Temperature-insensitive Microemulsions Research Articles

Physicochemical behaviors of cationic gemini surfactant (14-4-14) based microheterogeneous assemblies.

A comprehensive study of micellization and microemulsion formation of a cationic gemini surfactant (tetramethylene-1,4-bis(dimethyltetradecylammonium bromide; 14-4-14) in the absence or presence of hydrophobically modified polyelectrolyte, sodium carboxymethylcellulose (NaCMC), has been conducted by conductometry, tensiometry, microcalorimetry, and fluorimetry methods at different temperatures. Both critical micelle concentration and degree of ionization of the surfactant have been observed to increase with increasing temperature. The interfacial and thermodynamic parameters were evaluated. The standard Gibbs free energy of micellization (ΔGm°) is negative, which decreases with increase in temperature. Larger entropic contribution is observed compared to the enthalpy. The interaction of 14-4-14 with NaCMC produces coacervates which was determined from turbidimetry method. The pseudoternary phase behavior of the microemulsion systems comprising water (or NaCMC as additive), 14-4-14, isopropanol (IP) or n-butanol (Bu) as cosurfactant, and isopropyl myristate (IPM) were studied at 298 K. Phase diagrams reveal that IP derived microemulsions (in the absence of NaCMC) offer a large isotropic region compared to Bu-derived systems at comparable physicochemical conditions. Increasing the concentration of IP or Bu decreases the isotropic region in the phase diagram. NaCMC influences the microemulsion zone, depending upon its concentration, and type of cosurfactant and surfantant/cosurfactant ratio. Dynamic light scattering and conductometric measurements show the size of the droplet, threshold temperature of percolation, scaling parameters, and activation energy of the percolation process of 14-4-14/IP or Bu derived microemulsion systems without/with NaCMC at various physicochemical conditions. Bu exerts a greater effect to reduce θt than IP as a cosurfactant (in the absence of NaCMC) at comparable ω. On the other hand, IP showed better percolating effect than Bu in the presence of NaCMC. Bu and IP (as cosurfactant) and NaCMC (as additive) influenced the microemulsion droplet size (Dh) to different extents under comparable conditions. Temperature insensitive microemulsions have been reported at the studied temperature range (298–353 K). 14-4-14/IP (1:2)-derived microemulsion showed a fractured surface at fixed ω = 15, where ω is the water and surfactant molar ratio, and temperature (298 K); whereas, large scale mesospheres comprising multiple closely winded nanoslices and spheroid morphology were formed in 14-4-14/IP and 14-4-14/Bu microemulsions, respectively, in the presence of 0.01 g % NaCMC, at comparable conditions. These systems revealed good antimicrobial activity toward the strains of Gram-positive Bacillus subtilis and Gram-negative Escherichia coli bacteria at 298 K, and inhibitory effect was governed by ω, type of cosurfactant, and bacterial strains.

Physicochemical investigation of mixed surfactant microemulsions: Water solubilization, thermodynamic properties, microstructure, and dynamics

In this contribution, we report on a systematic investigation of phase behavior and solubilization of water in water-in-heptane or decane aggregates stabilized by mixtures of polyoxyethylene (20) cetyl ether (Brij-58) and cetyltrimethylammonium bromide (CTAB) surfactants with varying compositions in conjugation with 1-pentanol (Pn) at fixed surfactant(s)/Pn ratio and temperature. Synergism in water solubilization was evidenced by the addition of CTAB to Brij-58 stabilized system in close proximity of equimolar composition in both oils. An attempt has been made to correlate composition dependent water solubilization and volume induced conductivity studies to provide insight into the solubilization mechanism of these mixed systems. Conductivity studies reveal the ascending curve in water solubilization capacity-(Brij-58:CTAB, w/w) profile as the interdroplet interaction branch indicating percolation of conductance and the descending curve is a curvature branch due to the rigidity of the interface in these systems. The microstructure of these systems as a function of surfactant composition has been determined by dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) measurements. FTIR study reveals increase and decrease in relative population of bound and bulk-like water, respectively, with increase in Brij-58:CTAB (w/w). DLS measurements showed that the droplet hydrodynamic diameter (Dh) decreases significantly with the increase in Brij-58:CTAB (w/w). Further, the interfacial composition and energetic parameters for the transfer of Pn from bulk oil to the interface were evaluated by the dilution method. Formation of temperature-insensitive microemulsions and temperature invariant droplet sizes are evidenced in the vicinity of the equimolar composition. The results are interpreted in terms of a proposed mechanism.

