Conformational properties of short poly(oxyethylene) chains in water studied by IR spectroscopy

Abstract

Conformational properties of short-chain poly(oxyethylene) (POE) compounds in water have been studied by IR spectroscopy. The compounds examined are CH₃(OCH₂CH₂)_mOCH₃ (m = 1–4), CH₃CH₂(OCH₂CH₂)₂OCH₂CH₃, CH₃(OCH₂CH₂)_mOH (m = 2–4), CH₃CH₂(OCH₂CH₂)_mOH (m = 2 and 3), CH₃(CH₂)₃(OCH₂CH₂)_mOH (m = 2 and 3), H(OCH₂CH₂)_mOH (m = 1–4 and 6), PEG 600 and PEG 2000. The relative intensities of the gauche and trans conformational key bands of the C–C and C–O bonds have been measured with varying solution concentration, and the effects of the chain length and the terminal group on the conformation around the respective bonds have been studied. The relative population of the gauche conformation around the C–C bond increases significantly with increasing water fraction. The rate of increase in the gauche population does not depend practically on the length of the POE chain and the length of the terminal alkyl group as long as it is a butyl or shorter group. The terminal hydroxy group greatly restrains the preference of the gauche conformation around the C–C bond in water. The conformational change of the C–O bond with varying solution concentration is much smaller than that of the C–C bond. The population of the trans conformation around the C–O bond increases only slightly with increasing water fraction for the compounds with alkyl terminals, but no substantial conformational changes are observed for the compounds with hydroxy terminal(s). The conformational behaviour of the C–C bond suggests the presence of several types of hydrogen bonds between the POE chain and water. One is a hydrogen-bonded bridge by a water molecule between the adjacent ether oxygen atoms, and the others are bridges by one or more water molecules between the ether oxygen atoms separated by two oxyethylene units. The restrained gauche preference of the terminal O–CH₂–CH₂–OH segment may be accounted for by the destabilised hydration structure around the terminal part of the chain. The present results have shown the importance of both the hydrogen bonding and the structural matching between the POE chain and the water in the conformational stabilisation of the POE chain in water.