Equilibrium Surfactant Concentration Research Articles

General rheology of highly concentrated emulsions with insoluble surfactant

A general constitutive model is constructed and validated for highly concentrated monodisperse emulsions of deformable drops with insoluble surfactant through long-time, large-scale and high-resolution multidrop simulations. There is the same amount of surfactant on each drop, and the linear model is assumed for the surface tension versus the surfactant concentration. The surfactant surface transport is coupled to multidrop hydrodynamics through the convective–diffusive equation and the interfacial stress balance. Only the limit of small surfactant diffusivities is addressed, when this parameter does not affect the rheology. An Oldroyd constitutive equation is postulated, with five variable coefficients depending on one instantaneous flow invariant (chosen as the drop-phase contribution to the dissipation rate). These coefficients are found by fitting the model to five precise rheological functions from two steady base flows at arbitrary deformation rates. One base flow is planar extension (PE) ($\dot{\unicode[STIX]{x1D6E4}}x_{1},-\dot{\unicode[STIX]{x1D6E4}}x_{2},0$), the other one is planar mixed flow (PM) ($\dot{\unicode[STIX]{x1D6FE}}x_{2}$, $\dot{\unicode[STIX]{x1D6FE}}\unicode[STIX]{x1D712}x_{1}$, 0) with $\unicode[STIX]{x1D712}=0.16$. A small but finite $\unicode[STIX]{x1D712}$ (a precise choice in the range $\unicode[STIX]{x1D712}\sim 0.1$ is unimportant) provides a necessarily perturbation to exclude severe ergodic difficulties and abnormal, kinked behaviour inherent in simple shear for high drop volume fractions $c$, especially at small capillary numbers $Ca$ and small drop-to-medium viscosity ratios $\unicode[STIX]{x1D706}$. The database rheological functions are obtained for $c=0.45{-}0.6$, $\unicode[STIX]{x1D706}=0.25{-}3$ and surfactant elasticities $\unicode[STIX]{x1D6FD}=0.05{-}0.2$ (based on the equilibrium surfactant concentration) from long-time simulations by a multipole-accelerated boundary-integral code with $N=100{-}200$ drops in a periodic cell and 2000–4000 boundary elements per drop. The code is an extension from Zinchenko & Davis (J. Fluid Mech., vol. 779, 2015, pp. 197–244) to account for surfactant transport and Marangoni stresses. Massive drop cusping or (sometimes) drop break-up limit the range of $Ca$ from above in the base flows, but there is no substantial lower limitation owing to the absence of phase transition difficulties. At small $\unicode[STIX]{x1D706}$, even minimal surface contamination may have a strong effect on the rheology. The simulations remain accurate for quite strong drop interactions, when the PE emulsion viscosity is nine times that for the carrier fluid. The model validation against a steady PM flow with a different $\unicode[STIX]{x1D712}=0.5$ shows a very good agreement for various $Ca$, $c$ and $\unicode[STIX]{x1D706}$. In the three PE and PM time-dependent flow tests, the quasi-steady approximation is found to predict stresses poorly. In contrast, the combination of the steady-state results for PE and PM used in the present method to generate the Oldroyd parameters gives a model with much better predictions for these time-dependent flows.

Effects of a surfactant monolayer on the measurement of equilibrium interfacial tension of a drop in extensional flow

The effect of surfactant monolayer concentration on the measurement of interfacial surface tension using transient drop deformation methods is studied using the Boundary Integral Method. Emulsion droplets with a surfactant monolayer modeled with the Langmuir equation of state initially in equilibrium are suddenly subjected to axisymmetric extensional flows until a steady state deformation is reached. The external flow is then removed and the retraction of the drops to a spherical equilibrium shape in a quiescent state is simulated. The transient response of the drop to the imposed flow is analyzed to obtain a characteristic response time, τ s ∗ . Neglecting the initial and final stages, the retraction process can be closely approximated by an exponential decay with a characteristic time, τ r ∗ . The strength of the external flow on each model drop is increased in order to investigate the coupled effect of deformation and surfactant distribution on the characteristic relaxation time. Different model drops are considered by varying the internal viscosity and the equilibrium surfactant concentrations from a surfactant free state (clean) to high concentrations approaching the maximum packing limit. The characteristic times obtained from the simulated drop dynamics both in extension and retraction are used to determine an apparent surface tension employing linear theory. In extension the apparent surface tension under predicts the prescribed equilibrium surface tension. The error increases monotonically with the equilibrium surfactant concentration and diverges as the maximum packing limit is approached. In retraction the apparent surface tension under predicts the prescribed equilibrium surface tension depends non-monotonically on the equilibrium surfactant concentration. The error is highest for moderate surfactant concentrations and decreases as the maximum packing limit is approached. It was found that the difference between the prescribed surface tension and the apparent surface tension increased as the viscosity ratio decreased. Differences as large as 40% were seen between the prescribed surface tension and the apparent surface tension predicted by the linear theory.

