An Extension of the Phase Rule for Adsorption under Equilibrium and Non-Equilibrium Conditions

Abstract

An adsorption field is defined as a homogeneous part of a solid or liquid surface on which adsorption can occur. In presence of adsorbable components each field contains one or more surface phases. Adsorption experiments have demonstrated that in many cases all the intrinsic properties of an isolated surface phase are variables characterized by C+1 degrees of freedom (F) where C is the number of components. If this surface phase postulate (S.P.P.) applies to each surface phase in a system, then, for a system in a state of complete equilibrium, F has the value F0 = C+S —Pv—Ps+2 where S, Pv and Ps are the numbers of fields, volume phases and surface phases, respectively. By considering the possible mechanisms, such as interphase mobility and vapor interchange by which equilibrium may be attained, this phase rule is extended to states of partial equilibrium, for example, cases where the surface phases and the volume phases are at different temperatures. The effects of electric fields are considered. Non-equilibrium states, which may be divided into steady states and transient states are characterized by values of F greater than F0. Experiments on transients by which F can be determined may thus serve to determine S when it is otherwise unknown. If the geometrical arrangement of the surface phases is known, such experiments serve to determine the surface diffusion coefficient D, or the phase boundary resistance to diffusion. The applicability of the S.P.P. depends on a certain degree of intraphase mobility of the adatoms.