Investigations into the formation and characterization of microemulsions. II. Light scattering conductivity and viscosity studies of microemulsions

Microemulsions formed with water—xylene—sodium alkyl benzene sulfonate (NaDBS)-hexanol have been investigated using time-average light scattering conductivity and viscosity measurements at 25°C. Four different concentrations of NaDBS were used, namely, 5, 10.9, 15, and 20 wt% and the molar ratio of n-hexanol:NaDBS was kept constant at 3.4, 3.24, 3.4, and 3.4, respectively. The light scattering results showed that at low volume fractions of water, φ_H2O, the Rayleigh ratio, R₉₀ increased slowly with an increase in φ_H2O but above a critical volume fraction of water, φ_H2O^S, R₉₀ increased almost linearly with increase of φ_H2O, reached a maximum at another critical water volume fraction, φ_H2O^C, above which R₉₀ decreased with further increase in φ_H2O. The results were interpreted qualitatively in terms of the possible aggregate units formed. A quantitative analysis of the light scattering data was carried out using a procedure based on the use of a hard sphere model for particle interactions. Using this approach, the water droplet radius R was calculated as a function of φ_H2O for the four systems investigated. The results showed a linear increase of R with increase in φ_H2O. Approximate values of the water radii were also calculated from the interfacial area and these were found to be in full agreement with the radii obtained from light scattering measurements. The conductance κ showed a non-monotonic variation as the water concentration was increased. A maximum in the κ-φ_H2O was observed at a critical volume fraction of water φ_H2O^S above which κ decreased and then remained almost constant over a range of φ_H2O values. The conductance then sharply increased at another φ_H2O value, namely φ_H2O^C. This φ_H2O^C value was reduced with increase in NaDBS concentration. The conductance results indicate structural changes in the system as the water concentration increases and the transition observed correlate with those obtained from light scattering. Moreover, the low κ values found and the non-monotonic variation of κ with φ_H2O are indicative of the presence of definitive water cores with an external surfactant film which acts as a barrier for ion transport. Viscosity results showed the behavior normally encountered with concentrated dispersions, the relative viscosity η_r increasing exponentially with φ_H2O. The viscosity data were fitted to the Mooney equation. The results showed an increase in the Einstein coefficient with increase in NaDBS concentration indicating an increase in the hydrodynamic volume of droplets. This was attributed to the increase of the ratio of the surfactant layer thickness to the droplet core radius as the NaDBS concentration is increased.     

Preparation of monodisperse ellipsoidal polystyrene particles

A method is described for the preparation of monodisperse ellipsoidal particles of polystyrene in the colloidal size range. Monodisperse polystyrene particles were dispersed in a solution of polyvinyl alcohol. This dispersion was then allowed to form, by evaporation, a thin film of polyvinyl alcohol containing spherical polystyrene particles. Strips of this film were clamped into a metal frame, heated rapidly in an oil bath to 200°C and stretched to a predetermined extent in order to convert the spherical particles into ellipsoids; the film was then cooled. A wide range of axial ratios for a variety of initial particle sizes was obtained by this method.     

Preparation of core-shell polymer colloid particles by encapsulation

 By means of heterocoagulation anionic poly-[styrene] particles were coated with smaller electrosterically stabilised cationic particles of poly-[butyl methacrylate]. On heating the heterocoagulated units 45 °C above the glass transition temperature of poly-[butyl methacrylate], as predicted theoretically, the latter polymer spread over the surface of the poly-[styrene] particle to give a composite particle with a core-shell structure. It was found that the extent of packing of the small particles on the larger core particle was a critical feature of the coating process. Received: 12 September 1996 Accepted: 18 September 1996     

Reversible aggregation Part I. Reversible flocculation monitored by turbidity measurements

The aggregation of four separate polystyrene latices, ranging in diameter from 210 nm to ca. 1 m, using sodium chloride and magnesium sulphate as the electrolytes, has been examined turbidimetrically. The reversibility of the aggregation was examined using a dialysis technique. It was found that for large particle size latices, diameters 781 nm and 1 m, aggregation was reversible in sodium chloride even at concentrations up to 0.5 mol dm^–3. The aggregation of smaller particles was not reversible. The aggregation of large particles in magnesium sulphate solutions was not readily reversed by dialysis. The latter result does not appear to agree with theoretical predictions.Dedicated to Professor Dr. Armin Weiss on the occasion of his 60th birthday.     

