EXPERIMENTAL MEASUREMENTS OF INTERFACIAL TENSION BETWEEN LIQUID-LIQUID MIXTURES IN THE PRESENCE OF SURFACTANTS

Yuri Zeniti Sinzato¹; Nuno Jorge Sousa Dias¹; Francisco Ricardo Cunha¹

¹ University of Brasilia

doi:10.20906/CPS/COB-2015-0446

Resumo

The property interfacial tension between liquid-liquid interfaces is a crucial quantity on the study of emulsion flow rheology and characterization. The use of surfactant molecules on a membrane avoids for instance coalescence of the oil droplets by reducing the interfacial tension of the mixture. A transient effect occurs due to the redistribution of the surfactant molecules at the liquid-liquid interface. The present work carries out interfacial tension measurement as a function of the continuous phase components volume fraction which is equivalent to a liquid-liquid interface of different viscosity ratios. The dependence of the interfacial tension on the volume fraction of surfactants is also examined. The interfacial tension as a function of time is fitted by empirical power-law correlations. The experiments are carried out in a drop volume tensiometer, based on the balance between buoyancy and capillary forces on a drop that is detached from the edge of a capillary tube. The interfacial tension between mineral oil and glycerol/water solution is then measured for several values of glycerol volume fractions. The interfacial tension between oil and water is also measured for various concentrations of Span 80 (lipophilic surfactant) or Tween 80 (hydrophilic surfactant). Again, empirical correlations are proposed in order to fit the experimental data. The equilibrium time and the corresponding equilibrium interfacial tension are used as parameters of the correlations. The results show that the relaxation time for the surfactants distribution reaching steady state decreases with surfactants volume fraction. We determine experimentally the critical value of the surfactant volume fraction and its associated relaxation time. We can expect that beyond the critical concentration, instabilities like drop coalescence could be avoided during the synthesis of an oil-in-water emulsion.

Palavras-chave: interfacial tension; surfactants; emulsion; mixture