Thermal lens investigation of the temperature dependence of the refractiveindex of organo-aqueous solutions

The thermal lens technique has been used to experimentally determine the te mperature of maximum refractive index (TMRi) of low molarity aqueous methan ol solutions. The well-known decrease in the measured TMRi is observed with increasing concentration of methanol. At the TMRi, the temperature depende nce of refractive index (dn/dT) is zero, resulting in the absence of a ther mal lens signal. In this work, the observed changes in the TMRi are related to the changes in the expansion coefficient of the water and to a lesser e xtent, to changes in the polarisability coefficient of the water. It is als o seen that the enhancement in the thermal lens parameter upon the addition of methanol to water is related primarily to the change in expansion coeff icient of the aqueous solution. (C) 1999 Elsevier Science B.V. All rights r eserved.