Small angle and quasi-elastic neutron scattering studies on polymethylmethacrylate latices in nonpolar media

Poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in a hydrocarbon medium provide, as a polymer latex, a useful system for investigating the properties of a sterically stabilised concentrated dispersion. In the present work these latices were investigated by small angle coherent neutron scattering and by quasielastic incoherent scattering. The former technique was used to probe the dimensions of the surface layer of poly(12-hydroxystearic acid) and to determine the diameter of the core particle of poly(methylmethacrylate) using dilute dispersions. In concentrated dispersions it was used to determine the structure factor. The latter was compared with that expected for an interacting system of hard spheres and reasonable agreement obtained. The quasielastic technique was used to probe the translational diffusion of the poly(12-hydroxystearic acid) molecules on the surface and the diffusion of the total particles in the concentrated systems.     

Fluorescence quenching studies on poly(methyl methacrylate) particles : Matching of the refraction index of the stabilizer phase to that of the solvent with added carbon disulphide

Detailed information about the interior morphology of sterically stabilized colloid particles ought to be accessible from light-scattering measurements when the index of refraction of the stabilizer layer is matched to that of the diluent. When carbon disulphide is added to dispersions of poly(methyl methacrylate) (PMMA) latex particlesstabilized with poly(12-hydroxystearic acid) (PHSA) is dodecane, the scattered light intensity varies continuously with time; a permanent match of indices of refraction in this system is not achieved. Fluorescence quenching experiments with CS₂ as quencher were performed on similar particles containing 0.02 mol% anthracene groups covalently bound to the PMMA chains in order to obtain information about CS₂ swelling of these particles. Time-dependent quenching experiments indicate rapid panetration of CS₂ (< 10 min) into the 150-nm diameter particles. Stern-Volmer analysis of steady-state CS₂ quenching data yielded results typical for polymer systems containing fluorophores in a distribution of environments; the particle-bound anthracenes are not all equally accessible to quencher. Nevertheless, all the anthracene labels are quenched to some extent. It is concluded that within 10 min after exposure, CS₂ penetrates throughout the entire volume of the particles.     

Studies of particle-particle interactions using polystyrene latices and time-average light scattering

Dilute dispersions of monodisperse negatively-charged polystyrene latex particles, radii 161 Å, have been examined by time-average light scattering at various latex volume fractions and electrolyte concentrations. The latter were varied from the low value produced by maintaining mixed bed ion-exchange resin beads in the systems (ca. 10^–5 mol dm^–3) to the value of 5×10^–3 mol dm^–3 obtained by the addition of sodium chloride. From angular scattering measurements determinations of the structure factors were made; these were produced as a consequence of the particle-particle interactions in the system. By extrapolation of the structure factor to zero scattering angle, values were obtained for the osmotic compressibility and hence the osmotic pressure of the systems as a function of the latex volume fraction. It was found that the experimental data obtained could be interpreted in terms of a hard-sphere model for the particle-particle interaction. Good agreement was obtained provided that the particles were assigned a hard-sphere radius which was determined by the electrostatic repulsion between the particles.     

Nonionic latices in aqueous media

Nonionic latices in aqueous media prepared using methoxy polyethylene glycol methacrylate as a comonomer/stabiliser and ascorbic acid/hydrogen peroxide as the initiator have been examined for stability in the presence of the 21 electrolyte, barium chloride. These latices were found to be stable up to the concentrations of barium chloride examined, 0.75 mol dm^–3. Charge stabilised latices, on the other hand, were coagulated in 2.1×10^–2 mol dm^–3 barium chloride. The high stability of the nonionic latices was attributed to the grafting, during polymerisation, of methoxy polyethylene glycol chains to the surface of the core particle of polystyrene. The nonionic latices on concentration to high volume fraction gave highly ordered packing of the particles even in the presence of 0.1 mol dm^–3 sodium chloride.