Fixed Water Content Research Articles

A Rapid 3-Day Excipient Screening Methodology and its Application in Identifying Chemical Stabilizers for Solid Formulations with Mixed Mechanisms of Degradation.

This study outlines a practical approach for assessing chemical instability by heating the drug-excipient binary mixtures or multi-excipient formulations at 75°C for 3days before characterization. Differentiating itself from other excipient compatibility methods, our methodology necessitates a saturated aqueous slurry rather than arbitrarily fixed water content. This allows bulk and surface water in the excipient to contribute to drug degradation. The synergistic impact of surface water and elevated temperature expedites degradation kinetics, resulting in accelerated data generation. Among excipient compatibility methods available, our method is quantitative and merges with traditionally used methodologies. The devised nomograph enables extrapolation of shelf life at 20°C from experimental data obtained at 75°C. This methodology also helped identify stabilizers for the drug NVS-1 where traditional excipient compatibility programs had failed. Incorporation of monovalent salts, such as sodium/potassium chloride and sodium bicarbonate at 5% w/w, significantly enhanced the chemical stability of NVS-1, ensuring stable tablet formulations. Our hypothesis posits that stabilization is due to increased ionic strength in the slurry, which stabilizes an induced dipole within the polar NVS-1 drug. Additionally, the presence of ions in the moisture layer is anticipated to stabilize π-π stacking of two planar aromatic NVS-1 molecules. The expedited generation of experimental data allowed the identification of inorganic salts to supplement a standard excipient compatibility screening panel. Considering the economic implications of stability testing methodologies in effort, cost, and duration, a faster turnaround in chemical stability data enhances formulation selection. This ultimately facilitates the development of drug formulations with greater efficiency without delays.

Electrical conductivity of anhydrous and hydrous gabbroic melt under high temperature and high pressure: implications for the high-conductivity anomalies in the mid-ocean ridge region

Abstract. The electrical conductivity of gabbroic melt with four different water contents (i.e., 0 %, 2.59 wt %, 5.92 wt %, and 8.32 wt %) was measured at temperatures of 873–1373 K and pressures of 1.0–3.0 GPa using a YJ-3000t multi-anvil high-pressure apparatus and Solartron-1260 impedance spectroscopy analyzer. At a fixed water content of 2.59 wt %, the electrical conductivity of the sample slightly decreased with increasing pressure in the temperature range of 873–1373 K, and its corresponding activation energy and activation volume were determined as 0.87 ± 0.04 eV and −1.98 ± 0.02 cm3 molec.−1, respectively. Under the certain conditions of 873–1373 K and 1.0 GPa, the electrical conductivity of the gabbroic melts tends to gradually increase with a rise in water content from 0 wt % to 8.32 wt %, and the activation enthalpy decreases from 0.93 to 0.63 eV accordingly. Furthermore, functional relation models for the electrical conductivity of gabbroic melts with variations of temperature, pressure, and water content were constructed at high-temperature and high-pressure conditions. In addition, the dependence relation of the electrical conductivity of melts with the degree of depolymerization was explored under conditions of four different water contents at 1373 K and 1.0 GPa, and three previously available reported results on those of representative calc-alkaline igneous rock melts (i.e., dacitic melt, basaltic melt, and andesitic melt) were compared in detail. In combination with our presently acquired electrical conductivity data on gabbroic melt with four different water contents and the available data on polycrystalline olivine, the electrical conductivity of a gabbroic melt–olivine system with variation of the volume percentage of anhydrous and hydrous melts was successfully constructed by using the typical Hashin–Shtrikman upper-bound model. In light of the electrical conductivity of the gabbroic melt–olivine system with previous magnetotelluric (MT) results, we find that anhydrous and hydrous gabbroic melts can be employed to reasonably interpret the high-conductivity anomalies in the Mohns Ridge of the Arctic Ocean.

