Hydrophobic repulsion

The albumin molecule has different binding sites for anionic and cationic ligands, but van der Waals forces also contribute (p. 58). The extent of binding correlates with drug hydrophobicity (repulsion of drug by water). 

Water-Solute. Polar moieties of the solute cause reduced retention because they can hydrogen bond and dipole interact with the water, partially mitigating the hydrophobic repulsive force. In more familiar... 

Muller H J 1998 Extraordinarily thick water films on hydrophilic solids a result of hydrophobic repulsion Langmuir 14 6789-92... 

O/W side in the other (------) there is strong hydrophobic repulsion, and the preferred radius is on... 

According to previons literatnre, the extraction of 1-hexene in the system (1-hexene + n-hexane + [BMIM] [NO3] + H2O + AgN03) can be mainly attribnted to the metal-olefin complexation between silver nitrate and 1-hexene. The only difference between the two experiments carried ont in this section is the addition of salt. Therefore the existence of water enhances the metal-olefin complexation. The hypothesis of the mechanism of how the water improves the selectivity of 1-hexene and n-hexane separation can be related to the balance between the complex-IL attraction forces and the hydrophobic repulsion forces between the organics and water. It is believed that water has the ability to exclnde both olefin and paraffin from the IL thereby decreasing the overall distribntion coefficient. However the complexation between the olefin and the silver salt and the resulting attraction of this complex to the IL phase even in the presence of water allows for some 1-hexene to be separated. This therefore rednces the distribution coefficient while increasing selectivity. 

The behavior of methacrylic acid is quite different. Evidently the aggregates formed either in bulk or in solution do not affect the polymerization kinetics. Perhaps the methyl group causes irregularities in the orientation of the vinyl groups either by steric or by hydrophobic repulsion. 

This term, though not significant for most polymer films, accounts for the AB-type interactions in polar liquids (for example, for water it accounts for the hydrophobic repulsion with Sp >0, Ip 0.2-l nm). The entropic confinement effects in polymer films due to adsorption/grafting at the film-substrate interface is also taken into account by this term and for this case Ip Rg (radius of gyration of the polymer). 

A surprising result that emerged from the earlier work on hydrophobic interactions is that there is little or no tendency for nonpolar solute association at low concentration. For example, simulation results led Watanabe and Andersen to discuss the hydrophobic repulsion between krypton atoms in water.Calculated PMFs have typically supported a similar conclusion (discussed below). Aggregation of methane in water was observed to occur at higher concentrations, " suggesting that there may be cooperative aspects to the solute aggregation process. This conflicts, however, with results from Shimizu and Chan who explicitly observed anticooperativity in three-body hydrophobic interactions. ... 

As early as 1969, Wlieeler and Widom [73] fomuilated a simple lattice model to describe ternary mixtures. The bonds between lattice sites are conceived as particles. A bond between two positive spins corresponds to water, a bond between two negative spins corresponds to oil and a bond coimecting opposite spins is identified with an amphiphile. The contact between hydrophilic and hydrophobic units is made infinitely repulsive hence each lattice site is occupied by eitlier hydrophilic or hydrophobic units. These two states of a site are described by a spin variable s., which can take the values +1 and -1. Obviously, oil/water interfaces are always completely covered by amphiphilic molecules. The Hamiltonian of this Widom model takes the form... 

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

Approximately a minimum of 1 to 5,000 is required before complexation is no longer dependent on molecular weight for small anions such as KI and l-ariiLinonaphthaLine-8-sulfonate (ANS) (86,87). The latter anion is a fluorescent probe that, when bound in hydrophobic environments, will display increased fluorescence and, as expected, shows this effect in the presence of aqueous PVP. PVP, when complexed with Hl, shrinks in si2e as it loses hydrodynamic volume, possibly because of interchain complexation. ANS, on the other hand, causes the polymer to swell by charge repulsion because it behaves like a typical polyelectrolyte (88). 

The term "hydrophobic interaction" unfortunately implies some form of molecular repulsion, which, outside the van der Waals radii of a molecule, is quite impossible. The term "hydrophobic force" literally means "fear of water" force. The term hydrophobic has been introduced as an alternative to dispersive but means the same. It is not clear from the literature how the word hydrophobic originated, but it may have been provoked by the immiscibility of a dispersive solvent such as n-heptane with a very polar solvent such as water. 

What are the essential features of surfactant systems An important ingredient is obviously the repulsion between water and nonpolar molecules or molecule parts, the hydrophobic force. This interaction is however highly nontrivial, and its analysis is still an active field of research [4,22,23]. Qualitatively, it is usually attributed to the strong orientational and positional correlations between nonpolar molecules in solution and the surrounding water molecules. The origin of the interaction is therefore entropic free water forms a network of hydrogen bonds. In the neighborhood... 

These equations show that hydrophobic and steric (van der Waals) interactions are of prime importance in the inclusion processes of cyclodextrin-alcohol systems. The coefficient of Es was positive in sign for an a-cyclodextrin system and negative for a P-cyclodextrin system. These clear-cut differences in sign reflect the fact that a bulky alcohol is subject to van der Waals repulsion by the a-cyclodextrin cavity and to van der Waals attraction by the p-cyclodextrin cavity. 

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