Physicochemical studies of mixed surfactant microemulsions with isopropyl myristate as oil

The present study is focused on evaluation of interfacial compositions and thermodynamic properties of w/o mixed surfactant [(sodium dodecylsulfate, SDS/polyoxyethylene (23) lauryl ether, Brij-35)/1-pentanol (Pn)/isopropyl myristate (IPM)] microemulsions under various physicochemical conditions by the dilution method. The number of moles of Pn at the interface (nai) and bulk oil (nao), and various thermodynamic parameters [viz. standard Gibbs free energy (ΔGo→i0), standard enthalpy (ΔHo→i0), and standard entropy (ΔSo→i0) of the transfer of Pn from bulk oil to the interface] have been found to be dependent on the molar ratio of water to surfactant (ω), concentration of Brij-35 (XBrij-35), and temperature. Temperature-insensitive microemulsions with zero specific heat capacity (ΔCp0)o→i have been formed at specific compositions. The intrinsic enthalpy change of the transfer process (ΔH0)o→i∗ has been evaluated from linear correlation between ΔHo→i0 and ΔSo→i0 at different experimental temperatures. The present report also aims at a precise characterization on the basis of molecular interactions between the constituents and provides insight into the nature of the oil/water interfaces of these systems by conductivity and dynamic light scattering studies as a function of ω and XBrij-35. Conductivity studies reveal that incorporation of Brij-35 in non-percolating water/SDS/Pn/IPM systems makes them favorable for ω-induced percolation behavior up to XBrij-35⩽0.5. But further addition of Brij-35 causes a decrease in conductivity with increasing ω. Furthermore, the hydrodynamic diameters of the microemulsion droplets increase with increase in both XBrij-35 and ω. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (nai) and bulk oil (nao) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. nai,nai, standard Gibbs free energy (ΔGt0), standard enthalpy (ΔHt0), and standard entropy (ΔGt0) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (XBrij-58 or Brij-78), oil and temperature. A correlation between (ΔHt0) and (ΔSt0) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Characterization of Water/Sucrose Laurate/n‐Propanol/Allylbenzene Microemulsions

AbstractWater/n‐propanol/sucrose laurate/allylbenzene micellar systems were formulated and applied in the isomerization of allylbenzene in the presence of heterogenized derivatives of some platinum group catalysts. The ratio (w/w) of n‐propanol/surfactant studied herewith was 2/1. Temperature insensitive microemulsions were found. The microemulsions were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity, and isentropic compressibility. The densities increase with increases in the water volume fraction. Excess volumes of the microemulsions decrease for water volume fractions below 0.2, level off for water volume fractions between 0.2 and 0.6 then increase for water volume fractions above 0.6. Excess volumes of the studied micellar systems increase with temperature. Isentropic compressibilities increase with temperature for water volume fractions below 0.8 and decrease for water volume fractions above 0.8. Structural transitions from water‐in‐oil to bicontinuous to oil‐in‐water occur along the microemulsion phase. The particle hydrodynamic diameter of the oil‐in‐water microemulsions at the 0.95 water volume fraction was found to decrease with temperature.

Properties of microemulsions with sugar surfactants and peppermint oil

In this study, we report on the properties of water + propylene glycol/sugar surfactant/peppermint oil + ethanol. The sugar surfactants were sucrose monolaurate and sucrose dilaurate. The mixing ratios (w/w) of water/propylene glycol and that of ethanol/oil equal 2 and 1, respectively. U-type microemulsions were observed in the sucrose monolaurate while S-type microemulsions were observed in the dilaurate-based systems. Temperature-insensitive microemulsions were formulated using the two surfactants. Water volume fraction percolation thresholds were determined by the study of electrical conductivity and dynamic viscosity. The structural parameters that include the periodicity and the correlation length were estimated using small angle X-ray scattering. The periodicity increases linearly with the increase in the water content whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the microemulsions from water-in-oil to bicontinuous and inversion to oil-in-water upon dilution with water. The hydrodynamic radius of diluted microemulsions measured by dynamic light scattering increases with the increase in temperature. The area per polar head group decreases with the increase in temperature.

