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 °C), uncompressed films of insoluble amphiphilic molecules C₁₃H₃₉X, where the head group X contains one (CONH₂, 1) or two (CONHC₂H₄CONH₂, 2) amide groups, spontaneously form two-dimensional (2D) crystalline clusters over aqueous subphases containing soluble amide or carboxylic acid molecules. These crystallites were detected and their structures were studied using grazing incidence X-ray diffraction (GID). In the presence of subphases containing carboxylic acid (RCO₂H, R = H, CH₂Cl) at sufficiently high concentrations, a loss of diffraction signal was observed for 1, while amide and less concentrated acid subphases did not show such a destructive effect. The effect of the subphase solute molecules was understood in terms of the different ways in which the solutes hydrogen bond to the amide head groups of the amphiphiles. Both amide and acid solute molecules can form hydrogen-bonded cyclic dimers with the amide head groups. With an amide 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 less than full occupancy. At high acid concentration, and thus more extensive formation of cyclic dimers between carboxylic acid and amphiphilic 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 that there is no decrease in crystallinity over a 1 M formic acid subphase. The shape of the intensity profiles of the Bragg rods and the specular X-ray reflectivity measurements of 2 indicate formation of molecular trilayers.