Langmuir 1991, 7, 1843-1845
1843
Molecular Layering in a Liquid Adsorbed Film at Room Temperature D. Beaglehole Physics Department, Victoria University of Wellington, Wellington, New Zealand
E. Z. Radlinska, B. W. Ninham, and H. K. Christenson' Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, ACT 2601, Australia Received April 1, 1991. I n Final Form: June 28, 1991 We present the first observation of structural effects in a film adsorbed from vapor at room temperature. A new ellipsometrictechnique has enabled us to identify steps in the adsorption isotherm of the nonpolar liquid octamethylcyclotetrasiloxane on molecularly smooth mica. The transition from one layer to two layers of adsorbed molecules at a relative vapor pressure of 0.8 is particularly distinct. This layering transition shifts toward higher vapor pressure with decreasing temperature but no qualitative change is found on passing through the triple point of the bulk liquid. Such layering phenomena in adsorbed films have previously only been detected in films of inert and other simple gases at low temperatures.
Introduction The adsorption of vapors to solid surfaces1V2 is an important area of surface science and is closely related to wetting and spreadingsand to surface melting! The extent of adsorption depends on the strength and range of the potential between the substrate and the adsorbing molecules compared to the potential between the individual molecules. For substrates that interact strongly with the vapor species, adsorption increases without limit as saturation is approached (wetting), for intermediate substrates, adsorption stops at a finite film thickness (partial wetting), and for weakly interacting substrates, no adsorption occurs even a t saturation (nonwetting). Adsorption isotherms have been classified accordingly as type I,type 11,or type 1II.S In general, adsorption increases with temperature and intermediate strength substrates may show a wetting transition at some temperature, T,, at which the adsorption becomes infinite at saturation. This transition from a film of finite thickness to a thick wetting film at T, is an example of a surface phase transition.6J Lattice gas models7~*and density functional theoriesg as well as many experiments on the adsorption of simple gases to substrates like exfoliated graphite or single crystals at low temperaturesl+ls confirm this behavior with the added feature of layering, i.e. the successive condensation (1) Brunauer, S. The Adsorption of Gases and Vapors; Oxford Univeristy Press: London, 1943. (2) Dash. J. G. Films on Solid Surfaces; Academic Press: New York, 1975. (3) de Gennes, P. G. Rev. Mod. Phys. 1986,57,827. (4) Dash, J. G. Contemp. Phys. 1989,30,89. (5) Dash, J. G. Phys. Rev. B: Solid State 1977, 15, 3136. (6) Sullivan, D. E.;Telo da Gama, M. M. In Fluid Interfacial Phenomena; Croxton, C. A., Ed.; Wiley: Chichester, U.K., 1986. (7) Pandit, R.;Schick, M.; Wortis, M. Phys. Reu. B: Condens. Matter 1982,26, 5112. (8)de Oliviera, M. J.; Griffithe, R. B. Surf. Sci. 1978, 71,687. (9) Ball, P. C.; Evans, R. J. Chem. Phys. 1988,89,4412. (10) Prenzlow, C. F.;Halsey, G. D.J. Phys. Chem 1957,6I, 1158. (11) Larher, Y.; Millot, F. J. Phys., Colloq. 1977, 38, C4-189. (12) Hamilton..J. J.:.Goodstein. D.L. Phvs. Rev. B: Condens. Matter 1983, i8,3838. (13) Zhu, D.-M.;Dash, J. G. Phys. Rev. B Condens. Matter 1988,523, 11673. (14) Nham, H. S.;Hew, G. B. Phys. Res. B: Condens. Matter 1988, 38,5166. (15) Faul, J. W. 0.; Volkmann, U. G.; Knorr, K. Surf. Sci. 1990,227, 390.
