Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant Mixtures

The adsorption at the interface between an aqueous solution of several surface-active agents and another fluid (air or oil) phase is addressed theoretically. We derive the kinetic equations from a variation of the interfacial free energy, solve them numerically and provide an analytic solution for the simple case of a linear adsorption isotherm. Calculating asymptotic solutions analytically, we find the characteristic time scales of the adsorption process and observe the behavior of the system at various temporal stages. In particular, we relate the kinetic behavior of the mixture to the properties of its individual constituents and find good agreement with experiments. In the case of kinetically limited adsorption, the mixture kinetics is found to be considerably different from that of the single-surfactant solutions because of strong coupling between the species.Comment: 19 pages, 7 figures, to be published in Langmui

Phase-field modeling droplet dynamics with soluble surfactants

Using lattice Boltzmann approach, a phase-field model is proposed for simulating droplet motion with soluble surfactants. The model can recover the Langmuir and Frumkin adsorption isotherms in equilibrium. From the equilibrium equation of state, we can determine the interfacial tension lowering scale according to the interface surfactant concentration. The model is able to capture short-time and long-time adsorption dynamics of surfactants. We apply the model to examine the effect of soluble surfactants on droplet deformation, breakup and coalescence. The increase of surfactant concentration and attractive lateral interaction can enhance droplet deformation, promote droplet breakup, and inhibit droplet coalescence. We also demonstrate that the Marangoni stresses can reduce the interface mobility and slow down the film drainage process, thus acting as an additional repulsive force to prevent the droplet coalescence

Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant Mixtures

The adsorption at the interface between an aqueous solution of several surface-active agents and another fluid (air or oil) phase is addressed theoretically. We derive the kinetic equations from a variation of the interfacial free energy, solve them numerically and provide an analytic solution for the simple case of a linear adsorption isotherm. Calculating asymptotic solutions analytically, we find the characteristic time scales of the adsorption process and observe the behavior of the system at various temporal stages. In particular, we relate the kinetic behavior of the mixture to the properties of its individual constituents and find good agreement with experiments. In the case of kinetically limited adsorption, the mixture kinetics is found to be considerably different from that of the single-surfactant solutions because of strong coupling between the species.Comment: 19 pages, 7 figures, to be published in Langmui

Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces

We present a theory for the kinetics of surfactant adsorption at the interface between an aqueous solution and another fluid (air, oil) phase. The model relies on a free-energy formulation. It describes both the diffusive transport of surfactant molecules from the bulk solution to the interface, and the kinetics taking place at the interface itself. When applied to non-ionic surfactant systems, the theory recovers results of previous models, justify their assumptions and predicts a diffusion-limited adsorption, in accord with experiments. For salt-free ionic surfactant solutions, electrostatic interactions are shown to drastically affect the kinetics. The adsorption in this case is predicted to be kinetically limited, and the theory accounts for unusual experimental results obtained recently for the dynamic surface tension of such systems. Addition of salt to an ionic surfactant solution leads to screening of the electrostatic interactions and to a diffusion-limited adsorption. In addition, the free-energy formulation offers a general method for relating the dynamic surface tension to surface coverage without relying on equilibrium relations.Comment: 36 pages, latex, 10 figure

Sulfoxide Surfactants

A novel group of nonionic surfactants, which we term ester sulfoxides, derived from 2-hydroxy-4-(methylthio) butyric acid are characterized in this thesis. The physico-chemical properties, equilibrium and dynamic properties, and microemulsion behavior of these surfactants are investigated. Based on the physico-chemical properties, the sulfoxide nonionic surfactant molecules presented good surface activity, good foaming, wetting ability and laundry detergency performance. Equilibrium surface tensions were determined for a number of molecules. The Langmuir adsorption isotherm is satisfactory for describing the adsorption behavior of the sulfoxide surfactant at the air/water interface. Adsorption kinetics onto the air/water interface was studied for an ester sulfoxide molecule with 8 carbon atoms in the tail of the surfactant. Comparing the experimental dynamic surface tension profiles of the surfactant solutions with the diffusion-controlled kinetic model indicate that the adsorption of this surfactant molecule onto the air/water interface is diffusion-controlled for dilute solutions at 25°C. HLD parameters of longer-chain surfactants were obtained and compared to that of alcohol ethoxylates. Ester sulfoxides are much less temperature sensitive than alcohol ethoxylates, and have a temperature coefficient similar to that of ionic surfactants. The ester sulfoxide moiety was determined to be as hydrophilic as ~5 ethylene oxide units according to the Cc value of the HLD parameters

