The HLD-NAC Model for Mixtures of Ionic and Nonionic Surfactants

A methodology for measuring the characteristic curvature of technical-grade ethoxylated nonionic surfactants: the effects of concentration and dilution

Characterization of the behaviour of commercially available non-ionic surfactants has received considerable attention due to their efficacy in a variety of applications. The main challenge in the application of these types of surfactants is that the hydrophilicity of the surfactant varies with concentration and dilution due to the polydispersity of the ethylene oxide groups. The hydrophilicity of a surfactant can be quantified by the characteristic curvature (Cc) parameter of the hydrophilic–lipophilic difference (HLD) framework. In this work, a model based on natural logarithmic regression was developed to calculate the Cc value of commercial surfactants as a function of surfactant concentration by a fast and simple phase scan. The slope of the Cc curve and the measured Cc at a reference concentration were used to develop the model. The Cc values determined with the model agreed with the measured values from the phase scans. Furthermore, the linear mixing rule proved to be reliable for mixtures of polydisperse ethoxylated surfactants. Finally, the impact of the water-to-oil ratio on the Cc was evaluated and the implications were discussed.

Spontaneous Emulsions: Adjusting Spontaneity and Phase Behavior by Hydrophilic-Lipophilic Difference-Guided Surfactant, Salt, and Oil Selection

Spontaneous emulsion behavior has been difficult to predict and could be influenced by many variables including salinity, temperature, and chemical composition of the oil and surfactant. In this work, the hydrophilic-lipophilic difference (HLD) framework was used to predict the formation of spontaneous emulsions using a mixture of Span-80 and SLES surfactants. The spontaneity and emulsion behavior of different systems were modeled by estimating the HLD_mix. The influence of surfactant ratio, salinity, and oil type was investigated. Spontaneous emulsification could only be observed when the HLD_mix was between -0.96 and 1.04. Within this range, a negative HLD_mix resulted in a greater spontaneity to form o/w emulsion, and a w/o emulsion was more likely to form when the HLD_mix was positive. When the HLD_mix was close to 0 (between -0.22 and 0.56 in our systems), emulsions were formed in both the oil and aqueous phases with high spontaneity. A combined effect of ultralow interfacial tension, Span-80 micelle swelling, and interfacial turbulence due to Marangoni effects is likely the main mechanism of the spontaneous emulsification observed in this study. A synergistic reduction in interfacial tension was observed between Span-80 and SLES (<1 mN/m). When the HLD of the system was close to 0, a bicontinuous emulsion phase was formed at the oil-water interface. The bicontinuous emulsion broke-up over time due to the ultralow interfacial tension and interfacial turbulence, forming dispersed oil and water droplets. Results from this work provide a practical method to suggest what surfactant composition, salinity, and oil type could promote (or eliminate) the conditions favorable for spontaneous emulsification.

Atomistic Insights into the Droplet Size Evolution during Self-Microemulsification

Microemulsions have been attracting great attention for their importance in various fields, including nanomaterial fabrication, food industry, drug delivery, and enhanced oil recovery. Atomistic insights into the self-microemulsifying process and the underlying mechanisms are crucial for the design and tuning of the size of microemulsion droplets toward applications. In this work, coarse-grained models were used to investigate the role that droplet sizes played in the preliminary self-microemulsifying process. Time evolution of liquid mixtures consisting of several hundreds of water/surfactant/oil droplets was resolved in large-scale simulations. By monitoring the size variation of the microemulsion droplets in the self-microemulsifying process, the dynamics of diameter distribution of water/surfactant/oil droplets were studied. The underlying mass transport mechanisms responsible for droplet size evolution and stability were elucidated. Specifically, temperature effects on the droplet size were clarified. This work provides the knowledge of the self-microemulsification of water-in-oil microemulsions at the nanoscale. The results are expected to serve as guidelines for practical strategies for preparing a microemulsion system with desirable droplet sizes and properties.

