4
J . Phys. Chem. 1986, 90, 4-6
mixed. The decrease in extinction at 240 nm is shown in Figure 4a. Again we used eq 1 to construct a reciprocal plot against time (Figure 4b). The average value found after 11 runs was k4 = (2.2 f 0.3) X lo3 M-' s-I, for HBrO, starting concentrations ranging to 2 X M. from 2 X If we have another look at Figure 2 we see that the observed product spectrum (dashed line) agrees fairly well with the sum of the literature spectra of bromate and hypobromous acidIzJ4 (full line). This shows that the main products of the reaction are indeed equal parts of HBr0C and HOBr, and is an extra indication that the reaction we followed must be the disproportionation of HBr02 and not some other unidentified secondary or side reaction.
Discussion Spectrophotometrical measurements indicate that the rate constant for the disproportionation of bromous acid is k , = (2.2 f 0.3) X lo3 M-' s-'. The main source of error lies in the uncertainty about the extinction coefficients of starting compound (14) Bridge, N. K.; Matheson, M. S . J . Phys. Chem. 1960, 64, 1280.
and products. Measurements with ion-selective electrodes led to comparable results. Our data are in full agreement with the values reported by Noszticzius et al.' Thus it seems that from now on the LO set of rate constants as elaborated by Tyson' should be used for calculations and computer simulations. Available literature data on indirectly determined rate constants should be reinterpreted. More precise values for the rate constants of a number of elementary steps can be obtained by relatively simple laboratory experiments, since thermodynamic consistency requires that these reactions, too, must be some orders of magnitude slower than previously assumed. Finally we would like to emphasize that, no matter how useful computer simulations can be, satisfactory outcome of computations should not keep the scientist from checking his assumptions in the laboratory as well. Acknowledgment. Thanks are due to Dr. MiklBs Jaky and the Central Research Institute for Chemistry in Budapest for laboratory facilities, and to Prof. Endre Koros for useful comments. F.A.'s stay in Budapest was made possible by the kind cooperation of the University of Amsterdam and the L. Eotvos University.
Interactlons between Hydrocarbon Surfaces in a Nonpolar Llquld: Effect of Surface Properties on Solvation Forces Hugo K. Christenson Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, ACT 2601, Australia (Received: September 6, 1985)
The force between hydrocarbon surfaces has been measured in the model nonpolar liquid octamethylcyclotetrasiloxane. The surfaces were prepared by either (i) adsorbing hexadecyltrimethylammonium bromide (CTAB) from solution or (ii) depositing a compressed Langmuir-Blodgett film of dioctadecyldimethylammonium bromide (DOAB) on a mica substrate. The results show how surface roughness affects solvation forces, leading to a reduced range (compared to bare mica surfaces) for DOAB or a complete smearing out for CTAB, in which case a van der Waals type attraction is found. For both surfaces the contact adhesion is consistent with Lifshitz theory. The results indicate that the interaction of (uncharged) rough surfaces may be adequately described by a van der Waals potential
Introduction A number of recent publications have dealt with direct measurements of the force between molecularly smooth mica surfaces immersed in a range of nonpolar liquids.'-3 As predicted theoretically4 and by computer simulations of model liquids between solid walls,5 the interactions at small separations are dominated by structural or solvation effects arising from packing restrictions imposed by the two surfaces. In liquids consisting of near-spherical molecules of limited flexibility, such as tetrachloromethane or octamethylcyclotetrasiloxane (OMCTS), the solvation force is a decaying oscillatory function of surface separation with a period close to the mean molecular diameter of the liquid. About 10 oscillations are measurable before the amplitude becomes comparable in magnitude to the van der Waals force, which is found, as predicted theoretically: in the limit of large separations. Liquids with flexible molecules such as the n-alkanes show fewer oscillations, and at least for the longer chain homologues like tetradecane,' the period of the oscillations is close to the mean thickness of the alkyl chains. In polar and hydrogen-bonding liquids the (1) R. G. Horn and J. N. Israelachvili, J . Chem. Phys., 75, 1400 (1981). (2) H. K. Christenson, J . Chem. Phys., 78, 6906 (1983). (3) H. K. Christenson, Chem. Phys. Lett., 118, 455 (1985). (4) D. J. Mitchell, B. W. Ninham, and B. A. Pailthorpe, Chem. Phys. Lett., 51, 257 (1977). ( 5 ) J. H. Lane and T H. Spurling, Chem. Phys. Lett., 67, 107 (1979). (6) B. W Ninham, J . Phys. Chem., 84, 1423 (1980). (7) J. N . Israelachvili, J . Colloid Interface Sci., in press.
