Effects of the addition of resins on aggregation and stability of petroleum asphaltenes in hydrocarbon solutions are studied by dynamic light scattering. The average aggregate size was monitored in real time as a function of the concentration of the precipitant (heptane) and resins. It is shown that resins serve as inhibitors for asphaltene aggregation, shifting the onset of aggregation. However, the dependence of the onset on the concentration of resins has a tendency to saturate. The characteristic time of aggregation decreases exponentially upon increase of the precipitant concentration, while it grows linearly upon increase of the concentration of resins. A definition of the onset of asphaltene aggregation based on the time dependence of the aggregate-size growth is suggested. It is also shown for all the samples studied (with and without resins) that the aggregation is controlled by diffusion-limited kinetics. The size of the aggregates as a function of time follows a diffusion-limited-kinetics power law with an exponent α = 0.36 ± 0.04, which is related to the fractal dimension d f of asphaltene clusters as α = 1/(1 + d f).

Hydrotropes are substances containing small amphiphilic molecules, which increase solubility of nonpolar (hydrophobic) substances in water. Hydrotropes may form dynamic clusters (less or about 1 ns lifetime) with water molecules; such clusters can be viewed as “pre-micelles” or as “micellar-like” structural fluctuations. We present the results of experimental and molecular dynamics (MD) simulation studies of interfacial phenomena and liquid–liquid equilibrium in the mixtures of water and cyclohexane with the addition of a typical nonionic hydrotrope, tertiary butanol. The interfacial tension between the aqueous and oil phases was measured by Wilhelmy plate and spinning drop methods with overlapping conditions in excellent agreement between techniques. The correlation length of the concentration fluctuations, which is proportional to the thickness of the interface near the liquid–liquid critical point, was measured by dynamic light scattering. In addition, we studied the interfacial tension and water–oil interfacial profiles by MD simulations of a model representing this ternary system. Both experimental and simulation studies consistently demonstrate a spectacular crossover between two limits in the behavior of the water–oil interfacial properties upon addition of the hydrotrope: at low concentrations the hydrotrope acts as a surfactant, decreasing the interfacial tension by adsorption of hydrotrope molecules on the interface, while at higher concentrations it acts as a cosolvent with the interfacial tension vanishing in accordance with a scaling power-law upon approach to the liquid–liquid critical point. It is found that the relation between the thickness of the interface and the interfacial tension follows a scaling law in the entire range of interfacial tensions, from a “sharp” interface in the absence of the hydrotrope to a “smooth” interface near the critical point. We also demonstrate the generic nature of the dual behavior of hydrotropes by comparing the studied ternary system with systems containing different hydrocarbons and hydrotropes.

A B S T R A C THydrotropes are substances consisting of amphiphilic molecules that are too small to self-assemble in equilibrium structures in aqueous solutions, but can form dynamic molecular clusters H-bonded with water molecules. Some hydrotropes, such as low-molecular-weight alcohols and amines, can solubilize hydrophobic compounds in aqueous solutions at a mesoscopic scale ($100 nm) with formation of longlived mesoscale droplets. In this work, we report on the studies of near-critical and phase behavior of binary (2,6-lutidine-H 2 O) and quasibinary (2,6-lutidine-H 2 O/D 2 O and tert-butanol/2-butanol-H 2 O) solutions in the presence of a solubilized hydrophobic impurity (cyclohexane). In additional to visual observation of fluid-phase equilibria, two experimental techniques were used: light scattering and smallangle neutron scattering. It was found that the increase of the tert-butanol/2-butanol ratio affects the liquid-liquid equilibria in the quasi-binary system at ambient pressure in the same way as the increase of pressure modifies the phase behavior of binary 2-butanol-H 2 O solutions. The correlation length of critical fluctuations near the liquid-liquid separation and the size of mesoscale droplets of solubilized cyclohexane were obtained by dynamic light scattering and by small-angle neutron scattering. It is shown that the effect of the presence of small amounts of cyclohexane on the near-critical phase behavior is twofold: (1) the transition temperature changes toward increasing the two-phase domain; (2) longlived mesoscopic inhomogeneities emerge in the macroscopically homogeneous domain. These homogeneities remain unchanged upon approach to the critical point of macroscopic phase separation and do not alter the universal nature of criticality. However, a larger amount of cyclohexane generates additional liquid-liquid phase separation at lower temperatures.

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The possible role of biogeochemical processes in the transport of colloidal and pseudo-colloidal U, Np, and Pu during bioremediation of radionuclide- and nitrate-contaminated groundwater was investigated. In two laboratory experiments with water samples taken from contaminated aquifers before and post bioremediation, we found that microbial processes could cause clayed, ferruginous, and actinide colloids to coagulate. The main mechanisms are biogenic insoluble ferrous iron species formations (goethite, pyrrhotite, siderite, troilite, and ferrihydrite), the aggregation of clay particles by microbial metabolites, and the immobilization of actinides in the bacterial cells, large polymers, and iron and clayed sediments. This process decreases the risk of colloidal and pseudo-colloidal transport of actinides.