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In this update, we report progress made toward identifying how surfaces of different surface energies and, therefore, functionality affect the adsorption/deposition of asphaltenes from model crude oil solutions.  This work entails modifying SiO₂ coated QCM sensor crystal surfaces with different functionalities and monitoring how the adsorption of asphaltenes is affected using Quartz Crystal Microbalance with dissipation monitoring. (QCM-D).  To date, the published literature regarding the adsorption/deposition of asphaltenes on metal surfaces involves using various techniques such as UV-Vis spectroscopy and QCM-D to monitor the adsorption/deposition.  Much of the research in this field focuses mainly on the adsorption/deposition of asphaltenes onto metal surfaces and asphaltene-metal interactions.  The focus of our research differs in that we investigate how surface energy and surface functionality of a substrate affects the adhesion of asphaltene particles under flow conditions. 

QCM-D experiments were conducted using a Q-Sense E4 equipped with a single flow module.  The senor crystal oscillators used were 14 mm diameter, AT-cut, SiO₂ coated (50 nm) quartz crystal with a fundamental frequency of 5 MHz.  The sensor crystal was mounted on a Peltier element that gives accurate temperature control of +0.02^oC in the measurement chamber.  Prior to all experiments the SiO₂ sensor crystals were washed using 2% Hellman Ex^TM in water for 120 mins, rinsed with deionized water, and then blown dry with an N₂ stream.  Solutions of asphaltenes were prepared by dissolving the appropriate amount of asphaltenes in toluene to make solutions of 50 and 100 ppm.  The asphaltene samples were sonicated for 10 minutes to ensure all aggregates were dissolved.  These solutions were then allowed to equilibrate for 24 hours in the dark prior to analysis.  Before beginning the experiments the temperature of the flow cell was set to the surrounding room temperature which was 22+2^oC and solvent was allowed to flow through the measurement chamber for 1 hour to allow the instrument to stabilize.  For all experiments the solvent baseline was considered stable when the variation of the signal was less than +2 Hz.  Next, asphaltene solutions were introduced via Teflon tubing into the measurement chamber and the frequency was monitored continuously throughout the experiments.  The measurements were made using the third overtone (n = 3) frequency (15 MHz) due to its higher sensitivity and low signal-to-noise ratio.  The mass of the adsorbed layer was then calculated using the Sauerbrey equation. 

SiO₂ sensor crystals were functionalized with various chlorosilanes using the following procedure.  Prior to functionalization the SiO₂ sensor crystals were cleaned with 2% Hellman Ex^TM for 120 min, rinsed with deionized water, and blown dry with an N₂ stream.  Immediately after cleaning the SiO₂ sensor crystals they were immersed in a 1.0 M solution of chlorotrimethylsilane in dichloromethane for 1 hour.  Next, the sensor crystals were rinsed by immersing them in dichloromethane for 15-20 min.  After rinsing, the crystals were dried under vaccum for 12 hours.  Contact angle measurements were conducted in triplicate with a VCA Optima from AST Products.  Measurements were taken in air using purified water with a drop size of ~25 µL. 

Results and Discussion

QCM experiments were conducted on asphaltene solutions in toluene at concentrations of 50 and 100 ppm using SiO₂ coated sensor crystals.  These experiments were conducted in order to monitor asphaltene adsorption onto a SiO₂ surface and to determine if asphaltene concentration affected the amount of asphaltenes adsorbed. In addition, these experiments served as a reference point for all of the various surfaces investigated.  From the plots of frequency (Hz) versus time (sec) we do not observe a concentration dependence on the amount of asphaltenes adsorbed for the concentrations studied.   QCM experiments on these samples were run multiple times in order to check reproducibility.   A sample set of data is provided in Table 1. 

Table 1.  Summary of QCM analyses of 50 and 100 ppm asphaltene solutions on quartz

Several other surfaces were generated and experiments performed at concentrations of 50 and 100 ppm in toluene in order to observe if the modified surfaces affected asphaltene adsorption and to further investigate if asphaltene concentration had an effect on the amount of asphaltenes adsorbed.  In these experiments we did observe an increase in the amount adsorbed; however, we did not observe a concentration dependence on the amount of asphaltenes adsorbed. 

Critical surface tensions were calculated using contact angle data for the various surfaces analyzed.  Zisman plots were generated from this data by plotting the cosine of the contact anlges for the different solvents used versus surface tension (dyne/cm).  the critical surface tension values were determined by extrapolating the liner fit to the data to 1.   A plot of average adsorbed mass versus surface tension was produced, and a correlation was observed.