Water penetration into micelles

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

D.G. Whitten, J.B.S. Bonilha, K.S. Schanze and J.R. Winkle, Solubilization and water penetration into micelles and other organized assemblies as indicated by photochemical studies, in K.L. Mittal and B. Lindman (Eds.), Surfactants in Solution, Plenum, New York, 1984, pp. 585-598. 

A full discussion of water penetration into micelles is beyond the scope of this chapter. The results described above, and others employing longer chain keto -surfactants in other micelles and bilayers, indicated a trend toward less water penetration to the core of aggregates as the surfactant tail length increased, and as aggregate curvature decreased (bilayer formation). More data from FT-IR studies... 

Chemical-shift measurement has proven to be useful in determining cmc, surfactant chain conformation, and the extent of water penetration into micelle. Menger [39] found that as many as three methylene groups of an alkyl chain near the head group are hydrated in a micelle. Relaxation times are related to translational and rotational molecular motions. These also can give us informa-... 

Just as the extent of water penetration into micelles stiU remains a controversial topic, micellar internal viscosity (microviscosity) also suffers from a similar fate. As the determination of both micellar internal viscosity and the extent of water penetration in micelles involves an almost similar experimental approach, that is, the use of some kind of probe, it is not surprising to see that both topics still remain controversial in nature. 

Menger, F.M., Boyer, B.J. Water penetration into micelles as determined by optical rotary dispersion. J. Am. Chem. Soc. 1980, 702(18), 5936-5938. 

There is litde doubt that the surfactant headgroups and counterion species are excluded from the micelle core. There is, however, some debate concerning water penetration into the core. Evidence for water penetration into the micelle core generally comes from spectroscopic experiments employing probe molecules (22-23). The probe molecules have been found to lie in a partly hydrophilic environment, and this has been interpreted as indicating water penetration into the core. Recent NMR relaxation (24) and neutron scattering (25) data provide fairly unequivocal evidence for minimal water penetration into the micelle core, however. 

Higher molecular weight products, such as calcium dinonylnaphthalene sulphonate, can also be used as demulsifiers, to remove water from oil systems. Examples of the include sump oils in ships. If sea water penetrates into the lubricant system, the sulphonate will remove the water by first including it in a micelle which leads to subsequent aggregation into droplets which are deposited into the sump. The metal of the sump is further protected by the corrosion inhibiting properties of the sulphonate. 

In this micellar model the head groups and associated counterions are extensively hydrated, so that the Stem layer could be regarded as akin to a concentrated electrolyte solution [25]. However, water penetration into the micelle has been assumed to be limited to only the first few methylene groups of the hydrophobic moiety [12]. There is considerable evidence that water molecules at the micellar surface are not much less reactive than those in water [30-33]. 

An additional question is that of water penetration. Menger [35] attempted to construct a micelle using space-filling models, and found that if the chains are extended there will be extensive voids into which water molecules, or solutes, can penetrate. However, these voids will be reduced in size if some eclipsing is introduced into the hydrophobic alkyl groups. This porous-cluster model is supported by physical and chemical evidence of extensive water penetration into the micelle. 

The primary effect of pressure on a micellar system is a change in the CMC, which is closely related to changes in the partial molar volume of micelle formation, aggregation number, and water penetration into the micelles. In general, the CMC increases with pressure, undergoes a maximum and then tends to decrease again. 

The molecular dynamics calculations on sodium octanoate micelle in water reveal that the inner core of the micelle is devoid of water molecules, while there is considerable water penetration into the outer region of the core. Dynamic simulation of hydration of spherical micelles of CigEg in aqueous solution reveals that the micellar interface is separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only.3° Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydro-phobic core at low temperature. ... 

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