Hydrophobic effect, interfacial water

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

Certain fibers are easier to wash than others. The hardness of the fiber surface, which varies not only with the basic fiber but also with the surface finish, affects both soilability and soil removal. Generally, soft finishes pick up and retain soil more readily than hard finishes, since soft surfaces permit greater soil-surface contact. The hydrophobicity of the fiber surface also influences soilability and soil removal, and the more hydrophobic fibers show greater soil retention especially for hydrophobic soils such as oils. These effects are explained by the high-energy interface existing between hydrophobic fibers and water (59, 60). Many soil components lower the interfacial energy and therefore locate at the fiber-water interface. 

When nonpolar compounds are suspended in water their relative insolubility causes them to associate, diminishing the water-hydrocarbon interfacial area (a hydrophobic effect). This association is greater in water than in methanol and brings the reactive partners into close proximity, increasing the rate of reaction. Any additive that increases the hydrophobic effect will increase the rate. ... 

In the case of precipitated silica, great endothermic contribution due to the displacement of interfacial water by the hydrophobic tails of surfactants may obscure the evaluation of the effect of the chain length on the enthalpy of displacement. As raising the temperature reduced this contribution to a great extent (see Fig.5b), the appropriate calorimetric experiments were carried out at 308 K. The results are presented in Fig.lO. 

In fact, hydrophobic molecules such as hydrocarbons are excluded from water through the same mechanisms as outlined above. Because they cannot dissolve in water they must form an interface. The same forces that defy gravity to minimize the surface area of a water droplet drive the segregation of hydrophobic compounds to minimize their interfacial contact with water. This is the macroscopic manifestation of the hydrophobic effect. It is a complex phenomenon with both enthalpic and entropic components, but under physiological conditions the... 

The hydrophobic effect provides the major determinant of the CMC its contribution is proportional to the area (ah) of the non-polar chain removed from exposure to water when micelles form (see above) (18-20). (The hydrophobic contribution is given by a term in yah, where y = oil/water interfacial tension.) This leads to a logarithmic relationship between the CMC and alkyl chain length for linear surfactants, as follows ... 

For surfactants with other types of hydrophobic groups, the CMC values are less readily available because far fewer measurements have been made, especially on pure compounds. However, the oil/water interfacial tensions are 56 and 46 dyn/cm for fluorocarbon and polydimethylsiloxane oils respectively, compared to the value of 52 dyn/cm for hydrocarbon/water. Hence from the sizes of the hydrophobic groups (see Table 21.1 below), the magnitude of the hydrophobic effect, and hence their CMCs can be estimated. This clearly shows that the well-known lower CMCs for fluorocarbon surfactants compared to normal derivatives arise from the much larger size of fluorocarbons, rather than any magic structuring of water ... 

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