Diffusion Coefficients Of Surfactants Research Articles

Numerical simulation of enhanced throughput of surfactant in Lamadian oilfield

Based on the actual data of Lamadian oilfield in terms of geological structure, reservoir fluid physical properties and production dynamics, a numerical simulation geological model of CMG STARS chemical flooding module is established, which is consistent with the static and dynamic characteristics of the oil reservoir. In order to study the effect of enhanced throughput of surfactants, based on the mechanism of enhanced throughput of surfactants, the numerical model was established by defining parameters such as oil-water interfacial tension at different concentrations, residual oil saturation, oil-water relative permeability curve, capillary pressure, isotonic point change and surfactant diffusion coefficient in the formation, etc. The numerical simulation of surfactant enhanced throughput was carried out to optimize the surfactant enhanced throughput parameters in Lamadian oilfield, and the sensitivity sequence of each injection and production parameter to the surfactant enhanced throughput and oil was determined. The final recommended injection and production parameters are surfactant injection amount (10000m3), injection concentration (0.5%) and brazing time (2d). It is predicted that the cumulative oil increase can reach 1310m3, the initial daily fluid production is controlled at 80m3/d, the daily oil production can be increased by 10m3/d, and the water cut can be reduced by 10%.

An Analytical Model for Estimation of the Self-Diffusion Coefficient and Adsorption Kinetics of Surfactants Using Dynamic Interfacial Tension Measurements.

We report a new analytical approach to model the transient diffusion and adsorption kinetics of a surfactant at a liquid/liquid interface using dynamic interfacial tension data. The developed model combined with the Frumkin/Langmuir isotherm is used to reproduce the experimental data of dynamic interfacial tension and predict the surfactant diffusion coefficient from a bulk solution to an interface and its adsorption kinetics. Experimental data of the dynamic interfacial tension of toluene and heptol solutions at various concentrations of asphaltenes (a natural surfactant) were employed to examine the ability of the developed model to regenerate the dynamic interfacial tension data. The model enabled us to estimate the apparent diffusion coefficient and adsorption kinetics of asphaltenes at different concentrations. The results showed that the diffusive migration of asphaltene toward an oil/water interface decreases at its higher concentrations and increases at higher concentrations of an aliphatic solvent such as n-heptane. Furthermore, the results reveal that the adsorption rate of asphaltenes at the interface increases at higher concentrations of surfactants and the aliphatic solvent. The developed analytical model finds applications in the prediction of the diffusion and adsorption kinetics of surfactants using dynamic interfacial tension data.

Refining of nonionic surfactant micellization theory based on the law of mass action

Two approaches to determining critical micelle concentration (CMC) are assessed, i.e., from the inflection point in the curve for the concentration dependence of the degree of micellization and as K1/(1–n), where K is the constant of the law of mass action and n is the aggregation number. The latter approach makes the theory simpler, while the former explicitly expresses the critical degree of micellization via the aggregation number. The concentrations of monomers and micelles are analyzed as functions of the overall concentration of a surfactant in a micellar solution. These functions look much simpler in the graphical form as compared with their complex exact analytical representation. This has resulted in derivation of simple analytical approximations for these functions, with these approximations being useful for calculations. The concentration dependence of the surfactant diffusion coefficient has been considered based on these approximations. It turned out that this dependence not only provides the known method for determining the diffusion coefficient of micelles, but also gives the possibility in principle to determine the aggregation number from the slope of the dependence of the diffusion coefficient on the inverse concentration (counted from the CMC in the CMC units). This new method for determining the aggregation number has been tested using the literature data on the diffusion coefficient of penta(ethylene glycol)-1-hexyl ether in an aqueous solution.

