Volumetric and transport properties in microemulsions and the point of view of percolation theory

Alkyl polyglucoside vs. ethoxylated surfactant-based microemulsions as vehicles for two poorly water-soluble drugs: physicochemical characterization and in vivo skin performance

Two types of biocompatible surfactants were evaluated for their capability to formulate skin-friendly/non-irritant microemulsions as vehicles for two poorly water-soluble model drugs differing in properties and concentrations: alkyl polyglucosides (decyl glucoside and caprylyl/capryl glucoside) and ethoxylated surfactants (glycereth-7-caprylate/ caprate and polysorbate 80). Phase behavior, structural inversion and microemulsion solubilization potential for sertaconazole nitrate and adapalene were found to be highly dependent on the surfactants structure and HLB value. Performed characterization (polarized light microscopy, pH, electrical conductivity, rheological, FTIR and DSC measurements) indicated a formulation containing glycereth- 7-caprylate/caprate as suitable for incorporation of both drugs, whereas alkyl polyglucoside-based systems did not exhibit satisfying solubilization capacity for sertaconazole nitrate. Further, monitored parameters were strongly affected by sertaconazole nitrate incorporation, while they remained almost unchanged in adapalene-loaded vehicles. In addition, results of the in vivo skin performance study supported acceptable tolerability for all investigated formulations, suggesting selected microemulsions as promising carriers worth exploring further for effective skin delivery of model drugs.

Fabrication and stabilization of nanoscale emulsions by formation of a thin polymer membrane at the oil–water interface

Extremely stable O/W nanoemulsions are fabricated by effective assembly of an amphiphilic PEO-b-PCL copolymer at the oil–water interface during phase inversion, which favors formation of a thin PEO-b-PCL film at the interface.

Volumetric and Diffusion Properties of Water/Surfactant/n-Propanol/4-Allylanisole Micellar Systems

Sol-gel encaged [(C8H17)3NCH3][RhCl4] catalyses the double bond isomerization in the flavoring agent 4-allylanisole in aqueous microemulsions. In order to provide optimal composition of the reaction medium water/n-propanol/surfactant/4-allylanisole micellar systems were formulated. The surfactants were sodium dodecyl sulfate, cetyltrimethylammonium bromide, sucrose monolaurate, and polyethylene glycol (7) glyceryl cocoate. The ratio (w/w) of n-propanol/surfactant equals 2/1. The extent of the microemulsions region as function of temperature was determined. The micellar systems were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity and isentropic compressibility. The micellar densities increase with the increase in the water volume fraction. Ultrasonic velocities increase with the increase in water volume fraction up to 0.8 then decrease. Ultrasonic velocities increase with temperature for water volume fractions below 0.8 and decrease for water volume fractions above 0.8. Quantitative analysis of the volumetric parameters enabled the characterization of structural transition along the micellar phase. The particle hydrodynamic diameter of the oil-in-water systems was determined as function of temperature. The particle hydrodynamic diameter decreases in the case of the ionic surfactants while in the case of nonionic surfactants it increases.

Characterization of cephalexin loaded nonionic microemulsions

Water/propylene glycol/sucrose laurate/ethoxylated mono-di-glyceride/isopropyl myristate/peppermint oil U-type microemulsions were used to solubilize cephalexin. Microemulsion dilution and interfacial factors contributing to the cephalexin solubilization were evaluated. Cephalexin solubilization capacity increases with the increase in the aqueous phase volume fraction (φ) up to 0.4 then decreases. Electrical conductivity of drug loaded and drug free microemulsions increases with φ. The hydrodynamic radius measured by dynamic light scattering of the oil-in-water loaded microemulsions decreases with temperature. The microemulsions were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity and isentropic compressibility. The microemulsion densities increase with φ up to 0.8 then decrease. The excess volume decreases with φ up to 0.8 then stabilizes. Ultrasonic velocities increase with the increase in φ while isentropic compressibility decreases. Analysis of the volumetric parameters enabled the characterization of structural transition along the microemulsion phase region. The presence of water-in-oil, bicontinuous and oil-in-water microemulsions, at aqueous phase volume fractions below 0.2, between 0.3 and 0.7 and above 0.8, respectively were found. Interfacial properties and dynamic structure of the monolayer for drug loaded and drug free microemulsions, were studied by electron paramagnetic resonance spectroscopy employing the nitroxide spin probe 5-doxylstearic acid. The rigidity of the interface was affected by the water content and also the presence of cephalexin.

