A multistate model of monolayer adsorption of anisotropic molecules

Abstract

A multistate treatment of monolayer adsorption of anisotropie rod-like molecules which can assume several interconvertible states with different surface areas is developed by extending the Boehm-Martire analysis of rigid rod adsorption with 10ng-range dispersion interactions. The statistical mechanical behaviour of adsorption in these systems is determined when (1) the total dispersion interaction energy between any pair of adsorbed molecules is constant irrespective of their adsorbed states and (2). the total dispersion energy between an adsorbed molecular pair assumes two different values depending upon whether they are in the same or different states. Spreading pressure-density isotherms and molecular density distributions for each state are obtained for each assumption about the total dispersionenergy between molecular pairs. For case (1) the isotherms display a single van der Waals loop at sufficiently low temperatures corresponding to a transition between two adsorbed states having different densities and adsorbed state compositions. For case (2) several loops may appear in the surface pressure isotherms corresponding to transitions between different states of molecular adsorption. At low densities this sequence of transitions occurs between states with slightly different contact areas over rather small density ranges and is qualitatively suggestive of the existence of a broad, asymmetric coexistence region similar to those observed by Hawkins and Benedek and Kim and Cannell for the spreading pressure isotherms of dilute pentadecanoic acid monolayers.