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 eth er (C(i)E(j)) systems examined in order to locate the transition betwe en weakly structured mixtures and microemulsions. Using small angle ne utron 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 transiti on along two different experimental paths. One path begins with well-s tructured mixtures, and the effective chain length of the surfactant c ombination C(6)E(2)/C(4)E(1) is decreased by increasing the C(4)E(1) f raction. The other path starts with equal amounts of water and oil mix ed by the strong amphiphile C(8)E(3). The local structure of these ''g ood'' microemulsions is weakened by increasing the temperature and con comitantly the oil/water volume ratio approaching the upper critical e ndpoint. As in previous studies analyzing the scattering experiments q uantitatively permits determination of the amphiphilicity factor which is a measure of the strength of the surfactant. We confirm prediction s that the amphiphilicity factor measured at the wetting transition be comes more negative as the temperature interval between the transition and the critical endpoint decreases. (C) 1996 American Institute of P hysics.