Headgroup

Kuhl T Let al 1994 Modulation of interaction forces between bilayers exposing short-chained ethylene oxide headgroups Biophys. J. 66 1479-88... 

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 ... 

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

Figure C2.3.6. Illustration of micelle stmcture obtained by Monte Carlo simulations of model octanoate amphiphiles. There are 30 molecules simulated in this cluster. The shaded spheres represent headgroups. Reproduced by pennission from figure 2 of [35].

The idealized reverse micelle sketched in figure C2.3.1 is an aggregate of a double-tail surfactant. In such systems the solvent is more compatible with the lyophobic part of the surfactant than with the headgroup. This preference... 

Micelles are mainly important because they solubilize immiscible solvents in their cores. Nonnal micelles solubilize relatively large quantities of oil or hydrocarbon and reverse micelles solubilize large quantities of water. This is because the headgroups are water loving and the tailgroups are oil loving. These simple solubilization trends produce microemulsions (see section C2.3.11). 

Figure C2.3.14. Isolated surfactant modes of adsorjDtion at liquid-solid interfaces for a surfactant having a distinct headgroup and hydrophobic portion (dodecyltrimetlrylammonium cation) (a), (b) headgroup specific interaction (c), (d) hydrophobic tail interaction, (e),(f) headgroup and tail interactions.

The first part is the headgroup, which is responsibie for the bonding to the substrate surface, which may be by chemisoriDtion or physisorjDtion. 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

For ammonium surfactants there is evidence for the existence of an additional specific interaction between the headgroups of the surfactant and the aromatic solubilisate . This is in line with the observation that partition coefficients for benzene in CTAB solutions are much higher than those for... 

Studies on a large number of aromatic compounds have revealed that for CTAB the largest shift occurs for the alkyl chain protons near the surfactant headgroup, whereas in SDS nearly all proton signals are shifted significantly " ". For SDS, the most pronounced shifts are observed for protons around the centre of the chain. This result has been interpreted in terms of deeper penetration of... 

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