Current Research and Challenges in Bitumen Emulsion Manufacturing and Its Properties

The global increase of road infrastructure and its impact on the environment requires serious attention to develop sustainable and environmentally friendly road materials. One group of those materials is produced by using bitumen emulsion. However, there are still scientific and technical obstacles standing against its regular application. The bitumen emulsion formulation process and compositional optimization are subjected to a high number of degrees of freedom. Consequently, obtaining the desired product is mostly based on a series of random and tedious trials because of the enormous number of tests that are carried out to meet the required properties, such as emulsion stability, viscosity, droplet size (and distribution), and bitumen emulsion chemistry. Several pre-established formulation procedures have been presented in the literature. Some of them have technical limitations to be utilized for practical industrial application, whereas others are still not understood enough to be applied in bitumen emulsion formulation. Therefore, discussing some important issues in this field could be useful to offer a practical guide for bitumen emulsion manufacturers when trying to formulate a well-defined bitumen emulsion to best fit its use in pavement infrastructure rather than to simply to meet standard specifications. This review paper aims to enable the ultimate potential of bitumen emulsion by further reviewing the research progress of bitumen emulsion manufacturing and discussing the literature available up to now on this topic, in the realm of bitumen emulsion manufacturing and emulsion chemistry.

Advances of microemulsion and its applications for improved oil recovery

Microemulsion, because of its excellent interfacial tension reduction and solubilization properties, has wide range of applications in the petroleum industry, especially in improved oil recovery (IOR). Herein, the concept, types and formation mechanism of microemulsion were primarily introduced. Then, the preparation and characterization methods were illustrated. Additionally, several effect factors were elaborated specifically based on the composition of microemulsion. Finally, the application of microemulsion in IOR was addressed, including IOR mechanism analysis based on sweep efficiency and displacement efficiency, injection method (microemulsion flooding, in-situ microemulsion formation) and field tests. Furthermore, the current challenges and prospects of microemulsion on IOR were analyzed.

Predicting Spontaneous Emulsification in Saltwater Environments Using the HLD Model

Spontaneous emulsification of toluene with nonylphenol polyethoxylate (NPE) and sodium dodecylbenzenesulfonate (SDBS) surfactants in saltwater environments was studied. NaCl promoted the spontaneous emulsification of an otherwise non-spontaneous SDBS-toluene system. Dynamic light scattering and turbidity indicated that spontaneity increased with NaCl concentration. The mechanism of spontaneous emulsification was dependent on surfactant type; NPE emulsified via micelle swelling, and SDBS emulsified via nucleation and growth. Hydrophilic lipophilic difference (HLD) calculations were used to model spontaneous emulsification and spontaneity. As HLD approached zero, conditions became more favorable for spontaneous emulsification. Between HLD values of -2.4 and -2.05, samples transitioned from non-spontaneous to spontaneous. This study aids in predicting spontaneous emulsion formation in saltwater environments for applications in nanoemulsion formation and wastewater remediation.

Formulation Improvements in the Applications of Surfactant-Oil-Water Systems Using the HLDN Approach with Extended Surfactant Structure

Soap applications for cleaning and personal care have been used for more than 4000 years, dating back to the pharaonic period, and have widely proliferated with the appearance of synthetic surfactants a century ago. Synthetic surfactants used to make macro-micro-nano-emulsions and foams are used in laundry and detergency, cosmetics and pharmaceuticals, food conditioning, emulsified paints, explosives, enhanced oil recovery, wastewater treatment, etc. The introduction of a multivariable approach such as the normalized hydrophilic–lipophilic deviation (HLD _N) and of specific structures, tailored with an intramolecular extension to increase solubilization (the so-called extended surfactants), makes it possible to improve the results and performance in surfactant–oil–water systems and their applications. This article aims to present an up-to-date overview of extended surfactants. We first present an introduction regarding physicochemical formulation and its relationship with performance. The second part deals with the importance of HLD _N to make a straightforward classification according to the type of surfactants and how formulation parameters can be used to understand the need for an extension of the molecule reach into the oil and water phases. Then, extended surfactant characteristics and strategies to increase performance are outlined. Finally, two specific applications, i.e., drilling fluids and crude oil dewatering, are described.

Microemulsion Microstructure(s): A Tutorial Review

Microemulsions are thermodynamically stable, transparent, isotropic single-phase mixtures of two immiscible liquids stabilized by surfactants (and possibly other compounds). The assortment of very different microstructures behind such a univocal macroscopic definition is presented together with the experimental approaches to their determination. This tutorial review includes a necessary overview of the microemulsion phase behavior including the effect of temperature and salinity and of the features of living polymerlike micelles and living networks. Once these key learning points have been acquired, the different theoretical models proposed to rationalize the microemulsion microstructures are reviewed. The focus is on the use of these models as a rationale for the formulation of microemulsions with suitable features. Finally, current achievements and challenges of the use of microemulsions are reviewed.

