STRUCTURAL INVESTIGATION OF THE EFFECTS OF NONELECTROLYTES AND SURFACTANTS ON WATER BY THERMAL LENS SPECTROMETRY

Changes in the structure of water induced by nonelectrolytes and supramolecular systems were determined by measuring the thermal lens effect that was generated as a consequence of the solution absorption of the near-infrared laser irradiation. By measuring the thermal lens of the solutions with different types of solutes added, at different concentrations and different temperatures, two parameters, namely the temperature at which dn/dT = 0 (i.e., temperature of maximum refractive index, TMR) and the value which is proportional to d(2)n/dT(2) (where n is the refractive index and Tis the temperature), can be obtained. Insight into the structure of water and the effects of different solutes on its structure can be elucidated from these two parameters. Specifically, urea which is the (water) structure breaker lowers the TMR and d(2)n/dT(2) whereas tert-butyl alcohol which is the (water) structure former increases TMR and d(2)n/dT(2) values. These effects can be explained in terms of a two-state model of water structure. The information obtained was then used to study the effect of surfactants including CTAB, SDS, SE-12, and Brij-35. It was found that when the surfactants are in the micellar forms, they decrease the structure of water more efficiently than when they are in the monomeric forms. The molecular structure of the surfactant also plays an important role in the observed effect; namely, the disruption of the water structure by surfactants at concentrations below cmc's was found to decrease in the order Brij-35 >> CTAB > SE-12 > SDS. For micellar solutions, the effect by Brij-35 is the strongest while those by CTAB, SDS, and SE-12 are similar.