Strong Specific Hydroxide Ion Binding at the Pristine Oil/Water and Air/Water Interfaces

Abstract

Despite claims, based largely on molecular dynamics simulations, that the surface of water at the air/water interface is acidic, with a positive charge, there is compelling experimental evidence that it is in fact basic, with a negative charge due to the specific adsorption of hydroxide ions. The oil/water interface behaves similarly. The pH dependence of the zeta potentials of oil drops has been measured by two very different techniques: on a single drop in a rotating electrophoresis cell and on about 10(14) submicrometer drops in a 2 vol % emulsion by an electroacoustic method to give similar results with a sigmoidal pH dependence characterized by an isoelectric point at pH 2-3 and a half adsorption point about pH 5.5, or at 10(-8.5) M hydroxide ion. This indicates that hydroxide ion is absorbed much more strongly than other anions. The pH dependence of a single N(2) bubble has also been measured and has the same pH dependence, independently of whether HCl or HI is used to adjust the pH. These similarities between the pH dependences of the zeta potentials of air bubbles and oil drops, as well as those reported from streaming potentials on solid inert surfaces such as Teflon, indicate that water behaves similarly, with only subtle differences, at each of these low dielectric hydrophobic surfaces, with an isoelectric point of pH 2-4. In acidic solutions at pH's below the isoelectric point, the surface is indeed positive, consistent with spectroscopic observations of the adsorption of hydrogen ions.

Coalescence efficiency model including electrostatic interactions in liquid/liquid dispersions

“…al (1996) investigated emulsions of different non-polar oils (xylene, dodecane, hexadecane and perfluoromethyldecalin) and found practically the same zeta potential under identical experimental conditions for all systems. This result was confirmed by the investigations of Creux et al (2009) who found similar pH dependencies of the zeta potential of nitrobenzene, benzene, octane, decane and dodecane. Thus, the electrostatic effects appear to be nearly independent from the type of oil, and zeta potential measurements of one specific oil can be adopted universally.…”

“…As the type of oil has a minor influence on the pH dependency of the zeta potential, the presented model can be adapted to several investigations available in literature (e.g. Creux et al (2009), Gray-Weale and Beattie (2009) and Marinova et al (1996)) for different salts and conditions. If proper data is not available, the zeta potential can be measured and its pH dependency can be described with Stern adsorption isotherm and Grahame equation as presented in this work.…”

“…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).…”

Hofmeister effects in micromolar electrolyte solutions

“…The weak effect of OH − on interfacial ion populations is particularly intriguing because the negative potential of the air-water interface has been ascribed to strong OH − adsorption to water-hydrophobe interfaces in general. 51,63,64 Figure 4(a), which displays the ratio χ = P 127 /P 79+81 = I − /Br − as a function of NaY concentrations, reveals that ClO 4 − and NO 3 − have the largest depressing effects on P 127 and P 79+81 , respectively. From the relative affinities of Br − (f ≡ 1.0), NO 3 − (f = 1.4), I − (f = 3.1), and ClO 4 − (f = 19) for the air-water interface (previously measured in a similar setup), 6 we infer that I − and Br function of (NaI + NaBr) concentration, displaying a broad minimum at ∼50 μM, in the absence of added electrolytes ( Figure S2).…”

Tribocharged Polymer Surfaces: Solvent Effect on Pattern Formation and Modification

“…On the other hand, water provides adsorbed [OH ¹ (H 2 O) n ] ions, which bind to positive PE cations, as expected considering the preferential adsorption of negative water cluster ions on hydrophobic surfaces, as is well established in the literature. 15,16 It should be noted that in all cases solvent evaporation from the polymer surface lowers the surface temperature, facilitating water condensation and thus increasing the adsorption of negative water ion clusters. Lubrication of the sliding surfaces could certainly play a role in suppressing charged domain formation, but this should affect both positive and negative domains, which is different from the experimental results.…”