Formulation of Bio-Based Washing Agent and Its Application for Removal of Petroleum Hydrocarbons From Drill Cuttings Before Bioremediation

Drill cuttings from petroleum exploration and production sites can cause diverse environmental problems. Total petroleum hydrocarbons (TPHs) are a major pollutant from the use of polyolefin-based mud. As an alternative to incineration, this study investigated the application of surfactant-enhanced washing technology prior to bioremediation. The washing step was necessary because the initial TPH concentrations were quite high at approximately 15% (w/w). Washing agents were formulated by varying the concentration of lipopeptide biosurfactant (in foamate or cell-free broth), Dehydol LS7TH (fatty alcohol ethoxylate 7EO, oleochemical surfactant) and butanol (as a lipophilic linker) at different salinities. The most efficient formula produced a Winsor Type I microemulsion (oil-in-water microemulsion) with polyolefin and contained only 20% (v/v) foamate and 2% (v/v) Dehydol LS7TH in water. Due to the synergistic behavior between the anionic lipopeptides and non-ionic Dehydol LS7TH, the formula efficiently removed 92% of the TPHs from the drill cuttings when applied in a jar test. To reduce the cost, the concentrations of each surfactant should be reduced; thus, the formula was optimized by the simplex lattice mixture design. In addition, cell-free broth, at a pH of 10, containing 3.0 g/L lipopeptides was applied instead of foamate because it was easy to prepare. The optimized formula removed 81.2% of the TPHs and contained 72.0% cell-free broth and 1.4% Dehydol LS7TH in water. A 20-kg soil washing system was later tested where the petroleum removal efficiency decreased to 70.7% due to polyolefin redeposition during separation of the washing solution. The remaining TPHs (4.5%) in the washed drilled cuttings were further degraded by a mixture of Marinobacter salsuginis RK5, Microbacterium saccharophilum RK15 and Gordonia amicalis JC11. To promote TPH biodegradation, biochar and fertilizer were applied along with bacterial consortia in a microcosm experiment. After 49-day incubation, the TPHs were reduced to 0.9% by both physical and biological mechanisms, while the TPHs in the unamended samples remained unaffected. With the use of the formulated bio-based washing agent and bioremediation approach, the on-site treatment of drill cuttings could be conducted with an acceptable cost and low environmental impacts.

How to Use the Normalized Hydrophilic-Lipophilic Deviation (HLDN) Concept for the Formulation of Equilibrated and Emulsified Surfactant-Oil-Water Systems for Cosmetics and Pharmaceutical Products

The effects of surfactant molecules involved in macro-, mini-, nano-, and microemulsions used in cosmetics and pharmaceuticals are related to their amphiphilic interactions with oil and water phases. Basic ideas on their behavior when they are put together in a system have resulted in the energy balance concept labeled the hydrophilic-lipophilic deviation (HLD) from optimum formulation. This semiempirical equation integrates in a simple linear relationship the effects of six to eight variables including surfactant head and tail, sometimes a cosurfactant, oil-phase nature, aqueous-phase salinity, temperature, and pressure. This is undoubtedly much more efficient than the hydrophilic-lipophilic balance (HLB) which has been used since 1950. The new HLD is quite important because it allows researchers to model and somehow predict the phase behavior, the interfacial tension between oil and water phases, their solubilization in single-phase microemulsion, as well as the corresponding properties for various kinds of macroemulsions. However, the HLD correlation, which has been developed and used in petroleum applications, is sometimes difficult to apply accurately in real cases involving ionic–nonionic surfactant mixtures and natural polar oils, as it is the case in cosmetics and pharmaceuticals. This review shows the confusion resulting from the multiple definitions of HLD and of the surfactant parameter, and proposes a “normalized” Hydrophilic-Lipophilic Deviation (HLDN) equation with a surfactant contribution parameter (SCP), to handle more exactly the effects of formulation variables on the phase behavior and the micro/macroemulsion properties.

