Types Of Microemulsions Research Articles

Microemulsions: A Medium for Polymerization Reactions

AbstractAqueous and nonaqueous microemulsions containing either of the monomers styrene or hexyl methacrylate polymerize when initiated with potassium persulfate. Reactions in both types of microemulsions lead to stable, bluish dispersions of latex particles with diameters ranging from 40 to 90 nm. By substituting formamide for water, one can investigate the effect of the monomer solubility in the continuous domain on the microemulsion polymerization mechanism in a way independent of the monomer type. When the monomer solubility in the polar domain is high, the latex particles are larger and have a larger number of polymer chains than when the monomer solubility is low. If the monomer solubility in the continuous domain is greater than the overall monomer concentration in the microemulsion, the reaction mechanism shifts from that of a microemulsion polymerization to one of a solution polymerization in the presence of empty micelles.

Theory of Vesicles and Droplet Type Microemulsions: Configurational Entropy, Size Distribution, and Measurable Properties

AbstractA model of vesicles and droplet type microemulsions is presented. It is shown that the size distribution of the droplets (either vesicles or microemulsions) coexisting with excess fluid is determined in general by only two terms: the free energy of the interface between the drops and the continuous phase, and a size dependent entropic factor commonly referred to as ‘the entropy of mixing’. The last mentioned term arises from translational configurations of the droplets. In this work we use an estimate of the translational contribution as derived in earlier work. A conjecture of the finite size effects coupled to the entropy of the interfacial layer is made based on experimental data.We treat the summation over the size distribution as an ‘effective partition function’ from which all relevant measurable properties are calculated. The necessity of invoking a curvature dependence of the interfacial tension is proved. For vesicles, their average size is shown to scale as the volume fraction raised to a universal power, whereas their polydispersity is fully determined by a universal coefficient.Because of the existence of a finite preferred curvature, no such universal behavior is found for microemulsions. Approximate analytical expressions for their size, interfacial tension of the planar oil‐water interface, and polydispersity are obtained. A limit (though not a physical one) is found at which universal behavior is recovered.The model is compared to experimental data found in the literature. Quantitative comparison reveals full agreement at a consistency level. Good agreement is found even when using only a single free parameter in the theory.However, this comparison points to an additional size independent term in the size distribution.

Microemulsions in Supercritical Hydrochlorofluorocarbons

We report the properties of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) microemulsions formed in supercritical hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons, and flourocarbons. The fluids used in this study include compounds that are of low toxicity and flammability and that are expected to remain environmentally acceptable well into the next century (e.g., 1,1,1, 2-tetrafluoroethane (R134a) and chlorodifluoromethane (R22)). We show that it is possible to form a water-in-oil type of microemulsion in a low molecular weight HCFC (R22). In addition to these HCFCs, We also review the ability to form microemulsions in 14 other fluids (ethane, propene, propane, n-butane, n-pentane, n-haxane, isobutane, isooctane, difluoromethane, trifluoromethane, hexafluoroethane, sulfur haxafluoride, xenon, and carbon dioxide) at conditions just above or below the critical point (0.75 < T/T{sub c} <1.1) of the solvent. We report extensively the phase behavior of AOT and didodecyldimethylammonium bromide microemulsions formed in a supercritical HCFC, R22. We show that microemulsions in HCFCs are practical alternatives to other fluids, such as supercritical carbon dioxide. 56 refs., 7 figs., 1 tab.

Microemulsions as reaction media for synthesis of sodium decyl sulfonate 2. Role of ionic surfactants

Synthesis of sodium decyl sulfonate in various types of microemulsions based on nonionic surfactants have been reported in our previous paper. As a continuation of this study the effect of ionic surfactants with different counterions on the reaction profile was investigated. As expected, the electrical potential of microemulsion droplets significantly influenced the degree of conversion probably by affecting the diffusion of ionic species into the interfacial region where the reaction occurs. The reaction yield was also highly dependent on the type of counterions. Tetradecyl trimethyl ammonium acetate increased the yield, but sodium dodecyl sulfate and tetradecyl trimethyl ammonium bromide gave decreased conversion. The counterion effect was explained by the competitive association of ions with the surfactant monolayer at the oil/water interface.

