Temperature dependences of micellar molecular weight and radius of nonionic surface active agent in the aqueous solution

Abstract

The molecular weight and diffusion constant of poly(oxyethylene) dodecyl ether (C 12OE 9·3) micelles with an average of 9.3 oxyethylene group were determined from light-scattering measurements and free boundary experiments over a wide temperature range. It was found that micelles formed by C 12OE 9·3 were monodisperse in the temperature range studied. Plots of the logarithms of micellar molecular weight and micellar radii estimated from the diffusion constants against the temperature have a break point around 45°C. This temperature agrees with a theta point where the second virial coefficient obtained from light-scattering measurements is zero. Although the number of the molecules participating in micelle formation increases with temperature, the size of the micelle remains constant up to the temperature corresponding to the theta point, because the spherical micelle is packed with increasing tightness. The increase in the size and weight beyond that point may be attributed to a departure from spherical shape.