Thermophoresis in protein solutions

Thermophoresis, unlike thermal diffusion in simple mixtures, consists in particle drift induced by a temperature gradient delT. We show that thermophoresis in lysozyme solutions has a very distinctive behavior: particle motion can indeed be tuned from "thermophobic" (towards the cold) to "thermophilic" (along delT) by decreasing T. The observed temperature behaviour weakly depends on electrostatic effects, and rather suggests a primary role of hydrophobic interactions, further supported by comparison with the temperature dependence of lysozyme equilibrium solubility. Most of the observed features can be qualitatively understood by envisaging thermophoresis as a "microscopic Marangoni effect", due to thermally induced gradients of the interfacial free energy.