Surfactant Chemical Potential Research Articles

The evolution of equilibrium poly(styrene sulfonate) and dodecyl trimethylammonium bromide supramolecular structure in dilute aqueous solution with increasing surfactant binding

HypothesisIn the last 20 years, it has been demonstrated that oppositely charged polyelectrolyte-surfactant (PE-S) mixtures are prone to forming kinetically arrested non-equilibrium aggregates, which are present in the prepared mixtures from rather low surfactant-to-polymer-repeat-unit ratios. Practically, this means that the PE-S mixtures used for the structural investigations of the formed PE-S complexes are typically a mixture of the primary PE-S complexes and large non-equilibrium aggregates of close to charge-neutral complexes. ExperimentsIn this work, we present a unique approach that allows the preparation of PE-S mixtures in the equilibrium one-phase region (surfactant binding β, is typically below 80%) without forming non-equilibrium aggregates. We used this method to prepare equilibrium, non-aggregated complexes of sodium poly(styrene sulfonate) (NaPSS, Mw = 17 kDa) and dodecyltrimethylammonium bromide (DTAB) (β = 10 – 70%) both in water and in an inert electrolyte (100 mM NaCl). The evolution of the complex structure was monitored by small-angle X-ray scattering (SAXS) as a function of increasing surfactant binding (β), and the measured scattering data were fitted by suitable structural models on an absolute scale where concentrations, compositions, and scattering contrasts calculated from molecular properties are used as restraints. FindingsWe could show that at low binding (β < 30%), the system is a mixture of bare polyelectrolyte coils and NaPSS-DTAB complexes containing a closed surfactant associates of low aggregation number wrapped by the polyelectrolyte chain. Once all polymer chains are occupied by a micelle-like surfactant aggregate, the aggregation number increases linearly with increasing surfactant chemical potential. Using the structural insight provided by the SAXS measurements, we could fit the experimental binding isotherm data with a physically coherent, simple thermodynamic model. Finally, we also compared the stoichiometric NaPSS-DTAB precipitate's structure with the equilibrium complexes' structure.

Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation.

The influence of two salts (NaSCN and Na2SO4) on the micellization of a nonionic surfactant (1,2-hexanediol) is quantified using Raman multivariate curve resolution spectroscopy, combined with a generalized theoretical analysis of the corresponding chemical potential changes. Although the SCN- and SO42- anions are on opposite ends of the Hofmeister series, they are both found to lower the critical micelle concentration. Our combined spectroscopic and theoretical analysis traces these observations to the fact that in both salt solutions the ions have a greater affinity for (or are less strongly expelled from) the hydration shell of the micelle than the free surfactant monomer, as quantified using the corresponding chemical potentials and Wyman-Tanford coefficients. This probe-free experimental and theoretical analysis strategy may readily be extended to micelle formation processes involving other surfactants, salts, and cosolvents, as well as to other sorts of aggregation and binding processes.

2D Crystals in Three Dimensions: Electronic Decoupling of Single-Layered Platelets in Colloidal Nanoparticles.

2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spin-orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant's chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

The influence of micelle formation on the stability of colloid surfactant mixtures

The stability of colloidal dispersions can be severely affected by the presence of surfactants. Because surfactants can adsorb at colloidal surfaces as well as form micelles, one can expect an interplay between both phenomena. Using grand-canonical coarse-grained Monte Carlo simulations on surfactant solutions confined between two surfaces, we investigate how adsorption and micelle formation affects the effective interaction between two colloidal particles, and hence, the stability of the colloidal dispersion. For solvophilic colloidal surfaces, we observe a short-ranged oscillatory solvation pressure that is hardly affected by the presence of surfactants in the system. The effective surface-surface interaction, however, reveals a decrease in solvophilic stabilization as a function of surfactant chemical potential. For solvophobic surfaces, we find that the capillary evaporation observed in a confined pure solvent, is counteracted by the addition of surfactants. Around the critical micelle concentration (CMC), the surface-surface interaction even becomes repulsive, enhancing stabilization of the colloidal dispersion. In contrast, the formation of micelles at concentrations above the CMC causes an additional depletion effect, resulting in an effective attraction, which in turn can destabilize a colloidal dispersion.

