The effect of the hydrocarbon–water interface structure on the behavior of an emulsion stabilized with dodecanephosphonic acid

We studied the phase transition of petroleum ether/water/n-dodecanephosphonic acid (DPA) from W/O to W/O/W emulsion when the neutralization degree (nd) of dodecanephosphonic acid (DPA) with NaOH was changed. We concluded that a phase transition from W/O emulsion to W/O/W emulsion occurs when nd ≈ 0.47 due to a change in the structure of the polar layer at the oil/water interface from a tightly hydrogen-bonded layer to a less compact layer formed by –PO3H2 and –PO3H− groups in favor of an inversion. The W/O/W emulsion is probably caused by a partition of acid molecules in the water droplets interface inside the oil droplets, and monosodium phosphonate molecules at the external interface of oil droplets.