0022-3654/86/2090-0004$01.50/0
force is also dominated by structural effects at short In spite of the occurrence of solvation forces, it has been found experimentally that the contact adhesion between mica surfaces in nonpolar liquids is well predicted by the Lifshitz theory of van der Waals forces.2 A substantial body of knowledge is thus emerging on the forces between mica surfaces in nonpolar liquids, but little work has been done with other surfaces. Horn and Israelachvili have reported measurements of the force in OMCTS between mica surfaces made hydrophobic by adsorption of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB). I They concluded that the measured forces, at least beyond the second oscillation (from contact), were similar to those found between uncoated mica surfaces. Horn and Israelachvili did not, however, use adequate control of the water content of the liquid, and in light of the large quantitative effects of water on the forces between bare mica surfaces in nonpolar liquids,lJJOthis throws some doubt on the validity of the results. Recent measurements of the hydrophobic attraction in water have shown a large dependence of the strength of the interaction on the nature of the hydrocarbon In particular, (8) H. K. Christenson and R. G. Horn, Chem. Phys. Lett., 98,45 (1983). (9) H. K. Christenson and R. G. Horn, J . Colloid Interface Sci., 103, 50
(1985). (10) H. K. Christenson, J . Colloid Interface Sci., 104, 234 (1985). (1 1) J. N. Israelachvili and R. M. Pashley, J . Colloid Interface Sci., 98, 500 (1984).
0 1986 American Chemical Society
Letters surfaces coated with the double-chain quarternary ammonium surfactant dioctadecyldimethylammonium bromide (DOAB) give a much stronger and more long-range attraction than CTABcoated surfaces. DOAB is insoluble and forms a monolayer at the air-water interface and must be transferred to the mica surface as a Langmuir-Blodgett film. In this initial study it was decided to investigate the forces between hydrocarbon surfaces in OMCTS using two different surfaces: CTAB-coated and DOAB-coated mica. By comparison of these results with previous measurements with bare mica surfaces, it was hoped to gain insight into the relative importance of solid-liquid and liquid-liquid interactions as well as surface properties such as roughness or hydrocarbon-chain packing in determining the nature of solvation forces.
Experimental Section The force measurements were carried out with the surface forces apparatus designed by 1~raelachvili.l~The separation between two molecularly smooth mica surfaces (in this case coated with a monolayer of surfactant) in a crossed-cylinder configuration is measured to 0.1-0.2 nm with multiple-beam interferometry, and the force between the surfaces is determined to lo-' N by monitoring the deflection of a double-cantilever spring." The hydrocarbon surfaces were prepared as follows: The silvered mica sheets were glued to the supporting silica disks with the standard epoxy resin Epon 1004, and after mica-mica contact was measured in dry nitrogen the surfaces were either (i) dipped in a 9 X lo4 M aqueous solution of CTAB for a few minutes and withdrawn or (ii) slowly (0.2 mm/s) pulled up through a monolayer of DOAB spread on the air-water interface in an all-Teflon Langmuir trough. The monlayer was spread from a 19:l mixture of n-hexane and ethanol and compressed to a head-group area of -0.6 nm2 at 35 mN/m. On transfer from the air-water interface to the mica substrate the area per head group of the DOAB surfactant decreases slightly to 0.55 nm2, as measured with mica strips of known surface area. The advancing contact angles of water were found to be 60 f 2" (CTAB) and 93 f 2" (DOAB). The octamethylcyclotetrasiloxanewas analytical grade (>99% by GC) from Fluka AG and purified by double distillation from molecular sieve 4A under reduced pressure (bp N 80 "C) in an atmosphere of nitrogen. The CTAB was obtained from Ajax and recrystallized twice from ethanol-acetone. The DOAB was supplied in recrystallized form by Eastman and used as received. After drying in the apparatus over phosphorus pentoxide for 24 h, the surfaces were brought together in an atmosphere of dry nitrogen to determine the zero of contact D = 0. The surfaces were then separated, and a droplet of OMCTS ( V = 0.05 cm3) was injected between the surfaces. The system was kept dry by maintaining a vessel with phosphorus pentoxide in the measuring chamber. All measurements were carried out at 22 f 1 "C. Results The thickness of the surfactant monolayers was determined by comparing contact in dry nitrogen before and after adsorption or deposition. The values obtained were 1.5 f 0.2 nm for CTAB and 2.0 f 0.2 nm for DOAB. The uncertainty is largely due to the unknown thickness of a physisorbed layer on the mica in air or nitrogen.I4 The force measured between DOAB-coated mica surfaces in OMCTS is shown in Figure 1. It is a decaying oscillatory function of separation, with a periodicity of 0.75 f 0.05 nm and a range of measurable oscillations of about 4 nm. (For comparison, the dotted lines give the "force envelope" found with bare mica surfaces, obtained by drawing lines through adjacent maxima and minima, respectively.) The innermost minimum appears to occur at D = 0.6 f 0.1 nm, and the adhesion measured at this minimum (12) R. M. Pashley, P. M. McGuiggan, B. W. Ninham, and D. F. Evans, Science, 229, 1088 (1985). (13) P. M. Claesson, P. Herder, C. Blom, and B. W. Ninham, in preparation. (14) J. N. Israelachvili and G. E. Adam, J. Chem. Soc., Faraday Trans. I , 74, 975 (1978).
