Bulk water inherent structures

The solvent also acts as a dielectric medium, which determines the field diji/dx and the energy of Interaction between charges. Now, the dielectric constant e depends on the inherent properties of the molecules (mainly their permanent dipole moment and polarizability) and on the structure of the solvent as a whole. Water is unique in this sense. It is highly associated in the liquid phase and so has a dielectric constant of 78 (at 25 C), which is much higher than that expected from the properties of the individual molecules. When it is adsorbed on the surface of an electrode, inside the compact double layer, the structure of bulk water is destroyed and the molecules are essentially immobilized... 

Similar stndies have been made on ions in liquid ammonia (at 240K). The mean residence times of ammonia molecules in the second solvation shells of the ions stndied are longer than for water molecules 12.7ps compared to 2.6ps for Ag [116], 28.5 ps compared with 6.5ps for Co [117], but shorter in the case of Cu 3.2ps [118] compared with 7.7ps for water [91]. Molecular dynamics computer simulations of solutions of ions in liquid ammonia [119] yielded the self-diffusion coefficients of ammonia molecules, D/10 m s , in the solvation shells of 6.1 and of R 7.4, shorter than the value for ammonia molecules in the bulk liquid, 11.5 1.5. These studies thus indicate that K and R are structure breakers and Ag" and Co are structure makers regarding the inherent structure of liquid anunonia. 

If the purpose of a calculation is to probe the inherent properties of a molecule as a thing in itself, or of a phenomenon centered on isolated molecules, then we do not want the complication of solvent. For example, a theoretically oriented study of the geometry and electronic structure of a novel hydrocarbon, e.g. pyramidane [6], or of the relative importance of diatropic and paratropic ring currents [7], properly examines unencumbered molecules. On the other hand, if we wish, say, to calculate from first principles the pZa of acids in water, we must calculate the relevant free energies in water [8]. Noteworthy too is the fact that solvation, in contrast to gas phase treatments, is somewhat akin to molecules in bulk, in crystals [9]. Here a molecule is solvated by its neighbors in a lattice, although the participants have a much more limited range of motion than in solution. Rates, equilibria, and molecular conformations are all affected by solvation. Bachrach has written a concise review of the computation of solvent effects with numerous apposite references [10]. 

This is not the end of the story, however. A third type of effect can alter the selfassociation structure and is directly related to the surfactant and or alcohol inherent properties. For instance, straight-chain ionic surfactants would produce liquid crystals of the lamellar type unless the temperature is quite elevated. Thus, in most cases of ionic systems, a large amount of alcohol (as much as two or three times the surfactant amount on a mole fraction basis) is required to melt the liquid crystal into a microemulsion, particularly for middle-phase ones [33]. Note, however, that too much alcohol could be detrimental to a high-performance microemulsion because the alcohol molecules which are not playing a cosurfactant role at the interface would dissolve into the bulk of one or both excess phases, making them more compatible [i.e., the alcohol would make the water less polar and the oil more polar (depending on the alcohol, but most particularly, intermediate solubility ones such as secondary butanol or tertiary pentanol)]. This is, of course, a way to narrow the miscibility gap, but this time by favoring the formation of a cosolubilized random mixture of all molecules instead of a microemulsion structure [50,65]. 

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