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Category: Selected Topics

There has been a great deal of effort in developing equations of state that can be used to describe the thermodynamics and bulk phase equilibria of fluids nand fluid mixtures. Analytical theories have now been developed to provide a quantitative description of bulk fluids comprising molecules which interact with more complex interactions, such associating systems, amphiphiles, polymers, and electrolytes. The SAFT-type theories clearly show how modern equations of state are able to describe the fluid phase behavior of systems ranging from small strongly assocating molecules such as water, to long-chain alkanes, polymers, and electrolytes.

Though the study of inhomogeneous fluids has a long history from the early mechanical models of Laplace and Young to the present day molecular density functional theory, a quantitative description of the interfacial properties of inhomogeneous fluids such as the interfacial thickness, adsorption, wetting and the surface tension is relatively rare, espcially in the case of moelcules with more complex interactions. Interfacial systems are ubiquitous in living systems (cell membranes which control molecular transport and biological functions) and are of fundamental industrial importance in areas as diverse as detergency (surfactants and solubilization), food production (emulsions and colloids), cosmetics (structured phases), and optoelectronic devices (liquid crystals).

The most successful modern theory of inhomogeneous classical fluids is undeniably the density functional (DFT) method, in which the free energy of the system is expressed as a functional of the spacially varying single particle density. The equilibrium density profile of an inhomogeneous system corresponds to the profile which leads to the minimum of the free energy of the system. Once the density profile is known, the interfacial properties may be determined from simple thermodynamic identities.

Our group (in collaboration with the Multiphase Fluid Systems Group of Prof. George Jackson, Imperial College London, UK) has developed new density functional theories based on the SAFT free. In particular, we have studied the effect of the potential range, chain length, association energy and volume and different association schemes on the interfacial properites. Our main achievements are described below:

• Development of a DFT based on the free energy of the SAFT-HS.

• Effect of the molecular parameters on the interfacial properties of chainlike associating molecules.

• Development of a DFT based on the SAFT-VR free energy.

• Application of the SAFT-VR DFT formalims to deal with the interfacial properties of systems of industrial interest.