Small angle neutron scattering near the wetting transition: Discrimination of microemulsions from weakly structured mixtures

The wetting transitions of water, n-alkane, and n-alkyl polyglycol ether (CiEj) systems are examined in order to locate the transition between weakly structured mixtures and microemulsions. Using small angle neutron scattering (SANS) we determine the local structure and relate it to the phase behavior and wetting transitions observed by macroscopic measurements. We measure the SANS of the mixtures across the transition along two different experimental paths. One path begins with well-structured mixtures, and the effective chain length of the surfactant combination C6E2/C4E1 is decreased by increasing the C4E1 fraction. The other path starts with equal amounts of water and oil mixed by the strong amphiphile C8E3. The local structure of these ‘‘good’’ microemulsions is weakened by increasing the temperature and concomitantly the oil/water volume ratio approaching the upper critical endpoint. As in previous studies analyzing the scattering experiments quantitatively permits determination of the amphiphilicity factor which is a measure of the strength of the surfactant. We confirm predictions that the amphiphilicity factor measured at the wetting transition becomes more negative as the temperature interval between the transition and the critical endpoint decreases.