Correlation between micellar structure and cloud point in long poly(oxyethylene)n oleyl ether systems

Phase behavior, micellar structure, and cloud point have been investigated in water/long poly(oxyethylene)(n) oleyl ether (C18:1EOn) (n = 10. 7, 19.2, 30.1, and 50.8) systems. With increasing n, the discontinuous micellar cub ic phase (I-1) becomes increasingly stable, effectively replacing hexagonal and lamellar liquid crystalline phases. From NMR self-diffusion measuremen ts in the liquid micellar phase at lower concentrations, we found that mice lles at larger n remain spherical and interact to a good approximation as h ard spheres up to crystallization. In the cubic phase, small-angle X-ray da ta indicate a body-centered cubic arrangement. The aggregation number of th e micelles monotonically decreases with increasing EO chain length, but the micellar molecular weight first decreases and then turns to increase at a certain EO chain length because of the increase of molecular weight of the surfactant molecule. As a result, the cloud point first increases with incr easing EO chain length, reaches a maximum, and then slightly decreases as t he EO chain length is further increased. Comparison was also made of the cl oud-point behavior of poly(oxyethylene)(n) polymer systems with that of the C18:1EOn systems.