Soluble Surfactant Research Articles

Spontaneous emulsification of water in oil at appreciable interfacial tensions

Spontaneous formation of aqueous droplets in kerosene was observed, which was facilitated by the presence of an oil soluble surfactant: Span® 80 at concentrations above CMC. Kerosene/water interfacial tension under all conditions studied was not lower than 4mN/m. Therefore, ultra-low interfacial tension was not required for this process to occur spontaneously. The process was caused by a transfer of water molecules to swollen reversed micelles. The influence of both the surfactant concentration in the organic phase and NaCl concentration in the aqueous phase on spontaneous aqueous droplet formation was investigated. Nano-sizing analyse of the drops was performed, which showed the droplets sizes in between 100 and 400nm. It is proven that the presence of salt in the aqueous phase inhibits droplet formation. It is shown that big sessile aqueous droplets deposited on a hydrophobic substrate inside the kerosene phase were dissolved in kerosene through formation and growth of droplets, which form an aqueous film at the droplet base.

Effect of Soluble Surfactants on the Kinetics of Thinning of Liquid Bridges during Drops Formation and on Size of Satellite Droplets.

The results of an experimental study on thinning and breakage of liquid bridges during detachment of a drop from the tip of a capillary are presented for a series of surfactant solutions (including cationic, anionic, and nonionic surfactants) over a broad range of molecular masses, values of critical micelle concentration, and concentrations. The used experimental protocol revealed that the kinetics of the bridge thinning depends much more on the dynamics of adsorption at the surface of the drop before it destabilizes, rather than on the depletion of surfactant from the surface of the thinning bridge due to its stretching as the instability develops. The kinetics of the bridge thinning and the size of satellite droplets formed after the bridge breakage depend considerably on the surfactant concentration and the value of critical micelle concentration. It is proposed that the dynamic surface tension on the time scale of the drop formation can be used as an effective surface tension for the description of the bridge kinetics over the broad range of experimental conditions used.

Influence of Surfactants on Dip Coating of Fibers: Numerical Analysis

We numerically analyze the process of dip coating on fibers when the liquid to be deposited contains soluble surfactants. The predictions obtained are in excellent agreement with experimental results reported by different authors. Our results show that the thickness of the film deposited, and particularly the so-called surfactant thickening effect, changes its behavior when the coating speed exceeds certain limits; therefore, the effects produced by inertia forces are scrutinized to uncover, for the first time, the mechanisms inducing those changes.

Solutal Marangoni instability in layered two-phase flows

In this paper, the instability of layered two-phase flows caused by the presence of a soluble surfactant (or a surface-active solute) is studied. The fluids have different viscosities, but are density matched to focus on Marangoni effects. The fluids flow between two flat plates, which are maintained at different solute concentrations. This establishes a constant flux of solute from one fluid to the other in the base state. A linear stability analysis is performed, using a combination of asymptotic and numerical methods. In the creeping flow regime, Marangoni stresses destabilize the flow, provided that a concentration gradient is maintained across the fluids. One long-wave and two short-wave Marangoni instability modes arise, in different regions of parameter space. A well-defined condition for the long-wave instability is determined in terms of the viscosity and thickness ratios of the fluids, and the direction of mass transfer. Energy budget calculations show that the Marangoni stresses that drive long- and short-wave instabilities have distinct origins. The former is caused by interface deformation while the latter is associated with convection by the disturbance flow. Consequently, even when the interface is non-deforming (in the large-interfacial-tension limit), the flow can become unstable to short-wave disturbances. On increasing the Reynolds number, the viscosity-induced interfacial instability comes into play. This mode is shown to either suppress or enhance the Marangoni instability, depending on the viscosity and thickness ratios. This analysis is relevant to applications such as solvent extraction in microchannels, in which a surface-active solute is transferred between fluids in parallel stratified flow. It is also applicable to the thermocapillary problem of layered flow between heated plates.

Surface viscosity and Marangoni stresses at surfactant laden interfaces

We calculate here the force on a probe at a viscous, compressible interface, laden with soluble surfactant that equilibrates on a finite time scale. The motion of the probe through the interface drives variations in the surfactant concentration at the interface that in turn leads to a Marangoni flow that contributes to the force on the probe. We demonstrate that the Marangoni force on the probe depends non-trivially on the surface shear and dilatational viscosities of the interface indicating the difficulty in extracting these material properties from force measurements at compressible interfaces.

