Surface Concentration Of Surfactant Research Articles

Molecular dynamics simulation of oil displacement using surfactant in a nano-silica pore

Molecular Dynamics (MD) simulations of oil droplet displacement have been performed using pressure driven surfactant flooding at a typical reservoir condition (T=330 K and P=20 MPa). The behavior of the micellization of surfactant molecules has been validated. A micelle with a radius of 22.85 Å is formed by 60 anionic sodium dodecyl benzenesulfonate (SDBS) surfactant molecules in aqueous solution. Surfactant additions result in significant reduction of interfacial tension (IFT) for oil/water system and such reduction is dependent on surfactant surface concentration. The microscopic mechanism of IFT reduction is described. Interfacial thickness increases from 3.5 Å to 22.5 Å at T=300 K andP=1 atm after surfactant molecules are adsorbed at oil/water interface, indicating high miscibility of two phases and thus results in interfacial tension reduction; the calculated interface formation energy of a single surfactant molecule is −145.7 Kcal/mol, which means the additions of surfactant would lead to the decrease of system energy and thus a more steady system. For surfactant flooding simulation, oil droplet static contact angle increases with surfactant additions. The larger the static contact angle of oil droplet, the stronger the drop deformation and the higher the displacement speed. Limited deformation is observed as oil droplet detaches from the solid substrate. Compared with water flooding, surfactant additions can significantly increase oil displacement speed by up to 80 %.

Surfactant transport upon foam films moving through porous media

In the context of foam improved oil recovery or foam-based soil/aquifer remediation, a model is presented for evolution of surfactant surface concentration on foam films moving through porous media. The model exploits an analogy between surface transport behaviour in foam fractionation and surfactant transport behaviour for foam in porous media. Films either stretch as they move from pore throat to pore body, or shrink when moving from pore body to pore throat: this stretching/shrinking then influences surfactant concentration. In addition, Plateau borders at the edges of films can supply surfactant to films, or receive surfactant from them. The model is solved numerically and key parameters governing its behaviour are identified. Parameter regimes are encountered, corresponding to modest stretching or shrinkage rates, in which surfactant concentration on films is predicted to be closely coupled to surfactant concentration in Plateau borders. Alternate parameter regimes are encountered, corresponding instead to fast stretching or shrinkage rates, in which surfactant concentration on films is predicted to become independent of surfactant concentration in Plateau borders. Asymptotic solutions available in each of these regimes are compared with numerical solutions. A preliminary comparison between the model predictions and experimental data from literature is also described. The appeal of the model is that predictions can be made comparatively simply. Indeed the model can predict when foam films might become significantly depleted in surfactant, possibly leaving films liable to breakage, which would then be detrimental to foam in porous media applications.

Surfactant-laden film lining an oscillating cap: problem formulation and weakly nonlinear analysis

A surfactant-laden liquid film that lines the inside of an oscillating spherical cap is considered as a model of lung alveoli. Pulmonary surfactant solubility is described by Langmuir adsorption kinetics, modified by incorporating the intrinsic compressibility of the adsorbed monolayer. A novel boundary condition, supported by experimental data and scaling arguments, is applied at the rim. The condition enforces mass conservation of water and surfactant by matching the ‘large-scale’ dynamics of the alveolus to ‘small-scale’ equilibrium over mid-alveolar septa of small but finite thickness. Linear and weakly nonlinear analysis around the conditions in a non-oscillating cap indicates that the occurrence of shearing motion in the liquid is related to the non-zero film thickness over the rim, and shearing velocity at the interface is predicted an order-of-magnitude lower than the velocity of radial oscillation. Marangoni stresses dominate the interfacial dynamics, but capillary stresses affect significantly the interior flow field. In particular, they produce spatial modulations in flow rate, surface concentration of surfactant and wall shear stress, whose length scale varies with $Ca^{-1/3}$ , i.e. is determined by a balance between capillary and viscous forces. Non-zero adsorption kinetics modifies at first order only the amplitude and phase of surface concentration, but affects all other variables at second order. In particular, it sets a steady drift of surfactant away from the alveolus and towards the rim. Finally, an attempt is made to relate the present predictions to physiological findings about air flow and particle deposition inside alveoli, and about shear stress-inflicted damage in diseased lungs.