Binding Constants of Symmetric or Antisymmetric Electrolytes and Aggregation Numbers of Oil-in-Water Type Microemulsions with a Nonionic Surfactant

The effects of electrolytes (symmetric and antisymmetric) on the phase behavior, the apparent cloud point, the ζ-potential, the particle size, and the surface charge density of an oil-in-water type (O/W type) microemulsion forming in solutions of a nonionic surfactant (tetraethylene glycol monodecyl ether, C 10 -POE 4 )/n-decane/brine have been examined. Additions of NaSCN and/or Ca(SCN) 2 result in increasing the temperature where the O/W-type microemulsions formed, while additions of NaCl, NaNO 3 , CaCl 2 , and/or NH 2 CONH 2 decrease the temperature. When the concentrations of NaSCN and/or Ca(SCN) 2 increase, the apparent cloud points of the O/W-type microemulsion increase, while there is a decrease in the particle sizes of the microemulsion droplets. On the other hand, as the concentrations of NaCl, NaNO 3 , CaCl 2 , or NH 2 CONH 2 increase, the apparent cloud point of the microemulsion decreases slightly, but their particle sizes increase. The electrophoretic mobilities of the microemulsion droplets can be measured only in the systems containing NaSCN or Ca(SCN) 2 . Moreover, the ζ-potential and the surface charge density show negative values at a given salinity, though C 10 POE 4 does not possess a charged component in a molecule. Applying formulas described in this article, the adsorption of anion (SCN-) to the hydrophilic moieties of C 10 POE 4 microemulsions is shown to be greater than those of cations (Na + or Ca 2+ ). The binding constants (K) of anion and cation to the hydrophilic groups of C 10 POE 4 microemulsions are found to be K Na = 0 M -1 , K Ca = 0.2 M -1 , 10.0 M -1 < K SCN < 12.0 M -1 and the adsorption density of C 10 POE 4 molecules on the oil/water interface (N) is found to be N = 0.3 nm -2 ; the surfactant (C 10 POE 4 ) numbers per O/W-type microemulsion droplet decrease (a few thousands to a few hundreds) with the increasing concentration of NaSCN and/or Ca(SCN) 2 .

Synthesis of crystalline polyaniline

The synthesis of crystalline polyaniline base using a water-in-oil type microemulsion as a medium of polymerization is reported. The polymer exhibits well defined crystalline phase whose observed orthorhombic lattice parameters are a = 7.65, b = 5.75, c = 10.22Å and V = 450 Å 3. The polymer obtained has been characterized by XRD, FTIR, UV and Cyclic Voltammetry (CV) confirming the crystalline phase, surfactant stabilized nature, base form and dopability.

Thermodynamic Evaluation of the Polydispersity of Droplet Microemulsions

By exploiting our previously presented multiple chemical equilibrium approach to the theory of Winsor II microemulsions (Eriksson, J. C.; Ljunggren, S. Progr. Colloid Polym. Sci. 1990, 81, 41.) we derive relations which enable an evaluation from thermodynamic data of the polydispersity of Winsor II type microemulsions as well as of corresponding one-phase (ternary droplet) microemulsions. We find that, quite generally, a size polydispersity ( R /R max ) in the range 0.1-0.2 is to be anticipated for such microemulsion and, moreover, that the absolute width of the size distribution to a large extent is determined simply by the value of the interfacial tensio, γ∞, of the planar, surfactant-laden oil/water interface. The shape and surfactant film packing density flucuations also influence the peak width but to a lesser extent. Furthermore, we show how the curvature-related bending constant k 2 =2k c +k c can be evaluated from the overall volume fraction of droplets

