Top: dependence of the equilibrium surface potential φ_d(a) on grain radius a. Materials are NaCl (i); (ii) and silicate (iii); (iv) immersed in typical plasma conditions expected at r ~ 5.7R_J distance in the Jupiter system (see text). The results (i) and (iii) are obtained from the generalized J_s model (7) and compared to results (ii) and (iv) using the Sternglass approximation for SEE . Bottom, left: dependence of the charge-to-mass ratio q/m on grain radius a for NaCl grains corresponding to the results (i) and (ii) in Fig. 3 (top). Bottom, right: zoom for small | q/m | emphasizing the existence of a size range where the new SEE model leads to a different polarity of the grains.