An analytically solvable numerical approximation to Ward-Tordai equation applied to Langmuir adsorption

The calculation of surfactant surface concentration is a longstanding problem in surface science. In the simplest case of diffusion-controlled adsorption, assuming instantaneous adsorption and a stagnant bulk phase, a diffusion type partial differential equation (PDE) is obtained. Different numerical methods were used to solve this PDE, one of them was proposed by Ward and Tordai (1946) that obtained a simpler functional form, an integral equation that has been named after them. The Ward-Tordai equation was linearly discretized for Langmuir and Friedlich adsorption in this manuscript. It was possible to write the discretized equation as a second-degree polynomial that has analytical solutions, avoiding sophisticated numerical methods guaranteeing convergence. The solution was developed for Langmuir and Freundlich isotherms for planar and spherical surfaces and successfully described simulation and experimental data available in the literature.

Insights into interfacial behaviours of surfactant and polymer: A molecular dynamics simulation

Surfactants and polymers are two common chemicals used to stabilize the wet foam, but molecular-level insights into the structures, dynamics and hydrogen bonds of these agents and water at the surface are still lack. Herein a series of all-atom molecular dynamics simulations were conducted to explore the behaviors of surfactants/polymers at liquid films. Simulations confirmed that OPC water model can better reproduce the surface tension, and surface tension decreases while interface thickness increases as the increase of surfactant surface concentration. Our findings provide molecular insights on the morphology transitions of surfactant monolayers, which is dependent on the van der Waals energy among surfactants. Surfactant monolayers greatly retard the mobility of the interfacial water molecules, but such capability only exists nearby interfacial area. By adding polymers inside, water dynamics can be retarded further through the hydrogen bonding effects. In other words, polymers in the bulk solution produce a “bolt-like” effect which constrains the water around the chains.

Long-wave instability in thin heated films doped with soluble surfactants

Derived in this paper are evolution equations for the thickness and surface surfactant concentration pertaining to a thin liquid layer that is laden with a soluble surfactant as it flows under the influence of gravity down a heated incline. These equations account for variations in surface tension with surfactant concentration and temperature. A linear stability analysis and numerical simulations are carried out and discussed. An expression for the critical Reynolds number has been obtained and includes both thermocapillary and solutocapillary effects. The linear stability analysis was found to be in harmony with the nonlinear numerical simulations. A comparison with a weighted residual model also revealed good agreement.

Cooperative Effects Dominating the Thermodynamics and Kinetics of Surfactant Adsorption in Porous Media: From Lateral Interactions to Surface Aggregation.

Surfactant adsorption in porous media remains poorly understood, as the microscopic collective behavior of these amphiphilic molecules leads to nonconventional phenomena with complex underlying kinetics/structural organization. Here, we develop a simple thermodynamic model, which captures this rich behavior by including cooperative effects to account for lateral interactions between adsorbed molecules and the formation of ordered or disordered self-assemblies. In more detail, this model relies on a kinetic approach, involving adsorption/desorption rates that depend on the surfactant surface concentration to account for facilitated or hindered adsorption at different adsorption stages. Using different surfactants/porous solids, adsorption on both strongly and weakly adsorbing surfaces is found to be accurately described with parameters that are readily estimated from available adsorption experiments. The validity of our physical approach is confirmed by showing that the inferred adsorption/desorption rates obey the quasi-chemical approximation for lateral adsorbate interactions. Such cooperative effects are shown to lead to adsorption kinetics that drastically depart from conventional frameworks (e.g., Henry, Langmuir, and Sips models).

A molecular parameter to scale the Gibbs free energies of adsorption and micellization for nonionic surfactants

A long standing goal in the interfacial thermodynamics community is free energies of adsorption and micellization in terms of molecular parameters. Historically, correlations for CiEj's, a common class of nonionic surfactants, have been proposed between the number of ethylene glycol groups and/or the number of carbons and thermodynamic parameters, such as the critical micelle concentration (CMC), the equilibrium constant (K), and the maximum surface concentration (Γ∞). However, no such correlations to date are capable of capturing a large dataset of CiEj thermodynamic parameters. In this paper we propose the functional form of the critical micelle concentration and adsorption isotherm parameters in terms of molecular parameters for nonionic, CiEj surfactant chemistry. An extensive review of the literature is performed to generate interfacial thermodynamic parameters for a wide-range of CiEj chemistries. The data are used to parameterize and validate the molecular models. More specifically, we combine the well known linear dependence of CMC and K on the number of carbons, NC, with the theorized dependence on hydrophilic-lipophilic balance (HLB), represented as yphob, which is capable of collapsing all experimental data onto mastercurves. The resulting free energy models represent the first successful phenomenological theory which is able to collapse a broad range of experimental data for CiEj surfactants. Furthermore, the proposed models for the free energies of micellization and adsorption are capable of predicting the maximum surface concentration of CiEj surfactants at the air–water interface, thus providing a link between the physics of adsorption and self-assembly. This theory is a step towards engineering interfacial thermodynamics by chemical design which enables prediction of interfacial properties of novel surfactants without extrapolation. In addition, this paper summarizes the many years of experimental and theoretical understanding of surfactant structure property relationships for CiEj's and the resulting theory quantitatively explains many hypotheses about their interfacial thermodynamics.

