THERMAL LENS EFFECT IN ELECTROLYTE AND SURFACTANT MEDIA

A novel method, which is based on the thermal lens effect, has been developed for the simultaneous, sensitive, and accurate determination of the thermal conductivities and temperature coefficients of the refractive indexes of solutions. The k and dn/dT values determined by this method were then used to elucidate the mechanism of the thermal lens enhancement that is induced by electrolytes and/or surfactants. Electrolytes were found to provide up to a 2-fold enhancement in the thermal lens signal. The enhancement is due to the effect of the electrolyte not on the thermal conductivity but rather on the dn/dT of the solution. Higher enhancements (up to 8-fold) were achieved when surfactants were added to the solution. The micellization process does not seem to have any observable effect on this enhancement. Similar to the case of electrolytes, the enhancement is predominantly due to the effect of surfactants on dn/dT of solution. The change in dn/dT upon the addition of electrolytes or surfactants was deconvoluted into two parts. The first, which results from the difference in the specific refractivity and dn/dT of the electrolyte or surfactant solution as compared to that of pure water, has a stronger effect on the enhancement than the second, which is caused by the alteration of the hydrogen-bonding network of water.