of complete layers of molecules on the substrate with increasing relative vapor pressure,p/po. For films a t temperatures far below the bulk triple point T3, where the vapor is in equilibrium with bulk solid, the steps in the isotherm are discontinuous, indicative of a first-order phase transition as each successivelayer condenses. These firstorder transitions terminate at layer critical temperatures, above which the transitions become continuous and the steps increasingly more rounded. For a multilayer system theory predicts a roughening temperature, TR,greater than all the layer critical temperatures, above which the adsorbed film-vapor interface can no longer remain smooth. Complications in the phase behavior may be caused by incompatibilities in the lattice structure of the substrate and the adsorbed layers when these have a solidlike structure at low temperatures and from surface melting at temperatures close to but below T3. By contrast, previous measurements of the adsorption of vapors to solid surfaces at room temperature have yielded smooth isotherms without any indication of layering, or structural Given the rough and chemically heterogeneous substrate surfaces used in most studies and the often insufficient thickness resolution, this is not surprising. On the other hand, structural effects have been observed in many studies of thin films confined between two smooth solid surfaces. Direct measurements of the force between solid surfaces (mainly mica) across liquids have shown that a t small surface separations oscillatory solvation forces replace the attractive van der Waals forces predicted by continuum theory in almost all systems studied.22 Studies of frictional forces between surfacesz3have shown that these depend on the numbers of layers of molecules confined between the sliding surfaces. Recently, ellipsometric measurements of the thickness profiles of drops spreading on solid substrates have (16) Lando, D.;Slutsky, L. J. Phys. Reo. B Solid State 1970,2,2863. (17) Tadros, M. E.; Hu, P.; Adamson, A. W. J.Colloid Interface Sci. 1974,49,184. (18) Blake, T.D. J. Chem. SOC., Faraday Trans. I 1975, 71, 192. (19) Hu, P.; Adamson, A. W. J. Colloid Interface Sci. 1977,59, 605. (20) Lee, W. Y.; Slutsky, L. J. J.Phys. Chem. 1982,.86,842. (21) Gee. M. L.:. Healv. _ .T. W.: White. L. R. J. Colloid Interface Sci. 1989,131, ia. (22) Christenson, H. K. J . Dispersion Sci. Technol. 1988, 9, 171. (23) Israelachvili, J. N.; McGuiggan, P. M.; Homola, A. M. Science 1988,240, 189.
0743-7463/91/2407-1843$02.50/0 0 1991 American Chemical Society
1844 Langmuir, Vol. 7,No. 9, 1991 detected layering of the liquid molecules, at least during the dynamic spreading process.24 In this letter we show the first example of an adsorption isotherm at room temperature that exhibits layering phenomena. A combination of a molecularly smooth and very homogeneous substrate (mica), an accurate ellipsometric technique, and a nonpolar liquid with large molecules (octamethylcyclotetrasiloxane,OMCTS) has allowed us to observe effects due to molecular structure in the adsorbed film, both above and below the bulk triple point (==meltingpoint).
Materials and Methods
3.5 3 2.5
Letters
, t
1
..
OMCTS is a nonpolar liquid with near-spherical molecules (diameter = 0.8 nm) that has been extensively used as a model m liquid in experiments with mica on surface forces and f r i c t i ~ n . ~ - ~ ~ The triple point, T3,was measured to be 18 "C and the critical 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 temperature, T,,is 314 "C.26 The mica was glued with Araldite (to achieve approximate refractive index matching on the back PJP(0) side) to an anodized aluminum disk and, after cleaving, immeFigure 1. Ellipsometrically determined thickness D of octamdiately placed under vacuum in a speciallydesigned cell, enclosed ethylcyclotetrasiloxane (OMCTS) adsorbed to mica surfaces at by circulating thermostated (fO.O1 "C) water. OMCTS (Fluka 19 "C as a function of the relative vapor preBsure,p po,of OMCTS. AG, 99%) vapor was admitted and the gas pressure measured The solid line is the thickness of the adsorbe film expected with a transducer. The thickness of the adsorbed layers was from the Lifshitz theory of van der Waals forces (calculated for measured as a function of p / p o below and above T3. the mica-OMCTS-vapor system according to the method given The ellipsometrictechnique is described in detail elsewhere.Bn in ref 29). The dashed line gives the proposed isotherm at temA phase-modulated ellipsometer is used to measure the real and peratures far below the melting point of bulk OMCTS where the imaginary components of the complex ratio of the p-polarized transitions from zero to one and from one to two layers are and s-polarized reflection amplitudes, r p / r l ,for a given angle of expected to be discontinuous (i.e. first order). The step size (0.85 incidence. Through the use of angle-averaging (achievedby the nm) is close to the mean molecular diameter of OMCTS. use of a lens to give a convergent incident light beam), it is possible 4.5 to easily interpret shifts in the ellipticity signal in terms of a layer thickness without the severe complications caused by 4 interference effects from the back side of the mica. (Perfect refractive index matching at the back side is impossible because 3.5 of the anisotropy of micaam) The adsorbed layer thickness was deduced from the change in ellipticity from the bare mica value 3 upon the introduction of vapor. The thickness sensitivity in our measurements was about 0.005nm (usingbulk refractive indices). 2.5
d
Results Figure 1shows an adsorption isotherm obtained at 19 "C. The coverage is linear at low pressures and there is an inflection point around p / p o = 0.45 and a distinct step in the isotherm at p / p o = 0.8. Four separate measurements with four different mica sheets all showed such a step at the same pressure. The isotherm tends to a finite value a t saturation, as expected from the finite contact angle (