Sonoprocessing of oil: Asphaltene declustering behind fine ultrasonic emulsions

Despite the transition toward carbon-free energy carriers, liquid fossil fuels are expected to occupy an important market share in the future. Therefore, it is crucial to develop innovative technology for better combustion reducing the emissions of pollutants associated with their utilization. Water in oil (w/o) emulsions contribute to greener combustion, increasing carbon efficiency and reducing emissions. Water content, emulsions stability, and droplet size distributions are key parameters in targeting the efficient use of emulsions as combustibles. In particular, for fixed water content, the finer the emulsion, the better its beneficial effect on combustion. In this work, two emulsions, mechanically and ultrasonically generated, were compared. Cryogenic scanning electron microscopy (cryo-SEM) allowed the visualization of water droplets inside the oily matrix. No surfactants were added to the oil, due to its high asphaltenic content. Asphaltene molecular aggregates, namely clusters, act as natural surfactants stabilizing the emulsions by arranging at w/o interface and forming a rigid film. The asphaltenic rigid film is clearly visualized in this work and compared for the two emulsions. The results showed finer water droplets in the ultrasonically generated emulsion, together with a reduction in the thickness of the asphaltenic film. Ultrasonically induced cavitation favored the de-clustering (breakage of intermolecular forces) of asphaltene molecules. Thus, smaller clusters allowed to stabilize smaller water droplets resulting in an ultra-fine emulsion, which improves the combustion performances of the fuel.

Water-mediated adhesion of oil sands on solid surfaces at low temperature

Frozen oil sands adhere to the walls of mining fleets in winter, which is an undesirable phenomenon that reduces the delivery capacity of vehicles and wastes fuel for in-land transportation. For automated surface mining, an effective and scalable approach is needed to replace the current method for removing adhered oil sands by shovelling or steam cleaning. In this work, we identify ice formation at the interface of oil sands and the different substrates (steel, rubber, and bitumen/asphaltene coated steel) as the primary reason for the observed high adhesion strength of more than 1,000 kPa; in absence of ice formation, the adhesion strength of oil sand is smaller than 2 kPa. At temperature of −2.5 °C to −20 °C, the adhesion strength was measured by a force apparatus with thermal control as water content in oil sands matrix was varied from 4 to 14 wt%. The adhesion strength was found to increase linearly with water content. At a fixed water content, the adhesion strength was stronger at a lower temperature after a short freezing time of 5 min. The relationship between water content and the adhesion strength was rationalized by a theoretical model, based on the contact area between ice and the substrate. X-ray micro-computed tomography confirmed that water formed more capillary bridges with the substrate at a higher water content. To effectively reduce the adhesion of oil sands on the substrate, a method was proposed based on anti-icing by spraying a small amount of anti-freezing liquid (0.3 L/m2) on the substrate, which effectively reduced the adhesion strength to <2 kPa. The insights and proposed anti-freezing approach in this study may be applied to reducing adhesion of not only frozen oil sands, but wet granular materials in general.

Gypsum Mortars with Acacia dealbata Biomass Waste Additions: Effect of Different Fractions and Contents

In recent decades, interest in the eco-efficiency of building materials has led to numerous research projects focused on the replacement of raw materials with mineral and biomass wastes, and on the production of mortars with low-energy-consuming binders, such as gypsum. In this context, five different fractions (bark, wood, branchlets, leaves, and flowers) of Acacia dealbata—an invasive species—were evaluated as fillers for premixed gypsum mortars, at 5% and 10% (vol.) addition levels and fixed water content. Although these biomass fractions had different bulk densities (&gt;50% of variation), all the mortars were workable, although presenting different consistencies. As expected, dry density decreased with biomass addition, but, while mortars with addition at 5% presented a slight shrinkage, a slight expansion occurred with those with 10% addition. Generally, the mechanical properties decreased with the biomass additions even if this was not always proportional to the added content. The wood fraction showed the most positive mechanical results but flexural and compressive strengths of all the tested mortars were found to be higher than the lower standard limit, justifying further studies.

