Electron Paramagnetic Resonance Study of the Surface Hydration of Triton X-100 Micelles in Water with Added Monovalent Alkali Salts

The hydrophobic spin probe 2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl octadecanoate (TEMPO-stearate) was used to study the hydration of the polar shell of Triton X-100 micelles as functions of the concentration of the electrolytes KCl, NaCl, and LiCl and temperature. It was shown that the hydration of the polar shell of the Triton X-100 micelle decreases with both increasing electrolyte concentration and increasing temperature. The effect of Li+ on the hydration of the polar shell was found to be smaller than those of Na+ and K+, which have almost identical behavior. The effective water concentration decreases from 18.3 to 15.8 M for LiCl and from 18.3 to 13.9 M for NaCl and KCl when the concentration of the electrolyte in the Solution increases from 0 to 2.5 M. The dehydration of the polar shell was correlated to the average value of the cation hydration number Calculated from literature data; the greater the cation hydration number, the greater the dehydration for the same increase in electrolyte concentration. Also, it was shown that the Cloud point is strongly correlated to the hydration of the polar shell.