Diffusion of surfactant from a micellar solution to a bare interface. 1. Absorbing boundary.

We analyze dynamic adsorption of surfactant from a micellar solution to a rapidly created surface that acts as an absorbing boundary for surfactant monomers (single molecules), along which the monomer concentration vanishes, with no direct micelle adsorption. This somewhat idealized situation is analyzed as a prototype for situations in which strong suppression of monomer concentration accelerates micelle dissociation, and will be used as a starting point for analysis of more realistic boundary conditions in subsequent work. We present scaling arguments and approximate models for particular time and parameter regimes and compare the resulting predictions to numerical simulations of the reaction-diffusion equations for a polydisperse system containing surfactant monomers and clusters of arbitrary aggregation number.

Nonlinear dynamics in micellar surfactant solutions. I. Kinetics.

This is the first of a pair of articles that present the theory of kinetic and transport phenomena in micelle-forming surfactant solutions in a form that facilitates discussion of large deviations from equilibrium. Our goal is to construct approximate but robust reduced models for both homogeneous and inhomogeneous systems as differential equations for unimer concentration c_{1}, micelle number concentration c_{m}, average micelle aggregation number q and (optionally) aggregation number variance σ_{m}^{2}. This first article discusses kinetics in homogeneous solutions.

Nonlinear dynamics in micellar surfactant solutions. II. Diffusion.

We discuss diffusion in micellar surfactant solutions in a form appropriate for analyzing experiments that involve large deviations from equilibrium. A general nonlinear dynamical model for inhomogeneous systems is developed that describes the effects of diffusion and micelle kinetics as a set of coupled partial differential equations for unimer concentration, micelle number concentration, average micelle aggregation number, and, optionally, the variance of the micelle aggregation number. More specialized models are developed to describe slow dynamics in situations in which the system stays in a state of partial local equilibrium or full local equilibrium.

Using Microemulsion Phase Behavior as a Predictive Model for Lecithin-Tween 80 Marine Oil Dispersant Effectiveness.

Marine oil dispersants typically contain blends of surfactants dissolved in solvents. When introduced to the crude oil-seawater interface, dispersants facilitate the breakup of crude oil into droplets that can disperse in the water column. Recently, questions about the environmental persistence and toxicity of commercial dispersants have led to the development of "greener" dispersants consisting solely of food-grade surfactants such as l-α-phosphatidylcholine (lecithin, L) and polyoxyethylenated sorbitan monooleate (Tween 80, T).

Simulation of diblock copolymer surfactants. III. Equilibrium interfacial adsorption.

Monte Carlo simulations are used to study adsorption of highly asymmetric diblock copolymers to a polymer-polymer interface, and the results compared to self-consistent field theory (SCFT) predictions. The simulation model used here is a bead-spring model that has been used previously to study equilibrium and kinetic properties of spherical micelles [J. A.

Does the Solvent in a Dispersant Impact the Efficiency of Crude-Oil Dispersion?

Dispersants, used in the mitigation of oil spills, are mixtures of amphiphilic molecules (surfactants) dissolved in a solvent. The recent large-scale use of dispersants has raised environmental concerns regarding the safety of these materials. In response to these concerns, our lab has developed a class of eco-friendly dispersants based on blends of the food-grade surfactants, soy lecithin (L) and Tween 80 (T), in a solvent.

Simulation of diblock copolymer surfactants. I. Micelle free energies.

Semigrand hybrid Monte Carlo simulations are used to measure equilibrium properties of micelles formed in a simple bead-spring model of asymmetric A-B diblock copolymer surfactant molecules in an A homopolymer solvent, over a range of values of surfactant solubility. Simulations are used to accurately measure the free energy of formation of micellar clusters as a function of aggregation number over a wide range of values, and to characterize the crossover from spherical to rodlike micelle shape with increasing aggregation number. Dynamical properties of the same model are considered in an accompanying paper [Phys.

Simulation of diblock copolymer surfactants. II. Micelle kinetics.

Molecular dynamics (MD) simulations are used to measure dynamical properties of a simple bead-spring model of A-B diblock copolymer molecules, and to characterize rates and mechanisms of several dynamical processes. Dynamical properties are analyzed within the context of a kinetic population model that allows for both stepwise insertion and expulsion of individual free molecules and occasional fission and fusion of micelles. Kinetic coefficients for stepwise processes and micelle fission have been extracted from MD simulations of individual micelles.

Mechanism of Micelle Birth and Death.

