Surfactant headgroups

Aqueous micelles are 40-80 A diameter spherical aggregates which are dynamically formed from surfactants in water above a characteristic concentration, the CMC (9). Depending on the chemical structure of their hydrophilic headgroups, surfactants can be neutral or charged (positively or negatively). The alkyl chain of the surfactants typically contains between 5-20 carbon atoms. Micelles rapidly break up and reform by two known processes. The first process occurs on the microsecond time scale and is due to the release and subsequent reincorporation of a single surfactant from and back to the micelle. The second process occurs on the millisecond time scale and is ascribed to the dissolution of the... 

H.M. Kao, Y.W. Liao, C.C. Ting, Synthesis of cubic mesoporous silica SBA-1 with bulky headgroup surfactant cetyltripropylammonium bromide. Microporous and Mesoporous Materials, 2007, 98, 80-88. 

For example, whereas low area headgroup surfactants (high g values, g= 1) favour lamellar liquid crystal type phases or cubic phases with low curvature, such as the Ia-3d bicontinuous gyroidal structure, surfactants with higher area headgroups with lower g values favour strongly curved micellar shapes, such as cylindrical rods (MCM-41) or spheres (e.g. SBA-1, SBA-2). 

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. 

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... 

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.

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... 

From the apparent ionization degree it was concluded that the EO chain probably behaves as part of the headgroup. As with Aalbers [49], a low surface charge of the sodium alkyl ether carboxylate micelles was mentioned. The micelle aggregation number N increases with the C chain much more than for the corresponding nonionic surfactants. In the case of C8 there was no influence of temperature. A small decrease was found with increasing EO, but much smaller than in the case of nonionics. 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

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