Abstract:

Proton NMR measurements were conducted on a U-type model quaternary nonionic microemulsion contg. varying amts. of water and having a fixed wt. ratio of surfactant [octaethylene glycol mono-n-dodecyl ether; C12(EO)8], cosurfactant (pentanol), and oil (dodecane). Chem. shift and T1 relaxation time data were used to obtain information about the microemulsion properties and structure. A slow exchange was obsd. between the hydroxyls of water, C12(EO)8, and pentanol in microemulsions contg. up to 10 wt% water. This is precedence of the hydroxyl-water interaction over hydration of the ethylene oxide (EO) groups of the surfactant. The fraction of bound water was calcd. from the chem. shift and T1 data. For the model system, chem. shift measurements indicated an inversion from W/O to O/W microemulsion at about 55-60 wt% water, in agreement with the microviscosity values evaluated from T1 data: 1.1 cP (55 wt% water) compared to 4.7 cP (30 wt% water) and 12.7 cP (10.7 wt% water). The irregularities in the monotonic dependence of the chem. shifts on water content in the microemulsion at 18 and 55 wt% water were interpreted to possibly reflect structural changes that took place in soln. [on SciFinder(R)]