Influence of fixed charge concentration and water uptake on ion sorption in AMPS/PEGDA membranes

Water content and charge density strongly influence ion sorption in charged membranes. However, isolating the individual effect of each parameter on ion sorption can be difficult due to experimental challenges associated with independently controlling these two variables during membrane synthesis. This frustrates development and validation of fundamental models and structure/property relationships. In this study, three series of uncharged and negatively charged hydrogel membranes were prepared using poly(ethylene glycol) diacrylate (PEGDA) as a cross-linker and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as an ionic monomer. Water content and charge density were controlled by varying AMPS content and PEGDA chain length. Ion sorption in these materials was measured using aqueous NaCl solutions at concentrations ranging from 0.01 to 1.0 M. At constant charge density, salt sorption increased as water content increased. At fixed water content, salt sorption decreased as charge density increased. Salt sorption in membranes with the highest water content and charge density was predicted using the Donnan/Manning model, which accounts for solution and membrane phase thermodynamic non-idealities. However, this approach could not predict sorption in weakly charged and uncharged membranes, presumably due to additional membrane phase thermodynamic non-idealities associated with ion solvation and uncharged polymer segments. • High ion activity coefficients in uncharged membranes suggest ions have a much greater affinity for aqueous solution than swollen, uncharged membranes. • The Donnan/Manning model predicts experimental sorption well for highly hydrated and/or highly charged membranes. • Model predictions agree poorly with experimental data for membranes that are less hydrated, weakly charged, or uncharged due to phenomena not accounted for in Manning's theory (e.g., dielectric exclusion and non-idealities introduced by polymer segment-water-ion interactions).

Hydrothermal liquefaction of Prosopis juliflora biomass for the production of ferulic acid and bio-oil

The objective of this work was to study the hydrothermal liquefaction (HTL) of Prosopis juliflora biomass for the production of ferulic acid and bio-oil. Biomass was processed with various solvents (NaOH, KOH, HCl and H2SO4) to produce ferulic acid (FA). FA oxidation was carried out using the Nano ZnO catalyst to produce an optimum vanillin yield of 0.3 g at 70 °C with 0.4% catalyst loading for a time of 60 min. The spent solid residue was then processed using HTL at 5 MPa pressure and a temperature range of 240–340 °C. Various biomass loading (2.5 g to 12.5 g) was taken for a fixed water content of 200 mL. Bio-oil optimum yield was 22.5 wt% for 10 g/200 mL of biomass loading ratio. The optimum temperature was 300 °C for a processing time of 1 h. The catalyst showed the reusable capability of two three consecutive cycles.

Study on the Dynamic Constitutive Relationship of EPS Particles Light Weight Soil Based on Hardin–Drnevich Model

In order to research the dynamic stress–strain relationship of EPS (expanded polystyrene) particles light weight soil, under the condition of fixed water content and fixed curing age, the dynamic deformation properties of light weight soil with different mixing ratios are researched by dynamic triaxial tests. The results show that at the same cycle number and dynamic stress, the dynamic strain of light weight soil decreases with the increasing of cement content and the decreasing of EPS particles content. The dynamic bearing capacity of EPS particles light weight soil with cement content in the range of 10–20% and EPS particles content in the range of 0.14–0.86% is 1.5–3 times that of remolded soil. It indicates that light weight soil possesses obvious dynamic bearing capacity. In addition, when the vibration frequency increases from 0.5 to 6 Hz, the bigger the loading rate is, the more uneven the distribution of the pore water pressure in the soil is, and the less deformation can occur, which means that there is a greater constraint on the development of pore water pressure and deformation. Thus at the same dynamic strain, the dynamic stress of light weight soil under bigger vibration frequency is greater. The shape of dynamic backbone curves of light weight soil conforms to hyperbola, and its nonlinear dynamic response process can be described by Hardin–Drnevich model. Different stress state tests are used to verify the applicability of Hardin–Drnevich model. It is found that the relative errors between the measured values and calculated values of backbone curves are less than 15% by analysing the test data. It shows that Hardin–Drnevich model can be used to describe the dynamic deformation characteristics of EPS particles light weight soil.

