generally applicable to other systems involving anisotropic flexible chain molecules or to certain tvpes of molecules havine: anisotropic interaction potentials (such as water, leading to the attractive hydrophobic interaction). In conclusion, it has been shown that, for certain anisotropic flexible chain liquid molecules between two (23)Christenson, H.K.J. Phys. Chem. 1986,90,4.
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surfaces, there exists a hitherto unsuspected attractive force that appears to be associated with entropic orientational (or alignment) effects. This type of attractive entropic force may be conceptually similar to the attractive entropic depletion forcez4associated with changes in the density of solute molecules between two surfaces as they approach each other. The latter interaction is described by a scalar order parameter (the density) while the former should be described by a vector (orientational) order parameter. Depending on the nature of the liquid and its interaction with the surfaces, repulsive entropic orientational forces could also arise, as they do in the case of repulsive hydration force^,^^^^^ though one would expect attractive forces whenever the surface-induced ordering lowers the entropy of the liquid adjacent to it, as occurs for polymer melts26and water.lsal0 There are also many obvious practical implications of these findings, e.g., for interparticle interactions in anisotropic media, for the stability of colloidal systems, for the action of lubricants, etc. Further investigation of this extra attractive force is under way, the results of which will be presented in a future publication. Acknowledgment. We thank the U.S. Department of Energy for financial support under DOE Grant DEFG03-87ER45331,although this support does not constitute an endorsement by the DOE of the views expressed in this article. Registry No. CTAB, 57-09-0;DTAB, 70755-47-4; 2-methyloctadecane, 1560-88-9. (24)Joanny, J. F.;Leibler, L.; de Gennes, P. G. J.Polym. Sci., Polym. Phys. 1979,17,1073. (25)Israelachvili, J. N.In Physics of Complex and Supermolecular Fluids; Safran, S. A., Clark, N. A. Eds.; Wiley: New York, 1987;pp 101-113. (26)Silberberg, A. J. Colloid Interface Sci. 1988,125,14.
Influence of Electric Field at the Electrode/Electrolyte Interface on EXAFS Results Mahesh G. Samant IBM Research Division, Almaden Research Center, K32l802, 650 Harry Road, San Jose, California 95120 Received December 12, 1988. I n Final Form: April 7, 1989 The effect of the electric field at an electrode/electrolyte interface on the structural parameters evaluated from the EXAFS measurements is considered. The apparent near neighbor distance evaluated by EXAFS is shifted. For an electric field of 5 X 10' V/cm, a near neighbor distance of 2.500 8, will appear shorter or larger by 0.012 8, depending on the direction of the electric field. The shift is larger for longer distances, 0.018 8, at 3.000 A. Introduction One of the recent advances in probing structures at electrode/electrolyte interfaces under in situ conditions has involved the successful application of fluorescencedetected surface extended X-ray absorption fine-structure spectroscopy (SEXAFS)1-3and of surface X-ray diffrac(1)Blum, L.; Abruna, H.; White, J.; Gordon, J.; Borges, G.; Samant, M.; Melroy, 0. J. Chem. Phys. 1986,85,6732. (2)Samant, M. G.; Borges, G.; Gordon, J.; Blum, L.; Melroy, 0. J. Am. Chem. SOC.1987,109,5970.
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tion.4~~These techniques utilized hard X-rays, which have significant penetration depth in a condensed phase such as the electrolyte and have Provided direct information on the atomic di&nces and C r y s a o P P h i c s h ~ ~ t u r In e . the case of both of these techniques, surface sensitivity was (3)Melroy, 0.; Samant, M. G.; Borges, G.; Gordon, J.; Blum, L.; White, J.; Albarelli, M.; McMillan, M.; Abruna, H. Langmuir 1988,4 , 728. (4)Samant, M. G.; Toney, M.; Borges, G.; Blum, L.; Melroy, 0. J. Phys. Chem. 1988,92,220. ( 5 ) Samant, M. G.; Toney, M.; Borges, G.; Blum, L.; Melroy, 0. Surf. Sci. 1988,193,L29.
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