EFFECT OF COSOLVENT ON THE LATERAL ORDER OF SPONTANEOUSLY FORMED AMPHIPHILIC AMIDE 2-DIMENSIONAL CRYSTALLITES AT THE AIR SOLUTION INTERFACE

At low temperature (5-12-degrees-C), uncompressed films of insoluble a mphiphilic molecules C19H39X, where the head group X contains one (CON H2, 1) or two (CONHC2H4CONH2, 2) amide groups, spontaneously form two- dimensional (2D) crystalline clusters over aqueous subphases containin g soluble amide or carboxylic acid molecules. These crystallites were detected and their structures were studied using grazing incidence X-r ay diffraction (GID). In the presence of subphases containing carboxyl ic acid (RCO2H, R = H, CH2Cl) at sufficiently high concentrations, a l oss of diffraction signal was observed for 1, while amide and less con centrated acid subphases did not show such a destructive effect. The e ffect of the subphase solute molecules was understood in terms of the different ways in which the solutes hydrogen bond to the amide head gr oups of the amphiphiles. Both amide and acid solute molecules can form hydrogen-bonded cyclic dimers with the amide head groups. With an ami de subphase, such dimers lead to an extension of the hydrogen-bonding network of the crystallites, and thus enhance its stability, but acid molecules may also bind to the monolayer at low concentrations with le ss than full occupancy. At high acid concentration, and thus more exte nsive formation of cyclic dimers between carboxylic acid and amphiphil ic amide molecules, repulsive interactions between lone pair electrons on oxygen atoms of bound acid molecules inhibit formation of ordered arrays of these dimers and lead to a lack of diffraction signal. In 2, the second amide group strengthens the crystallites to the extent tha t there is no decrease in crystallinity over a 1 M formic acid subphas e. The shape of the intensity profiles of the Bragg rods and the specu lar X-ray reflectivity measurements of 2 indicate formation of molecul ar trilayers.