Self-Consistent Field Modeling of Adsorption from Polymer/Surfactant Mixtures

We report on the development of a self-consistent field model that describes the competitive adsorption of nonionic alkyl-(ethylene oxide) surfactants and nonionic polymer poly(ethylene oxide) (PEO) from aqueous solutions onto silica. The model explicitly describes the response to the pH and the ionic strength. On an inorganic oxide surface such as silica, the dissociation of the surface depends on the pH. However, salt ions can screen charges on the surface, and hence, the number of dissociated groups also depends on the ionic strength. Furthermore, the solvent quality for the EO groups is a function of the ionic strength. Using our model, we can compute bulk parameters such as the average size of the polymer coil and the surfactant CMC. We can make predictions on the adsorption behavior of either polymers or surfactants, and we have made adsorption isotherms, i.e., calculated the relationship between the surface excess and its corresponding bulk concentration. When we add both polymer and surfactant to our mixture, we can find a surfactant concentration (or, more precisely, a surfactant chemical potential) below which only the polymer will adsorb and above which only the surfactant will adsorb. The corresponding surfactant concentration is called the CSAC. In a first-order approximation, the surfactant chemical potential has the CMC as its upper bound. We can find conditions for which CMC < CSAC . This implies that the chemical potential that the surfactant needs to adsorb is higher than its maximum chemical potential, and hence, the surfactant will not adsorb. One of the main goals of our model is to understand the experimental data from one of our previous articles. We managed to explain most, but unfortunately not all, of the experimental trends. At the end of the article we discuss the possibilities for improving the model.

Modeling of Confinement-Induced Phase Transitions for Surfactant Layers on Amphiphilic Surfaces

A self-consistent field model is used to consider a solution of positively charged surfactants up to its critical micellization concentration adsorbing onto two surfaces in close proximity. Each surface mimics a polystyrene sulfonate interface; that is, hydrophobic properties are combined with a (fixed) negative charge. We observe large and sudden changes in adsorption as a function of separation, which are not normally considered when interpreting surface force measurements. The parameters are chosen such that the adsorbed surfactant layer is of a monolayer type when the surfaces are far apart. A typical interaction curve is presented for a fixed surfactant chemical potential, which is extracted from the set of adsorption isotherms each with a fixed slit width. When the slit width approaches the thickness of the two surfactant layers, a first-order phase transition takes place, which is driven by the unfavorable hydrophobic-water contacts. At the transition, the average orientation of the surfactants switches from a high concentration of tails at the surface to a bilayer configuration where tail profiles from both sides merge in the center. The headgroups are pulled slightly away from the surface. The interaction force jumps from a weak electrostatic repulsion at large distances (two effectively positively charged surface layers repel each other) to a strong electrostatic attraction at short distances (the central surfactant bilayer is attracted to the oppositely charged surfaces). The amount of adsorbed surfactants tend to decrease with decreasing distance between the surfaces but suddenly increases at the transition. Because of this, we anticipate that in surface force experiments, for example, there is a hysteresis associated with this transition: the forces and also the adsorbed amounts depend not only on the distance between the surfaces but also on the history if nonsufficient equilibration times are implemented.

Investigation of Surfactant Efficiency Using Dissipative Particle Dynamics

We have used dissipative particle dynamics (DPD) to simulate surfactant monolayers on the interface between oil and water. With a simple surfactant model, we investigate how variations in size and structure of surfactants influence their ability to reduce the interfacial tension. In particular, we studied the effect of branching of the hydrophobic tail. We found that branched surfactants are more efficient at the interface than linear ones only if the head groups are sufficiently hydrophilic to prevent the molecules from staggering. By combining DPD with a Monte Carlo method, we have imposed constant surfactant chemical potential and (normal) pressure in separate simulations of bulk and interface. From this, we can determine the bulk concentration needed to obtain a given interfacial tension. We found that higher concentrations of branched surfactants are required to obtain the same reduction of the interfacial tension. We argue that the stronger excluded volume interactions which make branched surfactants more efficient at the interface compared to their linear isomers at the same time make them less inclined to adsorb at the interface.