Microemulsions Formation on Water/Nonionic Surfactant/Peppermint Oil Mixtures

The water/nonionic surfactant/peppermint oil systems were studied in order to determine the one-phase microemulsion regions. The surfactants were of two types: hydroxylated (sugar esters) and ethoxylated (ethoxylated mono-di-glyceride and polyoxyethylene sorbitan monooleate). These surfactants were able to form microemulsions without the use of cosurfactants or cosolvents. The ternary phase behavior at 25, 37, and 45°C was explored to determine the effect of temperature on the amount of solubilized water in the microemulsions. For each system investigated, the total area of the one phase microemulsion region, AT, was estimated. Temperature insensitive microemulsions were observed in the sugar ester based systems. It was found that minor changes in the surfactant structure suffice to provoke a considerable change in the total monophasic area of the system. Hydrophilic sugar esters were able to solubilize higher quantities of water in peppermint oil compared to the polyoxyethylene type surfactants. These alcohol free microemulsions are promising for food, cosmetics and pharmaceutical applications.

Microstructure of Alkyl Glucoside Microemulsions: Control of Curvature by Interfacial Composition

The phase behavior of water/n-octane/n-octyl-β-d-glucopyranoside (C8G1)/1-octanol (C8E0) permits formulating temperature-insensitive microemulsions spanning the whole water−oil composition range. The types of microstructures formed along the trajectory of the middle-phase microemulsion are examined by NMR diffusometry, yielding the respective diffusion coefficients of all the components. The diffusion experiments provide clear evidence of the transition from oil-in-water to water-in-oil microemulsions via bicontinuous structures in a remarkably large range around phase inversion. Small-angle neutron scattering (SANS) along the same path confirm the picture. Furthermore, SANS curves on the absolute scale permit extracting the specific internal interface in the microemulsion as it passes through phase inversion. When the composition of the internal interface is known, the mean area per surfactant molecule is determined. It is found that as the interfacial film becomes increasingly rich in C8E0, that is, the...

A study of the microstructure of four-component sucrose ester microemulsions by SAXS and NMR

Sucrose esters form a class of surfactants with the important properties of being biodegradable, non-toxic and capable of forming temperature-insensitive microemulsions. Such microemulsions would be expected to suit a variety of food-based and pharmaceutical applications; however to date little is known about their structure and stability. In this study, the Winsor IV microemulsion systems composed of sucrose esters (SE)/1-butanol/water and oils such as n-dodecane, n-hexadecane and medium chain triglyceride (MCT), have been investigated using small angle X-ray scattering (SAXS), pulsed gradient spin echo (PGSE) NMR and viscosity measurements. The SAXS results for the sucrose monostearate (S1570) system at SE/MCT/1-butanol=1.5:1.1 clearly indicate that the periodicity d increases with increase in water content and is not sensitive to the nature of the oil. From the amphiphilicity factor, f a, and the correlation length, ξ, one can conclude that the n-dodecane-based microemulsion system is the most ordered. Microstructure investigation by PGSE NMR gives evidence of structural changes as the water content in the system increases. The oil self-diffusion remains unchanged when MCT serves as the oil phase. However, when the oil is paraffinic in nature ( n-dodecane and n-hexadecane) the self-diffusion coefficient indicates participation of the oil molecules at the interface. Surfactant self-diffusion is only weakly affected by the water content. The shorter chain oils ( n-dodecane and MCT) solubilize a maximum of 40 and 47 wt.% of water and cannot invert, while the long chain paraffinic ( n-hexadecane-based system) inverts into an O/W microemulsion. The viscosity of these microemulsions decreases with increasing water content. The absence of a yield stress in any of the samples studied, together with the linearity of the flow curves, is evidence that there are no relaxation processes in these microemulsions which show a non-Newtonian flow behavior.