A laboratory study of spilling breakers in the presence of light‐wind and surfactants

AbstractSpilling breaking waves in the presence of light‐wind and surfactants are studied experimentally in a wind‐wave tank. The breaking waves are mechanically generated with a single wave maker motion that produces a weak spilling breaker in clean water without wind. Separate experiments are performed with the same wave maker motion at different low wind speeds in clean water and in water with various concentrations of Triton X‐100 (soluble surfactant). The crest profiles of the waves along the center plane of the tank are measured with a laser‐induced fluorescence (LIF) technique that utilizes a high‐speed camera. In clean water with wind speeds lower than 2.3 m/s (the minimum wind speed of wind‐generated waves for clean water in our tank), the breaking of the waves is initiated with a similar bulge‐capillary‐waves pattern on the forward face of the wave crest as reported in Duncan et al. (1999). When the wind speed is above 3 m/s, wind waves are generated. The wind waves strongly affect the breaking process of the mechanically generated waves. It is found that the bulge‐capillary‐waves pattern is independent of the wind, but is dramatically affected by surfactants. The slope of the back face of the wave crest decreases with increasing wind speed. At the moment of incipient breaking, the distances between the leading edge of the bulge (called the toe) and the highest point of the wave crest in all cases are linearly proportional to the surface wind drift. After the fluid in the bulge slides down the front face of the wave, the maximum horizontal distance of the toe away from the crest increases as the wind speed increases.

Numerical study of surfactant effects on the buoyancy-driven motion of a drop in a tube

We present results of our numerical study of the effect of surfactants on buoyancy-driven motion of drops in a tube at intermediate Reynolds numbers. The drop and bulk phases are treated as incompressible Newtonian fluids and simulated using a front-tracking method. The steady shapes and velocity–volume curves for drops ranging in drop size from 0.2 to 1.3 of tube radius are determined numerically. At small Bond numbers, the velocity–volume curve shows a maximum before the velocity plateaus for large drops. As the Bond number increases, the maximum in the velocity–volume curve disappears with elongated, more streamlined drop shapes consistent with previous experimental studies. For increasing Weber numbers, the rear of the drop shows a flattening followed by the development of a negative curvature. Surfactants are modeled using a Langmuir equation of state in the adsorption–desorption limit and the effect of surfactant mass transfer, fractional coverage of surfactants, and the interfacial Peclet number on the velocity–volume curve is determined. Marangoni stress generated due to the non-uniform distribution of surfactants at the interface reduces drop mobility. Reduced drop mobility is more prominent for drop sizes that are comparable to the tube diameter and is maximum when mass transfer to and from the interface is inhibited. As the fractional coverage of soluble surfactants, or the interfacial Peclet number increases, larger Marangoni stresses are generated along the interface that lead to greater retardation of the drop motion.

Impact of pH and temperature on phase diagrams of different aqueous biphasic systems

The phase diagrams of aqueous biphasic systems impart a distinct idea regarding the feasibility of biphase formation by different water soluble substances at their optimum concentrations. Depending on nature of the components viz., the water soluble polymers, surfactants, salts, amino acids or ionic liquids, a general trend of the biphase formation with varying temperature, pH and concentration has been studied over the recent years. This critical review is an endeavor to assess the general trends of these phase forming components to form biphasic systems with varying conditions of temperature and pH in light of the reported phase diagrams. Suitable explanations for the mechanisms of such behavior have been sorted out. The avenue yet to be explored has been addressed as these systems have a tremendous potential to be the future platform to solve different analytical issues.

Investigating the role of a poorly soluble surfactant in a thermally driven 2D microfoam.

Foam drainage dynamics is known to be strongly affected by the nature of the surfactants stabilising the liquid/gas interface. In the present work, we consider a 2D microfoam stabilized by both soluble (sodium dodecylsulfate) and poorly soluble (dodecanol) surfactants. The drainage dynamics is driven by a thermocapillary Marangoni stress at the liquid/gas interface [V. Miralles et al., Phys. Rev. Lett., 2014, 112, 238302] and the presence of dodecanol at the interface induces interface stress acting against the applied thermocapillary stress, which slows down the drainage dynamics. We define a damping parameter that we measure as a function of the geometrical characteristics of the foam. We compare it with predictions based on the interface rheological properties of the solution.

