Isentropic compressibility and transport properties of CTAB-alkanol-hydrocarbon-water microemulsion systems

Acoustic spectroscopy: A powerful analytical method for the pharmaceutical field?

Acoustics is one of the emerging technologies developed to minimize processing, maximize quality and ensure the safety of pharmaceutical, food and chemical products. The operating principle of acoustic spectroscopy is the measurement of the ultrasound pulse intensity and phase after its propagation through a sample. The main goal of this technique is to characterise concentrated colloidal dispersions without dilution, in such a way as to be able to analyse non-transparent and even highly structured systems. This review presents the state of the art of ultrasound-based techniques in pharmaceutical pre-formulation and formulation steps, showing their potential, applicability and limits. It reports in a simplified version the theory behind acoustic spectroscopy, describes the most common equipment on the market, and finally overviews different studies performed on systems and materials used in the pharmaceutical or related fields.

Fourier transform infrared investigation on the state of water in reverse micelles of quaternary ammonium gemini surfactants C12-s-C12⋅2Br in n-heptane

The state of water in the reverse micelles of C12-s-C(12).2Br homologues has been investigated by Fourier transform infrared spectroscopy. The results showed that the solubilized water had four states: the quaternary ammonium head-group-bound, the Br--bound, the bulklike, and the free water. With increasing W0, the number of bulklike water per surfactant (nb) rapidly increased, which indicated swelling of the reverse micelle. The number of the head-bound water per surfactant (nN+) gradually increased. This was attributed to a reduction of the interfacial curvature, which permitted more water molecules to associate with the ionic heads of surfactants and also led to a part of n-hexanol being expelled from the interface and thus water filled up. Owing to the existence of n-hexanol in the interface, the head-bound water of the present system was smaller than that of AOT system at the same W0. The number of counterion-bound water per surfactant (nBr-) remained unchanged with W0. This was due to much smaller dissociation of the head of C12-2-C(12).2Br than that of AOT. With increasing s, unchanged nN+ is attributed to the comprehensive effects of enlarged head, which promotes the hydration, increased ionization degree, and reduced size of the water pool. Owing to increased ionization degree, nBr- increases with s.

Aggregation of quaternary ammonium gemini surfactants C12-s-C12⋅2Br in n-heptane/n-hexanol solution: Effect of the spacer chains on the critical reverse micelle concentrations

The critical reverse micelle concentrations of C(12)-s-C(12).2Br (s=2,3,4,5,6,8,12) in n-heptane/n-hexanol solutions, cmc(I)s, have been determined by absorption spectrum method using iodine as probe. The values of cmc(I)s are smaller than those of the critical micelle concentrations (cmc(aq)s) in aqueous solution and reach a maximum at s=4, which is similar to the variation of cmc(aq) with s. The variation of cmc(I) with s is attributed to the alteration in the spacer chain conformation, i.e. when the spacer is short enough, it may adopt stretch configuration and when the spacer is long, it tends to bend towards the continuous oil phase to achieve a more densely packing of the quaternary ammonium heads on the surface of the core.

Interfacial composition and structural parameters of water/C12-s-C12 x 2Br/n-hexanol/n-heptane microemulsions studied by the dilution method

The interfacial composition of the stable water/C12-s-C12 x 2Br/n-hexanol/n-heptane microemulsions has been studied in detail by dilution method. The results showed a marked maximum amount of the n-hexanol populating on the surfaces of droplets (represented as a = n(a)i/n(s), where n(a)i and n(s) are respectively the moles of n-hexanol and gemini surfactant on the surface of droplets) with increasing water content. At a constant level of water addition (the molar ratio of water to surfactant W0 = 20), a decreased with increasing the spacer length in the C12-s-C12 x 2Br molecule. The structural parameters of a w/o microemulsion were also estimated by analyzing the data of dilution experiments, and we found that the radius of the water pool was very sensitive to the increment of water content. The radius of the water pool varied from 0.74 to 5.35 nm with increasing W0 from 10 to 50. The variation extent reached 4.61 nm. In the cases of water/CPC/n-butanol/isopropyl myristate and water/CTAB/n-butanol/isopropyl myristate, however, the corresponding variation extents were only 1.22 and 1.68 nm, respectively, when increasing comparable water content. The ratio of N(a)/N(2C), where N(a) and N(2C) are respectively the average numbers of n-hexanol and the total average numbers of alkyl chains of gemini surfactant populating on per droplet surface, decreased obviously with increasing water content at W0 > 15. This indicated that C12-2-C12 x 2Br favored to form large droplets that were suitable to solubilize more water.

Dynamics of percolation and energetics in the clustering of water/AOT/oil microemulsions in the presence of ethanol amines

Temperature-induced percolation in water/AOT/oil microemulsions in the presence of mono-, di-, and tri-ethanol amines have been studied using conductometric measurements. The percolation temperature of water/AOT/oil microemulsions depends on the nature of the alkanol amine added. Mono- and di-ethanol amines hinder the percolation process, while triethanol amine promotes the process. Percolation studies were also conducted with varying ω = [H2O]/[AOT] values and varying chain lengths of continuous oil phase (C6–C10). The results indicate that increases in both ω and the chain length of the oil decrease the percolation temperature. The microemulsion systems have been analyzed in terms of percolation temperature, scaling equation parameters, and activation energies. The energetic parameters of the clustering process have also been determined employing the phase–separation model. The influence of alkanol amines on the percolation phenomenon has been rationalized in terms of the changes in fluidity of the interfacial layer, the viscosity of the water micropool, and the attractive interactions of the microemulsion droplets. The influence of the alkanol amine additives on the stated parameters was discussed in view of the individual effects of the alcohol and amine moieties on the properties of water/AOT/oil microemulsions.Key words: microemulsion, percolation, conductometry, alkanol amine, surfactant.

Conductivity of water-in-oil microemulsions stabilized by mixed surfactants

The electrical conductivity of D2O-in-n-heptane microemulsions stabilized by cationic/nonionic surfactant mixtures was studied as a function of D2O content, surfactant concentration, and surfactant mixture composition. The surfactants employed were cationic di-n-didodecyldimethylammonium bromide, DDAB, nonionic poly(oxyethylene) monododecyl ethers, C12EJ, with J=3-8 and 23, nonionic polymeric surfactants of the type PEO-PPO-PEO (Pluronic), and the reverse structure analogues (Pluronic R). Qualitative structural information was drawn from a comparison between the measured conductivity and that predicted by the charge fluctuation model for spherical droplets. The conductivity versus water content curves were found to be typical for water-in-oil systems composed of spherical droplets. From the effect of blending nonionic surfactant with DDAB on the measured conductivities, it was concluded that microemulsion conductivity is independent of the concentration of cationic surfactant (DDAB). This finding agrees well with theoretical microemulsion conductivity models.

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)).

Microemulsion-based media as novel drug delivery systems

Microemulsions are clear, stable, isotropic mixtures of oil, water and surfactant, frequently in combination with a cosurfactant. These systems are currently of interest to the pharmaceutical scientist because of their considerable potential to act as drug delivery vehicles by incorporating a wide range of drug molecules. In order to appreciate the potential of microemulsions as delivery vehicles, this review gives an overview of the formation and phase behaviour and characterization of microemulsions. The use of microemulsions and closely related microemulsion-based systems as drug delivery vehicles is reviewed, with particular emphasis being placed on recent developments and future directions.