Hydrophobic effect, interfacial

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

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. 

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. ... 

The nonpolar portion of surfactant ions has an important role in promoting the adsorption process because it increases the affinity of these organic ions to the interfacial region. The effect derives from mutual attraction between the hydrophobic tails as well as their tendency to escape from an aqueous environment. That mechanism is precisely the same one which causes the spontaneous formation of micelles in aqueous solution and is known as the hydrophobic effect [78]. In the case of surfactant adsorption, it is responsible for the formation of surface aggregates. However, it is not easy to accurately predict the shape and the size of such molecular associations in the same way that the structure of bulk aggregates can be determined from the geometry of the molecule. This is because the surface imposes different restrictions on the organization of the adsorbed layer. 

Hydrophobic Effect on the Surface Tension and Interfacial Tension... 

The latter values are approximately five times larger. This shows that the simple dependence of the hydrophobic effect on the number of carbon atoms becomes rather complicated when considering the interfacial properties. These differences thus may be suggestive of the differences in orientation of the aUcyl chains at the interfaces. This subject has been recently investigated by measuring surface tension and interfacial tension near the freezing point of the oil (alkanes) phase under supercooled measurements, as described further below. 

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 most specific role of liquid-liquid interfaces that we found is a catalytic effect in the solvent extraction of metal ions and interfacial complexation kinetics. Shaking or stirring of a solvent-extraction system generates a wide interfacial area or a high specific interfacial area defined as the interfacial area divided by the bulk phase volume. Almost all extractants, and an auxiliary ligand in some cases, are more or less interfacially active, since they have both hydrophilic and hydrophobic groups. Interfacial adsorption of the extractant or an intermediate complex at the liquid liquid interface can very effectively facilitate the extraction rate. In this chapter, the catalytic role of the interface in metal complexation will be discussed. 

A question often asked is whether the parabolic energy wells as predicted by Pieranski have an activation barrier that prevents the particle from falling in spontaneously. One can argue that, especially for a large spherical particle, upon its approach to the soft interface, the interface needs to deform and liquid has to drain. This event adds an activation barrier that needs to be overcome for the particle not to bounce off the interface, and clearly the interfacial tension between the two soft bulk phases (liquid-liquid and liquid-air) and the viscosity of both phases play key roles. Note that a potential hydrophobic effect [28] can counterbalance such a barrier because the dewetting of the liquid between a hydrophobic particle and the hydrophobic liquid phase, or air, stimulates long-range attraction and eases the adhesion process. 

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 ... 

The forces controlling surfactant interactions with polymers are identical to those involved in other solution or interfacial properties, namely, van der Waals or dispersion forces, the hydrophobic effect, dipolar and acid-base interactions, and electrostatic interactions. The relative importance of each type of interaction will vary with the natures of the polymer and surfactant so that the exact characters of the complexes formed may be almost as varied as the types of material available for study. 

In fact, the predominant forces determining association of amphiphiles in well defined structures (e.g., micellar, cylindrical, lamellar) are the hydrophobic effect, tending to associate chains together, and repulsions between head groups. The latter are of electrostatic origin for ionic surfactants and steric for non-ionic surfactants. These two forces tend respectively to diminish or increase the interfacial area per molecule at the water/chain interface. The result is an optimal interfacial area Uq. 

Due to the presence of the hydrophobic effect, surfactant molecules adsorb at interfaces, even at low surfactant concentrations. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. The surface activity of surfactants should therefore be considered a dynamic phenomenon. This can be determined by measuring surface or interfacial tensions versus time for a freshly formed surface, as will be discussed further below. 

Tanford developed the idea that micelle structure and surfactant solution properties depend on a delicate balance of forces, that is, the hydrophobic effect drives aggregation and is balanced by stabilizing interactions in the interfacial region of the aggregates between headgroups... 

---