Properties of surfactant monolayers in relation to microemulsion phase behaviour

The relationship between the properties of surfactant monolayers at oil-water interfaces and the phase behaviour in bulk of mixtures of oil + water + surfactant is discussed. Such monolayer properties include the spontaneous curvature, c o the interfacial tension, I γ, the elasticity K (or rigidity) associated with the mean curvature, and the elasticity K associated with the Gaussian curvature. The model system chosen for investigation is the anionic surfactant AOT + aqueous NaCl + n-alkane at 20°C. In such systems, inversion of microemulsion type from oil-in-water (o/w) to water-in-oil (w/o) is possible with increasing electrolyte concentration. The tension, γ, passes through an ultralow minimum value at conditions corresponding to the formation of three phases. Using small angle neutron scattering, we have determined the structure of surfactant-rich third phases (c o ~ 0) formed with the different alkanes. Lamellar phases consisting of surfactant monolayers separated alternately by oil and water appear with short alkanes, whereas L 3 and bicontinuous phases form in systems containing longer alkanes. The bending elasticity K has been measured for planar monolayers at the oil-water interface by ellipsometry. K is independent of salt concentration but depends markedly on alkane chain length N, falling from ~ 1 k BT for N < 11 to ~0.1 k BT for N = 14. This is discussed in terms of the differing extents of oil penetration into the surfactant chains. Higher rigidities favouring lamellar phases and lower rigidities favouring bicontinuous microemulsions are in line with the theoretical predictions of de Gennes and Taupin. Estimates of the constant K have been obtained in droplet microemulsions (w/o) from a knowledge of their size, K and γ. The sign of the constant is in agreement with the geometry of the phases formed in three phase systems. Finally, the ideas and concepts developed in the oil-water systems described above are used to explain the wetting behaviour by alkanes of AOT monolayers at the air-water surface.

Binding Constants of Inorganic Electrolytes and Aggregation Numbers of Oil-in-Water-Type Microemulsions with an Amphoteric Surfactant

The pH dependence of the ζ-potential and the particle size of the oil-in-water (O/W) type (lower phase) microemulsion forming in solutions of N α ,N α -dimethyl-N e -lauroyllysine (DMLL)/1-pentanol/octane/brine has been examined. The ζ-potential changes from positive to negative values with increasing pH. The particle sizes of the microemulsion droplets show two maxima, at pH 3 and pH 8, with increasing salinity. Moreover, the surface charge densities become negative in regions where the net charge of DMLL molecules becomes almost zero (the so-called isoelectric region). Applying formulas described in this article, the adsorption of chlorine ions to the hydrophilic groups of the amphoteric surfactant (DMLL) microemulsion is shown to be greater than that of sodium ions

EFFECT OF THE SURFACTANT ON ENZYMATIC HYDROLYSIS OF PALM OIL IN MICROEMULSION

ABSTRACT Lipase catalyzed hydrolysis of a triglyceride, palm oil, was performed in L2 type microemulsions based on different surfactants. Isooctane was used as hydrocarbon and the composition of the microemulsions was the same in all experiments. NMR self-diffusion measurements indicated that all microemulsions consisted of closed water droplets and that the structure did not change much during the course of the reaction. The hydrolysis was fast in the microemulsions based on branched-chain anionic or nonionic surfactant but very slow when a branched cationic or a linear nonionic surfactant was used. The cationic surfactant was found to form aggregates with the enzyme. No such interactions were detected with the other surfactants.

Pseudomonas cepacia lipase: Esterification reactions in AOT microemulsion systems

The activity of purifiedPseudomonas cepacia lipase has been investigated in esterification reactions of various aliphatic alcohols with natural fatty acids. The reactions were carried out in microemulsions formed in isooctane by bis(2ethylhexyl)sulfosuccinate sodium salt (AOT). The optima pH, T and water content (wo) for the enzyme activity in this type of microemulsions have been determined. Studies on the effect of various fatty acids and alcohols on the enzyme specificity have shown a preference of this lipase for palmitic and caprylic acid as well as for propanol, while reactions involving cyclic alcohols can not be catalyzed at all. The differences on the behavior of this lipase as compared to other lipases studied in microemulsion systems as well as in other systems are discussed.