Fish Waste Based Lipopeptide Production and the Potential Application as a Bio-Dispersant for Oil Spill Control

There is a growing acceptance worldwide for the application of dispersants as a marine oil spill response strategy. The development of more effective dispersants with less toxicity and higher biodegradability would be a step forward in improving public acceptance and regulatory approvals for their use. By applying advances in environmental biotechnology, a bio-dispersant agent with a lipopeptide biosurfactant produced by Bacillus subtilis N3-1P as the key component was formulated in this study. The economic feasibility of producing biosurfactant (a high-added-value bioproduct) from fish waste-based peptone as a nutrient substrate was evaluated. Protein hydrolyzate was prepared from cod liver and head wastes obtained from fish processing facilities. Hydrolysis conditions (i.e., time, temperature, pH and enzyme to substrate level) for preparing protein hydrolyzates were optimized by response surface methodology using a factorial design. The critical micelle dilution (CMD) value for biosurfactant produced from the fish liver and head waste generated peptones was 54.72 and 47.59 CMD, respectively. Biosurfactant product generated by fish liver peptone had a low critical micelle concentration of 0.18 g L-1 and could reduce the surface tension of distilled water to 27.9 mN/m. Structure characterization proved that the generated biosurfactant product belongs to the lipopeptide class. An alternative to the key surfactant dioctyl sulfosuccinate sodium (DOSS) used in Corexit 9500 has been proposed based on a binary mixture of lipopeptides and DOSS that exhibited synergistic effects. Using the standard baffled flask test, a high dispersion efficiency of 76.8% for Alaska North Slope oil was achieved at a biodispersant composition of 80/20 (v/v) of lipopeptides/DOSS. The results show that fish waste can be utilized to produce a more effective, environmentally acceptable and cost-efficient biodispersant that can be applied to oil spills in the marine environment.

Characterizing the Oil-like and Surfactant-like Behavior of Polar Oils

In this work, a bifunctional model was developed to fit and predict the phase inversion point (PIP) of microemulsions containing polar oils. This model incorporated the hydrophilic-lipophilic difference (HLD) equations, where HLD = 0 at the PIP. The model uses a Langmuir isotherm to account for the interfacial segregation of polar oils as a function of their concentration in the bulk oil phase. The segregated polar oil was treated as being surfactant-like, having a characteristic curvature (Cc). The polar oil in the bulk oil phase was characterized via an equivalent alkane carbon number (EACN). The Cc value was obtained considering deviations in the PIP at low polar oil concentrations. The EACN was determined considering PIP deviations at high polar oil concentrations. Naphthenic acid and dodecanol were used as model polar oils mixed with ionic and nonionic surfactants and nonpolar oils. The EACN of the polar oil was shown to be independent of the EACN of the nonpolar oil and likely independent of the surfactant. The Cc for dodecanol was likely independent of the surfactant used. For naphthenic acid, the Cc was independent of the nonpolar oil, and within a certain surfactant type (ionic, nonionic, or extended ionic), it was likely independent of the surfactant. For the naphthenic acid systems, the segregation predicted via the bifunctional model was consistent with experimental measurements of this segregation. Given that the bifunctional model only involves phase inversion experiments, it is a convenient method to determine the oil-like and surfactant-like nature of polar oils.

Hydrophilic-Lipophilic-Difference (HLD) Guided Formulation of Oil Spill Dispersants with Biobased Surfactants

The large-scale use of dispersants during the BP Horizon spill revealed various risks associated with these formulations, particularly the use of volatile organic compound (VOC) solvents linked to respiratory illnesses, and the poor biodegradability of surfactants. Previous attempts at solving these issues involved formulations of lecithin and polyethylene glycol ester of sorbitan monooleate (Tween® 80) that still required the use of a volatile solvent, ethanol. In this work, the Hydrophilic-Lipophilic Difference (HLD) framework was used to develop a lecithin formulation containing food-grade lipophilic (Glycerol MonoOleate – GMO- and sorbitan monooleate – Span® 80) and hydrophilic (polyglycerol caprylate) linkers in combination with a nonvolatile and mineral oil solvent with food additive status. The HLD parameters for lecithin, linkers, and oils were used to determine the lecithin-linker formulas that yielded HLD ∼0 (the surfactant phase inversion point), reaching interfacial tensions of 10−2 mN/m, and high emulsification effectiveness with diluted bitumen. This effectiveness was close to that obtained with a simulated dispersant, and superior to the lecithin-Tween® 80-ethanol formula. The lecithin-linker system produced 4–11 μm emulsified drops, sufficiently small to enhance the biodegradability of the dispersion.