A potentiometric study of the adsorption of a cationic surfactant onto laponite in water–methanol and water–dimethylsulfoxide mixtures

We studied the adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite in water–methanol and water–DMSO mixtures at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA + and Br −-selective electrodes. All of the isotherm shapes in the mixture were classical. Activity coefficients were determined for the transfer between water and hydroorganic mixtures. Comparison of the adsorption isotherms obtained for different surfactant chemical potentials showed that at higher concentrations of free surfactant, the aggregation state does not depend on the nature of the surrounding media. A study of the influence of the ionic strength showed that there is a common intersection point in each hydroorganic mixture. This point corresponds to the charge compensation point and depends on the nature of the surrounding medium.

Mesoscopic Simulation of Polymer−Surfactant Aggregation

The formation of a polymer−surfactant complex has been studied in bulk solution using dissipative particle dynamics simulation of a system containing polymer, surfactant, and water. Widom insertion is used to obtain the surfactant chemical potential. Two distinct modes of surfactant adsorption on the polymer have been observed, depending on the interaction mechanism. These are (a) continuous adsorption and (b) polymer−surfactant aggregation via discrete micelles. If the binding between polymer and surfactant is dominated by hydrophobic interaction (i.e., when the surfactant tails tend to adsorb on the polymer), continuous adsorption is found, leading to molecular bottlebrush or swollen cage conformations. If the binding between polymer and surfactant is dominated by the surfactant headgroups, micellar adsorption is found, leading to a necklace of micelles on a polymer backbone. For each interaction strength between polymer and surfactant headgroup, there is an interaction between polymer and surfactant ta...

Micellization in Model Surfactant Systems

Formation of micelles in model lattice surfactant systems was studied by a novel methodology based on grand-canonical Monte Carlo simulations. The methodology involves combining free-energy information from a series of simulations in small systems by histogram reweighting. The solution osmotic pressure as a function of overall volume fraction of surfactant shows a sharp break at the critical micelle concentration (cmc) at sufficiently low temperatures. Studies in larger systems at appropriate values of the surfactant chemical potential are used to investigate the size distribution of micellar aggregates. The methodology allows for a clear distiction between micellization and macroscopic phase separation. Two symmetric diblock surfactants have been considered in the present work. The cmc was found to increase with increasing temperature. The enthalpy change on micellization was determined to be proportional to the chain length of the surfactant. The mean micelle aggregation numbers were found to decrease a...

Demixing in Ternary Mixed Micelles

Micellar demixing yielding conjugate micellar phases that coexist in an aqueous solution is known for a very few surfactant systems even though the presence of two distinct types of micelles in solution has practical application. Here, on the basis of a pseudophase regular solution model for surfactant chemical potentials, we predict micellar demixing to be widely prevalent in ternary surfactant mixtures that contain anionic, cationic, and nonionic surfactants. While demixing of micelles in these ternary systems has not previously been reported, the prediction is confirmed here on the basis of surface tension measurements.

Thermodynamics of adsorption of dodecyltrimethylammonium bromide onto laponite in fused ethylammonium nitrate and its aqueous solutions

The adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite was studied in a molten salt at room temperature (θf = 14 °C), ethylammonium nitrate (EAN), and in mixtures of this salt with water at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA+ and Br−-selective electrodes. At low EAN concentrations (less than 1 mol L−1), the shape of the adsorption isotherm is classical and all curves share a common intersection point. In more concentrated salt solutions, the quantity of adsorbed amphiphile decreases with increasing EAN concentration. The activity coefficients of transfer of DTABr between water, EAN–water solutions and molten EAN were determined. A comparison of adsorption isotherms at a given surfactant chemical potential shows that the quantity of DTA+ adsorbed onto laponite is approximately four time less in molten salt than that in the 1 mol L−1 EAN aqueous solution. This may be caused by formation of an adsorbed layer constituted of both dodecyltrimethylammonium and ethylammonium ions.

