Influence of surfactant crystallization on the aggregation rate of dodecane-in-water nanoemulsions. Changes in the viscosity of the external phase

Abstract

Rates of aggregation of dodecane-in-water (d/w) nanoemulsions stabilized with 7.5 mM sodium dodecylsulfate (SDS) are evaluated beyond 500 mM NaCl. As in the case of hexadecane-in-water (h/w) emulsions, it is found that flocculation rates apparently decrease with the ionic strength, departing appreciably from theoretical predictions. Since many-particle simulations proved to be very time consuming, an accurate knowledge of experimental parameters is necessary prior to the realization of further evaluations. Here, the change in the viscosity of the aqueous solution due to the surfactant phase behavior is considered as a possible cause of referred phenomenon. The influence of the external viscosity on the outcome of the simulations is appraised using a quotient between the actual viscosity of the surfactant solution at a given salt concentration and its value in the absence of salt. For that purpose, the viscosity of 0.5 and 7.5 mM SDS solutions was measured between 300 an 900 mM NaCl for temperatures of 20 and 25 {\deg}C. It is concluded that the augment of the aqueous viscosity due to the formation of surfactant crystals diminish significantly the aggregation rate. Yet this decline seems insufficient to justify the observed reduction of kFC at 25 C. Nevertheless, it is noteworthy that a 5-degree change in the temperature of the surfactant solution causes a remarkable decrease of kFC . Additionally, a set of two-particle simulations is used here to illustrate the limitations of this methodology for the appraisal of the viscosity contribution. It is confirmed that such approach is only convenient to study the effect of the interaction potential on the aggregation rate, as it was formerly conceived by Fuchs [1936].