A surface aggregate phase transition is described. Atomic force microscopy has been used to image the equilibrium association of sodium dodecyl sulfate (SDS) and 1-dodecanol molecules at the interface between graphite and aqueous solutions. In pure SDS solutions, the molecules associate into long, parallel hemicylindrical surface aggregates over a concentration range from about one-third to at least 10 times the critical micelle concentration (cmc). Above the cmc, dodecanol has little influence on the surface aggregate structure, probably because dodecanol is partitioned into the bulk micelles. Below the cmc, dodecanol causes a transition from hemicylindrical aggregates to a two-phase mixture in which flat sheets coexist with swollen hemicylindrical aggregates. In this mixture, the hemicylindrical aggregates are preferentially located at and parallel to steps on the underlying graphite substrate. Under conditions where hemicylinders and flat sheets coexist, an increase in bulk dodecanol concentration results in an increase in surface coverage by flat sheets. No bulk solution changes were detected by NMR in the region where the surface phase transition was observed.

Substitution of a triethyl for a trimethyl headgroup in alkyltrimethylammonium or (ω-hydroxyalkyl)trimethylammonium bromide surfactants leads to a significant reduction in the Krafft temperature. This substitution allowed us to produce asymmetric bolaform surfactants that form micellar solutions at room temperature. It is reasoned that the proximity of the bromide counterion to the quaternary ammonium center stabilizes the hydrated crystal of the conventional and bolaform surfactants through Coulombic interactions. The bolaform surfactants experience further stabilization as a result of bonding between the hydroxyl functionality and the quaternary nitrogen and bromide. Substitution of ethyl for methyl groups in the headgroup increases the ion−ion and ion−dipole distances, lowering the stability of the hydrated crystal, which in turn lowers the Krafft temperature.

The adsorbed layer structure of tetradecylpyridinium, hexadecylpyridinium, heptadecafluorodecylpyridinium, and tetradecyltriethylammonium chloride and their mixtures on mica has been determined by AFM imaging. In addition, the composition of the mixed adsorbed layers has been measured, showing a significant surface enrichment of the pyridinium surfactants, particularly the partially fluorinated species. Shape transitions in the adsorbed layer are correlated with surface and bulk compositions and explained by consideration of the adsorption mechanism.

ω-Hydroxy quaternary ammonium bolaform surfactants form much smaller micelles in solution than the corresponding non-hydroxy (conventional) surfactants. Aggregation numbers for micelles formed by the ω-hydroxy surfactants, determined using 1 H NMR spectroscopy and steady-state fluorescence quenching, are very small (N ) 5-18). The micelle size is consistent with aggregates of about half the diameter of a conventional surfactant. This implies that both the quaternary ammonium and the terminal hydroxyl are positioned at the micelle/water interface. The critical micelle concentrations of the bolaform surfactants, determined using 1 H NMR and the conductivity method, are between 3 and 9 times greater than those of the corresponding non-hydroxy surfactants.