Interfaces Charged by a Nonionic Surfactant

Highly hydrophobic, water-insoluble nonionic surfactants are often considered irrelevant to the ionization of interfaces at which they adsorb, despite observations that suggest otherwise. In the present study, we provide unambiguous evidence for the participation of a water-insoluble surfactant in interfacial ionization by conducting electrophoresis experiments for surfactant-stabilized nonpolar oil droplets in aqueous continuous phase. It was found that the surfactant with amine headgroup positively charged the surface of oil suspended in aqueous continuous phase (oil/water interface), which is consistent with its basic nature. In nonpolar oil continuous phase, the same surfactant positively charged the surface of solid silica (solid/oil interface) which is often considered acidic. The latter observation is exactly opposite to what the traditional <i>acid–base mechanism of surface charging</i> would predict, most clearly suggesting the possibility for another charging mechanism

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor–acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge

Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions

Electric charging of colloidal particles in nonpolar solvents plays a crucial role for many industrial applications and products, including rubbers, engine oils, toners, or electronic displays. Although disfavored by the low solvent permittivity, particle charging can be induced by added surfactants, even nonionic ones, but the underlying mechanism is poorly understood, and neither the magnitude nor the sign of charge can generally be predicted from the particle and surfactant properties. The conclusiveness of scientific studies has been limited partly by a traditional focus on few surfactant types with many differences in their chemical structure and often poorly defined composition. Here we investigate the surface charging of poly­(methyl methacrylate) particles dispersed in hexane-based solutions of three purified polyisobutylene succinimide polyamine surfactants with “subtle” structural variations. We precisely vary the surfactant chemistry by replacing only a single electronegative atom located at a fixed position within the polar headgroup. Electrophoresis reveals that these small differences between the surfactants lead to qualitatively different particle charging. In the respective particle-free surfactant solutions we also find potentially telling differences in the size of the surfactant aggregates (inverse micelles), the residual water content, and the electric solution conductivity as well as indications for a significant size difference between oppositely charged inverse micelles of the most hygroscopic surfactant. An analysis that accounts for the acid/base properties of all constituents suggests that the observed particle charging is better described by asymmetric adsorption of charged inverse micelles from the liquid bulk than by charge creation at the particle surface. Intramicellar acid–base interaction and intermicellar surfactant exchange help rationalize the formation of micellar ions pairs with size asymmetry

Janus Particles in a Nonpolar Solvent

Amphiphilic Janus particles are currently receiving great attention as “solid surfactants”. Previous studies have introduced such particles with a variety of shapes and functions, but there has so far been a strong emphasis on water-dispersible particles that mimic the molecular surfactants soluble in polar solvents. Here we present an example of lipophilic Janus particles which are selectively dispersible in very nonpolar solvents such as alkanes. Interfacial tension measurements between the alkane dispersions and pure water indicate that these particles do have interfacial activity, and like typical hydrophobic, nonionic surfactants, they do not partition to the aqueous bulk. We also show that the oil-borne particles, by retaining locally polar domains where charges can reside, generate electric conductivity in nonpolar liquidsanother feature familiar from molecular surfactants and one commonly exploited to mitigate explosion hazards due to flow electrification during petroleum pumping and in the formulation of electronic inks

Behavior of Spherical Poly(2-acrylamido-2-methylpropanesulfonate) Polyelectrolyte Brushes on Silica Nanoparticles up to Extreme Salinity with Weak Divalent Cation Binding at Ambient and High Temperature

The colloidal stability of nanoparticles (NPs) stabilized by grafted polyelectrolyte (PE) brushes in concentrated divalent ion solutions, at either ambient or high temperature, is of interest in a wide variety of applications including medicine, personal care products, oil and gas recovery, reservoir imaging, and environmental remediation. Previous attempts to determine the length of PE brushes at these conditions have been limited by lack of colloidal stability particularly when divalent ions form complexes with the charges on the brushes. We find that brushes of highly acidic strong PE poly­(2-acrylamido-2-methyl­propane­sulfonate, AMPS) end-grafted to silica NPs provide colloidal stability at salinities up to 4.5 M CaCl₂ or NaCl. Thus, the brush behavior could be studied with dynamic light scattering (DLS) and the electrophoretic mobility by phase analysis light scattering (PALS) from the salt-free condition to the extreme salinities of 4.5 M. In monovalent NaCl solutions, the highly extended poly­(AMPS) brushes at low salt concentration (<i>C</i>_s) collapse monotonically with increasing <i>C</i>_s. On the other hand, in divalent CaCl₂ solutions the brushes underwent four distinct regimes of (i) a low <i>C</i>_s collapse regime, (ii) a relatively broad plateau regime (0.1 M ≤ <i>C</i>_s < 1 M), (iii) a weak reswelling regime, and (iv) a high <i>C</i>_s collapse regime. The novel behavior in regimes ii–iv may be attributed to weak interactions of the poly­(AMPS) brushes with Ca²⁺. We also find that the brushes are more extended at 90 °C as thermal energy weakens interchain bridging, which is consistent with the behavior of free polymer chains dissolved in CaCl₂ solutions at extreme salinities