Monte Carlo simulations for amphiphilic aggregation near a water phase transition

In this study we analyze the equilibrium and dynamical properties of a lattice model for amphiphilic aggregation in a waterlike associating solvent. The amphiphiles are described as flexible chains of interconnected sites in a body-centered cubic lattice, with hydrophilic and hydrophobic portions. The solvent molecules occupy a single site and resemble the water tetrahedral molecular structure, with the possibility of hydrogen-bond formation and different densities. Following the phase diagram of the solvent model, we are able to study the effects of a phase transition of the solvent in the micellar dynamics. By carrying out Monte Carlo simulations, we analyze the micelle aggregate size distribution curve, the critical micelle concentration, the surfactant diffusion coefficient, the residence time, and the exit/entering rates of the amphiphiles from/to aggregates of different sizes. We also investigate the dipolar reorientational time correlation function for interfacial water and water molecules in the solvent bulk, as well as the number of hydrogen bonds per molecule in both cases.

Properties of microemulsions based on mixed nonionic surfactants and mixed oils

The systems studied were water/sucrose laurate/ethoxylated mono-di-glyceride/isopropylmyristate/peppermint oil. The solubilization capacity of water in the oils is dependent on the surfactants and oils mixing ratios (w/w). The transport properties (electrical conductivity and dynamic viscosity) were studied as function of water volume fraction. Electric percolation phenomenon was observed in these systems and the water volume fraction percolation thresholds were determined. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the water-in-oil to bicontinuous and inversion to oil-in-water microemulsions that occur upon dilution with water. The diffusion coefficients of surfactants increase with the increase in the water volume fraction. The structural parameters studied by small angle X-ray scattering that include the periodicity and correlation length were estimated. The periodicity increases linearly with the increase in the water volume fraction whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. Cryogenic transmission electron microscopy images for diluted microemulsions revealed the presence of spheroidal droplets of up to 10 nm diameter.

Partitioning and Localization of Fragrances in Surfactant Mixed Micelles

AbstractThe localization and dynamics of fragrance compounds in surfactant micelles are studied systematically in dependence on the hydrophobicity and chemical structure of the molecules. A broad range of fragrance molecules varying in octanol/water partition coefficientsPowis employed as probe molecules in an aqueous micellar solution, containing anionic and nonionic surfactants. Diffusion coefficients of surfactants and fragrances obtained by Pulsed Field Gradient (PFG)‐NMR yield the micelle/water distribution equilibrium. Three distinct regions along the log(Pow) axis are identified: hydrophilic fragrances (log(Pow) < 2) distribute almost equally between micellar and aqueous phases whereas hydrophobic fragrances (log(Pow) > 3.5) are fully solubilized in the micelles. A steep increase of the incorporated fraction occurs in the intermediate log(Pow) region. Here, distinct micelle swelling is found, while the incorporation of very hydrophobic fragrances does not lead to swelling. The chemical structure of the probe molecules, in addition to hydrophobicity, influences fragrance partitioning and micelle swelling. Structural criteria causing a decrease of the aggregate curvature (flattening) are identified.2H‐NMR spin relaxation experiments of selectively deuterated fragrances are performed monitoring local mobility of fragrance and leading to conclusions about their incorporation into either micellar interface or micelle core. The tendencies of different fragrance molecules (i) to cause interfacial incorporation, (ii) to lead to a flattening of the micellar curvature and (iii) to incorporate into micelles are shown to be correlated.

Regular and irregular deswelling of polyacrylate and hyaluronate gels induced by oppositely charged surfactants

The deswelling kinetics of macroscopic polyacrylate (PA) gels in solutions of dodecyltrimethylammonium bromide (C 12TAB) and cetyltrimethylammonium bromide (C 16TAB), with and without added sodium bromide, as well as hyaluronate (HA) gels in solutions of cetylpyridinium chloride (CPC) are investigated. Additional data are also provided by small-angle X-ray scattering and microgel experiments. The purpose is to study the deswelling behavior of (1) regularly deswelling gels, for which the deswelling is successfully described using a core/shell model earlier employed for microgels, and (2) irregularly deswelling gels, where the gel turns into a balloon-like structure with a dense outer layer surrounding a liquid-filled core. For regularly deswelling gels, the deswelling of PA/C 12TAB is found to be controlled by diffusion through both stagnant layer and collapsed surface phase, while for PA/C 16TAB it is found to be controlled mainly by the latter. The difference in deswelling rate between the two is found to correspond to the difference in surfactant diffusion coefficient in the surface phase. Factors found to promote irregular deswelling, described as balloon formation, are rapid surfactant binding, high bromide and surfactant concentration, longer surfactant chain length, and macroscopic gel size. Scattering data indicating a cubic structure for HA/CPC complexes are reported.

