ABSTRACT
K. A. REZAEI GOMARI, A. A. HAMOUDA,* T. DAVIDIAN and D. A. F AGERLAND Petroleum Department, University of Stavanger, P. 0. Box 8002 Ullandhaug, N0-4036 Stavanger, Norway
Abstract-The effect of the pH and salt content on partitioning coefficient (K) and interfacial tension (1FT) for fatty acids dissolved in the n-decane/water system has been investigated. The fatty acids investigated here are divided into three categories: short chain (heptanoic acid), long chain (stearic and oleic acids), and naphthenic acids (cyclohexane-pentanoic and decahydronaphthalenepentanoic acids). A similar trend is obtained for both K and IFT when systems of these acids are tested at different pH values and salt contents. Minimum changes in K and 1FT are shown for systems containing short-chain fatty acids. Divalent cations (Mg2+) showed a larger effect in reducing both 1FT and K as a function of pH than the monovalent Na +. An estimation is made for the acid dissociation constant (pK.) of stearic acid using the partitioning coefficient (K-model) and IFT data (ionized surface group (ISG) model). The estimated pK. values for stearic acid are found to be 7.5 and 7 from K and ISG models, respectively. The humidified (presence of water film) calcite surface is shown to be altered to a more oil-wet at lower pH, and as the pH increases the contact angle decreases. The degree of alteration of wettability is dependent on the structure of the fatty acid and, hence, on K, IFT and pH.
