Model studies on the n-alkane emulsions stability

Stability of n-alkanes (from heptane to tetradecane) emulsions in 1 M n-propanol solutions were studied by photon correlation spectroscopy using ZetaPlus (Brookhaven Co.) instrument. Using the same apparatus, the effective diameter and zeta potentials after 5, 15, 30, 60, 120 min, and 1 and 2 days were measured for the same sample. Moreover, the effect of pH (3, 10 or 11), and Na+, Ca2+ and La3+ cation concentrations were examined. The results obtained show that the effective diameter depends on the n-alkane chain length. The ''odd'' n-alkanes forms the microdroplets of a bigger size than the ''even'' ones. Very minor effect of pH = 3 or pH = 10 was found for the tested decane and tetradecane emulsions. The Na+, Ca2+, and La3+ effect on the effective diameter depended on their concentrations and valency. As it might be expected, the highest effect appeared in the La3+ solutions. Zeta potentials of n-alkane emulsions in n-propanol solutions are negative and in most systems they range between - 20 and - 30 mV. The H+ ions practically do not influence the zeta potential but OH- ions increase its negative value to 40 - 50 mV. Sodium (10(-5)-10(-1) M) and calcium (10(-5)-10(-2) M) ions practically do not change the zeta potential of the droplets. However, La3+ cations at the smallest concentration already reverse the negative zeta potential of tetradecane droplets to positive. The results show that the emulsions are relatively stable, even up to two days. However, it should be kept in mind that the concentrations of the droplets could not be determined in these experiments. The possible mechanism of the emulsion stability is discussed.