Microemulsion-based electrosynthesis of polyparaphenylene

A new synthetic route for electro-depositing polyparaphenylene films based on an oil-in-water type microemulsion is described.

Recent Progress in Emulsion and Emulsion Technology

Recent progress in emulsions and emulsion technology used in many fields is summarized. This paper deals mainly with definition, properties of microemulsion and macroemulsion, their applications in cosmetic, medical, and food fields, preparation of middle phase microemulsions, preparation and properties of multiple emulsions, zeta-potential of microemulsions, emulsionliquid membranes, and flotation separation of O/W type microemulsion.

Microemulsions in three- and four-component systems with a catanionic surfactant: Octylammonium octanoate

The ternary surfactant system octylammonium octanoate-octane-water forms a reverse micellar solution phase (microemulsion) in addition to the lamellar liquid crystalline phase. Addition of the ionic surfactant sodium octanoate to the ternary system extends the stability of the solution region toward the water axis. The structure of the solution phase is investigated from the components' self-diffusion coefficients measured by the pulsed-gradient spin-echo FT-NMR method. In the ternary system without the ionic surfactant the measured oil diffusion is rapid and that of water and surfactant is very low; the system consists of water-in-oil type microemulsions. The water and oil diffusion coefficients in the microemulsions containing ionic surfactant are strongly dependent on sample composition, but that of the surfactant is low over the whole solution region. At low water content stable water droplets are present, but at high water concentration, relative self-diffusion coefficients of both water and oil are high and close to each other, indicating the presence of a bicontinuous type microemulsion. The solvation of oil molecules into the micelle is discussed in the light of the two-site and cell diffusion models.

Winsor transitions and interfacial film compositions in systems containing sodium dodecylbenzene sulphonate and alkanols

In alkane + water systems, containing the single chain surfactant sodium dodecylbenzene sulphonate (SDBS), inversion of microemulsion type (from o/w to w/o) cannot be effected by addition of salt alone. The addition of alkanols as cosurfactants can promote the Winsor I—III—II transitions and reduce the oil-water interfacial tension to ultralow values. The compositions of mixed monolayers of surfactant and alkanol have been determined tensiometrically and (in the case of butanol) by direct analysis. Long chain alkanols are more effective at promoting microemulsion phase inversion than the shorter homologues. The ratios of cosurfactant to surfactant at the microemulsion drop surfaces at the condition of minimum tension are 3.0, 1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The effects of alkane chain length on monolayer compositions incorporating butanol are discussed in terms of the differing extents of alkane penetration into the chain region of the film. Estimated rigidity constants for films containing SDBS and butanol are low (approximately 0.7 kT) in line with measured values reported recently for a number of surfactant+cosurfactant systems.

Incorporation of hydrocarbons into surfactant monolayers

Alkane incorporation into close-packed surfactant monolayers at oil-water interfaces can greatly influence the surface concentration of surfactant, depending on the chain length of the alkane. Such incorporation into monolayers stabilising microemulsion droplets in Winsor systems (microemulsions in equilibrium with excess of the dispersed phase) can cause an inversion of the microemulsion type from oil-in-water to water-in-oil or vice versa. Closely related to this are the effects which solubilised hydrocarbon can have on the cloud points of aqueous solutions of nonionic surfactants. It is difficult to measure quantitatively the extent of hydrocarbon incorporation into surfactant monolayers at oil-water interfaces, but we have determined the extent of uptake of a range of alkanes into monolayers of various surfactants at the air-water surface. In this paper we consider close-packed monolayers at both planar and curved (microemulsion droplet) interfaces, and the effects which incorporation of hydrocarbons into the monolayers has on system behaviour. The extent to which mixed alkane+surfactant monolayers resemble monolayers at oil/water interfaces is explored, and for one system we show how alkane incorporation into films at A/W surfaces directly mirrors microemulsion behaviour in the same system.

