Sizes of solvent molecules

Table 3.4 The sizes of solvent molecules (or prorated per mole)...

These discrete effects relating to local water structuring around packing defects cannot be captured properly by existing continuous models of the interfacial electrostatics [17]. This is mainly because such models are based on mean-force potential approximations to solvent interactions, where solvent degrees of freedom are averaged out, and this is clearly an inappropriate ansatz to deal with cavities of the size of solvent molecules themselves. 

Figure 15. Change of Xv with size of solvent molecule...

The spatial factor connected with the size of the colloidal bead dilfering by a few orders of magnitude from the sizes of solvent molecules. 

The effect of solvent is limited then to a cage effect and to solvation the first increases the rate coefficients of bi-molecular reactions, and becomes even more important as the size of solvent molecules becomes greater than that of the reactants it is also increased by increasing viscosity of the solvent. 

The considered above electrostatic models of ion interaction are, undoubtedly, simplified. Each ion is surrounded by the solvate shell, whose character and sizes are determined by the ion, its charge and radius, and sizes of solvent molecules and such their parameters as the dipole moment of their polar groups, structure and sizes of the molecule. The solvent, its solvating ability, and the influence on the ion interaction are not reduced to the medium with the dielectric constant e only. Similarly, the interaction of ions is not restricted by the formation of only the ion atmosphere ion pairs, triples, and associates of several ions appear in the solution. Ion pairs, which can be separated by the solvate shell or be in contact to form contact pairs, also differ in structure. As a whole, the situation is more complex and diverse than its description by the classical theory of interaction of spherical charges in the liquid medium of dielectrics. The solvating ability of the solvent is determined only in part by its dielectric constant. For aprotic solvents, the ability of their heteroatoms to be donors of a free pair of electrons for cations is very significant. The donating ability of Ihe solvent is characterized by its donor number DN, which for the solvent is equal to the enthalpy of its interaction with SbCls in a solution of 1,2-dichloroethane... 

There is no one best method for describing solvent effects. The choice of method is dependent on the size of the molecule, type of solvent effects being examined, and required accuracy of results. Many of the continuum solvation methods predict solvation energy more accurately for neutral molecules than for ions. The following is a list of preferred methods, with those resulting in the highest accuracy and the least amount of computational effort appearing first ... 

The energy of interaction between a pair of solvent molecules, a pair of solute molecules, and a solvent-solute pair must be the same so that the criterion that = 0 is met. Such a mixing process is said to be athermal. The solvent and solute molecules must be the same size so that the criterion... 

Since we are explicitly interested in the difference in the sizes of solvent and solute molecules, it is more appropriate to express the values of AU on a per unit volume basis rather than on a molar basis. Accordingly, in Eq. (8.41) we replace the total number of sites N by the total volume of the mixture V and write... 

In addition to an array of experimental methods, we also consider a more diverse assortment of polymeric systems than has been true in other chapters. Besides synthetic polymer solutions, we also consider aqueous protein solutions. The former polymers are well represented by the random coil model the latter are approximated by rigid ellipsoids or spheres. For random coils changes in the goodness of the solvent affects coil dimensions. For aqueous proteins the solvent-solute interaction results in various degrees of hydration, which also changes the size of the molecules. Hence the methods we discuss are all potential sources of information about these interactions between polymers and their solvent environments. 

Fawcett et a/.317-319 have studied the Hg/EtOH interface in the presence of various anions (BIv, CIO , Cl", Br , I-). The surface activity of the anions has been found to increase in the above order. The double-layer data for Hg/EtOH have been found to be similar to those for MeOH,127,293 with some difference attributable to the bigger size of EtOH molecules. The double-layer thickness has been found to differ from that expected from the real cross section of the solvent molecules. 

One can further elaborate a model to have a concrete form of /(ft), depending on which aspect of the adsorption one wants to describe more precisely, e.g., a more rigorous treatment of intermolecular interactions between adsorbed species, the activity instead of the concentration of adsorbates, the competitive adsorption of multiple species, or the difference in the size of the molecule between the solvent and the adsorbate. An extension that may be particularly pertinent to liquid interfaces has been made by Markin and Volkov, who allowed for the replacement of solvent molecules and adsorbate molecules based on the surface solution model [33,34]. Their isotherm, the amphiphilic isotherm takes the form... 

TP Gall, RC Lasky, EJ Kramer. Case II diffusion Effect of solvent molecule size. Polymer 31 1491-1499, 1990. 

In a homogeneous fluid the frictional resistance a particle experiences depends largely on its size and shape and on the nature of the solvent. For large molecules, where the slip factor (tendency of solvent molecules to adhere to solute) approaches infinity, the frictional resistance is... 

---