Tailoring the Properties of Calcium Aluminate Macro-Defect-Free Cements: From Brittle to Ductile Behavior

Cementitious macro-defect-free (MDF) materials can be obtained processing a mixture of a water-soluble polymer, namely polyvinyl alcohol (PVA) and the calcium aluminate cement. Based on process techniques typical of plastics, the MDF products can be obtained in complex forms and, thanks to the reduced water to cement ratio, w/c, their strength is very high compared to standard cements. However, these materials show both the sensitivity typical of water-soluble polymers, presenting a low glass transition temperature (Tg), and the brittle behavior typical of ceramic materials and therefore low toughness. The aim of this work is to modify the mechanical properties of MDF cements by tuning their unacceptable brittle behavior. Water solution of carboxylate styrene butadiene rubber is used as reactive additives, and different mixture formulations are tested changing the rubber-like content both with respect to the fixed water content and PVA amounts. It is shown that the modifications of the organic phase imply small or negligible effects on paste formation of different recipes as detected by the torque–time responses. From flexural and fracture toughness tests, a wide range of responses was observed, and depending on the particular recipe, the mechanical behavior can be tuned from a brittle to a ductile behavior.

Anisotropic high-temperature creep in hydrous olivine single crystals and its geodynamic implications

We compile the experimental data on deformation of hydrated olivine single crystal to determine the influence of water and pressure. We analyse the data from low pressure (P = 0.1 MPa and 0.1–0.3 GPa) high-resolution experiments as well as the results from higher pressures (2 to 6 GPa) containing larger uncertainties, using a flow law of the form ε̇i=ε̇idry+ε̇iwet with (i:slip system). The data are normalized to a common stress (150 MPa) and temperature (1573 K) and we determine the parameters water content exponent (r) and activation volume (V∗). We found largely different values of r and V∗ for different orientations (slip systems) implying that the influence of water and pressure is highly anisotropic providing largely different values of r and V∗. For the [110]c orientation where the [100](010) slip system is activated, we obtain r[100] = 0.35 ±0.08, and V∗[100] = 11.0 ± 3.0 cm3/mol, while for the [011]c orientation where the [001](010) slip system is activated, r[001] = 1.3 ± 0.30, and V∗[001] = 5.6 ± 3.6 cm3/mol (for a fixed water content). The highly anisotropic effects of water and pressure suggest that creep in olivine is not controlled solely by diffusion but also controlled by the density of jogs (or kinks) on dislocations that is controlled by water and pressure. We find that the easier slip system changes from the a-slip (slip along the [100] direction) at low water content and low pressure to the c-slip (slip along the [001] direction) at high water content and high pressure, suggesting that the c-slip is softer than the a-slip in most of the asthenosphere, whereas the opposite is true in most of the lithosphere. Implications of these results are discussed using a model of deformation of a polycrystalline aggregate made of anisotropic crystals. It is suggested that in the asthenosphere where most of plastic deformation occurs, short-term time-dependent small strain deformation associated with post-seismic and post-glacial deformation is dominated by the c-slip and strongly dependent on the water content, whereas the long-term large strain deformation associated with mantle convection is dominated by the a-slip that is only weakly dependent on water content.

