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Pore Scale Dynamics of Microemulsion Formation Evren Unsal, Marc Broens, and Ryan Troy Armstrong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00821 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016
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Pore Scale Dynamics of Microemulsion Formation Evren Unsal1*, Marc Broens1,2, Ryan T. Armstrong1† 1
Shell Global Solutions International, B.V., Rijswijk, The NETHERLANDS 2
†
Delft University of Technology, Delft, The NETHERLANDS
Current address: University of New South Wales, School of Petroleum Engineering, Sydney, AUSTRALIA
KEYWORDS. Microemulsion, microfluidics, phase behavior, solvatochromic dye, enhanced oil recovery
ABSTRACT. Experiments in various porous media have shown that multiple parameters come into play when an oleic phase is displaced by an aqueous solution of surfactant. In general, the displacement efficiency is improved when the fluids become quasi-miscible. Understanding the phase behavior oil/water/surfactant systems is important because microemulsion has the ability to generate ultra-low interfacial tension (5%) required it was economically not successful7-8. Over the decades, surfactant formulations improved significantly. Surfactant flooding, where low concentrations of surfactant ( 0 the surfactant exhibits more affinity for the oil, and if HLD < 0 the affinity is
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more favorable for aqueous phase. Changing the formulation variables, e.g. salinity, oil type, surfactant type, and/or temperature, can vary the HLD.
In this study, the phase behavior of the current surfactant-oil-water system was analyzed as a function of salinity. Aqueous solutions of surfactant and co-solvent with concentrations listed in Table 1 were prepared. The salinity of the solutions was varied by adding sodium chloride (NaCl) and for each salinity a 10 mL (Vtotal) test tube was prepared, which consisted of 5 mL of surfactant solution and 5 mL of n-decane (mixing ratio of 1:1). The contents of the tubes were shaken vigorously, and the tubes were allowed to sit stagnanate as the phases separated under the influence of gravity. Once equilibrated, the microemulsion phase formed an opaque middle layer that contained most of the surfactant and some water and oil.
The solubilization was analyzed by visually inspecting the tubes under light. The water and oil solubility ratios, γw or γo, were described as the volume of water, Vw, or oil, Vo, solubilized into the microemulsion phase per unit weight of surfactant. By definition, at the optimum salinity (HLD = 0) the surfactant had similar affinity to water and oil phases; consequently, γw equals γo and Vw equals Vo
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. The middle phase microemulsion was found in equilibrium with both
excess oil and aqueous phases. At low salinities (HLD < 0), the lower phase microemulsion was in equilibrium with the excess oil whereas at high salinities (HLD > 0) the upper phase microemulsion was in equilibrium with excess aqueous phase.
Microfluidic experiments (dynamic method). A microfluidic chip made of glass (Micronit, The Netherlands) was used to study in situ microemulsion formation (Figure 1). The chip
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featured a T-junction structure where two channels merged at 90° angle. Each channel was connected to a separate syringe pump (Harvard Apparatus, PhD Ultra, UK) for the injection of fluids. The single constitutive components of the microemulsion, i.e. the surfactant solution and n-decane were injected into the T-junction separately via the designated channels. The oil phase channel had a width of 100 micrometers and the aqueous phase channel was 200 micrometers wide. The channel depth was 40 micrometers, and it was uniform throughout the flow channel. The microfluidic chip was placed in the horizontal position under an inverted fluorescence microscope (Leica DM6000, Germany). The microscope could be operated in two modes, i.e. normal or fluorescent light.
Figure 1. Microfluidic setup. The microscope was focused on the T-junction, shown with yellow circle.
Phase identification by solvatochromic dye. Visualization of the in situ formation of the microemulsion was viable by using the fluorescent solvatochromic dye Nile Red. It is also known as Nile blue oxazone or 9-diethylamino-5H-benzophenoxazine-5-one (Cayman Chemical Company, USA). The dye is a lipophilic stain, highly oil-soluble, and is most commonly used for staining lipid droplets. The fluorescence of Nile Red is strongly influenced by the polarity of its environment and the emission spectrum can vary from deep red to strong yellow-gold color.
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Several studies on spectroscopic properties of Nile Red dye in aqueous surfactant and micellar solutions report that in the presence of surfactant it exhibits considerable absorption in the submicellar concentration region; the optical absorption spectra typically show a maximum emission in the range of 450-500 nm 52-54.
The cell-based assay Nile Red solution was diluted with n-decane (500x dilution by volume). Under fluorescent light, coloured n-decane emitted amber color; the surfactant solution did not emit any light as the dye was not water soluble. Their microemulsion emitted a bright red color due to the proximity of polar water molecules to the surfactant dissolved in the oleic phase.
4. Results 4.1.Phase behaviour studies (static method) The phase behavior of System I (Table 1) was determined by the salinity scan shown in Figure 2a. The NaCl concentration varied from 1.6% to 2.4% (w/w), with 0.1% increments. The optimum salinity was at 1.9% NaCl. The phase behaviour study was repeated for 0.5% and 2% surfactant concentrations, Systems II and III, respectively (Table 1). All tests were performed at room temperature. The optimum salinity did not significantly shift with surfactant concentration; all three systems had an optimum salinity of 1.9-2.0% NaCl. The main difference between the three systems was the amount of microemulsion formed between the aqueous and oil layer in the test tubes. System III had significantly less microemulsion than the other two systems with higher surfactant concentration. The results are summarized in Table 2.
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Figure 2. a) Salinity scan of System I with NaCl. Markings on the tubes indicate volume in mL, b) optimum salinity tube with definitions of Vw, Vo, and Vtotal.
Table 2. Effect of surfactant concentration on the physical properties of the middle phase. The viscosity is given for the optimum salinity at a shear rate of 7 s-1.
Surfactant system
% surfactant (w/w)
Optimum salinity (%NaCl)
I
4.0
1.9
II
2.0
III n-decane
Volume middle phase(%)
Viscosity of bulk aqueous phase(mPa.s)
Viscosity of middle phase(mPa.s)
37
40
3.8
2.0
21
1.7
4
0.5
2.0
5
0.98
6
--
--
--
0.85
--
Solubility measurements. The solubility ratios, γw and γo were determined by visually inspecting the tubes shown in Figure 2a. Where a middle phase formed, Vw was determined as the volume of the middle phase below the middle line, and Vo was the volume of the middle
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phase above the middle line. Vtotal was the total tube content volume; the middle line was the one half mark of the total volume, Vtotal (Figure 2b). The optimal salinity, defined as the salinity at which Vw was equal to Vo, was found to be 1.9% NaCl
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. Below optimum salinity, more than
half of the middle phase was positioned below the middle line (Vw>Vo), suggesting O/W type of emulsion. Above the optimum salinity, a larger fraction of the middle phase was positioned above the middle line (Vw
