HYDRATION AND STRUCTURAL-PROPERTIES OF A HOMOLOGOUS SERIES OF NONIONIC ALKYL OLIGO(ETHYLENE OXIDE) SURFACTANTS

The water sorbed by oligo(ethylene oxide) monododecyl ether of the hom ologous series from one up to eight oxyethylene groups and the repeat distance in the case of the lamellar phase at constant relative humidi ties (RH = 84.2 and 97.0%) were determined by the isopiestic method an d X-ray diffraction, respectively, at 25 degrees C. Further, deuterium NMR relaxation of deuterium oxide in large concentration range of sur factant dispersions in heavy water was measured at the same temperatur e. In the case of the lamellar phase the surface area of surfactant an d the thickness of the hydrophobic core were estimated from the repeat distance using the composition of the samples and the molecular volum es. For all surfactant dispersions the effective water binding energy was deduced from the deuterium relaxation times. It was found that the dependence of the water sorbed and of the surface area at constant re lative humidity as well as of the effective water binding energy on th e number of the oxyethylene groups can be described by linear function s. These findings are discussed with respect to the parameters which e xhibit gradual changes along the ethylene oxide chain and in the light of a dynamic picture. The water sorbed increases by 0.7 and 2.1 molec ules of water at the relative humidities of 84.2 and 97.0% (25 degrees C), respectively, the surface area of the surfactant by 3.7 Angstrom( 2) at the relative humidity of 97.0% (25 degrees C), and the effective water binding energy by 7 kJ/mol by adding one oxyethylene group.