Influence of geopolymer formulation parameters on the elastic and porous properties over a one-year monitoring

The effects of two formulation parameters, by changing the Si/Al molar ratio and the initial water content, were studied to understand their impacts on the aging of the metakaolin-based geopolymer. Five formulations were designed and resulting samples were conditioned under 100% relative humidity. Long-term mechanical characterizations (up to 360days) were conducted to measure the Young modulus and the compressive strength by ultrasound and compression tests, respectively. As well, porosimetry was performed using gravimetric technique (at 28-day term) and nitrogen sorption technique (up to 360days). At fixed Si/Al ratio when increasing the initial water content, results showed a decrease of mechanical parameters and the formation of larger porous network with an increasing effective pore diameter (from 5.3 to 11.1nm). Conversely, at fixed water content when Si/Al ratio increased, porous network characteristics were fairly identical with employed formulations, and mechanical parameters slightly increased. Also, during the seven first days, the mechanical parameter growth became slower when increasing Si/Al ratio.

Strength development of mortars using a magnesium silicate hydrate binder system under different curing conditions

A magnesium silicate hydrate (M-S-H) binder system was studied to develop low CO2 emission cementitious materials. The binder system contained 60% magnesium oxide and 40% silica fume. M-S-H mortar mixtures were prepared with a fixed water content of w/c=0.40, compared to a control mix using Portland cement (PC). A polymer-based superplasticizer was used to improve the workability of the M-S-H mixtures. Samples were cured at different moist and curing temperatures for compressive strength tests performed at 7, 28 and 90 days. It is found that curing conditions have a significant influence on the strength development of mortar mixtures using M-S-H binder systems. Ambient conditions increase early strength for M-S-H binders, however, reduces long-term strength over 90 days. This curing regime surprisingly results in higher strengths at 28 and 90 days age for M-S-H samples compared to PC. Heated treatment increases early strength, however, decreases the long-term strength of M-S-H samples over 90 days curing period. M-S-H binders have slow strength development compared to PC. The highest compressive strengths of M-S-H samples cured in different regimes at 7, 28, 90 days were 50, 53 and 71 MPa while PC samples cured in water at 21oC at these ages had higher strengths of 55, 75, and 86 MPa, respectively.

Ultrasonic investigation of the effects of composition on the volume fraction of bubbles and changes in their relative sizes in non-yeasted gluten-starch blend doughs

Interactions between water, gluten and starch during dough mixing alter the aeration properties of dough. Effects of composition on dough gas volume fraction and relative changes in bubble sizes of non-yeasted gluten-starch (G-S) blend doughs were investigated using density measurements and an ultrasonic transmission technique, respectively. At fixed water content, greater gluten content increased the air volume fraction, while frequency-dependent ultrasonic attenuation coefficient and phase velocity measurements indicated that the bubble sizes in the G-S doughs were larger. The latter outcome may be due to mixing to optimal conditions such that shorter mixing times for doughs of high gluten content lessened the number of bubble subdivision events during mixing. The effect of increased water content on the attenuation coefficient implied a decrease in mean bubble radius as elucidated using an ultrasonic model. Time evolutions of attenuation coefficient and phase velocity for G-S blend doughs had a similar trend to those of non-yeasted wheat flour doughs. However, the shifts in the frequency of the peaks observed in the ultrasonic parameters were noticeably slower for G-S blend doughs, implying that G-S blend doughs were more stable against disproportionation.

Preparation of latex nanoparticles using nanoemulsions obtained by the phase inversion composition (PIC) method and their application in textile printing

Abstract The low energy phase inversion composition (PIC) method was used for the formation of monomer based nanoemulsions from a system composed of monomer/Brij78/water. These nanoemulsions were used as templates for generating polymer and copolymer nanoparticles. Styrene was used as monomer for studying the phase diagram, where parameters such as the O/S ratio and water addition rate influence the formation of the nanoemulsion and the structure of the final polymer particles. Hexadecane was added to the oil phase in order to achieve stability against Oswald ripening. Nanoemulsions with a fixed water content of 80 wt% and O/S ratios from 0.25 to 9 generated polymer particles with diameters between 30 and 110 nm as determined from dynamic light scattering (DLS) and transmission electron microscopy (TEM). The polydispersity of the polymer nanoparticles was found to depend on the O/S ratio. Different copolymer latexes based on styrene, butyl acrylate and methyl methacrylate were prepared with a fixed O/S ratio 9 and water content 80% and applied successfully as binders for the inkjet printing of cotton fabrics and compared to corresponding copolymer latexes prepared by the high energy methods or the conventional methods.

