STRUCTURE INTERFACIAL ACTIVITY RELATIONSHIPS AND THERMAL-STABILITY STUDIES OF CERRO-NEGRO CRUDE-OIL AND ITS ACID, BASIC AND NEUTRAL FRACTIONS/

Acid, basic and neutral fractions from Orinoco Belt Crude Oil (Cerro N egro) were isolated by ion-exchange chromatography and characterized b y spectroscopic methods (FTIR, H- and 13C-NMR), titration with KOH and molecular weight measurements using vapor pressure osmometry (VPO). F or the evaluation of the interfacial tension, a toluene/water model sy stem was used. In general, it was determined that interfacially active compounds (natural surfactants) at high pH values are concentrated ma inly in the acid fractions, strong acid fraction (SAF) and total acid fraction (TAF) of the crude oil. By interfacial tensions determination s, it was concluded that the hydrophobic portion of the natural surfac tants has higher aromatic character and molecular weight whereas the h ydrophilic region showed higher acidity and concentration of polar sus tituents in comparison with the original Cerro Negro crude oil. Also i t was found that SAF has lower interfacial activity per molecule than TAF. The results were explained in terms of a better hydrophilic-lipop hilic balance of the TAF, (higher molecular weight, lower concentratio n of polar groups and higher total acid number) in comparison with the SAF. Finally, it was determined that Cerro Negro crude oil and its co mponents suffer important changes (decarboxylation reactions, etc.) at temperatures normally reached during distillation ( > 250 degrees C), therefore, their interfacial activities are also substantially change d as well. (C) 1998 Elsevier Science Ltd.