Determination of micellar characteristics by measuring quantities related to the surfactant chemical potential

A simple method of finding the critical micelle concentration (cmc) and the degree of counterion binding of surfactant micelles from the experimental dependence of the activity coefficient, the osmotic coefficient and the vapour-pressure lowering of a surfactant solution on concentration is described.

Stability criteria for emulsions.

The coalescence of monodisperse silicone oil-in-water emulsions stabilized with sodium dodecyl sulfate has been studied. We report the existence of a sharp destabilization threshold, controlled by surfactant chemical potential, osmotic pressure, and droplet diameter, at which the rate of coalescence increases dramatically. We present evidence that the stability of the emulsions can be characterized by two microscopic parameters: a minimum stable value of the surfactant chemical potential and a maximum value of the pressure exerted upon a droplet-droplet interface.

Flocculation and Coalescence of Oil-in-Warer Emulsions

AbstractEmulsions are commonly made of macrosized droplets dispersed in a continuous solvent. These systems are subject to two types of instabilities. One is reversible: flocculation, the other is irreversible: coalescence. The reversible flocculation induced by excess surfactant is studied. It is shown that an attractive interaction arises from depletion of surfactant micelles and leads to a fluid-solid phase transition. The sensitivity of this phase transition to the oil droplet diameter leads to a fractionated crystallisation method for purifying in size highly polydisperse emulsions. These well characterized systems are used to study the stability of the film against coalescence. Two distinct regimes of coalesence are found. One is driven by the surfactant chemical potential and occurs in a dilute emulsion. The other is driven by the water chemical potential and takes place in a concentrated emulsion which ressembles a biliquid foam.

A novel approach to the mechanics and thermodynamics of spherical micelles

A detailed mechanical theory for a spherical micelle is presented, based upon the notion that a micelle has the nature of an interface throughout. Accordingly, we presume that the (ensemble average) pressure tensor is generally anisotropic in a micelle except at r= 0 (for symmetry reasons). Following the Gibbs scheme for treating curved interfaces, the surface tension is usually defined with reference to the surface of tension (s.o.t.). In the case of a micelle, however, this method is inconvenient since the location of the s.o.t. is highly variable and difficult to assess a priori. An alternative approach is developed whereby the hydrocarbon core volume is employed to define the hydrophobic dividing surface (h.d.s.) located at r=R, which separates the internal (i) core part of the micelle from the exterior (e) water solution phase. In order to achieve a complete thermodynamic description it is then necessary to introduce an additional intensive variable, the interaction (disjoining) pressure πm. At the h.d.s., the Laplace equation can be written on the modified form pi–pe= 2γe/R–πm where pi is the average tangential pressure within the hydrocarbon core and γe is the surface tension originating outside r=R. πm is normally attractive (< 0) and is due chiefly to the hydrocarbon-chain packing constraints and the effect of curvature on the electrostatic interactions.Surfactant micelles are very small systems for which fluctuations of the aggregation number, N, are important. Hence the surface thermodynamics of micelles is calculated, taking account of the general principles of the thermodynamics of small systems developed by Hill. On this basis we show that, at phase equilibrium, the average surfactant chemical potential in the micellar state, 〈µN〉, (but not each µN separately) is equal to the monomer chemical potential in the intermicellar solution and, moreover, that the average net work expended to form a micelle with surface area A is equal to 〈(γe+πmR)A/3〉. At aggregation equilibrium this work is counterbalanced by free-energy gains of entropic origin caused by the dispersion of the micelles in the solution and their size fluctuations.Our model calculations for SDS micelles show that a consistent, quantitative theory of micelle formation can be obtained by using (i) Tanford's expressions for the solubility of hydrocarbons in water, (ii) the Poisson–Boltzmann/cell-model treatment of the electrostatics of a uniformly charged sphere, (iii) the macroscopic excess free energy, 50 mJ m–2, to account for the hydrocarbon-core/water contact and (iv) the conformation free-energy function for hydrocarbon chains packed in a spherical aggregate computed by Gruen and de Lacey. The large and negative πm components caused by factors (ii) and (iv) strongly promote micelle formation. The average pressure level in the hydrocarbon core is predominantly determined by the Laplace term 2γe/R and may typically amount to several hundred atmospheres, in agreement with experiments on the solubility of gases in micellar solutions.

