B. Bera

Analytic model for the electrowetting properties of oil-water-solid systems

The competitive wetting of oil and aqueous electrolytes on solid surfaces depends strongly on the surface charge of the solid-water and the water-oil interface. This charge density is generally not known a priori but changes as ions adsorb or desorb from or to the interfaces, depending on the composition of the fluid and the thickness of thin films of the aqueous phase that frequently arise on hydrophilic surfaces, such as minerals. We analyze the wettability of such systems by coupling standard Derjaguin-Landau-Verwey-Overbeek theory to a linearized charge regulation model. The latter is found to play an important role. By linearizing electrostatic interactions as well, we obtain a fully analytic description of transitions between different wetting scenarios as a function of the surface potentials at infinite separation and the charge regulation parameters of the two interfaces. Depending on the specific values of the regulation parameters, charge regulation is found to extend the parameter range of partial wetting and complete wetting at the expense of pseudopartial wetting and metastable wetting configurations, respectively. A specific implementation of the model is discussed for mica-water-alkane systems that was investigated in recent experiments.

Charge Control And Wettability Alteration At Solid-liquid Interfaces

Most solid surfaces acquire a finite surface charge upon exposure to aqueous environments due to desorption and/or adsorption of ionic species. The resulting electrostatic forces play a crucial role in many fields of science, including colloidal stability, self-assembly, wetting, and biophysics as well as technology. Enhanced oil recovery is an example of a large scale industrial process that hinges in many respects on these phenomena. In this paper, we present a series of experiments illustrating fundamental aspects of low salinity water flooding in well-defined model systems. We show how pH and ion content of the water phase as well as the presence of model polar components (fatty acids) in the oil phase affect the wettability (i.e. contact angle distribution) of oil-water-rock systems. Specifically, we discuss high resolution atomic force microscopy (AFM) experiments demonstrating the preferential adsorption of multivalent cations to mineral surfaces such as mica and gibbsite. Cation adsorption leads to increased and in some cases reversed surface charge at thesolid-liquid interface. In the case of charge reversal, the adsorption process can trigger a wetting transition from complete water wetting in ambient oil (i.e. zero water contact angle) in the absence to partial wetting in the presence of divalent cations. While already dramatic for pure alkanes as base oil, adding fatty acids to the oil phase enhances the effect of divalent ions on the oil-water-rock wettability even more. In this case, contact angle variations of more than 70° can be observed as a function of the salt concentration. This enhancement is caused by the deposition of a thin film of fatty acid on the solid surface. AFM as well as surface plasmon resonance spectroscopy measurement in a microfluidic continuous flow cell directly demonstrate that adsorbed Ca ions promote secondary adsorption of acidic components from the oil phase. The combination of the effects discussed provides a rational scenario explaining many aspects of the success of low salinity water flooding.

Cationic Hofmeister series of wettability alteration in mica-water-alkane systems

The specific interaction of ions with macromolecules and solid–liquid interfaces is of crucial importance to many processes in biochemistry, colloid science, and engineering, as first pointed out by Hofmeister in the context of (de)stabilization of protein solutions. Here, we use contact angle goniometry to demonstrate that the macroscopic contact angle of aqueous chloride salt solutions on mica immersed in ambient alkane increases from near-zero to values exceeding 10°, depending on the type and concentration of cations and pH. Our observations result in a series of increasing ability of cations to induce partial wetting in the order Na+, K+ < Li+ < Rb+ < Cs+ < Ca2+ < Mg2+ < Ba2+. Complementary atomic force microscopy measurements show that the transition to partial wetting is accompanied by cation adsorption to the mica–electrolyte interface, which leads to charge reversal in the case of divalent cations. In addition to electrostatics, hydration forces seem to play an important role, in particular for the monovalent cations.

Ion-induced Wetting Transition during Low Salinity Waterflooding

Low Salinity Waterflooding has been one of the most investigated and used methods of Enhanced Oil Recovery (EOR) in the past decades. Irrespective of the much debated mechanism of such waterflooding, a change in rock-wettability (from oil-wet to water-wet) definitely occurs during the process. We investigate optically the wetting of mineral surfaces by aqueous salt solutions in ambient oil. Monovalent salt solution (NaCl, KCl) at all pH and concentration shows an almost ‘complete’ wetting scenario on various surfaces while divalent salt solutions (CaCl2, MgCl2) above a threshold pH and concentration demonstrates partial wetting on mica. We attribute this wetting transition to the stronger ion-adsorption and subsequent charge reversal at mica/water interface in case of divalent cations. Using Ellipsometry we investigate the molecularly thin aqueous film between mineral and oil phases in such wetting scenarios, and measure the surface charge at the interface using zeta potential measurement technique. Surface Complexation modeling validates the surface charge reversal and we calculate the interface potential Φ in the aqueous film to demonstrate the gradual development of a contact angle (as Φmin). Making the oil more ‘realistic’ by adding polar fatty acid components in the oil phase makes the transition to partial wetting stronger as well as dramatic involving autophobing and self-propelling aqueous drops. We use Atomic Force Microscopy (AFM) to depict the complex adsorption process going on at oil/water as well as mineral/water interface aided by divalent cations. We have also investigated wetting at higher temperatures for the same system to emulate reservoir-like conditions where the combined effort of ion-adsorption and temperature control the conditions necessary for wetting transition.