Single-ion heat of transport in electrolyte solutions: a hydrodynamic theory

Specific salt effects on thermophoresis of charged colloids

We study the Soret effect of charged polystyrene particles as a function of temperature and electrolyte composition. As a main result we find that the Soret coefficient is determined by charge effects, and that non-ionic contributions are small. In view of the well-known electric-double layer interactions, our thermal field-flow fractionation data lead us to the conclusion that the Soret effect originates to a large extent from diffusiophoresis in the salt gradient and from the electrolyte Seebeck effect, both of which show strong specific-ion effects. Moreover, we find that thermophoresis of polystyrene beads is fundamentally different from proteins and aqueous polymer solutions, which show a strong non-ionic contribution.

Thermodiffusion in positively charged magnetic colloids: influence of the particle diameter

The Soret coefficient (ST) of positively charged magnetic colloids was measured as a function of the nanoparticles' diameter. The Z-scan technique and the generalization of the thermal lens model proved to be a reliable technique to measure ST. We show that ST is negative and increases with the particle's diameter, being best described by a functional dependence of the type ST∝d0. Potentiometric and conductometric experiments show that the particle's surface charge decreases as the temperature increases, changing the electrostatic interaction between the nanoparticles. The temperature gradient imposed in the ferrofluid by the Gaussian laser beam leads to the formation of the particle's concentration gradient. The origin of this phenomenon is discussed in terms of the decrease of the particle's surface charge in the hottest region of the sample and the thermoelectric field due to the inhomogeneous distribution of hydrogenous ions present in the colloidal suspension.

Seebeck effect in electrolytes

We study Seebeck effect in liquid electrolytes, starting from its simple neutral analog--thermodiffusion (so-called Ludwig-Soret or Soret effect). It is observed that when two or more subsystems of mobile particles are subjected to the temperature gradient, various types of them respond to it differently. In the case when these fractions, with different mobility parameters (Soret coefficients), are oppositely charged (a case typical for electrolytes), the nonhomogeneous internal electric field is generated. The latter field prevents these fractions from space separation and determines the intensity of the appearing Seebeck effect.

Charging of heated colloidal particles using the electrolyte Seebeck effect

We propose a novel actuation mechanism for colloids, which is based on the Seebeck effect of the electrolyte solution: Laser heating of a nonionic particle accumulates in its vicinity a net charge Q, which is proportional to the excess temperature at the particle surface. The corresponding long-range thermoelectric field E is proportional to 1/r(2) provides a tool for controlled interactions with nearby beads or with additional molecular solutes. An external field E(ext) drags the thermocharged particle at a velocity that depends on its size and absorption properties; the latter point could be particularly relevant for separating carbon nanotubes according to their electronic band structure.

Collective thermoelectrophoresis of charged colloids

Thermally driven colloidal transport is, to a large extent, due to the thermoelectric or Seebeck effect of the charged solution. We show that, contrary to the generally adopted single-particle picture, the transport coefficient depends on the colloidal concentration. For solutions that are dilute in the hydrodynamic sense, collective effects may significantly affect the thermophoretic mobility. Our results provide an explanation for recent experimental observations on polyelectrolytes and charged particles and suggest that for charged colloids collective behavior is the rule rather than the exception.

Thermophoresis and thermoelectricity in surfactant solutions

In electrolyte solutions, the differential migration of the ionic species induced by the presence of a thermal gradient leads to the buildup of a steady-state electric field. Similarly to what happens for the Seebeck effect in solids, the sample behaves therefore as a thermocell. Here, we provide clear evidence for the presence of thermoelectric fields in liquids by detecting and quantifying their strong effects on colloid thermophoresis. Specifically, by contrasting the effects of the addition of NaCl or NaOH on the Soret effect of micellar solutions of sodium dodecyl sulfate, we show that the presence of highly thermally responsive ions such as OH(-) may easily lead to the reversal of particle motion. Our experimental results can be quantitatively explained by a simple model that takes into account interparticle interactions and explicitly includes the micellar electrophoretic transport driven by such a thermally generated electric field. The chance of carefully controlling colloid thermophoresis by tuning the solvent electrolyte composition may prove to be very useful in microfluidic applications and field-flow fractionation methods.

Thermoelectric effect on charged colloids in the Hückel limit

We study the thermophoretic coefficient D(T) of a charged colloid. The non-uniform electrolyte is characterized in terms of densities and diffusion currents of mobile ions. The hydrodynamic treatment in the vicinity of a solute particle relies on the Hückel approximation, which is valid for particles smaller than the Debye length, a << [Formula: see text] . To leading order in the parameter a/[Formula: see text] , we find that the coefficient D(T) consists of two contributions, a dielectrophoretic term proportional to the permittivity derivative dvarepsilon/dT , and a Seebeck term, i.e., the macroscopic electric field induced by the thermal gradient in the electrolyte solution. Depending on the particle valency, these terms may take opposite signs, and their temperature dependence may result in a change of sign of thermophoresis, as observed in several recent experiments.

Transport in charged colloids driven by thermoelectricity

We study the thermal diffusion coefficient D{T} of a charged colloid in a temperature gradient, and find that it is to a large extent determined by the thermoelectric response of the electrolyte solution. The thermally induced salinity gradient leads in general to a strong increase with temperature. The difference of the heat of transport of coions and counterions gives rise to a thermoelectric field that drives the colloid to the cold or to the warm, depending on the sign of its charge. Our results provide an explanation for recent experimental findings on thermophoresis in colloidal suspensions.

Thermodiffusional transport of electrolytes in compact clays

The macroscopic Soret coefficient S(T) was measured for three porous media, namely mica, glass powder, and natural compact clay. At a mean temperature of T = 25 degrees C and with NaCl, S(T) for mica and glass powder was found to be equal to (3.1+/-0.7) x 10(-3) K(-1) and close to values for a free medium in agreement with theoretical predictions which are obtained under the assumption that the pressure gradient and the electric field are negligible on the pore scale. The main result is that for clay S(T) was found five times larger, presumably because of extra couplings with electrical phenomena. This latter measurement was confirmed by an independent technique based on the membrane potential.