Predictive Modeling of Microemulsion Phase Behaviour and Microstructure Characterisation in the 1-Phase Region (original) (raw)
Modelling interactions in microemulsion phases
Journal de Physique
2014 Nous présentons dans cet article une description unifiée des interactions dans les phases liquide isotrope et cristal liquide de microémulsions. Un modèle microscopique est utilisé pour tenir compte des termes d'énergie entropique, de courbure et d'interaction et on discute la stabilité relative des phases isotrope (microémulsion), lamellaire et hexagonale. Abstract. 2014 In this paper we present a unified description of the interactions in liquid isotropic and liquid crystal phases of microemulsions. A microscopic model taking into account the entropic, bending and interaction free energies is used to deduce theoretical phase diagrams and the relative stability of the isotropic (microemulsion), lamellar and hexagonal phases is discussed.
Microemulsions in oil-water-surfactant mixtures: Systematics of a lattice-gas model
Physical Review A, 1988
A lattice-gas model is constructed for oil-water-surfactant mixtures. The phase diagram of this model is obtained by using mean-field theory and Monte Carlo simulations aided by lowtemperature expansions. Microstructures, structure factors, and mean droplet lifetimes are also determined in some phases. Both two and three dimensions are studied, the former in more detail than the latter. It is shown that it is natural to interpret the paramagnetic phase in our model as a microemulsion. Our model is found to exhibit various properties that are in qualitative agreement with experimental observations of oil-water-surfactant mixtures: (1) two-and three-phase coexistence occurs between oil-rich, water-rich, and microemulsion phases along first-order phase boundaries or a triple line in certain regions of the phase diagram of our model; (2) the triple line, which ends in a tricritical point, is short and this leads to low oil-microemulsion and water-microemulsion interfacial tensions; (3) microstructures (including bicontinuous ones in three dimensions) and structure factors are similar to some experimental ones; (4) droplets in our microemulsion phase are long lived like their experimental counterparts; (5) long-lived, metastable phases, including long-period, lamellar, and glasslike phases, appear at low temperatures. The limitations of our model are discussed. Our study is compared with other studies of models of oil-water-surfactant mixtures.
Structure and dynamics in three-component microemulsions
The Journal of Physical Chemistry
NMR self-diffusion coefficients are reported for three-component micromulsions formed from didodecyldimethylammonium bromide/water/alkane. For hexane through tetradecane the oil diffusion coefficients are approximately half as large as those of the bulk oils and independent of composition. Therefore, the systems are oil continuous through the entire one-phase region. The diffusion coefficients for the surfactant are small and independent of composition. The water self-diffusion coefficients decrease with added water for hexane through dodecane and change in a manner consistent with the abrupt conducting-nonconducting transition known to occur in this system. The microemulsions are bicontinuous in the conducting regions and disconnected water-in-oil droplets in the nonconducting regions of the phase diagrams. The phenomena reported clearly demonstrate the interplay between forces due to oil penetration and those due to head-group interactions which control curvature and therefore microemulsion structure.
One-Dimensional Model for Oil-Continuous Microemulsions
Langmuir, 1996
A one-dimensional model for microemulsions is considered. In the model the microemulsion droplets are hard rods that interact through nearest-neighbor pair potentials. Besides the hard core repulsion the interaction presents a square well and a square barrier that have arbitrary width and strength. The thermodynamic and structure properties for this system are found via the isothermal-isobaric ensemble. The model predicts that, under certain conditions, the aggregation number increases with temperature, as is experimentally observed. When the temperature is lowered, the system structure is solid-like and it can be found in two phases with different characteristic wavenumbers.
Microemulsion Microstructure(s): A Tutorial Review
Nanomaterials
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.
