Dynamics of water surrounding micelles

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. 

Computer simulation studies have explored translational and rotational dynamics in micellar solutions and shown that both translational and rotational dynamics in the hydration layer of micelles (Stem layer) are significantly slower than that in the bulk. The dipole-dipole time correlation function (which measures the rotational dynamics) shows the appearance of a long-time tail of the time constant in the 100 ps range or above. The dependence of the rotational dynamics on the probe location has also been investigated and it was found that the dynamics becomes faster as the probe moves away from the surface [7]. 

The translational diffusion of water molecules at the micellar surface also slows down, but only by about 20%. Thus, the translational motion is less affected than the rotational motion. This is due to the fact that the average mean-square displacement (which measures the translational dynamics) is dominated by the fast-moving free water molecules in the layer, while the long time slow decay of orientational relaxation is found to be dominated by the bovmd water molecules. 

Water in and around micelles, reverse micelles, and microemulsions 

Hydrogen-bond lifetime analysis revealed that HBs between the polar head groups of the micelle and the water molecules are much stronger than those between two water molecules in bulk water and thus exhibit much slower dynamics - almost 13 times slower than that of bulk water. This result indicates the presence of quasibound water molecules on the surface. 

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