Binding of alkylpyridinium chloride surfactants to sodium polystyrene sulfonate

Binding of cationic surfactants to anionic polymers is well studied. However, the surfactant binding characteristics at very low concentration near the start of binding and at high concentration, where charge compensation may occur, are less well known. Therefore, the binding characteristics of hexadecyl-pyridinium chloride (C 16PC), dodecyl-pyridinium chloride (C 12PC) and decyl-pyridinium chloride (C 10PC) to sodium polystyrene sulfonate (SPSS) are investigated over a wide concentration range. The equilibrium surfactant concentration is measured with a surfactant-selective membrane electrode. The electrode shows good performance for the three surfactants, also in presence of SPSS. The observed critical micelle concentrations (CMCs) of the surfactants as a function of the ionic strength agree well with literature values and the CMC is not significantly affected by the presence of SPSS. For each surfactant, binding isotherms are obtained from depletion studies at three different NaCl concentrations. For C 16PC and C 12PC the binding isotherms at different NaCl concentration have a common intersection point (CIP) at relatively high surfactant concentration. For C 10PC the experimental accuracy is too small to observe a CIP. The CIP closely corresponds with the bound amount at the iso-electric point (IEP) of the surfactant–SPSS complex. At the IEP/CIP the charge of SPSS is compensated by bound surfactant ions. In region between the CIP and the CMC the polyelectrolyte charge is overcompensated by surfactant ions. Similar behavior has been found before with the same surfactants and humic acid as polyelectrolyte. At very low surfactant concentrations the C 10P + ions bind on isolated sites along the SPSS chain, the C 12P + ions form some dimers and the C 16P + ions form, already at the lowest concentration that could be measured, aggregates. At somewhat higher concentrations also C 10PC and C 12PC start to form small aggregates along the SPSS chain. The concentration at which this occurs is the critical aggregation concentration (CAC). Similarly as the CMC the CAC is a function of the surfactant chain length and the ionic strength. The size of the small aggregates increases with increasing chain length, but hardly depends on the ionic strength. With increasing surfactant concentration the formation of the aggregates along the SPSS chain continues. Electrode calibration at very low concentrations C 16PC and C 12PC can be improved when the SPSS–surfactant complex is used to buffer the solutions.

Remediation of a Diesel Contaminated, Sandy-Loam Soil Using Low Concentrated Surfactant Solutions (5 pp)

Surfactant enhanced ex-situ soil washing can be used to remediate diesel contaminated soils. Surfactants enhance the diesel removal from soils by two processes: mobilization and solubilization. Mobilization occurs at surfactant concentrations below the critical micelle concentration (CMC), while solubilization occurs at surfactant concentrations beyond the CMC of the surfactant. In this paper, the leaching of diesel from a self contaminated (10 000 mg diesel/kg) sandy-loam soil was studied using low concentrated non-ionic surfactant solutions of Tergitol NP-10 (10-6 to 10-3 mol/L). The surface tension of the supernatant solutions of soil suspensions in non-ionic surfactant (Tergitol NP-10) solutions (10-6 to 10-3 mol/L) was measured to determine the total surfactant concentration (surfactant sorbed onto the soil + surfactant in solution) at which micelles are present in the water phase of the soil suspension, or 'effective critical micelle concentration' (ECMC), and to calculate the sub-CMC sorption isotherm of Tergitol NP-10 onto the soil. The diesel removal was measured by soil leaching experiments with Tergitol solutions ranging from 10-6 to 10-3 mol/L. The sub-CMC sorption isotherm of Tergitol onto the soil could be described by the Freundlich equation. Even at very low surfactant concentrations (10-6 mol/L) the surfactant enhanced the diesel removal from the soil. Up to the surfactants' ECMC the diesel removal increased in a linear way with an increasing surfactant concentration in the wash water. A significant part (20%) of the diesel oil was removed in the presence of surfactant, but in absence of micelles. Beyond the surfactants' ECMC, the increase in diesel removal efficiency with increasing surfactant concentration was less pronounced. When the added surfactant concentration was increased to 10-3 mol/L, which corresponds to an equilibrium surfactant concentration in the supernatant solution (6.77.10-4 mol/L) above the CMC, the diesel oil removal was increased up to 50%. At this surfactant concentration emulsion formation was observed. Surfactant aided leaching of diesel from a self-contaminated, sandy-loam soil with surfactant solutions below their ECMC was able to remove in one step 20% of diesel from the soil by the mobilization mechanism. Succesive treatments or continuous leaching with surfactant solutions below their ECMC was not studied but is likely to reduce the diesel contamination further and at the same time avoid problems with emulsion formation.