Linking the Continuum and Molecular Scales of Adsorption Modeling for Non-Ionic Small Molecules and Homopolymers

Computational tensiometry and other quantitative adsorption predictions for small molecules and polymers are possible in the foreseeable future, but first, the application of the techniques to surfactant adsorption must be developed, and basic research is needed to identify the set of minimally required features of the molecular model that permits quantitative prediction. We take up the first challenge and apply three methods to three adsorption problems. In the first approach, we simulate poly(ethylene oxide) (PEO) oligomers and a model Tween 80 (polyoxyethylene sorbitan monooleate) molecule at the water/alkane interface. We use the weighted histogram analysis method (WHAM) to calculate interfacial potentials of mean force (PMFs) for PEG and Tween 80 using the atomistic GROMOS 53a6OXY+D and two coarse-grained (CG) MARTINI force fields. Because the force fields have not yet been validated for PEO adsorption to hydrophobic interfaces, we calculate PMFs for alcohol ethoxylates C12E2 and C12E8 and find agreement for the atomistic forcefield with reported semiempirical results, whereas for both CG force fields, PEO adsorbs too weakly to the hydrophobic interface. With the newly validated atomistic force field, we bracket the dilute adsorption free energy for a model Tween 80 molecule at the clean water/squalane interface. We also calculate the pressure–area isotherm and—with molecular thermodynamic theory and a simple transport model—demonstrate the transition from irreversible to reversible adsorption with increasing surface coverage, consistent with past experimental reporting. In the second approach, we sought to explain experiments that show relaxation of oil/water interfacial tension by adsorption of alkyl ethoxylate surfactants from water is delayed relative to diffusion-controlled adsorption. We examine possible causes of this delay. We argue that a theory implicating transient depletion near an adsorbing interface for suppressing interfacial relaxation is invalid. We find that re-dissolution of the surfactant in the oil droplet cannot explain the apparent interfacial resistance at short times. We also perform WHAM with molecular dynamics simulation and do not find any evidence of an energy barrier or low-diffusivity zone near the interface. Nor do we find evidence from simulation that pre-micellar aggregation slows diffusion enough to cause the observed resistance to interfacial adsorption. We are therefore unable to pinpoint the cause of the resistance, but we suggest that “dead time” associated with the experimental method could be responsible – specifically local depletion of surfactant by the ejected droplet when creating the fresh oil/water interface. In the third approach, we compute desorption rates for isolated polymers stuck to a solid wall with forward flux sampling (FFS). We interpret computed rates on the basis of a conjecture that a dimensionless desorption time scales with the equilibrium ratio of adsorbed surface amount to bulk concentration. We find that the dimensionless desorption time approaches the expected exponential scaling with the degree of polymerization multiplied by the mean field interaction between the monomer and the wall. However, we also find this strong adsorption scaling only becomes accurate for polymers which adsorb irreversibly on realistic timescales. We also find that excluded volume interactions and bending angle potentials shrink the desorption time and weaken the scaling of desorption time with N. For sufficiently weakly-adsorbing chains, the dimensionless desorption time becomes independent of N, suggesting a diffusion-controlled process overtakes the detachment process in importance.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144112/1/khuston_1.pd

Investigation of Asphaltene Adsorption at Liquid-Liquid and Liquid-Solid Interfaces

