Temperature dependence of the Soret coefficient of ionic colloids

The temperature dependence of the Soret coefficient $S_T\left(T\right)$ in electrostatically charged magnetic colloids is investigated. Two different ferrofluids, with different particles' mean dimensions, are studied. In both cases we obtain a thermophilic behavior of the Soret effect. The temperature dependence of the Soret coefficient is described assuming that the nanoparticles migrate along the ionic thermoelectric field created by the thermal gradient. A model based on the contributions from the thermoelectrophoresis and variation of the double-layer energy, without fitting parameters, is used to describe the experimental results of the colloid with the bigger particles. To do so, independent measurements of the $\zeta$ potential, mass diffusion coefficient, and Seebeck coefficient are performed. The agreement of the theory and the experimental results is rather good. In the case of the ferrofluid with smaller particles, it is not possible to get experimentally reliable values of the $\zeta$ potential and the model described is used to evaluate this parameter and its temperature dependence.

Figure 1

(a) Increment in the index of refraction of sample FF1 as a function of the particles' volume fraction $\mathrm{\Phi }$. (b) Temperature dependence of $dn/d\mathrm{\Phi }$ of sample FF1.

Figure 2

Time evolution of the thermoelectric potential in the electrolytic solutions (nitric acid) for $p\text{H}=2$. The cell mean temperature was $T=298$ K.

Figure 3

Mean-temperature dependence of the Seebeck coefficient of the electrolytic solution (nitric acid) with $p\text{H}=2$.

Figure 4

The $\zeta$ potential of sample FF1 as a function of the temperature.

Figure 5

Typical normalized transmittance of the matter lens induced in sample FF1 at 293 K (open circles), 303 K (open diamonds), and 328 K (crosses). Solid lines are best fits with Eq. (1).

Figure 6

Temperature dependence of the Soret coefficient of samples FF1 (open circles). The solid line is a plot of Eq. (6). The dashed line represents the fit with the equation $S_T\left(T\right)=S_T^{\infty }(1-e^{(T^{*}-T)/T_0})$ [17].

Figure 7

Temperature dependence of the Soret coefficient of samples FF2 (open circles). The solid line is a fit with Eq. (6). The dashed line represents the fit with the equation $S_T\left(T\right)=S_T^{\infty }(1-e^{(T^{*}-T)/T_0})$ [17].

Figure 8

Temperature dependence of $D_M$ of (a) sample FF1 and (b) sample FF2. Solid lines are linear fits.