Self-assembly of a water soluble perylene and surfactant into fluorescent supramolecular ensembles sensitive to acetylcholinesterase activity

A self-assembled supramolecular probe based on perylene diimide exhibits a ratiometric response to the enzymatic activity of AChE by a surfactochromic effect.

Preparation of Thermosensitive Microcapsules Containing Water Soluble Powder by Melting Dispersion Cooling Method

It was tried to prepare the thermosensitive microcapsules containing the water soluble solid powder by the melting dispersion cooling method and to establish the optimum preparation conditions. As a model water soluble solid powder, sodium hydrogen carbonate was adopted in order to generate carbon dioxide gas and as a thermosensitive shell material, olefin resin with the melting point of ca. 40°C was used. In the experiment, the concentration of olefin resin in the shell material solution was mainly changed together with the concentrations of the oil soluble surfactant species and the α-tocopherol as a modifier of shell. Addition of α-tocopherol into the shell material solution could prevent the core from breaking away during the microencapsulation process and result in the higher microencapsulation efficiency, because the dispersion stability of solid powder in the shell material solution could be increased due to the increase in affinity between the shell material solution and solid powder. Also, the microencapsulation efficiency increased with the concentration of olefin resin, became maximum at 50 wt% and then, decreased. The microcapsules were found to begin melting at 36°C and to generate carbon dioxide gas.

Analysis of improved Lattice Boltzmann phase field method for soluble surfactants

In this paper we present a novel Lattice Boltzmann model for immiscible fluids with soluble surfactants adsorbing at the interface with improved numerical and extended physical properties. The numerical improvements are based on the use of an analytical representation of a regularized delta-function in the surface free energy functional for the surfactant. Furthermore, the physics of the system have been extended to differential solubility of the surfactant combined with the use of Frumkin sorption behaviour. This enables the scheme to approach more realistic systems like foams and emulsions. This novel scheme is much superior in numerical stability than our previous scheme, based on a squared gradient approximation. Furthermore, we have observed the phenomenon of interface broadening under certain conditions. This phenomenon limits the surface pressure to about 30% of the capillary pressure of a bare droplet. It remains to be investigated whether this interface broadening reflects some physical effect, as has been observed for proteins.

Recycled polyamide mortars modified with non-ionic surfactant: physical and mechanical strength after durability tests

This paper presents an investigation into the incorporation of polyamide powder waste in mortars with a polymeric organic soluble non-ionic surfactant. Several samples were prepared by replacing the aggregate with amounts of 0, 25, 50, 75 and 100 % polyamide powder, to obtain lightweight materials. It was found that the presence of powder waste modifies the characteristics of fresh and hardened mortar while maintaining suitable properties such as workable life, water retention, vapour permeability and bonding in mixtures with up to 50 % of sand replaced by polyamide. The surfactant modifies the matrix microstructure and contributes to the hydration of the various cement phases. The assessed mechanical strength after performing the accelerating ageing tests remains sufficient with moderate amounts of waste, meaning that these mortars possess good overall durability. The experimental results showed that the addition of polyamide waste makes it possible to obtain lightweight materials more environmentally sustainable for use as masonry mortars.

DCl Transport through Dodecyl Sulfate Films on Salty Glycerol: Effects of Seawater Ions on Gas Entry.

Gas-liquid scattering experiments were employed to measure the entry and dissociation of the acidic gas DCl into salty glycerol coated with dodecyl sulfate ions (DS(-) = CH3(CH2)11OSO3(-)). Five sets of salty solutions were examined: 0.25 and 0.5 M NaCl, 0.25 M MgCl2, 0.25 M CaCl2, and artificial sea salt. DS(-) bulk concentrations were varied from 0 to 11 mM, generating DS(-) surface coverages of up to 34% of a compact monolayer, as determined by surface tension and argon scattering measurements. DS(-) surface segregation is enhanced by the dissolved salts in the order MgCl2 ≈ CaCl2 > sea salt > NaCl. We find that DCl penetration through the dodecyl chains decreases at first gradually and then sharply as more chains segregate to the surface, dropping from 70% entry on bare glycerol to 11% for DS(-) surface concentrations of 1.8 × 10(14) cm(-2). When plotted against DS(-) surface concentration, the DCl entry probabilities fall within a single band for all solutions. These observations imply that the monovalent Na(+) and divalent Ca(2+) and Mg(2+) ions do not bind differently enough to the ROSO3(-) headgroup to significantly alter the diffusive passage of DCl molecules through the dodecyl chains at the same DS(-) chain density. The chief difference among the salts is the greater propensity for the divalent salts to expel the soluble ionic surfactant to the surface.

