Influence of flocculation and coalescence on the evolution of the average radius of an O/W emulsion. Is a linear slope of R3 vs. t an unmistakable signature of Ostwald ripening?

The LSW theory of Ostwald ripening, predicts a linear variation of the cube of the average radius of a dispersion as a function of time (R(3)vs. t) [I. M. Lifshitz, V. V. Slyozov, J. Phys. Chem. Solids, 1961, 19, 35-50; C. Wagner, Z. Elektrochem., 1961, 65, 581-591]. It also envisages a left-skewed drop-size distribution with a cut-off radius of 1.5R. Consequently, non-linear changes of R(3)vs. t are usually ascribed to either a transient period of time (previous to the attainment of the asymptotic limit of ripening) or other destabilisation processes. Up to now the effect of Brownian motion on Ostwald ripening (OR) has not been considered, although it is by far the strongest limitation of the LSW theory. In this work we show the results of incorporating the algorithm of De Smet et al. for Ostwald ripening simulations [Y. De Smet, L. Deriemaeker, R. Finsy, Langmuir, 1997, 13, 6884-6888] to our emulsion stability simulations (ESS) code. In particular, the short-time evolution of a dilute dodecane/water nanoemulsion in the absence of stabilisers is studied. At high ionic strength, the simulations suggest that R(3) can change linearly with time during the transient period of Ostwald ripening, due to the flocculation and the coalescence of the drops. This behavior is confirmed by the experiments for t < 100 s. At low ionic strength a concave downward curve is observed, both theoretically and experimentally.