Structural Chemical Study of Nonionic Surfactant-Water Systems by Vibrational Spectroscopy

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For elucidation of various characteristics of binary systems of alkyloxyethylene nonionic surfactants and water, properties of the hydrophilic oxyethylene chain play an important role. In particular, the conformational behavior of the oxyethylene chain is one of the important factors for understanding the structure of the system. Accordingly, in this year of 1992, conformational behavior of noncyclic and cyclic oxyethylene chains was studied in detail. The model compounds used are alpha-methyl-omega-methoxyhexakis(oxyethylene) and 18-crown-6, respectively. By utilizing infrared spectroscopy in combination with normal coordinate calculations, hydration and conformational states of the oxyethylene chain were examined in an extensive concentration range. Conformation of the OC-CO group was elucidated on the basis of changes of the hydration structure. With addition of water to neat liquid, the gauche conformation of the OC-CO group is more stabilized with the formation of bridged hydrogen bonds between the ether oxygens of the oxyethylene unit and water molecules. The distance between the oxygens of the gauche OC-CO group is about 0.28 nm. This distance is, however, too short for a single water molecule to form a stable bridged hydrogen bond, but may be adequate to stabilize the gauche structure through proton-lone pair interactions.In this year, another study was also performed on polymorphic behavior of C_<16>E_3, which is one of the typical alkyloxyethylene nonionic surfactants. Changes in crystallinity and conformation with phase transformations were elucidated by Raman spectroscopy. This polymorphic phenomenon is closely related to the structure of various mesophases formed by nonionic surfactants in aqueous solution.