Using molecular-kinetic theory, the effects of the substrate surface energy on the time relaxation of the contact angle of squalane droplets are investigated. The studied surfaces are mixed Langmuir-Blodgett films composed of two kinds of molecules of equivalent length but with different end groups: n-tricosanoic and omega-tricosenoic acids. The two-dimensional miscibility of these chemical compounds over the complete range of relative concentration is discussed an the basis of various criteria. We show that the critical surface tension and the friction coefficient of the squalane molecules on these surfaces, calculated from the relaxation of the contact angle, are linear functions of the relative concentration of the omega-tricosenoic acid in the monolayers.