5 research outputs found
Image Charge Effects on the Formation of Pickering Emulsions
Vigorous mixing of an aqueous particle dispersion with
oil usually
produces a particle-stabilized emulsion (a “Pickering emulsion”),
the longevity of which depends on the particles’ wetting properties.
A known exception occurs when particles fail to adsorb to the oil–water
interface created during mixing because of a strong repulsion between
charges on the particle surface and similar charges on the oil–water
interface; in this case, no Pickering emulsion is formed. Here, we
present experimental evidence that the rarely considered electrostatic
image force can cause a much bigger hindrance to particle adsorption
and prevent the formation of Pickering emulsions even when the particle
interaction with the interface charge is attractive. A simple theoretical
estimate confirms the observed magnitude of this effect and points
at an important limitation of Pickering emulsification, a technology
with widespread industrial applications and increasing popularity
in materials research and development
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 polyÂ(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 liquidsî—¸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