Micelles hydrophobic effect

C. Tanford. The Hydrophobic Effect Formation of Micelles and Biological Membranes. New York Wiley, 1980. 

Studies described in earlier chapters used cellular automata dynamics to model the hydrophobic effect and other solution phenomena such as dissolution, diffusion, micelle formation, and immiscible solvent demixing. In this section we describe several cellular automata models of the influence of the hydropathic state of a surface on water and on solute concentration in an aqueous solution. We first examine the effect of the surface hydropathic state on the accumulation of water near the surface. A second example models the effect of surface hydropathic state on the rate and accumulation of water flowing through a tube. A final example shows the effect of the surface on the concentration of solute molecules within an aqueous solution. 

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],... 

Tanford C. The hydrophobic effect formation of micelles and biological membranes. New York John Wiley Sons, 1980. 

Although the notion of monomolecular surface layers is of fundamental importance to all phases of surface science, surfactant monolayers at the aqueous surface are so unique as virtually to constitute a special state of matter. For the many types of amphipathic molecules that meet the simple requirements for monolayer formation it is possible, using quite simple but elegant techniques over a century old, to obtain quantitative information on intermolecular forces and, furthermore, to manipulate them at will. The special driving force for self-assembly of surfactant molecules as monolayers, micelles, vesicles, or cell membranes (Fendler, 1982) when brought into contact with water is the hydrophobic effect. 

The binding constants between the anionic substrates and cationic micelles are large because of the combination of coulombic and hydrophobic effects so rate enhancements may be large even with dilute surfactant. There is binding with non-ionic and zwitterionic micelles despite the absence of coulombic attraction (Bunton et al., 1975). 

N-Alkylhydroxamic acid hydrolysis Methyl Violet + OH" Cl C12H25S03Na + H30+, CTABr + OH". An attempt made to separate electronic and hydrophobic effects on the micellar reaction Anionic and cationic micelles. Effect of surfactant structure examined Berndt el at., 1984 Malaviya and Katiyar, 1984... 

Brown and Darwent (1979) recently demonstrated that the hydrophobic effect is also important in the micellar activation of peroxide nucleophiles. Engberts and coworkers reported that proton abstraction by sulfinate anions is facilitated (103-104-fold) in the CTAB micelle (Jagt and Engberts, 1977 van Langkruis and Engberts, 1979). 

In general, the standard enthalpy of micellization is large and negative, and an increase in temperature results in an increase in the c.m.c. the positive entropy of micellization relates to the increased mobility of hydrocarbon side chains deep within the micelle as well as the hydrophobic effect. Hoffmann and Ulbricht have provided a detailed account of the thermodynamics of micellization, and the interested reader will find that their tabulated thermodynamic values and treatment of models for micellar aggregation processes are especially worthwhile. 

HYDROPHOBIC INTERACTIONS. These bonding interactions arise from the tendency of nonpolar side chains of amino acids (or lipids) to reside in the interior, nonaqueous environment of a protein (or membrane/ micelle/vesicle). This process is accompanied by the release of tightly bound water molecules from these apolar side-chain moieties. The hydrophobic effect is thermodynamically driven by the increased disorder i.e., A5 > 0) of the system, thereby overcoming the unfavorable enthalpy change i.e., AH < 0) for water release from the apolar groups. 

Following this, the thermodynamic arguments needed for determining CMC are discussed (Section 8.5). Here, we describe two approaches, namely, the mass action model (based on treating micellization as a chemical reaction ) and the phase equilibrium model (which treats micellization as a phase separation phenomenon). The entropy change due to micellization and the concept of hydrophobic effect are also described, along with the definition of thermodynamic standard states. 

Surfactant aggregation in an anhydrous, nonpolar medium differs in several important respects from aggregation in water. The most apparent of these differences is that the hydrophobic effect plays no role in the formation of reverse micelles. The amphipathic species are relatively passive in aqueous micellization, being squeezed out of solution by the water. In contrast, surfactant molecules play an active role in the formation of reverse micelles, which are held together by specific interactions between head groups in the micellar core. 

Tanford, C., The Hydrophobic Effect. The Formation of Micelles and Biological Membranes, 2d ed., Wiley, New York, 1980. (Undergraduate level. A classic reference by a pioneer on the hydrophobic effect on the relevance of surfactants to biological membranes.)... 

Both adsorption from solution and micellization occur as a result of the hydrophobic effect. To test the correspondence between these two effects. Rosen assembled AG° values for adsorption at the air-water interface and for micellization of a number of linear and branched surfactants. The following is a selection of these data ... 

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