Igor K. Yudin

Mechanisms of asphaltene aggregation in toluene–heptane mixtures

Abstract The kinetics of asphaltene aggregation in toluene–heptane mixtures has been studied by photon correlation spectroscopy (PCS). The PCS technique was adapted for investigation of opaque liquids. We examined the natural asphaltenes of various origin as well as the same ones with chemically modified metal-porphyrin kernels. The concentration of asphaltenes in toluene varied from 1 to 10 g/l. The theoretical model developed employs Smoluchowski fundamental approach. It allowed us to describe quantitatively the experimental curves for which the universal behavior was found. At asphaltene concentrations below the critical micelle concentration the diffusion limited aggregation (DLA) solely is observed. Above the CMC the reaction limited aggregation (RLA) occurs in the initial stage of particle growth. Then, the aggregation mechanism goes to the DLA. The results obtained prove asphaltene aggregation in hydrocarbon solutions is similar to that of typical colloids.

CROSSOVER KINETICS OF ASPHALTENE AGGREGATION IN HYDROCARBON SOLUTIONS

We have investigated the kinetics of asphaltene aggregation in toluene+n-heptane mixtures with a dynamic light-scattering method adapted to opaque fluids. Two types of aggregation kinetics have been observed: diffusion-limited aggregation and reaction-limited aggregation. At asphaltene concentrations in toluene below the critical micelle concentration of about 3gl−1 the aggregation kinetics appears to be solely limited by diffusion. Far above the critical micelle concentration reaction-limited aggregation takes place at least in the initial stage of particle growth. At a concentration of asphaltenes in toluene slightly above the critical micelle concentration, a crossover between these two types of aggregation kinetics has been observed. We propose a simple theoretical expression for the crossover kinetics and also a universal scaled description of the observed phenomena.

Mechanism of suppression of protein aggregation by α-crystallin.

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by α-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of α-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

A compact photon-correlation spectrometer for research and education

A compact photon-correlation spectrometer for basic and applied research in physics, chemistry, biology, medicine, engineering, and environmental technology as well as for educational laboratory courses in these subjects has been developed. The instrumental setup enables one to make absolute measurements of the sizes of particles suspended in liquids in the range from 0.001 to 5 μm. The measurements are fast, lasting usually from seconds to several minutes. Real-time size monitoring, such as of kinetic aggregation processes, is also possible. The optical arrangement of the spectrometer even makes it possible to measure light scattering in opaque systems which are characterized by strong light absorption. The quantity of the sample to be studied can be quite small, starting from 0.01 cm3. The system includes specially designed software for performing data interpretation and for implementing fitting procedures, for exchange of data with other programs, and for automation of measurements of long duration or of sequences of measurements. Measurements verifying the accuracy of the system are presented for latex suspensions, for aniline dye dissolved in water, and for dilute solutions of polystyrene in toluene.

Dynamic Light Scattering Monitoring of Asphaltene Aggregation in Crude Oils and Hydrocarbon Solutions

An overview of dynamic light scattering (DLS) studies of asphaltene aggregation in hydrocarbons and crude oils is presented. A special optical scheme, to make light scattering measurements in “nontransparent” light-absorbing colloids possible, has been designed and tested. The modified DLS is an effective technique for real-time monitoring of petroleum colloids. Combined with accurate viscosity measurements, DLS is a powerful tool for investigating the colloid nature of crude oils, heavy petroleum fractions, and asphaltene solutions. Various regimes of asphaltene aggregation have been investigated by DLS in crude oils and hydrocarbon mixtures. Crossover between reaction-limited aggregation and diffusion-limited aggregation has been observed in hydrocarbon solutions of asphaltenes. Asphaltene colloidal structures, originally persisting in some crude oils, have been detected. Using n-heptane as precipitant, we have studied stability of crude oils with respect to asphaltene aggregation. We have found that the oils with different aromatics/saturates ratios exhibit different aggregation kinetics. Formation of stable asphaltene aggregates, originating from ultrasonic agitation and surfactant addition, has been also studied. A DLS microrheology in a crude oil has been demonstrated. Crude oils and heavy petroleum fractions are complex multicomponent fluids exhibiting properties of either molecular solutions or colloids depending on the origin and on the properties under consideration. Non-Newtonian rheology of petroleum systems (especially those with high-content asphaltenes and paraffins) indicates the presence of a supra-molecular mesoscopic structure. However, the physical nature of this structure is not yet well understood. It is commonly accepted

Light-scattering study of petroleum asphaltene aggregation.

Dynamic light scattering with an original optical scheme has been used for the investigation of opaque (strongly light-absorbing) asphaltene colloids in crude oils and hydrocarbon mixtures. Diffusion-limited aggregation and reaction-limited aggregation as well as a crossover between these two regimes have been observed. A simple interpolation for the crossover kinetics is proposed. Asphaltene colloidal structures, originally persisting in crude oils, have been detected. Addition of a precipitant above a threshold induces asphaltene aggregation. Depending on the nature of the precipitant, different crude oils respond differently on its addition: (a) exponential-in-time growth of aggregates to huge flocks or (b) fast formation of stable-in-size particles.