In micellar surfactant solutions, changes in the total number of micelles are rare events that can occur by either of two mechanisms-by stepwise association and dissociation via insertion and expulsion of individual molecules or by fission and fusion of entire micelles. Molecular dynamics simulations are used here to estimate rates of these competing mechanisms in a simple model of block copolymer micelles in homopolymer solvent. This model exhibits a crossover with increasing degree of repulsion between solvent and micelle core components, from a regime dominated by association and dissociation to a regime dominated by fission and fusion.

Differential Effects of Formal and Informal Gambling on Symptoms of Problem Gambling During Voluntary Self-Exclusion.

Voluntary self-exclusion (VSE) programs enable problem gamblers to engage in a break from casino-based gambling. The current study analyzed the effects of a VSE program in British Columbia, Canada on problem gambling symptoms and the comparative reductions in problem gambling symptoms when participants abstained from gambling, continued to participate in non-casino based gambling, or attempted to violate their exclusion contract. 269 participants completed two telephone interviews over a 6-month period.

Design and Characterization of a PVLA-PEG-PVLA Thermosensitive and Biodegradable Hydrogel.

A set of poly(δ-valerolactone--d,l-lactide)--poly(ethylene glycol)--poly(δ-valerolactone--d,l-lactide) (PVLA-PEG-PVLA) triblock copolymers was synthesized and the solution properties were characterized using rheology, cryo-TEM, cryo-SEM, SANS, and degradation studies. This polymer self-assembles into a low viscosity fluid with flowerlike spherical micelles in water at room temperature and transforms into a wormlike morphology upon heating, accompanied by gelation. At even higher temperatures syneresis is observed.

Modulus- and Surface-Energy-Tunable Thiol-ene for UV Micromolding of Coatings.

Micromolding of UV-curable materials is a patterning method to fabricate microstructured surfaces that is an additive manufacturing process fully compatible with roll-to-roll systems. The development of micromolding for mass production remains a challenge because of the multifaceted demands of UV curable materials and the risk of demolding-related defects, particularly when patterning high-aspect-ratio features. In this research, a robust micromolding approach is demonstrated that integrates thiol-ene polymerization and UV LED curing.

Dispersion of oil into water using lecithin-Tween 80 blends: The role of spontaneous emulsification.

Lecithin-rich mixtures of the nontoxic surfactants lecithin and Tween 80 are effective marine oil spill dispersants, but produce much higher oil-water interfacial tension than other, comparably effective dispersants. This suggests interfacial phenomena other than interfacial tension influence lecithin-Tween 80 dispersants' effectiveness. The interface between seawater and dispersant-crude oil mixtures was studied using light microscopy, cryogenic scanning electron microscopy, and droplet coalescence tests.

Water-in-Oil Microstructures Formed by Marine Oil Dispersants in a Model Crude Oil.

DOSS (dioctyl sodium sulfosuccinate), Tween 80, and Span 80, surfactants commonly used in marine crude oil spill dispersants, have been mixed into a model oil at a total surfactant concentration of 2 wt %, typical for dispersant-treated oil slicks. These surfactant-oil blends also contained 0.5-1.

Cryogenic transmission electron microscopy study: preparation of vesicular dispersions by quenching microemulsions.

We previously showed that long-lived nanoemulsions, seeming initially vesicular, might be prepared simply by diluting and cooling (quenching) warm microemulsions with n-hexadecane with precooled water. In this paper, we confirm that these systems are vesicular dispersions when fresh, and they can be made with similar structures and compositional dependence using alkanes with chain lengths ranging from octane to hexadecane. The nanostructures of fresh nanoemulsions are imaged with cryogenic transmission electron microscopy (cryo-TEM).

Efficient dispersion of crude oil by blends of food-grade surfactants: Toward greener oil-spill treatments.

Effectiveness of oil spill dispersants containing lecithin/Tween 80 (L/T) blends in ethanol was measured as a function of L:T ratio, surfactant:solvent ratio, solvent composition, and dispersant:oil ratio (DOR) using baffled flask dispersion effectiveness tests. Optimal L:T ratios are between 60:40 and 80:20 (w/w); at higher L:T ratios, effectiveness is limited by high interfacial tension, while at lower L:T ratios, insufficient lecithin is present to form a well-packed monolayer at an oil-water interface. These optimal L:T ratios retain high effectiveness at low DOR: 80:20 (w/w) L:T dispersant is 89% effective at 1:25 DOR (v/v) and 77% effective at 1:100 DOR (v/v).

Nanoparticles Containing High Loads of Paclitaxel-Silicate Prodrugs: Formulation, Drug Release, and Anticancer Efficacy.