Dynamic interfacial properties of sugar-based surfactants: Experimental study and modeling

Careful investigation about dynamic adsorption properties of surfactant molecules has a tremendous interest for their practical applications. The intention of this article is to study the dynamic adsorption properties at air/water interface for 16 nonionic sugar-based surfactants with gradual structure variation. Based on obtained results the dynamic surface tension (DST) behaviour of glycolipids was found to depend on both hydrophobic chain length and bulk concentration of surfactant. Interestingly, the plot of the maximum rate of surface tension decrease against the surfactant bulk concentration showed a linear trend for all studied surfactants independently of their structure. Furthermore, it was confirmed that all molecules were gathered on one unique curve representing a plot of the limit elasticity versus the relative surface concentration of surfactant. The using approach provided further insight into the anticipation of the dynamic surface properties of sugar-based amphiphiles.

Adsorption Kinetics of a Cationic Surfactant Bearing a Two-Charged Head at the Air-Water Interface

We studied the dynamics of adsorption at the air-water interface of a cationic surfactant bearing two charges, Gemini 12-2-12, at concentrations below and above the critical micelle concentration (cmc). We used maximum bubble pressure and Wilhelmy plate techniques in order to access all time scales in the adsorption process. We found that the adsorption dynamics are controlled by diffusion at the initial stage of the adsorption process (milliseconds) and it is kinetically controlled by an electrostatic barrier (minute) approaching the equilibrium surfactant surface concentration. Between these two extremes, we found several relaxation phenomena, all following exponential decays with characteristic times spanning from one to hundreds of seconds. By means of time-resolved surface potential measurements, we show that these processes involve charge redistribution within the interfacial region. The surface tension data are analyzed and interpreted in the framework of the free energy approach.

Dependence of Surface Tension on Surface Concentration in Ionic Surfactant Solutions and Influences of Supporting Electrolyte Therein

Abstract It has been well-known that the addition of electrolytes causes the ionic surfactant solution to have a lower surface tension by stimulating the surface adsorption. When the surface concentration of an ionic surfactant remains constant, the solution with supporting electrolyte in the bulk displays a lower surface tension than a solution without electrolyte. From the surface perspective we investigate the dependence of the surface tension of a solution upon the surface concentration of ionic surfactant and the influences of the supporting electrolyte therein, by means of thermodynamics and molecular dynamics simulation. The derived thermodynamic formula and simulation results predict, that at a given surface concentration the supporting electrolyte can change the orientation of the ionic surfactant, which results in a lower surface tension. The conclusions can be useful for the investigation to the surface structure of ionic surfactant solutions and the effects of supporting electrolyte.

Spreading of aqueous droplets with common and superspreading surfactants. A molecular dynamics study

The surfactant-driven spreading of droplets is an essential process in many applications ranging from coating flow technology to enhanced oil recovery. Despite the significant advancement in describing spreading processes in surfactant-laden droplets, including the exciting phenomena of superspreading, many features of the underlying mechanisms require further understanding. Here, we have carried out molecular dynamics simulations of a coarse-grained model with a force-field obtained from the statistical associating fluid theory to study droplets laden with common and superspreading surfactants. We have confirmed the important elements of the superspreading mechanism, i.e. the adsorption of surfactant at the contact line and the fast replenishment of surfactant from the bulk. Through a detailed comparison of a range of droplets with different surfactants, our analysis has indicated that the ability of surfactant to adsorb at the interfaces is the key feature of the superspreading mechanism. To this end, surfactants that tend to form aggregates and have a strong hydrophobic attraction in the aggregated cores prevent the fast replenishment of the interfaces, resulting in reduced spreading ability. We also show that various surfactant properties, such as diffusion and architecture, play a secondary role in the spreading process. Moreover, we discuss various drop properties such as the height, contact angle, and surfactant surface concentration, highlighting differences between superspreading and common surfactants. We anticipate that our study will provide further insight for applications requiring the control of wetting.

