Creaming Stability of Flocculated Monodisperse Oil-in-Water Emulsions.

The influence of droplet flocculation on the creaming stability of monodisperse n-hexadecane oil-in-water emulsions was studied. The creaming velocity of emulsions with different droplet radii (0.43 and 0.86 µm), droplet concentrations (1-67 vol%), and sodium dodecyl sulfate (SDS) concentrations (7-80 mM) were measured. Depletion flocculation was observed in the emulsions when the aqueous phase SDS concentration exceeded a particular level ( approximately 40 mM for 0.43-µm droplets and approximately 15 mM for 0.86-µm droplets). Creaming was monitored by measuring the back-scattered light from an emulsion as a function of its height. The creaming velocity increased with increasing flocculation and decreased with increasing droplet concentration. These results have important implications for the formulation of emulsion-based materials. Copyright 2000 Academic Press.

Nondestructive Monitoring of Sucrose Diffusion in Oil-in-Water Emulsions by Ultrasonic Velocity Profiling.

The diffusion of sucrose through an optically opaque oil-in-water emulsion was monitored nondestructively by measuring the ultrasonic velocity as a function of height. Initially, a corn oil-in-water emulsion (0, 5, 10, 15, or 20 wt% oil) stabilized by Tween 20 (1 wt%) and xanthan (1 wt%) was placed in a measurement cell at 30 degrees C. A 20 wt% sucrose solution containing the same concentration of Tween 20 and xanthan as the aqueous phase in the emulsion was placed on top of the emulsion. The ultrasonic velocity of this two-layer system was measured as a function of sample height and time and then converted into sucrose and oil concentration-distance profiles using empirical calibration curves. The translational diffusion coefficient of the sucrose in the upper and lower layers was determined by fitting the experimental data to a Fickian diffusion model. The measured diffusion coefficients of the sucrose molecules decreased as the droplet concentration in the emulsion increased, indicating retardation of the sugar molecule movement. Ultrasonic profiling was also used to monitor the compression of the emulsion due to movement of water molecules into the upper layer. Copyright 1999 Academic Press.

Theoretical and Experimental Study of Spectral Reflectance and Color of Concentrated Oil-in-Water Emulsions.

The influence of droplet and dye concentration on the optical properties of oil-in-water emulsions has been investigated. The spectral reflectance and tristimulus color coordinates (L, a, b) of a series of n-hexadecane oil-in-water emulsions with the same median droplet diameters ( approximately 0.3 µm), but different droplet concentrations (0.25 to 38.3 wt%) and red dye concentrations (0 to 0.2 wt%), were measured. Spectral reflectances decreased with increasing dye concentration and decreasing droplet concentration and had troughs at the same wavelengths as the peaks in the absorption spectra of the dyes. Emulsion L-values ("lightness") decreased and a- and b-values ("chromacity measures") increased as the dye concentration increased or the droplet concentration decreased. There was good qualitative agreement between the measured spectral reflectance of the emulsions and that predicted by the Kubelka-Munk theory of diffuse reflectance. Excellent agreement between theory and experiment could be obtained using an empirically determined correction-factor that accounts for cuvette effects. Copyright 1999 Academic Press.

Ultrasonic Spectroscopy Study of Flocculation and Shear-Induced Floc Disruption in Oil-in-Water Emulsions.

Ultrasonic attenuation spectroscopy was used to study flocculation and shear-induced disruption of flocs in oil-in-water emulsions. The ultrasonic attenuation spectra (1 to 150 MHz) of a series of 10 wt% corn oil-in-water emulsions (r32 = 0.2 µm) were measured. Depletion flocculation was induced in the emulsions by adding different concentrations (0 to 0.2 wt%) of a nonadsorbing biopolymer (xanthan) to the aqueous phase. At low frequencies, the attenuation coefficient of the emulsions decreased with increasing flocculation due to overlap of the thermal waves generated by the droplets. These observations were in good agreement with a theory recently developed to account for the influence of droplet flocculation on the ultrasonic properties of emulsions. The ultrasonic technique was also used to monitor the breakdown of flocs under shear flow. The dependence of the ultrasonic properties of emulsions on flocculation means that ultrasonic attenuation spectroscopy can be used to study droplet interactions in concentrated emulsions. Copyright 1998 Academic Press.

Effect of Ethanol on the Solubilization of Hydrocarbon Emulsion Droplets in Nonionic Surfactant Micelles

Light scattering was used to study the effects of ethanol on the mass transport of hydrocarbon molecules from emulsion droplets to nonionic surfactant micelles. n -Hexadecane oil-in-water emulsions (10 wt%) with a mean droplet diameter of 0.97 &mgr;m stabilized by 2 wt% polyoxyethylene sorbitan monolaurate, were prepared. These emulsions were diluted to 0.04 wt% n -hexadecane using a series of aqueous solutions containing different concentrations of ethanol (0 to 20 wt%) and polyoxyethylene sorbitan monolaurate (0 or 2 wt%). The time dependence of the droplet concentration and size distribution were monitored at 30°C using light scattering. The molecular flux of the solubilization process increased with increasing ethanol concentration, but the maximum amount of oil solubilized per unit amount of surfactant was independent of ethanol concentration. The data are interpreted in terms of the molecular processes that occur during solubilization.

Effect of Colloidal Interactions on the Rate of Interdroplet Heterogeneous Nucleation in Oil-in-Water Emulsions

Pulsed nuclear magnetic resonance was used to monitor the crystallization of supercooled liquid droplets in 30 wt% n-hexadecane oil-in-water emulsions at 6°C. Crystallization was induced in the liquid droplets when solid droplets of the same material were present. The rate of induced crystallization increased as the concentration of free non-ionic surfactant (polyoxyethylene sorbitan monolaurate) in the aqueous phase increased from 0 to 14 wt%. Differential scanning calorimetry measurements indicated that free surfactant had no effect on crystal nucleation of individual droplets. These results indicate that the surfactant enhances induced crystallization by altering colloidal interactions between droplets. Creaming measurements showed that flocculation was enhanced in emulsions when the free surfactant concentration was increased. We propose that the presence of free surfactant micelles increases the attraction between droplets because of an osmotic effect, and this attraction facilitates the ability of solid crystals from one droplet to induce crystallization in an adjacent liquid droplet.