We have investigated particle size, interior structure, drug release kinetics, and anticancer efficacy of PEG-b-PLGA-based nanoparticles loaded with a series of paclitaxel (PTX)-silicate prodrugs [PTX-Si(OR)3]. Silicate derivatization enabled us to adjust the hydrophobicity and hydrolytic lability of the prodrugs by the choice of the alkyl group (R) in the silicate derivatives. The greater hydrophobicity of these prodrugs allows for the preparation of nanoparticles that are stable in aqueous dispersion even when loaded with up to ca.

Almost fooled again: new insights into cesium dodecyl sulfate micelle structures.

Replacing sodium with cesium as the counterion for dodecyl sulfate in aqueous solution results in stronger complexation and charge shielding, which should lead to larger micelles and ultimately to a cylindrical structure (cf. spheres for sodium dodecyl sulfate), but small angle X-ray scattering (SAXS) and small angle neutron scattering patterns previously have been interpreted with ellipsoidal micelle models. We directly image CsDS micelles via cryo-transmission electron microscopy and report large core-shell spherical micelles at low concentrations (≤2 wt %) and cylindrical micelles at higher concentrations (5.

Cryogenic electron microscopy study of nanoemulsion formation from microemulsions.

We examine a process of preparing oil-in-water nanoemulsions by quenching (diluting and cooling) precursor microemulsions made with nonionic surfactants and a cosurfactant. The precursor microemulsion structure is varied by changing the concentration of the cosurfactant. Water-continuous microemulsions produce initial nanoemulsion structures that are small and simple, mostly unilamellar vesicles, but microemulsions that are not water-continuous produce initial nanoemulsion structures that are larger and multilamellar.

The role of dispersants' dynamic interfacial tension in effective crude oil spill dispersion.

The dispersion effectiveness of dispersants containing Tween 80, Span 80, and dioctyl sodium sulfosuccinate (DOSS) was characterized using a modified Swirling Flask test, and was correlated with both initial and dynamic interfacial tension produced by those dispersants at an oil-water interface. Compositional trends in effectiveness were shown to be governed by: (1) initial oil-water interfacial tension observed upon dispersant-oil-saltwater contact; (2) rate of increase (or decrease) from the initial interfacial tension as DOSS was rapidly lost to the aqueous phase; and (3) gradually slowing kinetics of dispersant adsorption to the oil-water interface as Span 80 concentration was increased, which ultimately diminished dispersion effectiveness considerably even as dynamic interfacial tension remained <10(-3) mN/m. It is proposed that this third phenomenon results not only from the hydrophobicity of Span 80, but also from the dependence of mixed Tween-Span-DOSS reverse micelles' stability in crude oil on dispersant composition.

Flash nanoprecipitation: particle structure and stability.

Flash nanoprecipitation (FNP) is a process that, through rapid mixing, stabilizes an insoluble low molecular weight compound in a nanosized, polymer-stabilized delivery vehicle. The polymeric components are typically amphiphilic diblock copolymers (BCPs). In order to fully exploit the potential of FNP, factors affecting particle structure, size, and stability must be understood.

Population and size distribution of solute-rich mesospecies within mesostructured aqueous amino acid solutions.

Aqueous solutions of highly soluble substances such as small amino acids are usually assumed to be essentially homogenous systems with some degree of short range local structuring due to specific interactions on the sub-nanometre scale (e.g. molecular clusters, hydration shells), usually not exceeding several solute molecules.

Dopamine D2 receptor antagonism suppresses tau aggregation and neurotoxicity.

Background: Tauopathies, including Alzheimer's disease and frontotemporal dementia, are diseases characterized by the formation of pathological tau protein aggregates in the brain and progressive neurodegeneration. Presently no effective disease-modifying treatments exist for tauopathies.

Methods: To identify drugs targeting tau neurotoxicity, we have used a Caenorhabditis elegans model of tauopathy to screen a drug library containing 1120 compounds approved for human use for the ability to suppress tau-induced behavioral effects.

CryoSEM investigation of latex coatings dried in walled substrates.

Nonuniformities, such as heavy edges or "coffee rings", frequently develop as particulate coatings dry. One idea for avoiding these nonuniformities is to engineer the substrate edges. In this work, monodisperse latex coatings were deposited on substrates with photoresist walls around their edges.

Development of a carbon clad core-shell silica for high speed two-dimensional liquid chromatography.

We recently introduced a new method to deposit carbon on fully porous silicas (5 μm) to address some of the shortcomings of carbon clad zirconia (C/ZrO(2)), which has rather low retention due to its low surface area (20-30 m(2)/g). The method enables the introduction of a thin, homogeneous layer of Al(III) on silica to serve as catalytic sites for carbon deposition without damaging the silica's native pore structure. Subsequent carbon deposition by chemical vapor deposition resulted in chromatographically useful carbon phases as shown by good efficiencies and higher retentivity relative to C/ZrO(2).