Controlling Nonpolar Colloidal Asphaltene Aggregation by Electrostatic Repulsion

While aromatic chemicals are applied to petroleum oil systems to thermodynamically prevent asphaltene precipitation, amphiphilic dispersants can truncate the precipitation process and create stable suspensions of asphaltene colloids in the submicrometer size range. Bulk sedimentation and dynamic light scattering have shown that stabilizing dispersants inhibit colloidal asphaltene aggregation at approximately the same concentration as is needed to effectively slow bulk sedimentation. At the same time, these same types of dispersants can alter the electrostatic properties of colloidal asphaltenes in nonpolar suspensions. While electrostatic stabilization has been linked to aggregation dynamics in several types of colloidal systems, both aqueous and nonpolar, the complete linkage between electrostatic interactions and aggregation inhibition has yet to be shown in colloidal asphaltene suspensions. In this work, we present dynamic light scattering and electrophoresis measurements in colloidal asphaltene suspensions, using three different petroleum fluids and a dispersant which truncates asphaltene precipitation and colloidal aggregation by enabling uniform electrostatic charging at the colloidal asphaltene surface. ■ INTRODUCTION Asphaltenes, the largest and most aromatic component of petroleum fluids, can undergo a liquid−liquid phase separation which leads to complete precipitation, often clogging wellbores and pipelines. Thermodynamic inhibition and cleaning methods rely on complete asphaltene solubilization or redissolution of the asphaltene precipitate. Stabilizing dispersants, on the other hand, can operate in the regime of colloidal asphaltene suspensions. The nonpolar nature of petroleum systems requires special care in understanding and controlling the factors which can stabilize colloidal asphaltenes. Dispersant type plays an important role in determining how such stabilization occurs in nonpolar suspensions, which have been investigated on several fronts in colloidal science. Whereas ionic dispersants can electrostatically stabilize colloids in nonpolar suspension through the introduction of inverse micelles, nonionic dispersants can even stabilize colloidal suspensions at concentrations below their critical micelle concentration (cmc). Asphaltenes, defined as being soluble in aromatics such as toluene, and insoluble in medium-chain alkanes like heptane, have molecular-scale sizes in the nanometer range. The molecular size characteristics of asphaltenes are best investigated in equilibrium solutions of isolated aspahltenes fully dissolved in toluene, through techniques like neutron and X-ray scattering. Due to their propensity toward π−π interactions, asphaltene molecules associate; even when dissolved in toluene they can exhibit molecular sizes up to 25 nm or even larger with an increase in temperature. Fractionation by ultracentrifuge followed by X-ray scattering reveals associated asphaltene molecular sizes anywhere between 3 and 25 nm. Composition is of utmost importance in considering the nature of asphaltenes in a mixture. Adding heptane below the onset of asphaltene precipitation leads to an increase in asphaltene molecular size, while still maintaining molecular equilibrium of the asphaltenes in Heptol. For instance one study reports an increase from 7 to 20 nm upon the addition of 45% heptane. An excess of heptane, however, leads to complete precipitation and separation. In this case, asphaltenes are no longer in equilibrium with the solution; their association cascades into the colloidal scale, and dynamics must be considered. Asphaltenes undergoing precipitation in a slight excess of heptane exhibit colloidal aggregation over a time scale of hours, which can be greatly accelerated by the addition of even more heptane. Without any toluene to provide thermodynamic stabilization, the colloidal growth and aggregation of asphaltenes in a great excess of heptane can lead to complete separation within minutes. At the same time, evidence of stabilizing repulsive electrostatic interactions has recently been reported in nonpolar colloidal asphaltenes suspensions in heptane with the addition of dispersants. The effect of three different dispersants was measured: two proprietary nonionic dispersants and one wellknown anionic dispersant. This previous work was performed on suspensions of a model oil consisting of isolated asphaltenes dissolved in toluene and reprecipitated in heptane. The two nonionic dispersants effectively truncated colloidal asphaltene growth and increased electrophoretic mobility, all at concentrations less than their critical micelle concentrations in heptane. The nonionic dispersants adsorbed to negative charges on the asphaltene colloids, thereby promoting positively charged colloids. By contrast, while the ionic dispersant induced some charge in the suspension above its cmc, it did not not effectively truncate colloidal asphaltene growth. Micelles of the ionic dispersant induced only weakly negative charges on the asphaltene colloids. However, while the Received: April 3, 2012 Revised: May 31, 2012 Published: June 4, 2012 Article