We use a micrometer-scale optical beam deflection technique to measure the thermodiffusion coefficient DT at room temperature (approx=24 deg C) of dilute aqueous suspensions of charged polystyrene spheres with different surface functionalities.In solutions with large concentrations of monovalent salts,>=100 mM,the thermodiffusion coefficients for 26 nm spheres with carboxyl functionality can be varied within the range -0.9 x 10~(-7) cm~2 s~(-1) K~(-1)< DT < 1.5 x 10~(-7) cm~2 s~(-1) K~(-1) by changing the ionic species in solution;in this case,DT is the product of the electrophoretic mobility muE and the Seebeck coefficient of the electrolyte,S_e = (Q_c* - Q_A*/2eT,D_T = -S_e muE,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 ,DT is negative,independent of particle concentration and independent of the Debye length;DT = -0.73 ± 0.05 x 10~(-7) cm~2 s~(-1) K~(-1).
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