What are mixing rules for equations of state and why are they important?

Mixing rules combine pure component EOS parameters for mixtures. Classical van der Waals mixing rules: a_mix = sum_i*sum_j(x_i*x_j*a_ij), b_mix = sum(x_i*b_i), with a_ij = sqrt(a_i*a_j)*(1-k_ij). Binary interaction parameter k_ij is fitted to VLE data (typically 0-0.2). More sophisticated: Wong-Sandler rules (ensure correct second virial coefficient), Huron-Vidal (incorporate activity coefficient model at high pressure). Mixing rules are critical - same EOS with different mixing rules gives very different predictions. k_ij selection significantly impacts VLE accuracy.