Equating to zero    and consequently,  

Thus,                                                              (34)

and letting D_s = xD_m,

    (35)

It is seen from equation (35) that the optimum velocity is, directly proportional to the diffusivity of the  solute in the mobile phase. To a lesser extent it also appears to be inversely dependent on the particle diameter of the packing and the film thickness of the stationary phase. The film thickness of the stationary phase is determined by the physical form of the packing, that is, in the case of silica gel, the nature of the surface and in the case of a reverse phase, on the bonding chemistry.