Adsorption of different amphiphilic molecules onto polystyrene latices

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain–surface and chain–chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.

Application of ordered mesoporous silica in adsolubilisation of alcohols by conventional and gemini cationic surfactants

Application of ordered porous silica in adsolubilisation of primary and aromatic alcohols by conventional and gemini cationic surfactants has been tested in aqueous media at 298 K. Benzyldimethyldodecylammonium bromide (BDDAB) and two alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) surfactants (C 12C SC 12) with ethanediyl (C 2) or hexanediyl (C 6) spacer were adsorbed onto microporous SiAl32 d18 or mesoporous SiAl32 d50 aluminosilicate of the MCM-41 type (the framework Si:Al ratio is equal to 32; d18 or d50 refers to the mean pore diameter in angstroms). The reversibility of adsorption of both surfactant ions, i.e. the surface-active cation and its counter-ion, the ion exchange with sodium cations, the evolution of some other parameters with surfactant adsorption, such as the pH of the equilibrated supernatant, the electrophoretic mobility of the solid particles and the differential molar enthalpy of displacement, were first studied taking BDDAB and purely siliceous Si d29 as an example of the adsorption system. Adsolubilisation of 1-butanol (C 4OH) by BDDAB adsorbed onto SiAl32 d18 and SiAl32 d50 was quantified either as a function of the equilibrium alcohol concentration at a fixed surfactant content of 12 mmol kg −1 or as a function of the surfactant concentration from a 10 mmol kg −1 NaBr solution at a fixed alcohol content of 10 mmol kg −1. The uptake of phenol (PhOH) by C 12C SC 12–SiAl32 d50 systems was monitored as a function of the equilibrium surfactant concentration at a fixed alcohol content of 1.5 mmol kg −1. The individual adsorption isotherms of the surface-active cation and alcohol additive were supplemented by the measurements of the differential molar enthalpy of displacement in systems with and without additive. Transfer of solubilisate molecules from the aqueous phase to the interfacial aggregates contributes to the overall exothermicity of the displacement phenomenon. Compared to non-porous silica supports, ordered mesoporous aluminosilicates are much more efficient as adsolubilisation host systems for low molecular weight alcohols.

Effect of charged colloidal particles on adsorption of surfactants at oil–water interface

A change of oil/water interfacial tension in the presence of cationic or anionic surfactants in an organic phase was observed due to the addition of charged fine solids in the aqueous phase. The charged fine solids in the aqueous phase adsorb surfactants diffused from the oil phase, thereby causing an increase in the bulk equilibrium surfactant concentration in the aqueous phase, governed by the Stern–Grahame equation. Consequently, surfactant adsorption at the oil–water interface increases, which was demonstrated from the measured reduction of the oil–water interfacial tension. The increased surfactant partition in the aqueous phase in the presence of the charged particles was confirmed by the measured decrease in the surface tension for the collected aqueous solution after solids removal, as compared with the cases without solids addition.

The distribution of oleic acid between salbutamol base drug and different propellant blends.

The distribution of oleic acid between Salbutamol base drug and the solvent in metered-dose inhalers (MDI's) has been investigated. The equilibrium surfactant concentration in the drug dispersions has been determined using a colorimetric method. The samples examined contained Salbutamol base drug particles and oleic acid dispersed in different propellant blends of freon 11 and 12. The maximum equilibrium concentration observed depended on the propellant blend used. The propellant blend and the distribution of the surfactant affected the dispersion stability. The effect of the surfactant is illustrated by the adsorption isotherm for oleic acid onto the Salbutamol particles. The results are correlated with zeta-potentials and particle size measurements made on similar systems in order to characterize the properties of surfactant stabilized MDI's.