Tween-embedded microemulsions--physicochemical and spectroscopic analysis for antitubercular drugs

The microemulsion composed of oleic acid, phosphate buffer, ethanol, and Tween (20, 40, 60, and 80) has been investigated in the presence of antitubercular drugs of extremely different solubilities, viz. isoniazid (INH), pyrazinamide (PZA), and rifampicin (RIF). The phase behavior showing the realm of existence of microemulsion has been delineated at constant surfactant/co-surfactant ratio (K (m) = 0.55) with maximum isotropic region resulting in the case of Tween 80. The changes in the microstructure of Tween 80-based microemulsion in the presence of anti-TB drugs have been established using conductivty (sigma) and viscosity (eta) behavior. The optical microscopic images of the system help in understanding the effect of dilution and presence of drug on the structure of microemulsion. Partition coefficient, particle size analysis, and spectroscopic studies (UV-visible, Fourier transform infrared, and 1H NMR) have been performed to evaluate the location of a drug in the colloidal formulation. To compare the release of RIF, PZA, and INH from Tween 80 formulation, the dissolution studies have been carried out. It shows that the release of drugs follow the order INH>PZA>RIF. The kinetics of the release of drug has been analyzed using the Korsmeyer and Peppas equation. The results have given a fair success to predict that the release of PZA and INH from Tween 80 microemulsion is non-Fickian, whereas RIF is found to follow a Fickian mechanism.

Behavior of acetyl modified amino acids in reverse micelles: A non-invasive and physiochemical approach

The well-characterized, monodisperse nature of reverse micelles formed by sodium bis-(2-ethylhexyl)sulfosuccinate/water/isooctane and their usefulness in assimilating compounds of varied interests have been exploited to investigate the effect of acetyl modified amino acids (MAA) viz., N-acetyl-L-glycine (NAG), N-acetyl-L-aspartic acid (NAA) and N-acetyl-L-cysteine (NAC), on the water pool and physiochemical properties. Non-invasive techniques such as FTIR and UV-vis absorption spectroscopy have been employed to analyze the interactions of MAA with core water and the AOT headgroup. The micropolarities on both sides of AOT interface have further been investigated by UV-vis absorption probes, methyl orange (MO) and methylene blue (MB). The dynamics of water and temperature induced percolation process have also been studied. The MAA molecules have been found to assist the process with the increase in water content where as a contrary behavior has been observed with the increase in temperature. Conductivity results have been further rationalized in terms of scaling equations, which delineate the dynamic nature of the percolation process. The results have also been analyzed in the light of activation energy of the percolation process and thermodynamics of droplet clustering.

Incorporation of Antitubercular Drug Isoniazid in Pharmaceutically Accepted Microemulsion: Effect on Microstructure and Physical Parameters

PURPOSE

The purpose of present study is to formulate microemulsion composed of oleic acid, phosphate buffer, Tween 80, ethanol and to investigate its potential as drug delivery system for an antitubercular drug isoniazid.

MATERIALS AND METHODS

The pseudo-ternary phase diagram (Gibbs Triangle) was delineated at constant surfactant/co-surfactant ratio (Km 0.55). Changes in the microstructure were established using conductivity (sigma), viscosity (eta), surface tension (gamma) and density measurements. Dissolution studies and particle size analysis were carried out to understand the release of isoniazid from the microemulsion formulation. Further, partitioning studies and spectroscopic analysis (FT-IR and (1)H NMR) was performed to evaluate the location of drug in the colloidal formulation.