Phase Behavior and Microstructure of Nonaqueous Microemulsions. 2
Langmuir, 1995
The microstructure of nonaqueous microemulsions formed with propylene glycol, glycerol, three different alkanes, and pentaethylene glycol mono-n-dodecyl ether (C12E5) is probed with NMR self-diffusion measurements and small angle neutron scattering (SANS). At low oil concentrations, both NMR selfdiffusion and SANS results can be modeled in terms of a microstrucure of ellipsoidal oil-rich droplets with only excluded volume interactions. These droplet structures percolate to an oil-continuous structure as the volume fraction of oil in the microemulsions increases. Percolation thresholds measured as a function of alkane chain length are interpreted in terms of the phase behavior of the microemulsion and the strength of droplet interactions. LA9405612
Determining scaling in known phase diagrams of nonionic microemulsions to aid constructing unknown
Advances in Colloid and Interface Science, 2010
Microemulsions based on nonionic surfactants of the ethylene oxide alkyl ether type C m E n , have been studied thoroughly for around 30 years. Thanks to the considerable amount of published data available on these systems, it is possible to observe trends to make predictions of phase diagrams not yet determined. Strey and Kahlweit, and subsequently Sottmann and Strey, with coworkers have studied and published phase diagrams for systems with a fixed ratio of oil to water, varying the surfactant, the so-called Kahlweit fish-cut diagrams. Some properties of the phase diagrams can be scaled to become general and not system dependent. Here are shown two examples of scaling data from phase diagrams and the use of trends to determine phase diagrams, both inside and outside a dataset. The trends of microemulsions with fixed ratio of surfactant to oil, the so-called Lund-cut diagrams, are also investigated. The trends are used to determine a new phase diagram and this is compared with previously unpublished experimental data on C 12 E 5-Octadecane-Water system. The scalings and trends make it possible to get good estimations of many of the important properties of the phase diagrams, both temperatures and surfactant concentrations of interest, by investigating one sample in the 3-phase region of the balanced fish-cut diagram.
Dimensionless Equation of State to Predict Microemulsion Phase Behavior
Langmuir : the ACS journal of surfaces and colloids, 2016
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...
Microemulsions: Formation and stabilization
Journal of Colloid and Interface Science, 1973
The influence on the the formation of water-in-oil microemulsions of the chain length and cation of the surfactant and the nature of the solvent were studied. From NMR and free energy of adsorption of the alcohol, it was concluded that the alcohol-surfactant interaction is weak. Measurement of the change in the water-oil interracial tension (-~) while alcohol was injected into one of the phases was recorded. It was found that ~ may be temporarily lowered to zero while the alcohol diffused through the interface. It would therefore, be possible for a dispersion to occur spontaneously (while ~,~ = 0). The role of the surfactant would be to lower "rs and stabilize the system against coalescence.
The Journal of Chemical Physics, 1990
The continuous inversion from a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at higher temperatures within the one-phase channel of water (0.6% NaCl)-n-decane-AOT microemulsion system is investigated by small angle neutron scattering (SANS). At constant AOT (surfactant) weight fraction r of 12%, the structural evolution as a function of temperature takes place in different forms as the oil-towater weight fraction a is varied from 15 to 90 %. At low o-w weight fractions (a = 15 and 20 %) the microemulsions transform from a water-internal, oil-continuous structure at lower temperatures to an oil-internal, water-continuous droplet structure at higher temperatures jumping across an intermediate region of a lamellar phase (La)' However, at higher o-w weight fractions (a = 80 and 90 %) the evolution goes through a stage of percolation of the water droplets first into extended water clusters, then the structural inversion takes place probably through a transition of these water clusters into an entangled tubular structure. At equal oil-to-water volume ration (a = 40%), the structure can be described as bicontinuous at both low and high temperatures. In this case we are able to extract two lengths characterizing the structure from SANS data using different models for the scattering length density fluctuation correlation function of a bicontinuous microemulsion.
Phase Behaviour of Ph-Dependent Microemulsions At High Temperature and Salinities
Oil & Gas Science and Technology-revue De L Institut Francais Du Petrole, 1997
This paper describes the formulation principal for a model microemulsion system which exhibits pH dependent phase behaviour. The system investigated consists of octane, brine, alkyl ether carboxylic acid surfactants and short chain alcohols. The CMCs of these surfactants were lower in acid form of the surfactant (COOH) than for the salt form (COO-), also the micelles formed in acid solutions were smaller than for the salt. Furthermore the surface and interfacial tensions were found to increase with increasing pH. Increasing pH ionises the carboxylic acid head group thereby making the surfactant more hydrophilic. The effect of an increase in pH can be counterbalanced by increasing the electrolyte concentration. Measurements of ultralow interfacial tensions at different salinities and temperatures have been made using a spinning drop apparatus. Three phase microemulsion systems were studied as a function of temperature and pH. It was observed that the presence of ethylene oxide (EO) moiety in the surfactant molecule made the surfactants less sensitive to salinity than anionic surfactants. In addition, the carboxylic ionic head group made the surfactant more stable to temperature than simple EO nonionic surfactants. Thus these surfactants are more robust than either simple anionic or nonionic surfactants and thus these materials have potential in the field of surfactant flooding for tertiary oil recovery.