Stability and permeability of amphiphile bilayers

In this review the rupture and permeability of bilayers are considered on the basis of a mechanism of the formation of microscopic holes as fluctuations in the bilayers. The hole formation is treated as a nucleation process of a new phase in a two-dimensional system with short-range intermolecular forces. Free rupture and deliberate rupture (by α-particles) of foam bilayers (Newtonian black films) are discussed. A comparison is made between the rupture of foam and emulsion bilayers. Experimental methods for obtaining foam and emulsion bilayers from thin liquid films are considered. Methods for investigating the stability and permeability of foam bilayers, which are based on a microscopic model allowing the use of amphiphile solutions with very low concentrations, are described. Experimental dependences of the lifetime of bilayers, the probability of observing the foam bilayer in a foam film, the gas permeability of bilayers, etc. on the concentration of amphiphile molecules in the solution are reported. The influence of temperature and external impact (e.g. α-particle irradiation) have also been experimentally studied. A good agreement between theory and experiment is established, allowing determination of several characteristics of foam and emulsion bilayers obtained from ionics or non-ionics: the specific edge energy of bilayer holes, equilibrium surfactant concentration below which the bilayer is thermodynamically metastable, work for the formation of a nucleus hole, number of vacancies in the nucleus hole, coefficient of gas diffusion through the bilayer, etc. On the basis of the effect of temperature on the rupture of foam bilayers the binding energy of a surfactant molecule in the bilayer is determined. The adsorption isotherm of surfactant vacancies in the foam bilayer is obtained which shows a first-order phase transition. Some applications to scientific, technological and medical problems are considered. The foam bilayer is used as a model for investigating short-range forces in biological structures, the interaction between membranes and cell fusion. It is also shown that the foam bilayer is a suitable model for studying the alveolar surface and stability. On that basis a clinical diagnostic method is developed for assessment of the human foetal lung maturity.

Effect of temperature on the rupture of newton black foam films

The effect of temperature on the rupture of foam bilayers is studied experimentally. The foam bilayers are microscopic horizontal Newton black foam films of radius 0.25 mm in contact with an aqueous solution of surfactant. The dependence of the mean time for rupture of the bilayers on the concentration of surfactant (sodium dodecyl sulfate, NaDoS) dissolved in the solution, in the presence of 0.5 mol dm −3 NaCl, is obtained at 10, 22 and 30°C. The following thermodynamic parameters: size of the nucleus hole, nucleation work and equilibrium surfactant concentration are determined for the above temperatures according to the theory of rupture of foam bilayers by hole nucleation. Values of 3.1·10 −11 J m −1 for the specific edge free energy of bilayer circular hole and 6.7·10 −20 J for the binding energy of NaDoS molecule in the foam bilayer are calculated.

The effect of surfactant and polymer addition to a fuel grade peat: Adsorption, electrokineties and dewatering

Mechanical dewatering studies on a highly decomposed peat using cationic and anionic dewatering aids were carried out. Of these products, cationic additives show some promise for application. Dewatering was carried out by pressing at 2.06 MPa for 2.5 min with added cationic or anionic surface-active agent. The effect of adsorption of two simple cationic surfactants on the electrokinetic potential of the peat particles as a function of pH was determined. In both cases, a change in the sign of the electrokinetic potential occurred at an equilibrium surfactant concentration that is pH dependent. The cationic-surfactants and -polymer improved the efficiency of dewatering up to maximum value while the anionic polymer did not. The optimum value occurred, in the case of the simple surfactants, at concentrations where the zeta potential is close to zero. The effect of chain-length dependence on surfactant adsorption on peat is discussed.

Effect of Nonionic Surfactants on Aqueous Primidone Suspensions

The following interactions between the soluble surfactant, octoxynol 9, and the very slightly soluble, finely powdered drug, primidone, in aqueous suspension were investigated: adsorption/desorption of the surfactant, micellar solubilization of the drug, and deflocculation of its particles. The last effect, measured by the sedimentation volume of the suspensions, was also investigated for other octoxynols. The adsorption of octoxynol 9 on solid primidone was proportional to the equilibrium surfactant concentration up to the critical micelle concentration. It leveled off at higher concentrations, reaching saturation at completion of a close-packed surfactant monolayer. The adsorption was essentially completely reversible. The solubility of primidone in water was very slight; its micellar solubilization was even less extensive. The sedimentation volume of primidone suspensions decreased with increasing equilibrium concentration of octoxynol 9 and began to level off at the critical micelle concentration of 0.018%. At about twice that concentration, the sedimentation volume became almost constant, but reached its lowest value only at ⩽0.5%. Slow rotation of suspensions prior to sedimentation promoted flocculation and higher sedimentation volumes between 0.01 and 0.03% octoxynol 9. Octoxynols with higher hydrophilic-lipophilic balance than octoxynol 9 produced considerably larger sedimentation volumes at comparable concentrations, due to a lower surface activity and a lesser tendency to adsorb on primidone.

