By using molecular dynamics (MD) simulations and potential of mean force (PMF) calculations, we studied the stability of model acidic oil molecules (C9H19COOH or C9H19COO-) adsorbed on muscovite surfaces in aqueous solution. The muscovite surfaces are covered by different cations (Na+, K+, Mg2+, and Ca2+). It was found that Ca2+-covered muscovite surface significantly enhances the adsorption of C9H19COO- with adsorption Gibbs energy 1 order of magnitude higher than that of Na+-covered surface and 3 times higher than that of K+-covered surface. Furthermore, we found clear evidence that Ca2+ and K+ cause cation bridging, whereas Mg2+ and Na cause water bridging. The adsorption Gibbs energy is much higher for cation bridging than that of water bridging. The ion specific effect is not observed when the carboxyl group is protonated (i.e., C9PH9COOH) These results well explain the results of previous wettability and core flooding experiments and support their key findings that adsorption of Ca2+ cations induces a macroscopic wetting transition either on a flat mineral surface (in wettability experiments) or in a porous media (in core flooding experiments). The insight obtained in this study le-ads us to optimal design of low-salinity water flooding for enhanced oil recovery.