Reports: AC546310-AC5: Electrokinetic Behavior and Passage of Polyelectrolytes and Surfactants throughout Tortuous Micro/Nanopore Media

Martin A. Hubbe, PhD , North Carolina State University

Recent advances in understanding of electrokinetic measurements and surface characteristics of mesoporous materials allowed us to achieve important gains in evaluation of permeation of probe substances into model materials, including silica gel and cellulosic fibers.  Our project goal has been to better understand the potential roles of polyelectrolytes and surface-active agents in porous media for oil recovery.  Such understanding is needed to develop more effective oil recovery strategies, especially in cases where those strategies involve the use of polyelectrolytes in the flooding solutions.  Our results will help guide the formulation of smart additives to be injected into a wellbore and accomplish various objectives in a controlled way – by either promoting or resisting permeation of materials through different zones of a formation, or even in the lining adjacent to the wellbore.  Results of the present work are part of an effort to reveal ways in which relevant additives can be used to influence permeability, depending on medium pore diameter and various controllable factors.  We also are interested in potential use of mesoporous materials for the biosorption of spilled petroleum.  So the final phase of the work focused on practical applications of our findings – with particular attention to issues surrounding the collection of oil by mesoporous cellulosic sorbents. 

Progress during Years 1-3 of Project

We achieved significant progress in evaluating factors affecting permeation of probe molecules into very small pores, representing those that exist within a mineral bed.

  • Rates of penetration into the pores (nominal size 6 nm, 15 nm, and 30 nm) of silica gel were strongly controlled by polymer molecular mass and salt concentration.
  • Adsorption of very-low-mass polyelectrolyte approached equilibrium very slowly, and the adsorbed amount was proportional to the square-root of time. 
  • Streaming potential measurements were found to be a particularly sensitive way to detect slight changes in the degree of permeation of cationic polyelectrolyte into the interior of silica gel particles.  It was possible to detect significant desorption as a function of time by means of streaming potential, whereas the corresponding change in concentration of the bulk solution was below the detectable limit.  By comparing the results of streaming potential tests at highly contrasting levels of background electrolyte, it was possible to discriminate between charge effects at the surface of silica gel particles, vs. effects due to permeation of polyelectrolyte into the mesopores.
  • Changes in electrokinetic potential, resulting from progressive adsorption of cationic polyelectrolyte into nanoporous material, were found to be proportional to the logarithm of time, consistent with Fick’s law of diffusion.
  • By comparing results with the dialyzed and the as-received polymer, we confirmed a key hypothesis to explain a reversal of streaming potential of silica gel at very high levels of polyelectrolyte treatment.  We had earlier proposed that such reversal was due to a low-mass fraction in samples of high molecular weight polyelectrolytes, and that hypothesis was borne out by our further work.
  • The pH-dependency of polymer permeation was the opposite of what one would expect based on just the strength of an electrostatic driving force.  We were able to demonstrate that weakly attractive forces provide for the highest rates of permeation, giving a good balance between attraction and continued mobility.
  • Tests carried out at variable concentration of supporting electrolyte also supported the hypothesis that a relatively weak electrostatic attraction would be more favorable to permeation.

A breakthrough was achieved in observing, for the first time, a trend towards more negative streaming potentials following placement of silica gel conditioned with cationic polyelectrolytes into aqueous polyelectrolyte-free solution in the presence of salt.  The inferred polymer desorption is not ordinarily observed in cases where polyelectrolytes adsorb onto the outer surfaces of granular materials, but recent thermodynamic studies and simulation work have predicted such behavior in the case of pores that are smaller than the bulk-phase radius of gyration of a polyelectrolyte. 

Our preliminary observations indicated that the presence of a layer of a high-mass polyelectrolyte coating the exterior surface of a nanoporous material can inhibit penetration of lower-mass probe molecules having the same electrical charge. Understanding and controlling such phenomenon can help to influence the progress of different solutes permeating through different zones of an oilfield operation.

Further project work concentrated on kinetic effects, carrying out confirmatory tests in the form of adsorption isotherms.  One major review article has been published related to the field of work.  Three project-related peer-reviewed articles have been produced. 

Final Phase of Work: Practical Application

To derive further benefits of the work, application tests were carried out using selected cellulosic fibers as model mesoporous materials for sorption of simulated crude petroleum.  The following factors were considered: type of fiber (including type of pulping, lignin removal, etc.), whether the substrate was dry or pre-wetted by water, the purposeful deposition of hydrophobic lignin onto the surfaces of fibers, and derivatization  of fiber surfaces with long-chain alkyl agents.  All the dry fibers were highly effective for uptake of the “spilled oil”, though hydrophobization generally had a negative effect on oil uptake.  Hydrophobization by lignin deposition or alkylization favored oil uptake by pre-wetted fibers.

Impact of Work on Career of Investigator and Students

As a result of the PRF-funded project work, the principal investigators have been able to improve the fundamental depth of our research efforts.  The technical content of the current work – related to permeation of polyelectrolytes in narrow pore spaces – has also been beneficial to us in forging some new research initiatives in the areas of environmental remediation and in the efficient dewatering of cellulosic fiber suspensions.

The graduate student who has undertaken most of the project work has benefitted greatly from the opportunity.  In addition to learning to run gel permeation chromatography (GPC), streaming potential analysis, and adsorption isotherm experiments, she has become fluent in the analysis and interpretation of the data produced by such methods.  She has earned a Master of Science degree, and will be very well prepared to start PhD work. A graduate student and two undergraduate students benefited from participation in the final phase of research.

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