Competition of mesoscales and crossover to theta-point tricriticality in near-critical polymer solutions.

The approach to asymptotic critical behavior in polymer solutions is governed by a competition between the correlation length of critical fluctuations diverging at the critical point of phase separation and an additional mesoscopic length scale, the radius of gyration. In this paper we present a theory for crossover between two universal regimes: a regime with Ising (fluctuation-induced) asymptotic critical behavior, where the correlation length prevails, and a mean-field tricritical regime with theta-point behavior controlled by the mesoscopic polymer chain. The theory yields a universal scaled description of existing experimental phase-equilibria data and is in excellent agreement with our light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 2 x 10(5) up to 11.4 x 10(6). The experiments demonstrate unambiguously that crossover to theta-point tricriticality is controlled by a competition of the two mesoscales. The critical amplitudes deduced from our experiments depend on the polymer molecular weight as predicted by de Gennes [Phys. Lett. 26A, 313 (1968)]. Experimental evidence for the presence of logarithmic corrections to mean-field tricritical theta-point behavior in the molecular-weight dependence of the critical parameters is also presented.

Effect of α-crystallin on thermal aggregation of glycogen phosphorylase b from rabbit skeletal muscle

Thermal aggregation of rabbit skeletal muscle glycogen phosphorylase b (Phb) has been investigated using dynamic light scattering under conditions of a constant rate of temperature increase (1 K/min). The linear behavior of the dependence of the hydrodynamic radius on temperature for Phb aggregation is consistent with the idea that the rmal aggregation of proteins proceeds in the kinetic regime where in the rate of aggregation is limited by diffusion of the interacting particles (the regime of “diffusion-limited cluster-cluster aggregation”). In the presence of α-crystallin, a prote in exhibiting chaperone-like activity, the dependence of the hydrodynamic radius on temperature follows the exponential law; this suggests that the aggregation process proceeds in the kinetic regime where the sticking probability for colliding particles becomes lower than unity (the regime of “reaction-limited cluster-cluster aggregation”). Based on analysis of the ratio between the light scattering intensity and the hydrodynamic radius of Phb aggregates, it has been concluded that the addition of α-crystallin results in formation of smaller size starting aggregates. The data on differential scanning calorimetry indicate that α-crystallin interacts with the intermediates of the unfolding process of the Phb molecule. The proposed scheme of the rmal denaturation and aggregation of Phb includes the stage of reversible dissociation of dimers of Phb into monomers, the stage of the formation of the starting aggregates from the denatured monomers of Phb, and the stage of the sticking of the starting aggregates and higher order aggregates. Dissociation of Phb dimer into monomers at elevated temperatures has been confirmed by analytical ultracentrifugation.

Mechanism of suppression of dithiothreitol-induced aggregation of bovine α-lactalbumin by α-crystallin

The kinetics of dithiothreitol (DTT)-induced aggregation of alpha-lactalbumin from bovine milk has been studied using dynamic light-scattering technique. Analysis of the distribution of the particles formed in the solution of alpha-lactalbumin after the addition of DTT by size showed that the initial stage of the aggregation process was the stage of formation of the start aggregates with the hydrodynamic radius (R(h)) of 80-100nm. Further growth of the protein aggregates proceeds as a result of sticking of the start aggregates. Suppression of alpha-lactalbumin aggregation by alpha-crystallin is mainly due to the increase in the duration of the lag period on the kinetic curves of aggregation. It is assumed that the initially formed complexes of unfolded alpha-lactalbumin with alpha-crystallin were transformed to the primary clusters prone to aggregation as a result of the redistribution of the denatured protein molecules on the surface of the alpha-crystallin particles.

Colloidal Properties of Crude Oils Studied by Dynamic Light-Scattering

Colloidal properties and kinetics of asphaltene aggregation in three crude oils using a dynamic light-scattering method adapted to opaque fluids were investigated. The technique makes it possible to measure the size of particles suspended in nearly nontransparent liquid media. The studied native crude oils were found to have persisting colloidal particles. The observed particles are assumed to be asphaltene/resin aggregates. Using n-heptane as a precipitant, asphaltene aggregation kinetics in crude oils was studied. Experimental measurements of the particle size as a function of time in solutions with different oil/precipitant ratios are reported. The aggregation kinetics accelerates with increasing precipitant concentration. The oil sample with a large amount of paraffins is on the edge of instability and exhibits a slow asphaltene aggregation process without precipitant. Aggregation in the two other oils starts only at some threshold concentration of the precipitant, lasts a short time, and results in the formation of stable-in-size particles. The results obtained prove that dynamic light scattering is an effective method to test petroleum colloid stability.