Effect of surfactants on thin film spreading under influence of surface acoustic wave

For the spreading of thin and free film of a partially wetting liquid with insoluble surfactant under the influence of surface acoustic wave, the dimensionless evolution equations governing the spreading dynamics are derived. The evolution equations contain the film thickness and the surface concentration of insoluble surfactant. Assuming that the thickness of the thin film is much smaller than the wavelength of sound in the liquid, the sound leaking off the surface acoustic wave cannot be sustained in the liquid film, and the acoustic radiation pressure and attenuation of the acoustic wave in the solid are both weak. Then the films spreading under different physical mechanisms are observed by numerical simulation. The results show that the surface acoustic wave drives the liquid film to spread and move. When the capillary stress is weak and the liquid film spreading is mainly controlled by the drift induced by surface acoustic wave, the spreading process consists of rapid spreading stage and balancing stage, and the Marangoni effect caused by uneven distribution of surfactant makes the liquid film spread faster in the first stage. When the capillary stress and the drift jointly dominate film spreading, the spreading process contains three stages, i.e. spreading stage, contracting stage and balancing stage. The effect of surfactant accelerates the spreading process, but the existence of contracting stage makes it take longer for the film to reach equilibrium. In addition, the disjoining pressure used in this paper promotes the liquid film spreading, as well as the Marangoni effect. As the correlation coefficient between disjoining pressure and surfactant concentration, <i>α</i>, and the Marangoni number, <i>M</i>, increase, the maximum thickness and the spreading radius of liquid film change faster.

Possibilities of Using the Surface Concentration of Surfactant Mixtures for Wastewater Treatment

Correlation between the manifestation of synergic action of surfactants in the process of their adsorption at the binary solution–air interface and the efficiency of the surface concentration of surfactants has been found. The use of the parameter of intermolecular interaction of surfactants in mixed adsorption layers is shown to be expedient for predicting the surface concentration of surfactants during their extraction from multicomponent aqueous solutions and wastewater. The possibility of intensifying the flotation extraction of some ionogenic surfactants from aqueous solutions in the presence of Tweens has been confirmed.

Evaluation of adsorption of surfactant at a moving interface of a single spherical drop

Numerical simulations of contaminated bubbles or drops adopt a model of adsorption-desorption kinetics developed for quiescent systems. However, the model has rarely been validated due to the lack of experimental data of surfactant concentration at a moving interface. Hence, we experimentally investigated surfactant concentration at the moving interface of a spherical drop falling in a stagnant liquid containing surfactant using the velocity distributions measured by spatiotemporal filter velocimetry (SFV). The molar flux of surfactant to the interface was also evaluated by substituting the measured velocity and surfactant concentration into the conservation equation of surface concentration of surfactant to examine the applicability of the Frumkin–Levich model to a drop falling in a contaminated system. We confirmed that the evaluation of surfactant concentration using SFV is of great use in understanding adsorption-desorption kinetics at an interface in a contaminated system and for validation of adsorption and desorption models. The measured results showed that the Frumkin–Levich model is not applicable to a moving interface, whereas it is applicable to an immobile interface, that the dependence of the molar flux on the molecular weight of surfactant is not so strong, and that the ratio of the surface concentration of surfactant to the maximum concentration, i.e. the surface coverage, is an appropriate index for judging an applicable range of the Frumkin–Levich model.

Experimental evaluation of Marangoni stress and surfactant concentration at interface of contaminated single spherical drop using spatiotemporal filter velocimetry

Spatiotemporal filter velocimetry (SFV) was extended to Lagrangian measurements with boundary-fitted measurement areas, and was applied to flows about single spherical drops of glycerol-water solution falling in stagnant silicon oil under clean and contaminated conditions to examine its applicability to the estimation of the Marangoni stress and surfactant concentration at a moving interface. Effects of bulk concentration of surfactant on the velocity field, the Marangoni stress and the surface concentration of surfactant were discussed from the measured data. As a result, we confirmed that accurate velocity distribution in the vicinity of the interface measured by SFV enables us to evaluate interfacial velocity and interfacial shear stresses and to estimate the Marangoni stress, interfacial tension and surfactant concentration at the interface with the assumption of negligible surface viscosity. The flow inside the drop and the interfacial velocity become weak due to the Marangoni stress caused by the gradient of surfactant concentration at the interface as the bulk concentration of surfactant increases. These results demonstrate that SFV is of great use in experimental analysis of adsorption and desorption kinetics at a moving interface.

Combined Molecular Dynamics Simulation-Molecular-Thermodynamic Theory Framework for Predicting Surface Tensions.

