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PRF# 57060-ND10 Project Title: Mixing Oil and Water by Ionic Liquids: Stability Maps of Emerging Fluids in Oil Recovery Principal Investigator: Dr. Cecilia Leal, Materials Science and Engineering at Urbana-Champaign PROJECT GOALS: In this project, we investigate the ability of Ionic Liquids (ILs), instead of surfactants, to stabilize mixtures of oil and water. This fundamental science research can potentially impact the search for new fluids in enhanced oil recovery. The project is organized under two specific aims:

Specific Aim 1. The Oil/Water/IL phase map: investigation of the ternary phase diagram of Oil/Water/ILs using high molecular weight oils and two distinct ionic liquids: more hydrophilic ones and more hydrophobic ones. To samples where a microemulsion isotropic fluid is present, we will also investigate the effect of addition of inorganic salts to the phase equilibria.

Specific Aim 2. Ternary solutions under confinement: Investigate the phase behavior of Oil/Water/IL when confined between solid surfaces. ILs can display peculiar behavior under confinement. In this aim we will investigate the structures of the ternary mixtures, in particular the single-phase regions of Oil/Water/IL (that possibly resemble surfactant-based microemulsions) under confinement. This is important because real oil reservoirs are confining environments. The effect of inorganic salt addition will also be investigated.

MAJOR ACCOMPLISHMENTS: During this reporting period, we pursued many objectives of Specific Aim 2 and continue to expand new research directions. The main tools employed were: i) X-ray Scattering, ii) visual inspection of ternary mixtures, iii) microscopy, and iv) Surface Force Apparatus (SFA). The following work is under review at The Journal of Physical Chemistry Letters: “Mixing Oil and Water with Ionic Liquids: Bicontinuous Microemulsions Under Confinement” by Hojun Kim, Mengwei Han, Rosa M. Espinosa-Marzal* and Cecilia Leal*.

Significant results and findings – phase diagrams: Ternary mixtures of Water, Hexadecane, and a IL with equal affinity for water and oil (P66614Cl) were tested with respect to the ternary phase diagram. A number of different phase regions were identified visually. Importantly, a few samples displayed a three-phase region consistent with water and oil separated by an isotropic fluid which is a thermodynamically stable microemulsion. The ternary phase diagram is depicted below in Figure 1.

Figure 1. Ternary phase diagram of [P6 6 6 14]Cl, water, and hexadecane at 25 ºC (mole fractions). One, two, and three color symbols represent mono-, bi-, and triphasic solutions of [P6 6 6 14]Cl, wa-ter, and hexadecane. The triphasic solution ([P6 6 6 14]Cl / water / hexadecane = 0.75 / 0.05 / 0.2, mole ratio) marked with a red star was chosen for further studies.

Significant results and findings – microemulsion structure: X-ray scattering data together with phase diagram determination shown above revealed that the microemulsion is of the bicontinuous type with the oil phase separated by water-ionic liquid microdomains. Figure 2 shows X-ray scattering data probing short (WAXS) and intermediate (SAXS) correlation lengths.

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Figure 2. Structure analysis. a) WAXS scans of the three phases in [P6 6 6 14]Cl / water / hexadecane (0.75 / 0.05 / 0.2, mole fraction) solutions. Oil-rich (black), microemulsion (green), and water-rich (blue) phases are marked from top to bottom. The arrow indicates IL column distance. b) SAXS scans of bicontinuous microemulsion (green) and Teubner-Strey fitting curve (red). An average oil-water periodic distance of 7.5 nm and a positional correlation length of 1.8 nm are obtained. c) Graphic representation of the microemulsion bicontinuous structure. The hexadecane molecules are not shown to simplify the cartoon.

Significant results and findings – confinement: Under nanoconfinement at the SFA, we discovered that in contrast to traditional surfactant-based systems, there is a firmly surface-adsorbed ionic liquid film that aids to stabilize the microemulsion. The microemulsion displays an affinity to the confining surfaces forming greatly disordered, flexible, mobile and large (~80 nm) structures. Sandwiching of the microemulsion to thinner films induces rearrangements culminating into two well-ordered layers with slow dynamics. This behavior is markedly in contrast to previous knowledge on microemulsions with conventional surfactants and opens a new paradigm of research in ionic liquids. The schematics of what the SFA data revealed are summarized in Figure 3.

Figure 3. Illustration of the structure of the IL-based microemulsion confined between mica surfaces at separations a) D between ~11-100 nm, b) D between 7 and 11 nm, where a transition of the nano-confined mixture into a lamellar structure happens that is finally squeezed out in two steps, reaching the final structure in c); the detail shows the assumed composition of the squeezed-out microdomain in one of the steps. d) Composition of the compressible structure at separations smaller than ~ 7 nm.

FUTURE PLANS: In the remainder time of the proposal we will continue to use a combination of SAXS and ternary phase diagram determination of a few ILs to determine more systems in which microemulsions are observed. We anticipate that, similar to conventional microemulsions, ILs may stabilize isotropic fluids of oil and water with various types of interface structures. We anticipate that droplet based systems (either water-in-oil or oil-in-water) will be encountered depending on IL hydrophobicity. We will investigate the behavior of such microemulsions upon confinement and expect very different results depending on the structure of the microemulsions. Finally, we will inspect how added salinity affects the behavior of the microemulsions with and without confinement.

d) Interfacial composition D