David Darrell Miller

Chemical Mapping of Ceramide Distribution in Sphingomyelin-Rich Domains in Monolayers

The incorporation of ceramide in phase-separated monolayers of ternary lipid mixtures has been studied by a combination of atomic force microscopy (AFM), fluorescence, and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Replacement of a fraction of the sphingomyelin by ceramide in DOPC/SM/cholesterol monolayers leads to changes in the SM-cholesterol-rich liquid-ordered domains. AFM shows the formation of heterogeneous domains with small raised islands that are assigned to a ceramide-rich gel phase. ToF-SIMS provides conclusive evidence for the localization of SM and ceramide in ordered domains and shows that ceramide is heterogeneously distributed in small islands throughout the domains. The results indicate the utility of combining AFM and ToF-SIMS for understanding compositions of phase-separated membranes.

Phase Behavior of Aqueous Mixtures of Anionic and Cationic Surfactants Along a Dilution Path

Because of the strong electrostatic interactions between the oppositely charged head groups present in cationic/anionic surfactant mixtures, the geometric packing ratio can effectively be tailored by varying the ratio of cationic to anionic surfactant. This additional degree of freedom makes available a rich variety of microstructures. Here we explore the behavior of aqueous mixtures of cetyl trimethylammonium tosylate and sodium dodecylbenzene sulfonate along a dilution path with surface tension, conductivity, and quasielastic light scattering measurements. We show a transition in surfactant structure from monomeric surfactant to vesicles to micelles coexisting with vesicles as surfactant concentration increases.

Video- and computer-enhanced light microscopy of three-dimensional colloidal structures

The ability of video- and computer-enhanced microscopy to directly visualize colloidal structures in real time has allowed colloid scientists to discover and characterize many new structures and processes. Low contrast surfactant microstructures are particularly amenable to this technique. Recent applications include the discovery and characterization of vesicles in mixtures of cationic and anionic surfactants, the mapping of the phase behavior of superwetting trisiloxane surfactants, the observation of the effects of counter-ion exchange on the dynamics of vesicle structure, and the characterization of the stability of novel reverse vesicles. The coupling of a video microscope and a digital image processor enables the three-dimensional tracking of colloidal particles. As a result, hydrodynamic radii, particle densities, mobilities, and pair potentials are readily accessible. Image processing techniques also allow us to use colloidal dispersions as simulators of molecular nucleation and growth processes.