Temperature-insensitive microemulsions formulated from octyl monoglucoside and alcohols: potential candidates for practical applications

Temperature-insensitive microemulsions formulated from octyl monoglucoside and alcohols: potential candidates for practical applications

Temperature-insensitive microemulsions and dilution ripening

Single-phase microemulsions are thermodynamically stable dispersions of an oil in water (O/W) or water in oil (W/O) and are an attractive means of formulating agrochemical products (bicontinuous systems of mutually dispersed oil and water are also possible). Two areas of interest are reported in this summary. (i) Temperature sensitivity – a disadvantage of many microemulsion systems is that the phase boundary of the single-phase region is often highly dependent upon temperature and electrolyte concentration. The aim of this work was the production of temperature-insensitive systems, the criterion used being that the radius of curvature of the droplets at the solubilisation boundary was independent of temperature. (ii) The application of microemulsion systems will involve dilution into water to form the spray system and, as such, an understanding of how the droplets grow under these conditions is of importance. Factors affecting droplet growth processes occurring upon dilution into water have been investigated.

Temperature Insensitive Microemulsions

This paper is concerned with the preparation of temperature insensitive microemulsions stabilized using mixtures of the ionic surfactant sodium bis(2-ethylhexyl)sulphosuccinate (AOT) and the nonionic surfactant n-dodecyl pentaoxyethylene glycol ether (C12E5). The temperature dependence of microemulsions arises from the temperature sensitivity of the preferred monolayer curvature, and this is similar in magnitude but opposite in sign for AOT and C12E5 monolayers. Thus, as is well-known for mixed ionic/nonionic systems, monolayers containing a mixture of AOT and C12E5 show a reduced temperature sensitivity. In this paper we develop a simple phenomenological model for the solubilization, phase inversion, and oil−water tension properties of such systems. The model successfully describes systematic experimental results for solubilization, phase inversion temperatures, and tensions for a wide range of composition and temperature. In addition to predicting conditions for which the microemulsions are temperature ...

Effect of Temperature on the Phase Behavior of Ionic–Nonionic Microemulsions

Microemulsions are formed over a wide range of temperatures in brine/sodium dodecyl sulfate/polyoxyethylene dodecyl ether (C12EO2or C12EO3)/decane systems. In a dilute region, lamellar liquid crystal is present instead of bicontinuous microemulsion at the composition at which the HLB of the mixed surfactant is optimum in the given system. The monomeric solubilities of C12EO2or C12EO3in oil and the mixing fraction of C12EO2or C12EO3in the mixed surfactant layer were determined using the geometrical relation of three-phase tie triangle containing the liquid crystal phase in the composition tetrahedron and were compared at 35 and 60°C. The change in the HLB composition with temperature is well described by increasing the monomeric solubility and decreasing the mixing fraction of nonionic surfactant in the surfactant layer. As a result, temperature-insensitive microemulsions are formed in the SDS–C12EO3system.

Temperature-Insensitive Microemulsions in a Sucrose Monoalkanoate System

Middle-phase microemulsion (surfactant phase) is formed in a water/sucrose monododecanoate (SE)/hexanol/decane system. Based on the geometrical relations of a particular three-phase triangle in the quaternary system, the monomeric solubility of hexanol in the excess oil phase and the mixing fraction of SE and hexanol at the water–oil interface inside the microemulsion were obtained. Both values are almost unchanged with increasing temperature. This phase behavior is completely different from that for a system including an ordinary polyoxyethylene-type nonionic surfactant and resembles that in a brine/ionic surfactant/cosurfactant/oil system. On the other hand, the solubilization capability of the SE–hexanol mixtures decreases at higher temperatures due to the decrease in the compatibility of the microemulsion with oil. However, the mixing ratio of mixed surfactant for the maximum solubilization is not greatly changed. Consequently, temperature-insensitive microemulsions can be attained in the present SE system. The effect of oil or surfactant chain length on the phase behavior is also discussed.