AGGREGATION AND MICROSTRUCTURE IN AQUEOUS-SOLUTIONS OF THE NONIONIC SURFACTANT C(12)E(8)

Mixtures of water and of the nonionic surfactant C(12)E(8) (octaxoy-et hyleneglycol monododecylether) have been investigated by time-resolved fluorescence quenching (TRFQ) and transmission electron microscopy at cryogenic temperature (cryo-TEM) to obtain information on the size an d shape of the surfactant aggregates and on the microstructure of the mixtures. The studies were performed at surfactant contents and temper atures where the micelles grew significantly and also where the system s undergo phase transitions from micellar-to-cubic upon increasing sur factant content or decreasing temperature and from cubic-to-hexagonal- to-micellar upon increasing temperature. No rapid change or discontinu ity was observed in the variation of the aggregation number with the s urfactant content or temperature upon crossing the micellar-to-cubic p hase boundary nor at the approach of the hexagonal phase from the cubi c phase, The results confirmed that the cubic phase consists of sphero idal micelles forming a three-dimensional array. The aggregation numbe rs at high surfactant content or temperature can be much larger than t hat of the minimum spherical micelle for a surfactant with a dodecyl c hain, suggesting that the micelles should be anisotropic. However, cry o-TEM showed that in the micellar phase the micelles are always sphero idal. These apparently inconsistent results are explained in terms of a partial mixing of the surfactant dodecyl chains and octaoxyethylene head groups which allows for spheroidal micelles of aggregation number much larger than for a surfactant with a dodecyl chain. The results s how extensive exchange of material at 40 degrees C, taking place most likely via temporary merging of micelles in micelle clusters then pres ent in the system. (C) 1997 Academic Press