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The coalescence of liquid drops induces a higher level of complexity comparedto the classical studies about the aggregation of solid spheres. Yet, it iscommonly believed that most findings on solid dispersions are directlyapplicable to liquid mixtures. Here, the state of the art in the evaluation ofthe flocculation rate of these two systems is reviewed. Special emphasis ismade on the differences between suspensions and emulsions. In the case ofsuspensions, the stability ratio is commonly evaluated from the initial slopeof the absorbance as a function of time under diffusive and reactiveconditions. Puertas and de las Nieves (1997) developed a theoretical approachthat allows the determination of the flocculation rate from the variation ofthe turbidity of a sample as a function of time. Here, suitable modificationsof the experimental procedure and the referred theoretical approach areimplemented in order to calculate the values of the stability ratio and theflocculation rate corresponding to a dodecane-in-water nanoemulsion stabilizedwith sodium dodecyl sulfate. Four analytical expressions of the turbidity aretested, basically differing in the optical cross section of the aggregatesformed. The first two models consider the processes of: a) aggregation (asdescribed by Smoluchowski) and b) the instantaneous coalescence uponflocculation. The other two models account for the simultaneous occurrence offlocculation and coalescence. The latter reproduce the temporal variation ofthe turbidity in all cases studied (380 \leq [NaCl] \leq 600 mM), providing amethod of appraisal of the flocculation rate in nanoemulsions.

Nanoemulsion stability: experimental evaluation of the flocculation rate from turbidity measurements