Carboxylic compounds in crude oils are responsible of the acidity of the crudes and lead to high corrosion rates. The total acid number (TAN) quantifies acidity of crude oil and values higher than 0.5 define the crude oil as acid. Among the methods used to reduce the TAN of crude oils, those involving ionic liquids (ILs) are recently increasing. Most of involved ionic liquids are basic compounds that neutralize the naphthenic acid. In this work the ionic liquid [EMIM] [EtSO4] acting as extraction solvent was investigated to reduce the TAN value of a synthetic mixture. Obtained reductions were up to 25% within the covered experimental conditions. Extraction temperature improves TAN reduction but only within a short temperature range, with the best results obtained around 50 degrees C. Investigation covers ionic liquid to crude oil ratios ranging 0.1-1 g/g and the highest ratios lead to the highest TAN reductions. UNIFAC, COSMO-RS and COSMO-SAC models allows describing liquid-liquid equilibria for such multi component systems. UNIFAC model required the fitting of interaction parameters for the pair IL-COOH and lead to accurate descriptions for the phase fractions and TAN values of both phases. COSMO models described an overestimation of the TAN reduction for the organic phase and a higher acid content in the IL phase. However, COSMO models shown being quite dependent on the way to obtain the a-profile, and accuracy of the calculated values clearly improved when cation and anion were computed together allowing the shielding of the basic areas in the anion. Consequently, predictions carried out with such models for these complex mixtures should be considered with precaution.