Interdroplet attractive forces in AOT water-in-oil microemulsions formed in subcritical and supercritical solvents

The van der Waals attractive interactions between aqueous droplets in water-in-oil type microemulsions have been investigated for a range of continuous-phase solvents including the alkanes from methane to isooctane and the noble gases, krypton and xenon. Hamaker constants for water droplets with surfactant shells of the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in subcritical and supercritical solvents were calculated by using Lifshitz theory and the resulting interaction potential calculations qualitatively account for many features of the phase behavior of these systems.

Cloud Points, solubilisation and interfacial tensions in systems containing nonionic surfactants

AbstractNonionic surfactants above their critical micelle concentration in aqueous solution phase separate, at the cloud point, as the temperature is raised as a result of concommitant changes in micellar size, shape and interactions. We discuss the ways in which the cloud point changes in the presence of various additives. If (sufficient) alkane is solubilised in aqueous surfactant (or water in an alkane solution) single phase microemulsions result. If further alkane (or water) is added separation into two phases occurs. Depending on temperature, the separation results from either attainment of the natural curvature of the surfactant film coating the microemulsion drops, or from interactions between droplets. In two phase (oil+aqueous) systems, one phase is a microemulsion and the other the excess of the dispersed (droplet) phase. The type of microemulsion obtained (i.e. the hydrophile‐lipophile balance of the system) depends on the temperature, and can be understood in terms of changes in the natural curvature of the surfactant films with temperature.

Solubilisation of hydrocarbons in surfactant monolayers

Hydrocarbons which do not spread macroscopically on aqueous surfactant solutions can nevertheless be incorporated (solubilised) in the adsorbed surfactant films, producing mixed monolayers. The surface concentration of alkane in these systems can be obtained semi-quantitatively from the lowering of surface tension caused by the solubilisation. For surfactant solutions above the c.m.c., alkane uptake is found to decrease with increasing alkane chain length, but to increase with increasing surfactant chain length. For a given surfactant, not only is the molar uptake of alkane greater the smaller the alkane, but also the volume uptake. Solubilisation does not significantly affect the surface concentration of surfactant, but rather it increases the monolayer thickness. As close-packed monolayers are progressively diluted, the alkane solubilisation is found to rise initially but then to fall rather rapidly when the surface concentration of surfactant is reduced to ca. half that at the c.m.c. The contact angles, θ, of all the allkanes on dodecyltrimethylammonium bromide solutions close to the c.m.c. are very low. As the surfactant monolayer is diluted, θ begins to rise rapidly when the monolayer becomes less than ‘close-packed’, finally approaching the value for alkane on pure water. For the system comprising sodium diethylhexylsulphosuccinate (AOT), aqueous NaCl and cyclohexane, increase in the salt concentration up to 0.025 mol dm–3 compresses the surfactant monolayer and as a ressult, hydrocarbon solubilisation is substantially reduced. Above this salt concentration solubilisation remains constant. Inversion of microemulsion type in this same system also occurs at 0.025 mol dm–3 NaCl and so it appears, for this system at least, that the hydrocarbon pentration is very similar at the oil/water and the air/water interfaces.

Dynamic light scattering observation of droplet aggregation in a Winsor type W/O microemulsion system

We have performed scattered light intensity autocorrelation measurements on a Winsor type microemulsion system composed of brine, cyclohexane, SDS and a mixture of 1-butanol and 1-pentanol. At high cosurfactant concentration, where the microemulsion phase was considered to consist of individual, spherical water-in-oil droplets of relatively low droplet volume fraction, the autocorrelation functions were observed to be essentially single exponential, as expected. Above a certain droplet volume fraction, however, additional decay modes were observed to enter the correlation data. These modes were interpreted to be due to rotation and/or internal motion of droplet aggregates.