The HLD-NAC Model for Extended Surfactant Microemulsions
Journal of Surfactants and Detergents, 2012
It has been confirmed that the structure of the alkyl group of an extended surfactant plays an important role in defining its interfacial properties. Alkyl groups containing a higher degree of b-branching (C2-branching) produce microemulsions with a larger characteristic length (n, the extent of solubilization in middle phases). This effect is explained on the basis that b-branching increases the hydrophobicity of the surfactant and decreases the optimal salinity of the microemulsion. Higher salinities produce a dehydration of the surfactant groups that lead to shorter extent of the interactions with the oil and the water. Larger characteristic lengths are desirable if the objective of the formulation is obtaining greater solubilization of oil and water, and lower interfacial tensions. Large characteristic lengths are, in most cases, associated with high interfacial rigidities, which are undesirable if rapid coalescence is required. However, mixtures of branched and linear extended surfactants produce large characteristic lengths and lower interfacial rigidities. The HLD-NAC model is able to reflect the experimental trends in solubilization of oil and water. The differences between the predictions of the model for the solubilization of oil and water in Type I and II formulations, respectively, highlight the complexities in the conformation of extended surfactants, particularly their PO groups, at oil-water interfaces and the need for advanced scattering techniques to evaluate these conformations.
Interfacial phase transitions of microemulsions
In this paper we study the interfaces between phases in a phenomenological model of a microemulsion that is in equilibrium simultaneously with an oil-rich and a water-rich phase. The tensions and chemical-composition profiles of the interfaces are calculated. We ask whether the oil-water, oil-microemulsion and microemulsion-water tensions uow, u r n and u r n are related by now < uom+amw or by uow = uom+umw In the former case the microemulsion phase does not wet the oil-water interface, whereas in the latter it does. We find separate ranges of values of the model's parameters in which each possibility is realized, while the microemulsion is a middle phase related symmetrically to the oil and water phases. When a parameter that breaks that symmetry is varied and a critical endpoint of the three-phase equilibrium is approached, an originally non-wet oil-water interface becomes wet (while an originally wet interface remains wet). The transition is of first order, accompanied by a change in interfacial structure. A microscopic lattice model of such three-phase equilibria is also described. In its context we raise (but do not fully answer) the same questions that we treated in the earlier phenomenological model.
Microscopic model for microemulsions. II. Behavior at low temperatures and critical point
The Journal of Chemical Physics, 1989
Low temperature properties and critical behavior for the more simple versions of the microemulsion model introduced in Ref. 1 are investigated. For the lattice case with nearestneighbor interactions an analytic asymptotic expression for the surface tension at the oilmicroemulsions-water coexistence is found for low T. For both lattice and continuum cases the critical point is determined, and we show how an oscillating phase (microemulsion) may be formed below the critical temperature. A. Ciach and J. S. HGlye: Microscopic model for microemulsions. II
The Phase Behavior and Microstructure of Efficient Cationic–Nonionic Microemulsions
Journal of Colloid and Interface Science, 2001
A surfactant mixture of didodecyldimethylammonium bromide (DDAB) and n-alkyl polyglycol ethers (C i E j ) can make efficient microemulsions of decane and water. Increases in surfactant efficiency by up to a factor of four are realized as 2% C 8 E 3 is replaced with DDAB. As little as 6% of an appropriate surfactant mixture can microemulsify equal masses of oil and water. The increase in DDAB concentration causes the spacing of the bicontinuous domains of oil and water to decrease and the correlation length of the surfactant monolayers to increase. These changes in structural parameters, as detected by small-angle neutron scattering, are in quantitative accord with theoretical calculations of changes in structure as a result of electrostatic stiffening of elastic membranes. Although the reported changes in microstructure are consistent with predictions, they alone cannot explain the observed large increases in overall surfactant efficiency. C 2001 Academic Press
A monolayer model of the interfacial region of microemulsions
Colloid & Polymer Science, 1988
Mixed monolayers of tetradecanol and oleic acid at the water-air interface were studied to provide a static "related structure" featuring the interface between water and oil of water-dodecane microemulsions. The films of pure components as a function of temperature show a strong area contraction between 25 ~ and 30 ~ caused by a change in the head groups hydration. This agrees with similar discontinuities found for some properties of the microemulsion in the same temperature range. At the water-air interface, the composition range of tetradecanol/oleic acid mixtures with the highest thermodynamic stability corresponds to the same stability range of the water-in-dodecane-potassium oleate microemulsions. Adsorption isotherms of tetradecanol and hexanol at the dodecane-water interface were studied to compare the surface behaviour of the two alcohols; the results indicate that the two alcohols have very similar two-dimensional surface phases and adsorption energies.