Neutron Reflectometry of Quaternary Gemini Surfactants as a Function of Alkyl Chain Length: Anomalies Arising from Ion Association and Premicellar Aggregation

We have measured the structure and properties of a series of dicationic quaternary ammonium compounds alpha,omega-bis(N-alkyl dimethyl ammonium)hexane halides (C-n-C-6-C-n) for values of the alkyl chain length n of 8, 9, 10, 11, 12, and 16, and a series of a,w-bis(N-alkyl dimethyl ammonium)diethylether halides (C C2OC2-C-n) for values of n of 8, 12, and 16, as well as C-8-C-12-C-8 and C-12-C-10-C-12 at the air/water interface. Although the critical micelle concentration (CMC) in the two series decreases in the normal way, that is, logarithmically, with increasing chain length, the limiting surface tension at the CMC and the limiting area per molecule both increase with chain length, in the opposite direction from comparable single chain surfactants. The structures of the surface layers, which were determined by neutron reflectometry, indicate that the anomalous behavior of the surface tension and area are probably caused by poor packing of the gemini side chains between adjacent molecules. Comparison of the directly determined surface coverage using neutron reflectometry and the apparent coverage determined by application of the Gibbs equation to surface tension data gives an experimental measurement of the prefactor in the Gibbs equation, which should be 3 for these geminis. It was found to vary from about 3 for the two C-16 geminis down to about 1.5 for the two C-8 geminis. We have devised a simple quantitative model that explains this variation and earlier observations that the Gibbs prefactor for C-12-C-10-C-12 (n varying from 3 to 12) is around 2. The model is consistent with the conductivity, NMR, and fluorescence measurements of other authors. This model shows that both dimerization and ion association are required to explain the surface tension behavior of cationic gemini bromide surfactants and that, in many cases, the prefactor itself varies with concentration.