DOI: 10.1021/la1033965 16941 Langmuir 2010, 26(22), 16941–16948 Published on Web 10/13/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Particle Charging and Charge Screening in Nonpolar Dispersions with Nonionic Surfactants Carlos E. Espinosa, Qiong Guo, Virendra Singh, and Sven H. Behrens* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology 311 Ferst Drive NW, Atlanta, Georgia 30332, United States Received August 26, 2010. Revised Manuscript Received September 29, 2010 The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only because of the high energetic cost associated with introducing electric charge in nonpolar environments. Nevertheless, some surfactants referred to as “charge control agents” are known to raise the conductivity of liquids with low electric permittivity and to mediate charge stabilization of nonpolar dispersions. Here we study an example of the particularly counterintuitive charging and electrostatic interaction of colloidal particles in a nonpolar solvent caused by nonionic surfactants. PMMA particles in hexane solutions of nonionic sorbitan oleate (Span) surfactants are found to exhibit a field-dependent electrophoretic mobility. Extrapolation to zero field strength yields evidence for large electrostatic surface potentials that decay with increasing surfactant concentration in a fashion reminiscent of electrostatic screening caused by salt in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via measurements of the particles’ pair interaction energy. The latter is obtained by liquid structure analysis of quasi-2-dimensional equilibrium particle configurations studied with digital video microscopy. In contrast to the behavior reported for systems with ionic surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant’s critical micelle concentration. 1. Introduction Electric surface charging of colloid particles is a common phenomenon in aqueous environments, and the resulting electrostatic particle interaction, mediated by small ions in solu- tion, is the primary stabilizing mechanism for many colloidal dispersions. 1 Apolar liquids, by contrast, are often considered charge-free because of the large energetic cost associated with the introduction of localized charge. 2 The self-energy of a spherical ion of diameter d ion and valency z in a medium of dielectric constant ε, for example, is given approximately by u B ¼ ðzeÞ 2 4πεε 0 d ion ð1Þ where e is the elementary charge and ε 0 the electric permittivity in vacuum. In water, with its dielectric constant of ε ≈ 80 at room temperature and a hydration layer effectively increasing the ion size, this electrostatic energy is comparable in magnitude to the available thermal energy. In nonpolar liquids such as alkanes (ε ≈ 2), by contrast, typically u B =k B T ¼ z 2 λ B =d ion . 1 ð2Þ even for monovalent species (z =1), and thus the occurrence of small ions is largely suppressed. Here k B T is the thermal energy scale (the product of Boltzmann’s constant and absolute temperature), and λ B is the Bjerrum length of the medium, around 28 nm for nonpolar liquids, as opposed to 0.7 nm for water. In spite of these energetic constraints, charging phenomena in nonpolar liquids do occur, and often, although not always, they are associated with the presence of surfactants. 3 Micellar aggre- gates of these surfactants (“reverse micelles”) are believed to play a particularly important role in increasing the effective size of small ions, thus lowering their self-energy, by incorporating them in the polar micelle core. Two surfactant-mediated charging effects have often been observed: a dramatic increase in the electric conductivity of oils and the promotion of particle charging in nonpolar dispersions. These effects have been utilized in many applications, such as the prevention of flow electrification during petroleum transport, 4,5 the stabilization of crude oil against asphaltene deposition, 6 the formulation of liquid detergents, 2 liquid electrostatic toners, 7 and electrorheological fluids, 8 as well as in the formulation of drugs carriers for inhalation, 9 the development of electrophoretic displays, 10-12 the assembly of new crystalline materials, 13,14 and the functionalization of solid surfaces. 15 Meanwhile, our under- standing of the underlying mechanisms is still very incomplete. A review article by Morrison from 1993 provides an excellent overview of the confusing body of evidence gathered by that time. 3 Later studies using advanced experimental tools such as *To whom correspondence should be addressed. (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989. (2) Van der Hoeven, P. C.; Lyklema, J. Adv. 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