Surfactants and interfacial phenomena

Surfactants and interfacial phenomena

Sorption/diffusion behaviour of anionic surfactants in polyacrylamide hydrogels: from experiment to modelling

The sorption and diffusion processes of anionic surfactants with different chain length through polyacrylamide hydrogels with low swelling degree have been studied by electrical conductivity measurements. The multicomponent equilibrium equation has been used to model the sorption isotherms of different anionic surfactant in the hydrogels. Such isotherms show that initial rapid sorption of unimer surfactant into the membranes occurs, suggesting that non-freezing water can be involved in these interactions. In aqueous solution, at concentrations near and above the critical micelle concentration an anti-co-operative region is found. The diffusion coefficients of the anionic surfactants inside the hydrogel matrix show that the mobility of diffusing surfactant entities is dependent on cross-linker concentration and chain length. The Cukier hydrodynamic model and the free volume theory as modified by Peppas and Reinhart were applied to explain the dependence of the diffusion coefficients of surfactant on surfactant concentration inside the hydrogel. The hydrodynamic model was applied with success to the more hydrophilic surfactant, sodium 1-octanesulfonate, showing that the diffusion coefficients, D, increase when the resistance to hydrodynamic medium decreases; when the surfactant chain length increases (sodium dodecyl sulfate and sodium 1-hexadecane sulphonate) the variation of D with the free volume can only be understood considering the sieving effect produced by the surfactant inside gel.

Anomalous Surfactant Diffusion in a Gel of Chemically Cross-Linked Ethyl(hydroxyethyl) Cellulose

The interactions of sodium dodecyl sulfate (SDS) with chemically cross-linked gels of ethyl(hydroxyethyl) cellulose (EHEC) were studied. Above the so-called critical association concentration (cac), binding of SDS gives rise to an increased swelling of the EHEC gels. The binding of SDS to the gels was measured with flame emission analysis of the sodium ion. The self-diffusion of the surfactant ion (DS) in the gels was studied by the NMR pulsed field gradient spin-echo technique. Both experiments were performed on gels swollen to equilibrium in SDS solutions of varying concentrations. Comparisons with the DS diffusion in a solution of non-crosslinked EHEC were also made. In the EHEC solutions the observed spin-echo decays for DS were always describable in terms of a single surfactant diffusion coefficient (Gaussian diffusion). In contrast, the DS diffusion in the gels above the cac (SDS) was clearly non-Gaussian, or anomalous. The echo decays in the gels could be fitte to a log-normal distribution of diffusion coefficients. When the time during which the diffusion was measured was increased, the width of the distribution increased, while the average diffusion coefficient remained constant. An increase in the width of distribution was also seen when the SDS concentration was increased. The anomalous diffusion is ascribed to inhomogeneities in the gel. (Less)

A Sorption-Kinetic Model for Surfactant-Driven Spreading of Aqueous Drops on Insoluble Liquid Substrates

Spreading of aqueous drops on hydrocarbon liquids occurs only when particular surfactants are added to the droplets above a critical concentration. For surfactant solutions of didodecyl ammonium bromide (DDAB) in water spreading over mineral oil, rates of droplet expansion are much slower than those corresponding to pure liquids spreading over immiscible liquid substrates with the same initial spreading coefficients. We present a sorption-kinetic model to explain quantitatively the spreading histories for aqueous DDAB droplets on mineral oil. Due to surfactant transport limitations, spreading occurs only when enough surfactant arrives at the dilating lens surfaces to establish a slightly positive, but near-zero spreading coefficient. We solve the convective diffusion equation for a cylindrical disk-like lens under the integral constraint of a constant surfactant adsorption density corresponding to a near-zero spreading coefficient. All observed spreading behavior is correctly portrayed by the proposed sorption-kinetic model including final equilibrium lens formation and spreading rates that are sensibly independent of drop volume, but are strongly dependent on drop surfactant concentration. Quantitative agreement is found with the experimental spreading data for a surfactant diffusion coefficient of 6×10−12 m2/s and an effective adsorption rate constant of 6.5×10−7 m/s. Both values prove physically reasonable. The sorption-kinetic model provides a new mechanism for understanding slow surfactant-driven spreading.

