Primitive hydrophobic effects

Hydrophobic and hydrophilic are categories of solvation effects in aqueous liquids. Classical ions such as Na" " or polar molecules such as NH3 are easily recognized hydrophilic solutes. In contrast, the interactions of hydrophobic solutes or groups with water molecules do not display classic electrostatic or specific chemical interactions. Primitive hydrophobic solutes are inert gases and simple hydrocarbons that are sparingly soluble in water. However, much of the interest in hydrophobic effects is associated with more complex solutes that contain both hydrophobic and hydrophilic moieties. Surfactant species, for example the decanoate anion, include both hydrophobic and hydrophilic parts and are called amphiphilic. 

Another primitive hydrophobic effect is the effect of water on the association of simple hydrocarbon molecules in aqueous solution. We will below give some discussion and an example of association of pairs of inert gas atoms, focusing on the function that gives the mean forces between such a pair upon differentiation, i.e, the pair potential of the average forces. These issues broaden the topic from hydration of solitary hydrophobic solutes to the interactions associated with disruption of the water structure by more than one hydrophobic solute in proximity. A further extension of the study of hydrophobic interactions is the treatment of conformational equilibrium of simple, flexible non-polar molecules in aqueous solution. 

This organization of our subject beginning with primitive hydrophobic effects leaves to an important subsequent stage the integration of valid molecular theories of primitive hydrophobic effects into the more complicated settings of molecular biophysics. 

An important part of the puzzle is that the most characteristic hydrophobic effects, the unfavorable entropies and large heat capacity changes, seem to be largely independent of the molecular details of solute-solvent interactions within broad families. This is an awkward point for computational chemistry that naturally invests great effort in accurately describing intermolecular interactions before entropies are considered. This point emphasizes the utility of studying model problems, primitive hydrophobic effects in the first place and modelistic expressions of those effects. It is helpful to identify the minimum that must be included in the model in order to get the interesting behavior and only after that to include all features actually present in specific cases. 

This section discusses theories and calculations that have been used in molecular modeling of primitive hydrophobic effects. There is a basic schism among approaches that have been pursued. One approach is to model hydrophobic effects empirically on the basis of experimental solubilities without direct consideration of solute-water molecular interactions. Hydrophobic effects extracted on the basis of empirical fitting of solubilities are often called hydrophobicities (see... 

Direct use of equation (2) or equation (4) to study primitive hydrophobic effects started in 1990 with the realizations that these formula can be directly feasible for atomic-sized solutes in water at the moderate conditions of greatest interest and that when feasible these results give information of direct conceptual relevance to molecular theories. We anticipate later discussion by asserting here that insights drawn from these direct calculations have recently lead to new theories that promise to the extend the range of applicability of molecular ideas beyond the primitive hydrophobic effects that is the topic of subsection 4.1.3. 

Stillinger analyzed the SPM and proposed a revised scale particle model (RSPM) that incorporates the measured surface tension in addition to the measured pressure, in this way correcting the most direct error of the SPM. The RSPM deserves note because it is more accurate for primitive hydrophobic effects than the SPM but not more difficult to use. 

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