Hydration and order in different aggregates of a poly(ethylene oxide) alkyl ether in aqueous solution. An ESR spin probe study

The local parameters (hydration, order, viscosity) in different aggregates (micelle, hexagonal and lamellar liquid crystals) in the phase diagram of poly(oxyethylene)[5]-dodecylether (C12E5) in water have been measured by EPR with two series of spin probes: x-DSA, (x = 5, 7, 10, 12 and 16) and CAT n (n = 4, 8, 11 and 16), A rather complete map of hydration ([H2O]/[EO]) along the PEO chains has been determined. It shows that the hydration of the PEG regions is very low near the PEO/hydrocarbon interface in all phases and increases towards the outer region of the PEO shell with a gradient which becomes steeper iii micellar phase in comparison with the lamellar phase. Temperature increase reduces the hydration levels at all measured sites. The order of the chains decreases by dilution from the lamellar to the micellar phase, but it is preserved within the frame of the lamellar phase at two different concentrations. The order degree, S, decreases in the series 5- > 7- > 10- > 12- > 16-doxyl, irrespective of temperature in all types of aggregates. This trend, together with decreasing hydration values, points to the insertion of the stearic acid chain in the aggregates along the normal to the interface. In samples quenched in liquid nitrogen, however, bending of the stearic acid chain was observed in a number of cases.