  While the surface charge of solid particles is a direct consequence of their
synthesis, the one of suspended oil drops depends on the adsorption equilibrium
of the surrounding molecules. The presence of salt raises the polarity of the
water phase, favoring the salting out of the surfactant from the aqueous
solution and increasing its surface excess. Yet, the electrolyte also screens
the resulting surface charge of the drops [Debye-H\"uckel, 1923]. As a result,
the electrostatic surface potential increases with the ionic strength until the
saturation of the interface and then decreases. This behavior produces a
maximum previously observed in hexadecane-in-water nanoemulsions
[Urbina-Villalba, 2013; 2015]. Here, the variation of the surface potential of
two dodecane-in-water (d/w) nanoemulsions is evaluated as a function of the
sodium chloride concentration. As expected, maximum values are obtained for
concentrations of 0.5 and 7.5 mM sodium dodecylsulfate (SDS). However, the
surface excess of dodecane drops shows an intermediate behavior between the
adsorption equilibrium predicted by macroscopic adsorption isotherms, and the
one previously found on hexadecane drops. The stability of the prepared
emulsions was followed monitoring the change in the average radius of the
dispersions during five minutes, six times the lapse of time employed in a
typical evaluation of the aggregation rate. In the case of 7.5 mM SDS, the
smallest change in size coincides with the maximum surface potential found (40
mM NaCl). This is not observed for 0.5 mM SDS. Three regimes of destabilization
are found to exist depending on the ionic strength of the aqueous phase. They
correspond to the prevalence of: 1) solubilization/ripening, 2) aggregation,
and 3) crystal precipitation.
