Ftir Studies on Phase Transitions of N-Octadecanoyl-L-Alanine and Zinc Octadecanoyl-L-Alaninate Lb Films

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Abstract
The phase transitions of N-octadecanoyl-L-alanine and zinc octadecanoyl-L-alaninate LB films have been investigated through variable temperature FTIR spectroscopy. Hydrogen-bonding interactions between the enantiomeric head-groups strengthens the interactions between the alkyl chains so that the transition temperature of the N-octadecanoyl-L-alanine LB film (Tc = 122 °C) is nearly twice as high as that of the same-chain-length stearic acid LB film (Tc = 65 °C). The intermolecular hydrogen-bonding interactions in the zinc octadecanoyl-L-alaninate LB film are greatly intensified in comparison to that in the N-octadecanoyl-L-alanine LB film; the relatively weak hydrogen-bonding interaction in the latter is easily disrupted around its transition temperature (Tc = 122 °C) together with an abrupt increase of va(CH2) stretching frequency, while the intensified intermolecular hydrogen-bonding interaction in the former weakens step by step with temperature along with a monotonic increase of va(CH2) stretching frequency, the zinc octadecanoyl-L-alaninate LB film thus exhibits a different phase behavior from the pure enantiomer LB film. Therefore, the strength of the intermolecular hydrogen bond is closely related to the interchain interaction and the film phase behavior.