Particle charging in nonpolar media
Liquid toners are insoluble, charged, coloured particles suspended in a
nonconductive liquid. The particles are significantly smaller than dry toner
particles (< 3 mm) and hence are capable of producing very high resolution
images. (A diagram showing the components of a conventional toner is
given in Figure 1.)
Dispersants for toner particles must be nonconductive, to avoid discharging
the latent electrostatic image, chemically inert, relatively non-viscous and
volatile. The isoparaffinic hydrocarbons meet the majority of these
requirements. These are highly branched alkanes with carbon skeletons
ranging from C10 to C15. Dispersing agents (or resins) are added to provide
colloidal stability, aid fixing of the image and provide charged or charging
sites for the particle surface.
Homopolymers such as polyethylene and copolymers such as those of
methacrylic acid or styrene are examples of suitable dispersing agents.
Particle charging agents are added to control the magnitude and sign of the
surface charge of the particles. These can range from alkylated aryl
sulfonates to succinnimides to divalent metal carboxylates. The pigment base
is usually chosen for its colour but size, insolubility and dispersability
can also influence the choice.
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The main sources of surface charge in aqueous media are ionization of
surface groups, adsorption of ions and ion exchange at the surface . These
types of mechanisms also apply in nonaqueous solvents of moderate to high
dielectric constant where there is a significant degree of ionization.
In hydrocarbon media, such as the isoparaffins, the extent and nature of
ionization is less readily defined, although conductance measurements have
clearly shown that ionization does occur . For surfactant species, it has
been shown that inverse micelles can form and critical micelle
concentrations (CMC) can be determined for some species . These micelles
tend to have smaller aggregation numbers (» 30) than those in aqueous media.
The CMC’s also tend to occur at lower surfactant concentrations and tend to
be poorly defined . In fact, there is considerable debate in the literature
regarding the mechanism of micelle formation in nonpolar media . Critical
micelle concentrations are possible for some surfactant species but others
may form micelles by a stepwise aggregation process . Formation of micelles
in nonpolar media is likely to be dependent on trace amounts of water
present.
Investigations of charged pigment surfaces are usually based in part on the
principle of electrophoretic migration of charged particles, for example,
microelectrophoresis, laser Doppler electrophoresis and streaming potential.
However, it is important to quantify the effects of electric field strength,
Joule heating, polarization effects and the presence of impurities such as
water. For example, adsorbed water can heavily influence the sign and
magnitude of the surface charge . Oxide surfaces are usually positive when
suspended in solutions of Aerosol-OT, but can be negative when water is
rigorously excluded .
From a theoretical perspective, the low dielectric constants of hydrocarbons
are usually assumed to be too small to permit significant dissociation of
electrolytes. Hence, electrostatic stabilization via the formation of
soluble layers around the particles is not possible . However, relatively
high surface potentials (measured as zeta potentials) are common in nonpolar
media . Even though the solvent may contain a low dielectric constant (e »
2) the low double layer capacitance implies that only a few charges are
required to obtain high surface potentials. This does not necessarily imply
that electrostatic stabilization will be a dominant effect as would be
expected in aqueous media.
A sufficiently strong electrostatic force requires not only a high surface
potential, but also a large potential gradient across the double layer.
Since double layers in nonpolar media solvents can range from 1 to 10 mm,
the presence of an intermediate concentration of ions in the form of a
partially dissociated surfactant can assist the electrostatic stabilization
via the formation of a greater potential gradient. Therefore, for
electrostatic stabilization to be a major contributor, the electric double
layer must be thick. However, since the potential gradient is usually low,
this is equivalent to a low field strength so that repulsions between
particles is low, but long range . So, if the particle concentration is low,
electrostatic stabilization may be effective. DLVO theory can be used to
estimate dispersion stability in nonpolar solvents if the surface potential,
Hamaker constant and ionic strength of the medium are known . Of these, the
ionic strength is the most difficult to estimate since the number and
valency of all species must be known. This is, of course, much simpler to
quantify in aqueous systems.
In real systems such as toners, particle concentrations are high
(inter-particle distances are small) and double-layer effects alone will not
stabilize the dispersion. Therefore, dispersing agents are usually added to
provide steric stabilization. This mechanism is based on adsorption of
polymers onto pigment surfaces to reduce the van der Waals attractive forces
. Steric stabilization can be described in terms of the free energy of
repulsion. Napper has classified the enthalpic and entropic contributions
that lead to overall steric stabilization.
Our group is currently involved in a major research program that is
addressing some of the above research areas.