Oil-Induced Viscoelasticity in Micellar Solutions of Alkoxy Sulfate

Rheological properties of the solution of an extended surfactant, sodium alkoxy sulfate (C₈-(PO)₄-(EO)₁-SO₄Na), are investigated as a function of the presence of various paraffinic oils over a range of salt conditions in the Winsor III microemulsion region at oil fractions where the microemulsion is "oil-starved". The addition of as small as 3 vol % alkane to 2 wt % surfactant solutions at salt concentrations where the oil-water interfacial tension is minimized induces a sudden shift in the rheological behavior. The solution viscosity increases by 5 orders of magnitude, with solid-like behaviors (G' > G″) being observed in the entire frequency region investigated (0.01-100 rad/s). Commonly, in the cases where wormlike micelles are present in the solution, alkanes are believed to be solubilized in the core of micelles, leading to a radial growth of the cylindrical part of the wormlike micelle, resulting in a drop of end-cap energy (E_C) and micelle length and a reduction in viscosity. In this study, however, the addition of oil causes the formation of wormlike micelles. The viscosity of solubilized-oil samples does, however, decrease with an increase in incorporated oil volume. We hypothesize that this "abnormal oleo-responsive" viscoelastic behavior is related to a spacer of intermediate hydrophilicity, that is, polypropylene oxide (PO) segment of the alkoxy sulfate, being inserted between the hydrophobic tail and hydrophilic head (the ethoxylated sulfate segment) of the extended surfactant. The addition of a small amount of oil likely extends the PO moiety and increases the tail length of the surfactant in the aggregates as well as reducing the headgroup size, driving the formation of wormlike micelles from a solution that initially had a viscosity consistent with the absence of such structures.

Solid-Liquid-Liquid Wettability of Surfactant-Oil-Water Systems and Its Prediction around the Phase Inversion Point

Surfactant-oil-water (SOW) systems are important for numerous applications, including hard surface cleaning, detergency, and enhanced oil-recovery applications. There is limited literature on the wettability of solid-liquid-liquid (SLL) systems around the surfactant phase inversion point (PIP), and the few references that exist point to wettability inversion accompanying the microemulsion (μE) phase inversion. Despite the significance of this phenomenon and the extreme changes in contact angles, there are no models to predict SLL wettability as a function of proximity to the PIP. Recent works on SLL wettability in surfactant-free systems suggest that SLL contact angles can be predicted with an extension of Neumann's equation of state (e-EQS) if the interfacial tension (IFT or γ_o-w) is known and if there is a good estimate for the interfacial energy between the wetting phase and the surface (γ_{S-wetting liquid}). In this work, IFT predictions for SOW systems around the PIP were obtained via the combined hydrophilic-lipophilic difference (HLD) and net-average-curvature (NAC) framework. To test the hypothesis that the combined HLD-NAC + e-EQS can predict wettability inversion around the PIP, with a given γ_S-μE, the contact angles (measured through the light oil phase, θ_O) for the μE of sodium dihexyl sulfosuccinate-toluene-saline water system were measured on high surface free energy (SFE) materials (glass, stainless steel, and mica) and on polytetrafluoroethylene (low SFE) around the PIP. Considering that at the PIP, most systems have a contact angle of 90°, an estimated γ_S-μE = 1/4γ_o-w@PIP was found to be suitable for the systems considered in this work and for systems presented in the literature. The largest deviations between the predictions and the experimental values were found in the positive HLD range (surfactant in the light oil phase). Although there is room for improvement, this framework can estimate the wetting behavior of SOW systems starting solely from formulation parameters.

Improving emulsion formation, stability and performance using mixed emulsifiers: A review

The formation, stability, and performance of oil-in-water emulsions may be improved by using combinations of two or more different emulsifiers, rather than an individual type. This article provides a review of the physicochemical basis for the ability of mixed emulsifiers to enhance emulsion properties. Initially, an overview of the most important physicochemical properties of emulsifiers is given, and then the nature of emulsifier interactions in solution and at interfaces is discussed. The impact of using mixed emulsifiers on the formation and stability of emulsions is then reviewed. Finally, the impact of using mixed emulsifiers on the functional performance of emulsifiers is given, including gastrointestinal fate, oxidative stability, antimicrobial activity, and release characteristics. This information should facilitate the selection of combinations of emulsifiers that will have improved performance in emulsion-based products.