Lipid nanocapsules containing the non-ionic surfactant Solutol HS15 inhibit the transport of calcium through hyperforin-activated channels in neuronal cells

Hyperforin is described as a natural antidepressant inhibiting the reuptake of neurotransmitters and also activating cation channels. However the blood–brain barrier limits the access to the brain of this biomolecule. To circumvent this problem it was envisaged to encapsulate hyperforin into biomimetic lipid nano-carriers like lipid nanocapsules (LNCs). When testing the safety of 25 nm LNCs it appeared that they strongly blocked hyperforin-activated Ca2+ channels of cultured cortical neurons. This inhibition was due to one of their main component: solutol HS15 (polyoxyethylene-660-12-hydroxy stearate), a non-ionic soluble surfactant. Solutol HS15 rapidly depresses in a concentration-dependent manner the entry of Ca2+ through hyperforin-activated channels without influencing store-operated channels. This effect is mimicked by Brij58 but not by PEG600, indicating that the lipid chain of Solutol HS15 is important in determining its effects on the channels. The inhibition of the Ca2+ fluxes depends on the cellular cholesterol content; it is stronger after depleting cholesterol with methyl-β-cyclodextrin and is nearly absent on cells cultured in a cholesterol-rich medium. When chronically applied for 24 h, Solutol HS15 slightly up-regulates the entry of Ca2+ through hyperforin-activated channels. Similar observations were made when testing 25 nm lipid nanocapsules containing the surfactant Solutol HS15. Altogether, this study shows that Solutol HS15 perturbs in a cholesterol-dependent manner the activity of some neuronal channels. This is the first demonstration that LNCs containing this surfactant can influence cellular calcium signaling in the brain, a finding that can have important clinical implications.

Simulations of impinging droplets with surfactant-dependent dynamic contact angle

An arbitrary Lagrangian–Eulerian (ALE) finite element scheme for computations of soluble surfactant droplet impingement on a horizontal surface is presented. The numerical scheme solves the time-dependent Navier–Stokes equations for the fluid flow, scalar convection–diffusion equation for the surfactant transport in the bulk phase, and simultaneously, surface evolution equations for the surfactants on the free surface and on the liquid–solid interface. The effects of surfactants on the flow dynamics are included into the model through the surface tension and surfactant-dependent dynamic contact angle. In particular, the dynamic contact angle (θd) of the droplet is defined as a function of the surfactant concentration at the contact line and the equilibrium contact angle (θe0) of the clean surface using the nonlinear equation of state for surface tension. Further, the surface forces are included into the model as surface divergence of the surface stress tensor that allows to incorporate the Marangoni effects without calculating the surface gradient of the surfactant concentration on the free surface. In addition to a mesh convergence study and validation of the numerical results with experiments, the effects of adsorption and desorption surfactant coefficients on the flow dynamics in wetting, partially wetting and non-wetting droplets are studied in detail. It is observed that the effects of surfactants are more in wetting droplets than in the non-wetting droplets. Further, the presence of surfactants at the contact line reduces the equilibrium contact angle further when θe0 is less than 90°, and increases it further when θe0 is greater than 90°. Nevertheless, the presence of surfactants has no effect on the contact angle when θe0=90°. The numerical study clearly demonstrates that the surfactant-dependent contact angle has to be considered, in addition to the Marangoni effect, in order to study the flow dynamics and the equilibrium states of surfactant droplet impingement accurately. The proposed numerical scheme guarantees the conservation of fluid mass and of the surfactant mass accurately.

