Free energy oxide-solution interface

Free Energies of Electrical Double Layers at the Oxide-Solution Interface... 

The aim of this paper is not to add to the current debate but to present a simple graphical method of analysing the free energy of formation of the electrical double layer at the oxide/solution interface ( 1). This will provide a simple way of visualizing the complementary roles of chemical reactions or surface properties of... 

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

For the four chnical anesthetics, d i,(z) exhibits minima at the interface. Thus, all these molecules are inter-facially active. However, the degree to which they tend to accumulate at the interface differs. For cyclopropane and nitrous oxide, the depths of the interfacial minima in measured with respect to the free energy in hexane, is smaller than 1 kcal/mol. The minima for isoflurane and desflurane are considerably deeper, approximately 2-2.5 kcal/mol. The interfacial activity of the solutes can be related to their polarity. Isoflurane and desflurane have an average permanent dipole moment of approximately 2 D. In contrast, cyclopropane is a symmetrical molecule with no permanent dipole moment, and the dipole moment of nitrous oxide is only 0.2 D. In spite of their small dipole moments, however, these molecules should be considered as weakly polar rather than non-polar. They are involved in non-negligible electrostatic interactions with water, that are primarily due to higher molecular multipole moments arising from substantial partial charges on the atoms. 

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