Interfacial Tension and Phase Behavior of Surfactant Systems

Abstract The conditions necessary for optimum low tension and phase behavior at high surfactant concentrations are compared with those required at low surfactant concentrations, where solubilization effects are not usually visible. Major differences in tension behavior between the high and low concentration systems may be observed when the surfactant used contains a broad spectrum of molecular species, or if a higher molecular weight alcohol is present, but not otherwise in the systems studied. We compared the effects of a number of aliphatic alcohols on tension with phase behavior. An explanation of these results, and also of other observed parameter dependences, is proposed in terms of changes in surfactant chemical potential. Surfactant partitioning data is presented that supports this concept. Introduction Taber and Melrose and Brandner established that tertiary oil recovery by an immiscible flooding process should be possible at low capillary process should be possible at low capillary numbers. In practice, the required capillary number, which is a measure of the ratio of viscous to capillary forces governing displacement of trapped oil, may be achieved by lowering the oil/water interfacial tension to about 10(-3) dyne/cm, or less. Subsequent research has identified a number of surfactants that give tensions of this order with crude oils and hydrocarbon equivalents. Interfacial tension studies tended to fall into two groups. Work at low surfactant concentrations, typically 0.7 to 2 g/L, has established that a crude oil may be assigned an equivalent alkane carbon number. Using pure alkanes instead of crude oil has helped the study of system parameters affecting low tension behavior. Important parameters examined include surfactant molecular structure, and electrolyte concentration, surfactant concentration, surfactant molecular weight, and temperature. At higher surfactant concentrations, interfacial tension has been linked to the phase behavior of equilibrated systems. When an aqueous phase containing surfactant (typically 30 g/L), electrolyte, and low molecular weight alcohol is equilibrated with a hydrocarbon, the surfactant may partition largely into the oil phase, into the aqueous phase, or it may be included in a third (middle) phase containing both water and hydrocarbon. Low interfacial tensions occur when the solubilization of the surfactant-free phase (or phases) into the surfactant-containing phase is maximized. Maximum solubilization and minimum tensions have been shown to be associated with the formation of a middle phase. Both the high and low surfactant concentration studies have practical importance because even though a chemical flood starts at high concentration, degradation of the injected surfactant slug will move the system toward lower concentrations. This study investigates the relationship between tension minima found with low concentration systems, and low tensions found with equivalent systems at higher surfactants concentrations, particularly those in which third-phase formation occurs. Many of the systems studied here contain a low molecular weight alcohol, as do most surfactant systems described in the literature or proposed for actual oil recovery. Alcohol originally was added to surfactant systems to help surfactant solubility, but can affect tensions obtained with alkanes, and with refined oil. Few systematic studies of the influence of alcohol on tension behavior exist. Puerto and Gale noted that increasing the alcohol Puerto and Gale noted that increasing the alcohol molecular weight decreases the optimum salinity for maximum solubilization and lowest tensions. The same conclusions were reached by Hsieh and Shah, who also noted that branched alcohols had higher optimum salinities than straight-chain alcohols of the same molecular weight. Jones and Dreher reported equivalent solubilization results with various straight- and branched-chain alcohols. In this study, we fix the salinity of each system and instead vary the molecule; weight of the hydrocarbon phase. SPEJ P. 242