CO2-Soluble Ionic Surfactants and CO2 Foams for High-Temperature and High-Salinity Sandstone Reservoirs

The sweep efficiency of CO2 enhanced oil recovery can be improved by forming viscous CO2-in-water (C/W) foams that increase the viscosity of CO2. The goal of this study is to identify CO2-soluble ionic surfactants that stabilize C/W foams at elevated temperatures up to 120 °C in the presence of a high salinity brine using aqueous phase stability, static and dynamic adsorption, CO2 solubility, interfacial tension, foam bubble size, and foam viscosity measurements. An anionic sulfonate surfactant and an amphoteric acetate surfactant were selected to achieve good thermal and chemical stability, and to minimize adsorption to sandstone reservoirs in the harsh high-salinity high-temperature brine. The strong solvation of the surfactant head by the brine phase and surfactant tail by CO2 allows efficient reduction of the C/W interfacial tension, and the formation of viscous C/W foams at high salinity and high temperature. Furthermore, the effect of temperature and methane dilution of CO2 on foam viscosity was eva...

Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface.

We report on experiments that probe photosensitized chemistry at the air/water interface, a region that does not just connect the two phases but displays its own specific chemistry. Here, we follow reactions of octanol, a proxy for environmentally relevant soluble surfactants, initiated by an attack by triplet-state carbonyl compounds, which are themselves concentrated at the interface by the presence of this surfactant. Gas-phase products are determined using PTR-ToF-MS, and those remaining in the organic layer are determined by ATR-FTIR spectroscopy and HPLC-HRMS. We observe the photosensitized production of carboxylic acids as well as unsaturated and branched-chain oxygenated products, compounds that act as organic aerosol precursors and had been thought to be produced solely by biological activity. A mechanism that is consistent with the observations is detailed here, and the energetics of several key reactions are calculated using quantum chemical methods. The results suggest that the concentrating nature of the interface leads to its being a favorable venue for radical reactions yielding complex and functionalized products that themselves could initiate further secondary chemistry and new particle formation in the atmospheric environment.

Enhanced physical stabilization of fenofibrate nanosuspensions via wet co-milling with a superdisintegrant and an adsorbing polymer

Drug nanoparticles in suspensions can form aggregates leading to physical instability, which is traditionally mitigated using soluble polymers and surfactants. The aim of this paper was to explore common superdisintegrants, i.e., sodium starch glycolate (SSG), croscarmellose sodium (CCS), and crospovidone (CP), as novel class of dispersants for enhanced stabilization of fenofibrate (FNB), a model BCS Class II drug, suspensions. FNB was wet-milled with superdisintegrants along with hydroxypropyl methylcellulose (HPMC), a soluble adsorbing polymer, in a stirred media mill. For comparison, FNB was also milled in the presence of HPMC and/or SDS (sodium dodecyl sulfate) without superdisintegrants. Laser diffraction, scanning electron microscopy, viscometry, differential scanning calorimetry, and powder X-ray diffraction were used to characterize the suspensions. The results show that 2% HPMC along with 1% SSG or 1% CCS mitigated the aggregation of FNB nanoparticles significantly similar to the use of either 5% HPMC or 1% HPMC–0.075% SDS, whereas CP was not effective due to its low swelling capacity. CCS/SSG enhanced steric–kinetic stabilization of the FNB suspensions owing to their high swelling capacity, viscosity enhancement, and physical barrier action. Overall, this study provides a mechanistic basis for a novel method of formulating surfactant-free drug nanosuspensions with co-milled superdisintegrants.

Stable finite element approximations of two-phase flow with soluble surfactant

A parametric finite element approximation of incompressible two-phase flow with soluble surfactants is presented. The Navier–Stokes equations are coupled to bulk and surfaces PDEs for the surfactant concentrations. At the interface adsorption, desorption and stress balances involving curvature effects and Marangoni forces have to be considered. A parametric finite element approximation for the advection of the interface, which maintains good mesh properties, is coupled to the evolving surface finite element method, which is used to discretize the surface PDE for the interface surfactant concentration. The resulting system is solved together with standard finite element approximations of the Navier–Stokes equations and of the bulk parabolic PDE for the surfactant concentration. Semidiscrete and fully discrete approximations are analyzed with respect to stability, conservation and existence/uniqueness issues. The approach is validated for simple test cases and for complex scenarios, including colliding drops in a shear flow, which are computed in two and three space dimensions.