Asphaltenes, as indigenous components in crude oils, are believed to play an important role in the petroleum production and processing industry. For example, asphaltene molecules can adsorb onto the water / oil interface like the amphiphiles and hinder water droplet coalescence, resulting in the stabilization of water-in-oil emulsions. Also, in the upstream production, depending on the temperature and pressure, asphaltenes can precipitate and deposit onto the reservoir rocks, the wellbores and the pipelines, leading to the change in reservoir rocks’ surface wettability and the blockage of the production facilities. Hence, it is crucial to understand the mechanism of asphaltenes adsorption at the water-oil interface and their deposition onto the solid surfaces, and investigations are reported here on both these adsorption and deposition phenomena. Defined by their solubility (soluble in aromatic solvents but insoluble in n-alkanes), the asphaltenes form a class of crude oil components with a wide distribution in molecular structures and this brings about polydispersity in surface activity, adsorption behaviors, and aggregate formation. The effects of polydispersity in mixtures of asphaltene molecules affect their dilatational rheology. This aspect of their behavior has often been neglected and is discussed here in the context of multi-component diffusional model simultaneously captures both dynamic interfacial tension and dilatational rheology behavior, with the same parameters, e.g., for composition and interfacial activity. The numerical analysis developed based on the diffusional mixture model reached the similar conclusions as the previous fractionation experiments that a minor fraction of asphaltene mixture (less than 10%) has a much higher surface activity than the bulk of them. This was confirmed by a recent study where the interfacially surface-active asphaltene fraction was separated from the remaining asphaltenes and this fraction was found to be enriched with larger molecules with a higher amount of heteroatoms such as oxygen and sulfur compared to the remaining bulk asphaltenes. The simulation results also supported the hypothesis that the long-term decay of surface tensions observed for asphaltenes-covered interfaces was a diffusion-controlled adsorption process and not a result of molecular reorganization. The applicability of Langmuir isotherm in study of asphaltenes adsorption kinetics was then discussed and justified by considering the reversibility at different scales (pendant drop experiment and emulsions) and introducing the BET model for multilayer adsorption. Asphaltenes can also precipitate and deposit onto solid surfaces depending on a wide range of factors such as temperature and pressure. A preliminary investigation is presented on solubility effects on asphaltene adsorption from crude oils onto stainless steel surfaces using quartz crystal microbalance with dissipation (QCM-D) techniques. The kinetics of deposition at different concentrations was examined and the sizes of the primarily deposited asphaltene molecules were estimated from the initial adsorption kinetics. The numerical analysis of the experimental data using a theoretical two- step deposition model was attempted, and the optimized mean aggregate size number proved to be quite close to the reported values for some rock types in the earlier adsorption studies. Despite asphaltene precipitation increases with the increasing heptane percentage, the deposition of asphaltenes was found to reach the maximum at the solubility of 70 vol% heptane content. The performance of a commercial model inhibitor was then assessed under different conditions using the developed experiment metrics and the inhibitor was found to be able to reduce the maximum deposition amount at the solubility with 70% heptane fraction, which happened to be the condition that generates the largest amount of deposition. The information related to the solubility effect and the inhibitor performance is essential to help the industry in the assessment of the operating conditions for a better management of asphaltene-related flow assurance problems in crude oil recovery

Dynamic Surface Tension of Surfactants Through Separation of Diffusion and Dilation Effects

Surfactants are biphilic monomers that adsorb to interfaces and have the ability to reduce interfacial tension. From this reduction in interfacial tension, surfactants are able to function as emulsifiers, detergents, and wetting agents among a variety of other things. Despite their ubiquity surfactants and surfactant behaviors, in particular those in highly dynamic environments, are not yet well characterized. In these environments, surfactants undergo significant distortions to droplet, bubble, and foam geometries which induce changes to the surface tension. This establishes a need to understand how surfactants respond to distortions of geometry through diffusion and dilational effects. To address this need, we characterize the dilational interfacial rheology of a pulsating pendant bubble in water-surfactant mixtures using the nonionic surfactants TWEEN 20 and TWEEN 40. This study considers two separate but concurrent processes: dilation of the bubble interface and diffusion of surfactants to the bubble interface. Through a timescale comparison of both of these separate processes, a new way to contextualize surfactant adsorption behavior and bubble mechanics was developed based on a single dimensionless Peclet number. This dimensionless parameter accurately describes changes to the surface tension for different surfactants and concentrations through a combination of both diffusion and dilation effects. We hope that a detailed analysis consisting of a single dimensionless number capable of capturing the effects caused by the combination of both diffusion and dilation will better inform a selection of surfactants for specific industries and applications

Structure-kinetics relationships in micellar solutions of nonionic surfactants

Micellar surfactant solutions are highly complex systems containing aggregates of different shapes and sizes all in dynamic equilibrium. I have undertaken an investigation into the kinetic processes that occur in micellar surfactant solutions subjected to both bulk perturbations and close to expanding surfaces. Supporting information regarding the equilibrium properties of surfactant micelles has been acquired using several experimental techniques including small-angle neutron scattering (SANS) and pulsed field gradient spin echo (PFGSE) nmr. Bulk exchange kinetics between micelles and monomers in solution have been investigated using both numerical modelling and stopped-flow dilution experiments. My results show that conventional theories of monomer-micelle exchange kinetics apply only under very limited conditions. In order to understand how micellesolutions respond to large perturbations from equilibrium a different approach is required. I have hypothesised an alternative monomer-micelle exchange mechanism. This hypothesis has been tested using numerical modelling and comparison of theoretical predictions with the results of stopped-flow perturbation experiments. These experimental results are consistent with my hypothesis. In addition to bulk exchange kinetics, I have also undertaken a detailed experimental investigation of adsorption kinetics from micellar systems on the millisecond timescale. Again my results indicate that conventional theoretical approaches are incomplete and I suggest an alternative adsorption pathway that should be included in future theories of adsorption from micellar surfactant solution