Development of a carboxymethyl chitosan functionalized nanoemulsion formulation for increasing aqueous solubility, stability and skin permeability of astaxanthin using low-energy method

In this research, firstly astaxanthin (ASX)-loaded nanoemulsions (NEs) were produced using a convenient low-energy emulsion phase inversion method. The optimised ASX-NEs were prepared in the presence of Cremophor^® EL and Labrafil^® M 1944 CS, with a surfactant-to-oil ratio of 4:6. The ASX-NE droplets were spherical with a mean droplet diameter below 100 nm and a small negative surface charge. The system was stable without alteration of mean droplet diameter for three months. Then, the ASX-NE was functionalised with carboxymethyl chitosan (CMCS) through direct CMCS (0.02%) incorporation during the preparation process. The ASX chemical stability and skin permeability increased in the following order: ASX solution control < ASX-NE < CMCS-ASX-NE. Cell viability assays on L929 cells revealed low cytotoxicity of blank NE, ASX-NE and CMCS-ASX-NE in the range from 5 to 500 μg mL^-1. In conclusion, the CMCS-ASX-NE might be a promising delivery vehicle in dermal and transdermal products.

Dimensionless Equation of State to Predict Microemulsion Phase Behavior

Prediction of microemulsion phase behavior for changing state variables is critical to formulation design of surfactant-oil-brine (SOB) systems. SOB systems find applications in various chemical and petroleum processes, including enhanced oil recovery. A dimensional equation-of-state (EoS) was recently presented by Ghosh and Johns1 that relied on estimation of the surfactant tail length and surface area. We give an algorithm for flash calculations for estimation of three-phase Winsor regions that is more robust, simpler, and noniterative by making the equations dimensionless so that estimates of tail length and surface area are no longer needed. We predict phase behavior as a function temperature, pressure, volume, salinity, oil type, oil-water ratio, and surfactant/alcohol concentration. The dimensionless EoS is based on coupling the HLD-NAC (Hydrophilic Lipophilic Difference-Net Average Curvature) equations with new relationships between optimum salinity and solubility. An updated HLD expression that includes pressure is also used to complete the state description. A significant advantage of the dimensionless form of the EoS over the dimensional version is that salinity scans are tuned based only on one parameter, the interfacial volume ratio. Further, stability conditions are developed in a simplified way to predict whether an overall compositions lies within the single, two-, or three-phase regions. Important new microemulsion relationships are also found, the most important of which is that optimum solubilization ratio is equal to the harmonic mean of the oil and water solubilization ratios in the type III region. Thus, only one experimental measurement is needed in the three-phase zone to estimate the optimum solubilization ratio, a result which can aid experimental design and improve estimates of optimum from noisy data. Predictions with changing state variables are illustrated by comparison to experimental data using standard diagrams including a new type of dimensionless fish plot. The results show that the optimum solubilization ratio and salinity using the tuned dimensionless EoS are within average errors of 2.44% and 1.17% of experimental values for the fluids examined. We then use the dimensionless equations and thermodynamic first-principles to derive the constant in Huh's equation for interfacial tension prediction.

How to Attain an Ultralow Interfacial Tension and a Three-Phase Behavior with a Surfactant Formulation for Enhanced Oil Recovery: A Review. Part 2. Performance Improvement Trends from Winsor's Premise to Currently Proposed Inter- and Intra-Molecular Mixtures

The minimum interfacial tension occurrence along a formulation scan at the so-called optimum formulation is discussed to be related to the interfacial curvature. The attained minimum tension is inversely proportional to the domain size of the bicontinuous microemulsion and to the interfacial layer rigidity, but no accurate prediction is available. The data from a very simple ternary system made of pure products accurately follows the correlation for optimum formulation, and exhibit a linear relationship between the performance index as the logarithm of the minimum tension at optimum, and the formulation variables. This relation is probably too simple when the number of variables is increased as in practical cases. The review of published data for more realistic systems proposed for enhanced oil recovery over the past 30 years indicates a general guidelines following Winsor's basic studies concerning the surfactant-oil-water interfacial interactions. It is well known that the major performance benefits are achieved by blending amphiphilic species at the interface as intermolecular or intramolecular mixtures, sometimes in extremely complex formulations. The complexity is such that a good knowledge of the possible trends and an experienced practical know-how to avoid trial and error are important for the practitioner in enhanced oil recovery.