Droplet-droplet interactions investigated using a combination of electrochemical and dynamic light scattering techniques. The case of water/BHDC/benzene:n-heptane system.

In this contribution the electrochemistry of [Fe(CN)6](4-/3-) as the probe molecule was investigated in benzyl-n-hexadecyldimethylammonium chloride (BHDC) reverse micelles (RMs) varying the composition of the external solvent (benzene:n-heptane mixtures) and the surfactant concentration, at a fixed water content and probe concentration. The electrochemical and dynamic light scattering results show that in water/BHDC/benzene:n-heptane systems the aggregate sizes increase on increasing BHDC concentration. This behavior was unexpected since it is known that for water/BHDC/benzene RM systems keeping the water content constant and the surfactant concentration below 0.2 M, the droplet sizes are independent of the concentration of the surfactant. We explain the results considering that on changing the external solvent to benzene:n-heptane mixtures, RMs tend to associate in clusters and equilibrium between free RMs and droplet clusters is established. A model is presented which, using electrochemical and dynamic light scattering data, allows calculating the aggregation number of the RMs, the number of RMs that form the droplet clusters and the standard electron transfer heterogeneous rate constant.

Reversible water weakening of quartz

We have performed looped pressure-stepping deformation experiments at the same temperature (800°C) and strain rate (2×10−6s−1) on a milky quartz crystal and a quartzite that deform by dislocation creep. Our results demonstrate that flow strengths at fixed water content depend on fH2O, and that the dependence is reversible. Thus, rheology of quartzite can be predicted if fH2O is known. In addition, the effect of fH2O on strength of quartzite is greater than that of the milky quartz single crystal. The differing sensitivities are due to differing effects that water has on deformation and recovery within grain interiors and at (or near) grain boundaries. These results indicate that, as temperature increases with depth and the dominant recovery mechanism of dislocation creep changes from recrystallization at grain boundaries to intragranular recovery, dependence of quartz creep on fH2O may also decrease.

Interfacial composition and characterization of a quaternary water-in-oil mixed surfactant (cationic of different alkyl chain lengths + polyoxyethylene type nonionic) microemulsions in absence and presence of inorganic salts

In this contribution, phase behavior and the Schulman's cosurfactant titration of a quaternary water-in-oil microemulsion, formed from equimolar cationic [alkyltrimethylammonium bromide (CnTAB)] and nonionic [polyoxyethylene (20) cetyl ether (C16E20)] surfactants, n-pentanol (Pn) and n-heptane or isopropyl myristate, have been studied along with the variation in alkyl chain length of the cationic surfactant, Cn (n=12, 14, 16 and 18) at a fixed water content (ω=25) and temperature (303K). The synergies in single-phase microemulsion zone, interfacial composition and the spontaneity of formation of mixed surfactant microemulsions have been observed with increase in Cn (n=12→18). Further, the effect of the variation of Cn (n=12→18) on the interface vis-à-vis interdroplet interaction, morphology and the dynamics of confined water of these systems have been examined by means of viscosity, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) measurements. The increase in the Cn (n=12→18) leads to shrink the droplet size as well as increase in the droplet numbers. Subsequently, the change in droplet size affects the states of water organization (bulk and bound) inside the pool. Additionally, the influence of different inorganic salts [normal salt (NaCl), precursor salt for nanoparticle synthesis (NiCl2, FeCl3), water structure affective salt (KF, KI)] on the transfer process of Pn (oil→ interface) at equimolar composition (of both surfactants) has been investigated. FTIR measurement (via D2O probing) indicates that the presence of salts significantly modifies the water structure in confined environment.

