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Magnetic Susceptibility Studies on a Lyotropic Liquid Crystal System – The Role of Bound Water

The bound water in aqueous micellar systems like the Cesium perfluoroprooctanoate (CsPFO)/water has been extensively studied using molecular simulations. However, suitable experimental techniques to probe the bound water (hydrogen bonded water molecules to the micellar surface) in these systems are lacking, as the hydration layer is very thin. The free or bulk water magnetic susceptibility is related to the number of hydrogen bonds between the water molecules. Hence, one can employ the magnetic susceptibility studies to probe the bound water effectively when the micellar aggregation number is small. With this view, we have carried out magnetic susceptibility studies on CsPFO/water system in the isotropic phase for various concentrations. We find that the susceptibility increases with decrease in temperature, exhibits a maximum and then decreases linearly. The decrease in susceptibility with decrease in temperature below the maximum can be related to the aggregation number of the micelles. On the other hand, the increasing trend in the susceptibility at higher temperatures (where the aggregation number of the micelles is small) may be attributed to the bound water. Further, we find that with decreasing concentration of CsPFO, the susceptibility maximum shifts towards the low temperature side indicating that the bound water extends to lower temperatures. From our magnetic studies, we infer that at very low concentrations of CsPFO (< 15 wt%), the bound water can be present even at room temperature. This is in agreement with the reported molecular simulation studies carried out for the same concentration of CsPFO.