Transport of nanoscale latex spheres in a temperature gradient

We use a micrometer-scale optical beam deflection technique to measure the thermodiffusion coefficient DT at room temperature (≈24°C) of dilute aqueous suspensions of charged polystyrene spheres with different surface functionalities. In solutions with large concentrations of monovalent salts, 2.100 mM, the thermodiffusion coefficients for 26 nm spheres with carboxyl functionality can be varied within the range -0.9 × 10 ^-7 cm ² s ^-1 K ^-1 < D _T < 1.5 × 10 ^-7 cm ² s ^-1 K ^-1 by changing the ionic species in solution; in this case, D _T is the product of the electrophoretic mobility μ _E and the Seebeck coefficient of the electrolyte, S _e = (Q* _c -Q* _A)/2eT, D _T = -S _e μ _E, where Q* _C and Q* _A are the single ion heats of transport of the cationic and anionic species, respectively. In low ionic strength solutions of LiCl, ≲5 mM, and particle concentrations ≲2 wt%, D _T is negative, independent of particle concentration and independent of the Debye length; D _T = -0.73 ± 0.05 × 10 ^-7 cm ² s ^-1 K ^-1.