RESULTS

Physico-chemical analysis of microemulsion system showed the occurrence of structural changes from water-in-oil to oil-in-water microemulsion. It has been observed that the microemulsion remained stable after the incorporation of isoniazid (in terms of optical texture, pH and phase separation). The changes in the microstructure of the microemulsion after incorporation of drug was analyzed on the basis of partition studies of isoniazid in microemulsion components and various parameters viz pH, sigma, eta,gamma. In addition, the particle size analysis indicates that the microemulsion changes into o/w emulsion at infinite dilution. The spectroscopic studies revealed that most of the drug molecules are present in the continuum region of an o/w microemulsion. Dissolution studies infer that a controlled release of drug is expected from o/w emulsion droplet. In the present system the release of isoniazid from microemulsion was found to be non-Fickian.

CONCLUSION

The present Tween based microemulsion appears beneficial for the delivery of the isoniazid in terms of easy preparation, stability, low cost, sustained and controlled release of a highly water soluble drug.

Temperature-induced percolation behavior of AOT reverse micelles affected by poly(ethylene glycol)s

The influence of poly(ethylene glycol)s additives viz. mono- (EG), di- (DEG), tri- (TEG), tetra- (TeEG) and poly(ethylene glycol)-400 (PEG-400) on temperature-induced electrical percolation of water/AOT/isooctane microemulsion system has been investigated. The composition of microemulsion systems has been kept constant to omega=22 and [additive] = 0.1 M w.r.t. dispersion medium. The effect of increase in the non-polar continuum (S= [Oil]/[AOT]) is indicated by increase in the percolation threshold, theta(c). The findings have been elaborated in terms of validity of scaling laws in the light of the dynamic percolation theory. The activation energy of the process, DeltaEp, has been estimated from Arrhenius plots. Pseudophase concept of the micellar aggregation has been utilized to assess the thermodynamics of clustering of the nanodroplets. The state of trapped water in the micellar core and the corresponding interactions with the AOT head group has been visualized through 1H NMR and FTIR analysis. Results show that at higher omega (>16.0), encapsulated water behaves like free or the bulk water.

Computer simulation of electrical conductivity of colloidal dispersions during aggregation

The computation approach to the simulation of electrical conductivity of colloidal dispersions during aggregation is considered. We use the two-dimensional diffusion-limited aggregation model with multiple-seed growth. The particles execute a random walk, but lose their mobility after contact with the growing clusters or seeds. The two parameters that control the aggregation are the initial concentration of free particles in the system p and the concentration of seeds psi. The case of psi=1, when all the particles are the immobile seeds, corresponds with the usual random percolation problem. The other limiting case of psi=0, when all the particles walk randomly, corresponds to the dynamical percolation problem. The calculation of electrical conductivity and cluster analysis were done with the help of the algorithms of Frank-Lobb and Hoshen-Kopelman. It is shown that the percolation concentration phi c decreases from 0.5927 at psi=1 to 0 at psi --> 0. Scaling analysis was applied to study exponents of correlation length v and of conductivity t. For all psi>0 this model shows universal behavior of classical 2d random percolation with v approximately t approximately 4/3. The electrical conductivity sigma of the system increases during aggregation reaching up to a maximum at the final stage. The concentration dependence of conductivity sigma(phi) obeys the general effective medium equation with apparent exponent ta(psi) that exceeds t. The kinetics of electrical conductivity changes during the aggregation is discussed. In the range of concentration Pc(phi)<p<0.9527 the time of percolation cluster formation pi c decreases with increasing phi.

