LONDON-VANDERWAALS AND EDL EFFECTS IN THE COALESCENCE OF OIL DROPS .2. IONIC-STRENGTH AND PH EFFECTS

Coalescence of n-hexadecane oil drops (70-100 mu m in diameter) suspen ded inside an aqueous medium of varying pH and ionic strength is repor ted. The drops were produced in situ by micropipettes inside aqueous-p hase-filled fine capillaries (100-160 mu m, i.d.). At constant pH valu es of 5.7 and 10.9 the ionic strength of the aqueous medium was change d with KCl so that the Debye length (kappa(-1)) varied between 1.6 to 362 nm and 0.36 to 14 nm, respectively. Visual observations showed coa lescence when kappa(-1) was less than similar to 5.0 nm. The DLVO theo ry together with previously reported measurements of critical thicknes s of rupture of aqueous thin films also predicts coalescence under the se conditions. The effect of change in pH at constant ionic strength ( kappa(-1) = 5.0 nm) was also studied. While theory predicts coalescenc e at all pH values with constant kappa(-1), coalescence time increases with pH. At similar conditions of high ionic strength, the coalescenc e times at very low pH are lower than at intermediate pH by two orders of magnitude, suggesting that pH is more important than ionic strengt h in controlling the coalescence of the oil drops. (C) 1995 Academic P ress, Inc.