STM investigations of three N-alkyl fatty acid amide molecules have been carried out to get information of their molecular arrangement on a highly oriented pyrolytic graphite surface. With variable positions of amide along the alkyl chain, complex lattices with different lattice constants were observed. Besides the lattices with a repeat unit matching one or two molecular lengths, a lattice with a repeat unit corresponding to three molecular lengths was found. In addition, the portion of different lattices depends on the length of the shorter alkyl chain. DFT-D calculations point to interactions of antiparallel oriented dipoles due to the amide group, which are distance dependent and thus larger for shorter N-alkyl chains.

The influence of the urea and amide group in the alkyl chain of methyl nonadecanoate on the surface properties is investigated and compared. For that purpose, the ureas CH3O2C-(CH2)m-NHCONH-(CH2)n-CH3 (n + m = 14) [1 (m = 2), 3 (m = 3), and 5 (m = 4)] and the amides CH3O2C-(CH2)m-NHCO-(CH2)n-CH3 (n + m = 15) [2(m = 2), 4 (m = 3), and 6 (m = 4)] were synthesized. The pi/A isotherms of the ureas show up to the attainable temperature of 313 K no LE phase, which indicates a very stable LC phase. The amides exhibit a two phase plateau region, with the exception of 2. The different behavior is connected with the hydrogen bond energies, which are stronger with the ureas in the LC than in the LE phase, whereas those of the amides have a similar strength in both phases. The effect of hydrogen bonds in self-assembled molecules of N,N'-dialkylurea CH3-(CH2)m-NHCONH-(CH2)n-CH3 (m + n = 14) [7 (n = 2)] was visualized by STM at the octylbenzene/graphite interface. Compound 7 forms a lamella structure with a periodicity of one molecule length. The tilt angle of 86 degrees +/- 2 degrees to the edge of the lamella points to a nearly orthogonal arrangement of the molecules. It indicates two equivalent bonds between the aza-hydrogens and the carbonyl oxygen. A similar arrangement is proposed for the LC phase of the ureas at the air/water interface.

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The influence of a hydrogen bond donor and acceptor in the hydrophobic part of an amphiphile on the monolayer stability at the air/water interface is investigated. For that purpose, the amide group is integrated into the alkyl chain. Eight methyl octadecanoates have been synthesized with the amide group in two orientations and in different positions of the alkyl chain, namely, CH3O2C(CH2)m NHCO(CH2)n CH3 (n + m = 14): 1 (m = 1), 3 (m = 2), 5 (m = 3), 7 (m = 14); and CH3O2C(CH2)m CONH(CH2)n CH3: 2 (m = 1), 4 (m = 2), 6 (m = 3), 8 (m = 14). The monolayers have been characterized by their pi/A isotherms, their temperature dependence and Brewster angle microscopy (BAM). Amphiphile 1 with the amide group close to the ester group (m = 1) behaves like an unsubstituted fatty acid ester, while 3, 5, and 7, with the amide group in an intermediate and terminal position, exhibit a two-phase region. The amphiphiles 2, 4, 6, and 8, with a reversed orientation of the amide group, all exhibit a two-phase region with higher plateau pressures and lower collapse pressures than those of 1, 3, 5, and 7. For 7 and 8, domains of the liquid condensed (LC) phase are visualized by BAM in the two-phase region. The liquid expanded (LE)/LC-phase transitions are all exothermic with enthalpies deltaH ranging from -31 to -12 kJ/mol. Comparison with other bipolar amphiphiles indicates that the LC phase is better stabilized by the hydroxy and dihydroxy groups than by the amide group. For model compounds of 1-4, optimized conformers in the LE and LC phases have been determined by density functional theory (DFT) calculations.