On the Temperature Percolation in a w/o Microemulsion in the Presence of Organic Derivatives of Chalcogens

The course of temperature percolation in a w/o microemulsion system comprising water/bis(2-ethylhexyl) sulfosuccinate sodium, AOT/isooctane affected by the presence of additives has been investigated. Additives, viz., organic derivatives of chalcogens including dipyridyl diselenide (Py2Se2), diphenyl diselenide (Ph2Se2), and dipyridyl ditelluride (Py2Te2), have been assimilated in the reverse micellar system. Formulations have been studied in terms of (i) the concentration variation of additives, (ii) the change in omega (= [H2O]/[AOT]), and (iii) the change in the nonpolar continuum, S (= [oil]/[AOT]). Phenyl derivatives hinder the percolation, whereas the pyridyl derivative in moderate amounts favors the phenomenon. The estimated values of the critical exponents are lower than those predicted by the dynamic percolation theory. The association model has been implemented to access the thermodynamic parameters of droplet clustering. Pyridyl compounds are expected to alter the rigidity of the surfactant monolayer, which could help to promote the attractive interdroplet interaction. FT-IR spectroscopy has been used to elucidate the changes occurring in the core water in the presence of organic derivatives of chalcogens as the droplet size is increased. Results have been rationalized in terms of the alteration in the physicochemical behavior of the water/AOT/isooctane microemulsion in the presence of additives.

Phase diagram and physical properties of a waterless sodium bis(2-ethylhexylsulfosuccinate)- ethylbenzene-ethyleneglycol microemulsion: an insight into percolation

Volumetric and transport studies have been carried out for the nonaqueous ternary microemulsion system containing sodium bis(2-ethylhexylsulfosuccinate) aerosol-OT (AOT), ethylbenzene (EB), and ethyleneglycol (EG). The results obtained for the conductivity sigma are presented over a wide range of volume fraction of dispersed phase phi and different molar concentration ratio omega=[EG]/[AOT]=2-10 at 30 degrees C and discussed in context of percolation theory. The variation of sigma with respect to temperature (T=10-60 degrees C) shows an increase in the conductance values but no percolation-type phenomenon is observed. The measurements of viscosity, density, and ultrasonic velocity have also been carried out to understand the behavior of this nonaqueous microemulsion system. The phase behavior of the microemulsion is sensitively dependent on the EG to AOT molar ratio. A simple structural model has been applied for the calculation of the various parameters, i.e., aggregation number (n), core radius (r(n)), and surface number density of the surfactant molecule at interface (alpha(s)).

Characterizing colloidal structures of pseudoternary phase diagrams formed by oil/water/amphiphile systems

Two pseudoternary phase diagrams were constructed using ethyl oleate, water, and a surfactant blend containing poly (oxyethylene 20) sorbitan monooleate and sorbitan monolaurate with or without the cosurfactant 1-butanol. Two colloidal regions were identified in the cosurfactant-free phase diagram; a microemulsion (ME) and a region containing lamellar liquid crystals (LC). The addition of 1-butanol increased the area in which systems formed microemulsions and eliminated the formation of any liquid crystalline phases. Samples that form the colloidal regions of both systems were investigated by freeze-fracture transmission electron microscopy and by viscosity and conductivity measurements. The three techniques were compared and evaluated as characterisation tools for such colloidal systems and also to identify transitions between the colloidal systems formed. A droplet ME was present at a low water volume fraction (phi w) in both systems (phi w < 0.15) as revealed by electron microscopy. At higher phi w values, LC structures were observed in micrographs of samples taken from the cosurfactant-free system while the structure of samples from the cosurfactant-containing system was that of a bicontinuous ME. The viscosity of both systems increased with increasing phi w to 0.15 and flow was Newtonian. However, formation of LC in the cosurfactant-free system resulted in a dramatic increase in viscosity that was dependent on phi w and a change to pseudoplastic flow. In contrast, the viscosity of the bicontinuous ME was independent of phi w. Three different methods were used to estimate the percolation threshold from the conductivity data for the cosurfactant-containing system. The use of nonlinear curve fitting was found to be most useful yielding a value close to 0.15 for the phi w.