Aileen Lozsán | PubMed Health

PubMed2006

Theoretical estimation of stability ratios for hexadecane-in-water (H/W) emulsions stabilized with nonylphenol ethoxylated surfactants.

The effect of steric interactions on the stability of oil-in-water emulsions is studied here by means of emulsion stability simulations (ESS). For this purpose, a new steric potential based on a modification of the one formerly proposed by Vincent et. al. is employed. The parameters of the calculation correspond to hexadecane in water emulsions stabilized with nonylphenol ethoxylated surfactants of different chain lengths (NPEm). Stability ratios (W) were calculated using the half life time of the number of drops per unit volume of these systems. A functional relationship between W and the repulsive potential barrier, (DeltaV), similar to the one previously found by Prieve and Ruckenstein for electrostatically stabilized suspensions was obtained. However, according to our simulations there exists a threshold for the stability of emulsions with respect to coalescence which is approximately located around 12.7 k(B)T.

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PubMed2005

Steric interaction between spherical colloidal particles.

The reliability of the Derjaguin approximation for the calculation of the mixing term between sterically stabilized colloidal particles is studied. For this purpose, the steric potential obtained from the experiment of Doroszkowski and Lambourne [J. Polym. Sci., Part C: Polym. Symp. 34, 253 (1971)] is regarded as an exact result. Several analytical expressions corresponding to the mixing term of the steric potential are tested. Vincent et al. [Colloids Surf. 18, 261 (1986)] obtained four of them using the Derjaguin approximation along with different profiles for the volume fraction of segments in grafted polymer layers. As will be shown, the exact calculation of the volume of interaction between two spheres with adsorbed polymer layers already leads to a considerable improvement of the theoretical prediction for the simplest case of constant spatial distribution of polymer monomers. This equation is also better than the four additional expressions that result from using Bagchi's formalism [J. Colloid Interface Sci. 47, 86 (1974)] with similar segment profiles. The deviations of Bagchi's formalism can be substantially minimized using Flory-Krigbaum theory instead of the Flory-Huggins formalism for the calculation of the free energy of mixing. The equations derived here for the steric potentials were derived for particles of distinct radii.

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