SANS From Tetradecylpyridinium Bromide Based Microemulsions

Small-angle neutron scattering is used to investigate tetradecyl pyridinium bromide/pentanol/heptane/heavy water microemulsions in the water-in-oil (W/O) microemulsion phase diagram region. The heavy water content is increased while the other components (surfactant, cosurfactant, hydrocarbon) are kept constant. With use of a simple model assuming spherical micelles interacting with a hard-sphere potential (Percus-Yevick model), aggregate sizes and packing fractions have been extracted and found to agree with values determined from the mixing conditions. These experiments clearly show the transition from the single-particle (heavy water + Stern layer droplet) scattering regime at low water concentration to the mixed single/interdroplet scattering regime when the intermicellar distance becomes comparable to the size of the micelles. © 1990 American Chemical Society

Calorimetric Observations of the Transition of Spherical to Rodlike Micelles with Solubilized Organic Additives

Incremental calorimetric titration of pentanol into aqueous solutions containing 3.5% cationic (tetradecyltrimethylammonium bromide, TTMAB, in 0.05 M NaBr) or anionic (sodium dodecyl sulfate, SDS, in 0.28M NaCl) surfactant and solubilized additives induced the transformation of spherical to rodlike micelles. The effect of the additive on the transition has been quantified as the change in the concentration of pentanol required to cause the transition divided by the concentration of additive, with a negative value indicating a cooperative effect of promoting the transition and a positive value indicating a retarding effect. Aliphatic hydrocarbon additives retarded the transformation for both surfactants, with the exception of a small cooperative effect for cyclohexane on TTMAB. Alkylbenzenes promoted the transition for TTMAB, as did benzene, toluene, and ethylbenzene for SDS, but the propyl and butyl derivatives showed the opposite effect. © 1993 American Chemical Society

Dynamic Behavior in Isotropic Regions of Tetradecylpyridinium Bromide/pentanol/heptane/water System

Self-diffusion coefficients from pulsed-gradient spin-echo NMR are reported for four components of the tetradecylpyridinium bromide-85% heptane 15% pentanol-water pseudoternary system. Measurements were taken throughout the inverted microemulsion region and also in a small isotropic region beyond the domain of lamellar liquid crystals. Boundaries for the smaller isotropic region were also delineated. Observations of the self-diffusion coefficients for water relative to those of the surfactant, oil, and alcohol show several distinct structural transitions within the water-in-oil region of the phase diagram. The smaller isotropic region exhibits a complete inversion of phase relative to the water-in-oil region. Conductivity measurements were used to further clarify the NMR data. © 1990

Hall-effect mobility for a selection of natural and synthetic 2D semiconductor crystals

We present a DC-AC Hall-effect analysis on transition-metal-dichalcogenides comprising natural crystals of molybdenum disulfide and tungsten diselenide; and synthetic crystals of hafnium diselenide, molybdenum ditelluride, molybdenum diselenide and niobium-doped molybdenum disulfide. We observe a wide range of Hall mobility and carrier concentration values with either a net electron or hole carrier type. The synthetic niobium-doped molybdenum disulfide crystal exhibits a net hole carrier type and a carrier concentration approximately two orders of magnitude higher than a non-intentionally doped natural molybdenum disulfide crystal, with an equivalent reduction in Hall mobility. This synthetic niobium-doped molybdenum disulfide crystal also shows a significantly reduced resistivity when compared to the other crystals. Secondary ion mass spectrometry shows higher counts of niobium in the intentionally-doped synthetic niobium-molybdenum disulfide crystal, in addition to various other high contamination counts in both the natural and synthetic molybdenum disulfide crystals, correlating well with the significant range of high resistivity observed. Compared to silicon, the resistivity in these contaminated TMD materials reduces less rapidly with increasing equivalent carrier concentration levels, and the resistivity is higher in magnitude by a factor of approximately 4-10 when compared to silicon, which in turn reduces the achievable Hall mobility by at least a similar factor. It is therefore suggested that more controlled growth methods of TMD materials which lead to significantly reduced contamination elements and levels, with improved stoichiometry, could potentially provide a significant increase in Hall mobility assuming no change in carrier properties