Headgroup environment, micellar solutions

Solvation dynamics (SD) studies of micellar solutions have reported a timescale which does not match with the dynamics either when the probe is inside the bulk water or when it is inside the bulk hydrocarbon (core). This indicates that the probe used resided neither in the bulk water nor in the dry core region, but was located in the Stem layer. Bhattacharyya and co-workers have studied SD in several micelles and found that the SD in the Stem layer of the micelles is three orders of magnitude slower than that in bulk water (in bulk the relaxation time is on the sub-picoseond timescale) [6]. The components that could cause solvation in the Stem layer of micelles are the polar or ionic headgroups of the surficants, the counterions, and the water molecules. In such an environment water motion could be severely restricted, giving rise to the slow component of SD. 

However, it was noted that the discrepancy between the micellar rate constant and the rate constant in the model compound solution was largest for 4, which was known to be more sensitive toward hydrophobic interactions than 5. In addition, the solvatochromic Ex30 probe indicated a much more hydrophobic environment and other authors similarly found systems for which a model solution only mimicking the surfactant headgroups was insufficient to reproduce properties of the micellar pseudophase. The Hammett p-value for hydrolysis of substituted 1-benzoyl-1,2,4-triazoles 6a-f could similarly not be reproduced using a model solution mimicking ionic interactions only. " ... 

Surfactants Two other classes of host molecule, with, however, flexible cavities, are amphiphilic micellar and vesicular assemblies. In a simple view, these molecules bear a hydrophilic headgroup (e.g., ionic, zwitter ionic, or non-ionic) and a hydrophobic tail (carbon hydrogen chain). Depending on the solvent environment they organize in 3-dimensional surface-active assemblies. For example, in aqueous solutions their alignment... 

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