Characterization of complex oil emulsions is critical yet challenging
both in science and in many industrial applications. Here we demonstrate
for the first time the use of flow cytometry as a fast method for
characterizing complex, polydisperse oil–water emulsions. Owing
to our interest in understanding how the presence of specific ions
might affect the properties of oil–water emulsions including
size, polydispersity, and complexity, we present a systematic study
of oil emulsions in deionized water and various brines of different
ionic strength. Forward scatter (FSC) and side scatter (SSC) intensities
associated with detailed statistics were judiciously combined to provide
a better understanding of these complex systems. We find that the
type and concentration profiles of ions around the oil droplets affect
significantly the properties of the emulsion. Weakly hydrated cations
NH4
+ and Ca2+ appear to be more effective
in screening the charge of oil droplets compared to the monovalent
Na+ and divalent Mg2+ ions, respectively. As
a result, coalescence and formation of larger droplets are seen in
the case of NH4Cl and CaCl2 compared to NaCl
and MgCl2, respectively. In addition, weakly hydrated anions
such as Cl– can come closer to the oil surface and,
thus, decrease the effective screening that the Na+ ions
provide as compared to SO4
2– ions, which
leads to more stable emulsions in NaCl compared to Na2SO4. In addition to these specific findings, the work demonstrates
the utility of the technique as a new tool for characterizing oil
emulsions in a wide spectrum of fields ranging from food to oil and
gas applications.