Langmuir 1987,3, 1051-1056 on its molecular packing density and structural rigidity. Densely packed monolayers of long-chain amphiphiles could be produced on solids by spontaneous self-assembly from solution, behaving as efficient impenetrable barriers with respect to the passage of the aqueous permanganate ion. Permeation of ionic species from aqueous media through such solid-supported monolayers is demonstrated to be a defect-controlled pr0~ess.l~ The barrier efficiency of highly ordered monolayers of this type is, ultimately, determined by their long-term structural stability, which is, in turn, a function of their specific mode of binding to the solid surface. Thus, the performance of covalently bonded monolayers (silanes) is remarkably superior to that of analogous ionic films (acid salts), the differences between the two types of films correlating with former's structural stability versus the latter's deterioration upon prolonged contact with the solution of the penetrating species. Complete monolayers of unsaturated long-chain acid salts prepared for the present study by spontaneous self-assembly (SA) from solution were found to be less penetrable than their LB counterparts. This is a consequence of the higher degree of perfection and structural stability of the SA monolayers. Taken together, the present and previously reported' spectral and wettability data suggest a rather intricate mechanism for the wetting of acid salt films by water and aqueous KMn04. The wetting process obviously involves both penetration through voids across the film and lateral diffusion of the wetting liquid in the monolayer-substrate interface. This requires sufficient surface mobility of the film-forming molecules. However, as demonstrated by the
1051
IR data, wetting of a film-covered surface by the KMn04 solution is not necessarily accompanied by detectable alterations of the initial film structure or by significant oxidation of intralayer double bonds. It thus appears that the passage of the wetting liquid across the film occurs through a limited number of pinhole defects, while its lateral diffusion in the fh-substrate interface is mediated by the coupled motion of relatively large numbers of film-forming molecules. Electrostatic factors might also play a role in the wetting of the ionic films. Finally, the nonwettable character of the silane monolayers, including the incomplete ones, is, obviously, a consequence of their surface immobility, resulting from intralayer and layerto-surface covalent bonding. The present results emphasize the key roles of film structural stability, besides its structural perfection, in the engineering of efficient monolayer barriers.17 The usefulness of "penetration-controlled" reactions, in conjunction with wettability observations, as indicators of the penetrability of monolayer films is demonstrated. Acknowledgment. We gratefully acknowledge the support of this work by grants from the Schmidt Fund and the European Research Office of the U.S.Army. Registry No. Mn04-, 14333-13-2; KMnO,, 7722-64-7; (t13)Ca(u)si,103941-63-5;ZnSe, 1315-09-9;(t-13)Czz(u), 506-33-2; Czo, 14923-81-0;OTS,112-04-9;CI8,Cd salt, 2223-93-0. (17) These conclusions are confirmed by recent results of an electrochemicalstudy of the water permeation through organized SA monolayers on gold electrodes."
Liquid-Flow Microcalorimetry of Surfactant Adsorption onto Sandstone. 1. Experimental Method and Initial Results N. M. van Os* and G. Haandrikman KoninklijkelShell-Laboratorium,Amsterdam (Shell Research B. V.), 1003 AA, Amsterdam, Netherlands Received December 16, 1986. I n Final Form: May 18, 1987 Studies of surfactant adsorption from aqueous solution onto sandstone by means of a new LKB liquid-flow microcalorimeter,a liquid-flowadsorption cell, and an auxiliary W spectrophotometer have demonstrated the possibility of determining simultaneously the enthalpy of displacement of solvent by solute and the adsorption isotherm. As an example, this paper discusses the adsorption of sodium p(2-decyl)benzenesulfonate from aqueous solution onto crushed Bentheim sandstone at 30 OC. Experimental values of the molar enthalpy of displacement (corrected for the heat of dilution) are compared with similar data obtained from the literature. Introduction Surfactant adsorption on reservoir minerals is a point of concern in the chemical flooding of oil reservoirs to enhance oil recovery. Quite simply, it involves a waste of costly material; moreover, adsorption of a surfactant can change the concentration of the injected solution to the point where the process ceases to be effective. We are carrying out studies to establish relationships between the chemical structure of surfactants and relevant adsorptive properties such as the amount adsorbed and the Gibbs free energy of adsorption. 0743-7463/87/2403-1051$01.50/0
Of the terms constituting the Gibbs free energy of adsorption, the entropic contribution is not an easily accessible quantity and still has to be estimated;' by contrast, efforts to determine the enthalpic term by microcalorimetry1-l6appear to be making good progress. Depending (1) Rouquerol, J. Pure Appl. Chem. 1986,57,69-77. ( 2 ) Denoyel, R.; Rouquerol, F.; Rouquerol, J. In Adsorption from Solution; Ottewil, R. H., Ed.; Academic: New York, 1983. (3)Noll, L. A.; Burchfield, T. E.; Good, W. D. Fifth Annual DOE Symposium on Enhanced Oil & Gas Recovery & Improved Drilling Technology, Paper B-9,August 1979,Tulsa, OK.
0 1987 American Chemical Society
1052 Langmuir, Vol. 3, No. 6, 1987
van Os and Haandrikman
on the type of calorimeter and the nature of the system Table I. Size Distribution of Core Material being studied, authors have used the flow method,24~6Js15 crushed the batch method,5v9 or the immersion m e t h ~ d , ~the *~~'~~~ Bentheim glass particles average pros and cons of which have been discussed in detail by grain size, mm % % (cum.) % % (cum.) Rouquerol.' >0.355 7.2 7.2 5.8 5.8 However, not all such experiments yield the enthalpy 10.8 18.0 7.0 12.8 0.30-0.355 of adsorption, a fact which has sometimes been overlooked 19.7 37.7 14.7 27.5 0.25-0.300 by workers in this field. Rouquerol' has remarked that 16.2 53.9 14.7 42.2 0.212-0.25 17.9 71.8 22.9 65.1 0.18-0.212 all experiments, except those performed by the immersion 18.9 90.7 24.8 89.9 0.15-0.18 method, involve the displacement of solvent molecules 0.126-0.15 8.1 98.8 from the surface of the adsorbent by molecules of solute 0.105-0.126 and that the experimental heat effect (taking into account 0.4 0.5 0.0904.105 99.9 10.1 }loo.o the amount adsorbed and the dilution enthalpy) is in fact 0.070-0.090 0.2 the enthalpy of displacement rather than the enthalpy of