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 5
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0 1 2 3 4 5 6 7 8 9 10 D (nm) Figure 1. Force as a function of separation between mica surfaces coated with the double-chain surfactant dioctadecyldimethylammonium bromide (DOAB) in octamethylcyclotetrasiloxane (OMCTS). The measured force F is normalized by the radius of curvature R of the surfaces and is proportional to the energy per unit area (,!?/A) between parallel, flat surfaces. The mean periodicity of the oscillatory solvation force is 0.75 f 0.05 nm,close to the mean molecular diameter of 0.8 nm. The dotted lines give the force envelope (see Results section) for uncoated mica surfaces in OMCTS (from ref 2). Note that the innermost minimum appears to occur at D = 0.6 nm. The surfaces moved in to D = 0.3 nm under a load of 100 mN/m. The height of the force maximum at 1.2 nm was 22 mN/m.
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Figure 2. Force as a function of separation between CTAB-coated surfaces in octamethylcyclotetrasiloxane(OMCTS). The surfaces jump into contact from D = 2.5 0.1 nm under the influence of a weak attraction. The dashed line denotes an inaccessible region of the force curve where dF/dD > k, the spring constant. The adhesion at contact is proportional to the energy between flats in contact which is twice the interfacial energy.
*
was 5 mN/m, and 3 mN/m in a second experiment (not shown). The surfaces was observed to move in by a further 0.2-0.3 nm under a load of 100 mN/m. The force measured between CTAB-coated mica surfaces in OMCTS is shown in Figure 2. There is a weak attraction resulting in an inward jump (when the gradient of the force exceeds the spring constant') at a separation of 2.5 f 0.1 nm, and the surfaces come directly into contact ( D = 0.1 f 0.1 nm). The
6 The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 adhesion measured at contact is 6 i 1 mN/m. Similar results were obtained in a second experiment, although the adhesion was only 4.5 f 0.5 mN/m.
Discussion The solvation force measured between DOAB-coated surfaces in OMCTS is qualitatively similar to that found between bare mica surfaces. The periodicity of the oscillations with the two different surfaces is, within experimental uncertainty, the same, and the decay of the oscillatory force is similar. The only significant differences lies in the range of the force; with DOABcoated surfaces the number of measurable oscillations is half of that observed between mica surfaces. That this is not simply due to a difference in dielectric or other continuum properties of the hydrocarbon compared to that of the mica is demonstrated by the dramatic change found when DOAB is replaced by CTAB. The force measured between the CTAB-coated surfaces is reminiscent of a van der Waals force, which has not previously been found for any nonaqueous liquid between uncoated mica surfaces. It is, however, not possible to determine the shape of the force curve in the region of instability (dotted line in Figure 2),l and the measurements prove only that there are no repulsive maxima and no minima deeper than the one at contact in this regime. Unfortunately, it is not a straightforward matter to calculate an experimental Hamaker constant from the inward jump at 2.5 nm. At this separation it would be necessary to perform a complete triple-layer analysis (including the mica substrate) with effects of r e t a r d a t i ~ n . ’ ~ The measured contact adhesion, however, can more readily be compared with that expected from the Lifshitz theory, since both retardation and the mica may be safely ignored for sufficiently small separations. The measured force F between curved surfaces of radius R is related to the energy per unit area E between parallel, flat surfaces via the Derjaguin approximation.’ E(D) = F ( D ) / 2 r R
(1)
The measured adhesion thus gives an energy per surface of the hydrocarbon-OMCTS interface of 0.4-0.5 mJ/m2 (for the two experiments). If one models the hydrocarbon surface as a (liquid) hexadecane layer and disregards the mica, a simple calculation of the nonretarded Hamaker constant A from Lifshitz theory16 J (at 295 K). The van der Waals energy yields A = 5 X between two flat surfaces is given by E(D) = A/12rD2
(2)
To obtain a theoretical value of the adhesion, one needs to employ some “cutoff distance” Dofor the surfaces in contact. This is, of course, a somewhat arbitrary quantity, but the value Do = 0.165 nm has been found to predict surface tensions from Hamaker constants to better than 20% for a range of non-hydrogen-bonding liquids.” Inserting this value together with the calculated Hamaker constant in eq 2 gives E ( D ) = 0.5 mJ/m2, in good agreement with the experimentally determined value. The main difference between the two monolayers lies in the packing of the hydrocarbon chains. The minimum head-group area of CTAB at the air-water interface is about 0.45 nm2.I8 If this value is used as a reasonable estimate for the area per CTAB molecule on the mica substrate and compared to the measured value for DOAB (0.55 nm2), it can be seen that the area per hydrocarbon chain is much larger for CTAB (0.45 nm2) than for (15) B. W. Ninham and V A. Parsegian, J . Chem. Phys., 52,4578 (1970). (16) D. B. Hough and L. R. White, Adu. Colloid Interface Sci., 14, 3 ( 1980). ( 1 7) J. N. Israelachvili, “Intermolecular and Surface Forces”, Academic Press, New York, 1985. (18) J. Weise, G. Zografi, and A. P. Simonelli, J . Pharm. Sci., 63 380 (1974).
