Transport in Charged Colloids Driven by Thermoelectricity

Abstract

We study the thermal diffusion coefficient DT 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 co-ions 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.PACS numbers: 66.10.C, 82.70.-y,47.57.JIntroduction. Colloidal suspensions in a non-uniform electrolyte show a rich and surprising transport behavior. Upon applying an electric field or a chemical or thermal gradient on a macromolecular dispersion, one observes migration of its components and a non-uniform distribution in the stationary state. The physical mechanisms of electrophoresis and diffusiophoresis are well understood [1,2] and widely used in biotechnology and microfluidic applications [3,4].The situation is less clear concerning transport driven by a thermal gradient. There is no complete description for the underlying physical forces, and even the sign of the thermophoretic mobility lacks a rationale so far. It had been known for a while that in some colloidal suspensions the particles move to the cold, and in others to the warm, corresponding to a positive and negative Soret effect, respectively [5][6][7]. Recent experiments on aqueous solutions of lysozyme protein [8,9], polystyrene beads [9-12], micelles [11], DNA [13], and Ludox particles [14] revealed a surprisingly similar temperature dependence in the range T = 0...80• C. In all cases, an inverse Soret effect occurs at low T , changes sign at some intermediate value T * , and seems to saturate above 50 • C. On the other hand, a large negative thermophoretic mobility has been reported for charged latex spheres in a buffered solution at weak acidity and low salinity [10]; adding LiCl or NaCl results in a change of sign and a transport velocity that depends significantly on the cation. These features strongly suggest a single mechanism related to the electric properties of the colloid; the relevance of the thermoelectric effect for colloidal suspensions has been pointed out recently [10].A thermal gradient modifies the solute-solvent interactions and drives the particle at a velocity [15]

Protein-binding assays in biological liquids using microscale thermophoresis

“…Although the theory of thermophoresis is still under debate, all theoretical descriptions agree that the molecular size and various surface parameters, such as charge and hydrophobicity, have an influence on Soret coefficient S T . Recent theoretical approaches also include the influence of the Seebeck effect: ions in the buffer move along a thermal gradient and give rise to an electric field, which in turn moves the molecules by electrophoresis [31][32][33] . As the buffer systems throughout this work used NaCl to set the ionic strength, the resulting Seebeck contribution was suggested to be small 33 .…”

Hydrodynamic Boundary Effects on Thermophoresis of Confined Colloids

“…In the case of thermophoresis, the corresponding effect on the local temperature gradient is small, because of the relatively weak thermal conductivity contrast at the particle-solvent-wall interfaces [22]. A more complex situation occurs if ion currents are relevant for the slip velocity, e.g., through the Seebeck effect [20,23] or a permittivity change due to phase separation [24,25]. Fig.…”

Thermomolecular Orientation of Nonpolar Fluids

“…We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.Thermal gradients are responsible for a wide range of non equilibrium effects, electron transport (thermoelectricity) [1], mass transport in suspensions (thermophoresis) [2][3][4][5][6], mass separation in liquid mixtures [7][8][9] and nucleation and growth of colloidal crystals [10]. Recently it has been shown that temperature gradients can induce orientation in polar fluids, a physical effect that is supported by Non Equilibrium Thermodynamics Theory (NET) and that can be explained in terms of the coupling of a polarization field and a temperature gradient [11].…”