Effect of water and guar gum content on thermal properties of chestnut flour and its starch

Thermal properties of chestnut flour and chestnut starch at several water content (40, 50, 60 and 95%, flour basis, f.b.) as well as the influence of guar gum (0.5, 1.0, 1.5 and 2.0%, f.b.) on both raw materials at fixed water content (50%, f.b.) were determined by differential scanning calorimetry (DSC). Thermal properties of guar gum–water systems at several guar content (0.5, 1.0, 1.5 and 2.0%, w/w) were also obtained by DSC. Results indicated that the water content and the presence of guar gum had a significant impact on the thermal properties of chestnut flour and its starch. For each endothermic curve, the values of onset (To), peaks (Tp1, Tp2) and final (T1) temperatures decreased linearly with increasing water content. Experimental data were successfully (R2 > 0.997) described following the Flory equation. A reverse trend was observed in the enthalpy values. Thermal properties of chestnut flour and chestnut starch were suppressed by the presence of guar gum even at the lowest concentrations employed (0.5%, f.b.). The guar gum addition to the assayed systems promoted a starch gelatinization delay and the enthalpy values showed a threshold content above 1.0% of guar gum. Analyses of aqueous guar gum mixtures showed that the existence of a second transition in chestnut starch systems can be successfully explained by means of hydrocolloid–starch interactions, whereas in the flour other interactions should be taken into account.

Rheological characteristics of debris-flow material in South-Gargano watersheds

In recent years, debris flows have represented a severe natural hazard in South-Gargano watersheds (Puglia Region, Southern Italy). Hill slopes erosion, caused by the inadequate protection of the degraded forests, produces large amounts of soil and debris that are conveyed downstream during heavy rainstorms. The involved material is characterized by limestone fragments and blocks in a sandy-silt matrix. In this paper, the rheological properties of such debris-flow materials have been investigated. Eight specimens of particulated sediments of particle diameter of d ≤ 4 mm have been analysed using a rheometric tool for large particle suspensions, the ball measuring system (BMS). The influence of sediment concentration on the bulk rheological behaviour has been evaluated at concentrations by volume obtained in fully water-saturated conditions (volumetric sediment concentration C V ~ 0.8) and with fixed water content (C V = 0.42). The rheological data were fitted to the Bingham, Herschel–Bulkley, and O’Brien and Julien models to define viscosity and yield stress dependency on sediment concentration, to provide viscosity information for hazard mitigation as well as for comparison to other debris-flow events.

The Influence of Bilayer Composition on the Gel to Liquid Crystalline Transition

We report molecular dynamics simulations of bilayers using a united atom model with explicit solvent molecules. The bilayer consists of the single tail cationic surfactant behenyl trimethyl ammonium chloride (BTMAC) with stearyl alcohol (SA) as the cosurfactant. We study the gel to liquid crystalline transitions in the bilayer by varying the amount of water at fixed BTMAC to SA ratio as well as by varying the BTMAC to SA ratio at fixed water content. The bilayer is found to exist in the tilted, L(beta') phase at low temperatures, and for the compositions investigated in this study, the L(beta') to L(alpha) melting transition occurred in the temperature range 330-338 K. For the highest BTMAC to SA composition (2:3 molar ratio), a diffuse headgroup-water interface is observed at lower temperatures, and an increase in the d-spacing occurs prior to the melting transition. This pretransition swelling is accompanied by a sharpening in the water density variation across the headgroup region of the bilayer. Signatures of this swelling effect which can be observed in the alkane density distributions, area per headgroup, and membrane thickness are attributed to the hydrophobic effect. At a fixed bilayer composition, the transition temperature (>338 K) from the L(beta') to L(alpha) transition obtained for the high water content bilayer (80 wt %) is similar to that obtained with low water content (54.3 wt %), confirming that the melting transition at these water contents is dominated by chain melting.