Letters the double-chain DOAB (0.28 rim2). (When multiplied by the measured thickness of the monolayers, these values give reasonable volumes per surfactant molecule of 0.7 nm3 (CTAB) and 1.1 nm3 (DOAB).) In order to maintain close packing of the chains, as would be required in air or water by surface energy constraints, a much larger tilt or degree of disorder must occur for the CTAB chains. In the nonpolar liquid, however, the interfacial energy is very small and solvent molecules might easily penetrate into the monolayer. The surface would become diffuse or rough and more so for the CTAB than the DOAB layers. In the case of CTAB this surface roughness is sufficient to completely smear out the solvation forces-no packing restrictions can be imposed by such a “soft” surface. With DOAB layers the effect is smaller but leads to a substantially reduced range of the solvation forces compared to that found between uncoated mica surfaces. Because of solvent penetration,the monolayers would swell in the nonpolar liquid, and this could account for the observation that the innermost minimum is at D = 0.6 nm. A second possibility which may, at least in part, account for the differences is the state or degree of order in the monolayer. The Langmuir-Blodgett film of DOAB is in a well-defined state when deposited on the mica whereas the CTAB molecules adsorb onto the mica in a much more disorderly fashion. It is possible that the CTAB layer thus has more holes, protruding chains, etc., than does the DOAB layer. By similar arguments to those above one might expect this to lead to a smearing out of structural effects. The adhesion at contact for the CTAB surfaces (4.5 and 6 mN/m) is close to the force at the innermost minimum for the DOAB surfaces ( 3 and 5 mN/m). The interfacial energies are thus similar and, as shown above, close to the predictions of Lifshitz theory for both surfaces. This is hardly surprising; the surface energy is half the difference between the energy at infinite separation and the energy of the surfaces in contact and is independent of details of the force law between the surfaces. It remains to discuss why the present results with CTAB are different from those obtained by Horn and Israelachvili. Preliminary measurements both with CTAb and DOAB layers on controlled addition of water indicate that there is a large effect toward an increase in structure with increasing amounts of water. It therefore seems likely that the earlier results were due to the presence of trace amounts of water. Investigations currently in progress aim to further clarify the effect of water on these and other hydrocarbon surfaces in nonpolar liquids. These results show how oscillatory solvation forces are reduced in range when the surfaces are rough or permeable to the liquid. If the roughness or solvent penetration becomes too large, the solvation force vanishes and a van der Waals type attraction is measured in nonpolar liquids. The use of a van der Waals interaction potential for the interpretation of experiments with particles stabilized by terminally grafted alkane chains in hydrocarbons may thus be justified in some cases.lg A similar smearing-out effect may account for why the oscillatory hydration force observed between molecularly smooth mica surfaces in water20 is not found between lipid bilayers adsorbed on mica.2’x22 With the rough surfaces of lipid head groups only a monotonic repulsion is measured. Acknowledgment. I am grateful to Professor B. W. Ninham for suggesting the use of the DOAB surfactant as a model hy-
drocarbon surface. Support was provided under the Australian National Energy Research Development and Demonstration Program which is administered by the Commonwealth Department of National Development and Energy. (19) J. Edwards, D. M. Everett, T. O’Sullivan, I. Pangalou, and B. Vincent, J . Chem. SOC.,Faraday Trans. I , 80, 2599 (1984). (20) R. M. Pashley and J. N. Israelachvili, J . Colloid Interface Sci., 101, 511 (1984). (21) R. G. Horn, Eiochim. Eiophys. Acta, 778, 224 (1984). (22) J. Marra and J. N. Israelachvili, Biochemistry, 24, 4608 (1985).
