Phase Transitions in Confined Molecularly-Thin Films
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fHASIE TRANSI•$IONS UN CONFIMUM
MOILIECU LARILY-TIIN FLMS
JOHN H. CUSHMAN*, D. J. DIESTLR', AND MKSCHOIN *1150 Lilly Hall, Purdue Univ., W. Lafayette, IN 47907-1150 "**Dept. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583 •*Universitit Witten/Herdecke, Naturwissenschaftliche Fakultit, Inst. Experimentalphysik, Stockumer Strasse 10, 5810 Witten, Germany AMSTRACT Phase changes in Lennard-Jones (IJ) and Stockmayer fluids confined between two parallel fcc (100) planes of rigidly fixed IJ atoms are studied by means of mixed isostress-isostrain canonical and isostrain grand-canonical Monte Carlo methods, and a microcanonical ensemble molecular dynamics technique. A nonequilibrium continuum thermodynamic derivation of the constitutive laws for the basic slit-pore model is developed and an equilibrium statistical thermodynamics anolog is reviewed. Independent constitutive parameters are chosen from the list T, p, EA or their thermodynamic duals n1,N, cij where T is absolute temperature, p is the chemical potential, EL is the infinitesimal strain tensor, N is the particle number, ri is the entropy and rij is the Piola stress tensor. Relaxation in monolayer films is studied via mean square displacement of an atom. The vicinal atoms are found to diffuse anomolously slow in accordance with a power-law relation. H¶TRODUCflON Microporous systems are ubiquitous throughout nature. Probably the most prevalent of such systems are natural clays', clay soils, shales and biological membranes 2 . Engineered microporous systems such as pillared clays 3 and some ceramics have numerous commerical uses. Isolated micropores such as those commonly studied in molecular tribology4 (e.g. two molecularly flat mica sheets which are separated by a few fluid molecular dyameters) are of considerable scientific interest. A main reason microporous systems are under intensive investigation is because of the anomolous character imparted by the walls of a micropore to the vicinal fluid 5 -8. Laboratory studies of such fluids invariably involve micropores created by natural minerals. On the other hand, computational experiments (virtual experiments) have been performed with numerous materials. Virtual experiments in the late 1980's led to a number of profound discoveries concerning anomalous phase behavior. To a significant extent the phase diagram is governed by the crystallagraphic structure of the pore walls, the width of the micropore, the relative registry of the two surfaces of the pore (we confine our attention to slit-pores), and the ability of the vicinal fluid to epitaxially associate with the surfaces. Thermodynamically the state of a simple vicinal fluid is governed by the chemical potential, p, the temperature T, and the potential field induced by the pore walls. This field can be described by specifying an infinitesimal strain tensor 9 , E, which describes the relative registry of the pore walls, and by the crystal structure of the walls. Rather than specifying E and p one can alternatively specify some or all of Mat. Res. Soc. Symp. Proc
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