Charging of Oil−Water Interfaces Due to Spontaneous Adsorption of Hydroxyl Ions

Abstract

The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their ζ-potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil−water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation−association equilibrium of the water molecules, adsorb at the oil−water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this phenomenon are discussed. The ζ-potential decreases in magnitude when poly(oxyethylene) chain nonionic surfactants are adsorbed at the interface.

Partitioning of Clay Colloids at Air–Water Interfaces

“…These previous studies indicate that the air-water interface is effectively negatively charged under all of our test conditions. It is also worth noting that very similar ζ(pH) values were obtained for oilwater interfaces by Marinova et al (15).…”

“…In the absence of surfactants, zeta potentials for the air-water interface are negative (13,14). The pHdependent negative charge at the air-water interface is attributed to hydrogen bonding of hydroxyl ions to interfacial water molecules (15,16). Thus, the dominant influence of the repulsive electrostatic force prevents partitioning of negatively charged colloids at air-water interfaces, except when surface potentials are low and ionic strength is high (11).…”

Coalescence efficiency model including electrostatic interactions in liquid/liquid dispersions

“…The sample preparation was done in a similar manner to the procedure applied by Marinova et al (1996). The pH of the water continuous phase (ultrapure water with a resistivity of 18.3 MΩ·cm, produced by the purification system Merck Millipore Milli-Q Gradient) was adjusted with hydrochloric acid (Merck 1.09970 p.a.)…”

“…Due to the sensitive unbuffered aqueous solution, the pH was measured by a pH-meter (WTW pH197) and indicator strips (VWR ProLabo Dosatest 3 zones). Due to the minor effect of dissolved carbon dioxide (CO2 / HCO3 -/ CO3 2-) reported by Marinova et al (1996), a purging of CO2 with N2 was omitted. Nevertheless, all serial dilutions and samples were closed as fast as possible to minimize the contact with the atmosphere and an additional dissolution of CO2.…”

“…Assuming different partition coefficients of the ions and applying the Albertsson model (Pfennig and Schwerin, 1998), it was shown that only the relative magnitude of the partition coefficients may determine the potential difference at the surface. Another widely used assumption is the preferred accumulation or adsorption of specific ions at the interface (especially hydroxide ions (Beattie and Djerdjev, 2004;Beattie and Gray-Weale, 2012;Beattie et al, 2005;Creux et al, 2009;Franks et al, 2005;Gray-Weale and Beattie, 2009;Marinova et al, 1996)), whether it is described by Gibbs (Lyklema, 2000) or Langmuir monolayers (Marinova et al, 1996;Tian and Shen, 2009). Other research groups doubt this concept of ion adsorption and attribute the created potential to the immobilization and orientation of water dipoles at the surface (Chibowski et al, 2005;Vacha et al, 2011) or to the deprotonation of fatty acid impurities in the oil Cabane, 2012a, 2012b).…”