Method of moments for computational microemulsion analysis and prediction in tertiary oil recovery

We discuss the application of Helfrich's surface torque density concept to microemulsion design and analysis from three different angles: (i) from the point of view of coarse-grained molecular simulations, using Dissipative Particle Dynamics, including charge interactions and added salt, (ii) using an approximate double-film model for the surface, and (iii) comparison with formulation approaches. The simulations use that the surface torque can be calculated unambiguously from the stress profile, provided the surface is tensionless. Very good agreement is found on predicting optimal salinity (or the absence of that) for a range of surfactants: dioctyl sodium sulfosuccinate, various twin-tailed sulfonates and sodium dodecyl sulfate. The simulations are very fast, on par with times for experiments, thus they could lead to a practical tool for discovery of more efficient surfactants, although much remains to be done with respect to other important variables: oil composition, surfactant mixtures, aggregation in solution, and so on. The microscopic model (second approach) is highly approximate: it is essentially based on two opposing swelling tendencies, that are both of osmotic nature. In accordance with the model, the tails are swollen by the oil and the charged head groups are confined in a salty layer in Donnan equilibrium with the salt solution. In this way, the surface interactions are purely entropic. The comparison of the film model with existing formulation approaches (third approach) covers the interfacial tension minimum, Winsor R theory, quantitative structure property relations (QSPR), hydrophilic-lipophilic deviation (HLD), HLD-net average curvature, and temperature coefficients. Using the surface torque analysis, we succeed in deriving in an ab initio way QSPR empirical coefficients that have been known for decades, but until now, have been obscure in origin.

Lecithin-linker microemulsion gelatin gels for extended drug delivery

This article introduces the formulation of alcohol-free, lecithin microemulsion-based gels (MBGs) prepared with gelatin as gelling agent. The influence of oil, water, lecithin and hydrophilic and lipophilic additives (linkers) on the rheological properties and appearance of these gels was systematically explored using ternary phase diagrams. Clear MBGs were obtained in regions of single phase microemulsions (μEs) at room temperature. Increasing the water content in the formulation increased the elastic modulus of the gels, while increasing the oil content had the opposite effect. The hydrophilic additive (PEG-6-caprylic/capric glycerides) was shown to reduce the elastic modulus of gelatin gels, particularly at high temperatures. In contrast to anionic (AOT) μEs, the results suggest that in lecithin (nonionic) μEs, the introduction of gelatin “dehydrates” the μE. Finally, when the transdermal transport of lidocaine formulated in the parent μE and the resulting MBG were compared, only a minor retardation in the loading and release of lidocaine was observed.

The hydrophobicity of silicone-based oils and surfactants and their use in reactive microemulsions

In this work, for the first time, the Hydrophilic-Lipophilic Difference (HLD) framework for microemulsion formulation has been applied to silicone oils and silicone alkyl polyether surfactants. Based on the HLD equations and recently introduced mixing rules, we have quantified the hydrophobicity of the oils according to the equivalent alkane carbon number (EACN). We have found that, in a reference system containing sodium dihexyl sulfosuccinate (SDHS) as the surfactant, 0.65 centistoke (cSt) and 3.0 cSt silicone oils behave like n-dodecane and n-pentadecane, respectively. Silicone alkyl polyether surfactants were found to have characteristic curvatures ranging 3.4-18.9, exceeding that of most non-ionic surfactants. The introduction of methacrylic acid (MAA) and hydroxyethyl methacrylate (HEMA) to the aqueous phase caused a significant negative shift in HLD, indicative of an aqueous phase that is less hydrophilic than pure water. The more hydrophobic surfactants (largest positive curvatures) were used in order to compensate for this effect. These findings have led to the formulation of bicontinuous microemulsions (μEs) containing silicone oil, silicone alkyl polyether and reactive monomers in aqueous solution. Ternary phase diagrams of these systems revealed the potential for silicone-containing polymer composites with bicontinuous morphologies. These findings have also helped to explain the phase behavior of formulations previously reported in literature, and could help in providing a systematic, consistent approach to future silicone oil based microemulsion formulation.