Modeling surfactant removal in foam fractionation: I — Theoretical development

Surfactant removal by foam fractionation was theoretically analyzed by treating surfactant mass transfer from a bulk solution onto a moving bubble as a convective diffusion process. A mathematical model expressing the removal rate as a function of pertinent parameters was developed. The parameters included superficial air velocity, bubble size, bubble rising velocity, surfactant diffusion coefficient, and concentration. The model indicated that surfactant removal can be described as a first-order process whose rate increases with the diffusion coefficient, the superficial air velocity and the surfactant concentration in the bulk solution, but decreases with bubble size and bubble rising velocity. Based on the assumption of a completely mixed reactor within a section of a foam fractionator, a simplified model for foam fractionation under batch operation conditions was also presented. This simplified model provides a theoretical basis according to which the diffusion coefficient of surfactants can be determined and the model itself can be verified with experimental data.

Mobilities in cubic liquid crystals (“ringing gels”) determined by NMR self-diffusion measurements

Diffusion coefficients in the ternary surfactant system consisting of surfactant (tetradecyldimethylaminoxide, C 14DMAO), hydrocarbon (C 6H 12), and water have been determined by the pulsed field gradient spin-echo NMR method. Concentrations were varied along two sections of the ternary phase diagram. The transition from a micellar phase (L 1) that consists of spherical micelles into a cubic liquid crystalline phase (“ringing gel”) is studied as the volume fraction of the micelles is increased at constant hydrocarbon surfactant ratio. The mobility of the solvent, water, is only moderately decreased for this series of samples. In contrast, the diffusion coefficient of the hydrocarbon is decreased by two orders of magnitude, and the one of the surfactant by three orders of magnitude, when passing from the micellar phase into the cubic phase. In the micellar phase, the observed diffusive motion ( D ∼ 1–3 × 10 −11m 2 s −1) is due to transport of the micelles. This conclusion is reached from the fact that the diffusion coefficients of surfactant and hydrocarbon are equal, and approximately two orders of magnitude lower than that of the neat hydrocarbon liquid. In agreement with expectations, a linear decrease of the diffusion coefficient with the volume fraction of the micelles is observed. In contrast, the mobility of the micelles is strongly reduced in the cubic liquid crystal. The dynamics observed in the latter phase is due to two processes. Hopping of hydrocarbon molecules between neighboring micelles is characterized by a residence time of ∼100 μs, and diffusion coefficients of 3–5 × 10 −13 m 2 s −1. The exchange of surfactant molecules between the micelles occurs on a slower time scale (∼1 ms), with diffusion coefficients on the order of 3 × 10 −14 m 2 s −1.

Binary diffusion of micellar electrolytes

Binary diffusion coefficients determined by a modified Harned conductimetric technique are reported for aqueous sodium dodecyl sulfate at 25°C from 0.0018 to 0.47 M. As the concentration is increased through the critical micelle concentration of 0.008 M, the measured diffusion coefficients decrease rapidly from about 8 × 10 −6 to 2 × 10.6 −6 cm 2 s −1. Just above the CMC the diffusion coefficients pass through a minimum then increase, reaching 5 × 10 −6 cm 2 s −1 at 0.2 M. The measurements are interpreted by analyzing the effects of micellization on (a) the free energy gradient driving the diffusion process and (b) the mobility of the diffusing electrolyte. The sudden drop in the surfactant diffusion coefficient at the CMC is a thermodynamic effect analogous to the vanishing diffusion coefficient at a critical mixing point due to zero free energy gradients. The increase in the diffusion coefficient for sodium dodecyl sulfate above the CMC is due to a 10-fold increase in the mobility of the micellized electrolyte.