ABSTRACT Conformation of α-n-alkyl-ω-hydroxyoligo (oxyethylene) surfactants CH3 (CH2)n-1 (OCH2CH2)mOH (CnEm) in the crystalline solid state is described. Raman spectra of a homologous series of 71 CnEm compounds with n = 1-16 and m = 1-8 were analyzed and their molecular conformations were determined. The spectra were analyzed on the basis of normal coordinate treatment and by the aid of conformation-spectrum correlations previously derived. The molecular conformations of the CnEm surfactants show interesting dependence on the lengths of the oxyethylene and alkyl chains. As the number of oxyethylene units increases, the conformation of CnEm with n ≥ 5 changes from highly extended forms (the γ and γT forms) to helix dominative forms (the β and α forms). This conformational transition takes place at m = 3. Being coincident with the transition point, the compounds of C8E3, C12E3, and C16E3 crystallize into either the γ form or the β form depending on the solidification conditions. For the compounds with shorter alkyl chains (n ≤ 4), the molecules take helical conformations and their conformational properties are substantially not affected by the oxyethylene chain length. The conformational behaviour of the CnEm compounds is elucidated by the conformational competition between the alkyl chain which favors the extended structure and the oxyethylene chain which favors the helical structure. The classical model of the meander conformation of the oxyethylene chain is not likely to exist.
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