A molecular modeling approach is presented with a focus on quantitative predictions of the surface tension of aqueous surfactant solutions. The approach combines classical Molecular Dynamics (MD) simulations with a molecular-thermodynamic theory (MTT) [ Y. J. Nikas, S. Puvvada, D. Blankschtein, Langmuir 1992 , 8 , 2680 ]. The MD component is used to calculate thermodynamic and molecular parameters that are needed in the MTT model to determine the surface tension isotherm. The MD/MTT approach provides the important link between the surfactant bulk concentration, the experimental control parameter, and the surfactant surface concentration, the MD control parameter. We demonstrate the capability of the MD/MTT modeling approach on nonionic alkyl polyethylene glycol surfactants at the air-water interface and observe reasonable agreement of the predicted surface tensions and the experimental surface tension data over a wide range of surfactant concentrations below the critical micelle concentration. Our modeling approach can be extended to ionic surfactants and their mixtures with both ionic and nonionic surfactants at liquid-liquid interfaces.

Analysis of the behavior of low-molecular cationic and high-molecular anionic surfactants on the interface of phases «binary aqueous solution – air»

Specific features of the behavior of hexadecylpyridinium bromide and sodium carboxymethyl cellulose at the interface between solution-air phases at different weight ratios of components in solutions, pH of the medium at a constant concentration of cationic surfactant are studied. The composition of adsorption layers and the parameter of intermolecular interaction of surfactants in them are calculated. Optimal conditions for surface concentration of cationic surfactants are proposed

Effect of concentration-dependent disjoining pressure on drainage process of vertical liquid film

For the drainage under the gravity of a vertical foam film containing insoluble surfactant, an improved concentration-dependent disjoining pressure model is formulated based on the published experimental results. The lubrication theory is used to establish the evolution equations of the film thickness, the surface concentration of insoluble surfactant, and the surface velocity, and the evolution characteristics of the film under different disjoining pressures are simulated numerically. The results show that the drainage process of a vertical liquid film generally undergoes two stages:the first stage is the thick film stage and the gravity plays a leading role in the drainage process; the subsequent stage is the thin film stage, the effects of capillary pressure and disjoining pressure increase gradually, and the disjoining pressure dominates the evolution of the film. The disjoining pressure effect is closely related to surfactant type and the correlation strength between the surfactant concentration and electrostatic repulsion force of disjoining pressure. For the ionic surfactant, electrostatic repulsion force increases with the increase of the surfactant concentration, but it is opposite for the nonionic surfactant. It is likely that the free hydroxide ions, which are considered to render the surface negatively charged, are partly adsorbed by the nonionic surfactant. So the surface charge of the foam film decreases as the concentration of the nonionic surfactant increases, resulting in a decrease in electrostatic repulsion. Therefore, some ionic surfactants can improve the stability of liquid film drainage and slow down the drainage process, while the effects of some nonionic surfactants are opposite. When the disjoining pressure is positively correlated with surfactant concentration, with the increase of correlation strength coefficient α, the thinning and drainaging processes of the film tend to slow down, hence the stability of the film is enhanced. When the disjoining pressure is negatively correlated with surfactant concentration, with the increase of the absolute value of α, the drainage process of the film is accelerated and the risk of film rupture is augmented. The results obtained in this paper are consistent with some of the experimental results, indicating that the concentration-dependent disjoining pressure is indeed an important factor in maintaining the stability of foam film containing some certain anionic or nonionic surfactants. The improved concentration-dependent disjoining pressure model established in this paper could not explain the phenomena of parts of cationic nor non-ionic surfactant film in drainage experiments. It can be inferred that the structure of surfactant molecule, the more detailed disjoining pressure model and the coupling of the disjoining pressure and surface elasticity should be considered in the future work.

Surfactant transport onto a foam film in the presence of surface viscous stress

Surfactant transport onto a foam film in the presence of surface viscosity has been simulated as a model for processes occurring during foam fractionation with reflux. A boundary condition is specified determining the velocity at the end of the film where it joins up with a Plateau border containing surfactant rich reflux material. The evolutions of surface velocity and surfactant surface concentration on the film are computed numerically using a finite difference method coupled with the material point method. Results are analysed both for low and high surface viscosities. Evolution is comparatively rapid when surface viscosity is low, but the larger the surface viscosity becomes, the slower the surface flow, and the lower the surfactant surface concentration on the film at any given time. For a large surface viscosity, the surface concentration of surfactant is maintained nearly uniform except at positions near the Plateau border where the velocity and surfactant concentration fields need to adjust to satisfy the boundary condition at the end of the film. The boundary condition imposed at the end of the film implies also that a drier foam (i.e. smaller radius of curvature of the Plateau border) leads to less surfactant transport onto the films. Moreover, the shorter the film length is, also the shorter the characteristic time for surfactant transport onto the film surface. Thinner films however give longer characteristic times for surfactant transport.