Investigations of amino acid-based surfactants at liquid interfaces

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Title: Investigations of amino acid-based surfactants at liquid interfaces
Author: Yang, Dengliang
Abstract: Herein are presented collective studies of amino acid -based surfactants , also known as lipoamino acids , at liquid interfaces . Chapter III describes an investigation of domain morphology of N -Stearoylglutamic acid (N -SGA ) Langmuir monolayers at the air /water interface by epifluorescence microscopy . Anisotropic feather -like domains were observed in L -enantiomeric monolayers while symmetric circular domains were found in racemic N -SGA monolayers . At a surface pressure of 30 mN /m the enantiomeric domains melted at 31 ? ?C while the racemic domains melted at 27 ? ?C . This result is exactly opposite to the behavior found in bulk crystals where the racemate melts at a higher temperature . These results were explained in terms of different molecular packing and hydrogen bonding between bulk crystals and two -dimensional thin films for enantiomeric and racemic compounds . Chapter IV summarizes the investigations of hydrogen bonding in N -acyl amino acid monolayers by vibrational sum -frequency spectroscopy (VSFS ) . The intermolecular hydrogen bonding interaction between the adjacent molecules through amide -amide groups in N -stearoylalanine (N -SA ) is characterized by an NH stretch peak at 3311 cm -1 . This is the first time that the amide NH stretching signals have been detected with the VSFS technique . A similar peak was detected at 3341 cm -1on N -SGA monolayer . The higher frequency indicates that the H -bond strength is weaker due to the larger size of the glutamic acid residue . The NH stretch mode can thus be used as a fingerprint of hydrogen bonding among amide -amide groups . A peak at 3050 cm -1 due to hydrogen bonding among carboxyl groups was also resolved from the VSFS spectra . Molecular models of intermolecular hydrogen bonding were proposed .
URI: http : / /hdl .handle .net /1969 .1 /2689
Date: 2005-11-01

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Investigations of amino acid-based surfactants at liquid interfaces. Available electronically from http : / /hdl .handle .net /1969 .1 /2689 .

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