The Cooperative Binding Isotherms of Sodium Alkanesulfonates to Poly(1-methyl-4-vinylpyridinium chloride)

Abstract Binding isotherms of nonane-, decane-, and undecane- sulfonate ions to poly (1-methyl-4-vinylpyridinium chloride) were determined by using nitrobenzene-membrane electrode. It was shown that an increase in alkyl chain length is accompanied not only by a rapid increase in the cooperativity parameter but by a regular decrease in an equilibrium surfactant concentration at half binding.

Nucleation mechanism of rupture of newtonian black films: II. Experiment

A mechanism of rupture of Newtonian black films is considered on the basis of the classical nucleation theory (D. Kashchiev, and D. Exerowa, J. Colloid Interface Sci. 77, 501 (1980)). This mechanism is experimentally tested by studying the mean life time τ of a microscopic Newtonian black (NB) foam film as a function of the surfactant concentration C. Ionic surfactants and the nonionic decyl-4-ethylene oxide (C 10 (EO) 4) and nonylphenol-20-ethylene oxide (NP20) are used to stabilize the investigated films. The τ( C) dependence is confronted with the theoretical expression and the following parameters of the nucleation mechanism of film rupture are determined: nucleation work, number of vacancies in the nucleus, hole specific peripheral energy, lateral bond energy between nearest surfactant molecules in the film, supersaturation, critical surfactant concentration for NB film formation, and equilibrium surfactant concentration at which the NB films is infinitely stable. By studying the probability of NB film rupture, it is shown that from a certain moment on the nucleation rate is time-independent.

The Binding of Sodium Dodecyl Sulfate to Lysozyme in Aqueous Solutions. II. The Effect of Added NaCl

Abstract The isotherms for the binding of sodium dodecyl sulfate (SDS) to lysozyme have been investigated as a function of the concentration of added NaCl at pH 5.8 and 25 °C by a potentiometric method utilizing a surfactant-selective electrode. The equilibrium surfactant concentration at which the binding begins to occur increases with increasing NaCl concentration. The equilibrium surfactant concentration giving the same binding number above 10, on the contrary, decreases with increasing NaCl concentration. Thermodynamic analysis shows that there takes place not only the exchange between chloride ion (Cl− ion) and dodecyl sulfate ion (DS− ion) but also the binding of DS− ion to the positively charged site unoccupied by Cl− ion. The addition of NaCl does not change the equilibrium SDS concentration at which the binding number begins to exceed 10. This equilibrium concentration is regarded as a characteristic one at which the affinity between lysozyme and SDS can be evaluated. The value of standard free energy change upon DS− ion binding and the number of Na+ ions bound to a DS− ion which forms a cluster on lysozyme were estimated from a thermodynamic equation based on the phase separation model.

Adsorption of a nonionic surfactant by cotton

The adsorption and desorption of polyoxyethylated 1-dodecanol containing an average of 14 ethylene oxide (EO) units on cotton was studied over a concentration range of two decades in distilled water at 25°C. The method consisted in equilibrating cotton fabric or chopped fiber with surfactant solutions and determining the equilibrium surfactant concentration by surface tension or by the cobalt blue colorimetric analysis. Surface tension is more affected by surfactant molecules of few EO units, whereas the colorimetric method emphasizes the more highly polyoxyethylated surfactant molecules; the observed discrepancy between the two methods (10%–30%) was caused by preferential adsorption of surfactant of shorter EO chain length onto cotton. The adsorption was almost completely reversible. The adsorption isotherm leveled off at 35 mg. surfactant/100 g. cotton near the critical micelle concentration. In view of the specific surface area of cotton, this level of adsorbed surfactant is of the order of the amount required to cover the cotton completely with a close-packed monolayer of surfactant molecules lying flat against the substrate.