Innovations in Supercritical Fluids - ACS Publications - American

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Dramatic Density-Induced Structural Changes in Microemulsions Formed in Near-Critical and Supercritical Solvents J. Zhang and J. L. Fulton Chemical Sciences Department, Pacific Northwest Laboratory, Battelle Memorial Institute, P.O. Box 999, 902 Battelle Boulevard, Richland, WA 99352 We demonstrate that the secondary structure of a microemulsion can be altered by changing the density of the continuous phase solvent. These changes in the microemulsion structure lead to dramatic changes in the physical properties of the system, such as the solution viscosity or electrical conductivity. For giant, rod-like micelles formed using the surfactant L-α-phosphatidylchloline (L -α-lecithin) in propylene, the system viscosity changes by three orders-of-magnitude with small changes in the system pressure. For the system of spherical micelles formed with didodecyldimethylammonium bromide (DDAB), the micelle clustering at low density increases electrical conductivity of the solution by three orders-of-magnitude. The microemulsion systems in near- or supercritical fluids are of importance for understanding reaction mechanisms and for modelling polymers. We also report results for a sodium dodecyl sulfate (SDS) aqueous solution forming normal micelles that can incorporate a small amount of fluid in the micellar core. For this system, changes in the primary structure can be induced by altering the amount of supercritical fluid in the microemulsion core by changing the fluid density. We establish correlations between the measured physical properties and the spectroscopic results. Many studies over the last eight years (1-3) have explored the properties of microemulsions formed in near- and supercritical solvents of the short chain alkanes(e.g., ethane, propylene). The existence of these phases in CO2 systems has also been reported (4-6). These microemulsions generally consist of ultra-small droplets of water surrounded by a shell of surfactant molecules. These structures are generally spherical and have sizes in the range from 10 to 100 nanometers. More recent studies have demonstrated that other aggregate geometries (e.g., rods and ellipses) are possible and have also shown how the microemulsion secondary 0097-6156/95/0608-0111$12.00/0 © 1995 American Chemical Society Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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structure (the spatial distribution of droplets) can be changed by altering the density of the supercritical fluid phase (3c,3d). Generally, the primary structure, that is the size and shape of the droplet, is not affected by the properties of the continuous phase solvent. For the systems where the bending energy of the interfacial film (7) is dominant, changes in fluid density have little effect on the primary structure. In this paper, we demonstrate that the secondary structure of the microemulsion can change appreciably in response to small changes in the near- or supercritical fluid density. These structural changes can lead to quite dramatic changes in the microemulsion physical properties, such as the solution viscosity and the electrical conductivity. These changes are attributed to the changes in the inter-aggregate attractive interactions that are controlled by the density of the continuous phase solvent. Thus, a remaining challenging problem is to derive details of the mechanisms of these density-induced changes in the microemulsion structures and to show how they manifest the dramatic changes in the system physical properties. In this article, we also present our results for a sodium dodecyl sulfate (SDS)/ethylene/water normal micelle system where changes in the fluid density do affect the primary structure. In contrast to the reverse microemulsions (3c,3d), for this normal micelle system, ethylene is incorporated into the cores of the micelle as the fluid density is increased. The primary structure of the microemulsion droplets is altered in response to a change in the overall volume fraction of the "oil" (ethylene) microphase. Because of the unique characteristics of supercritical fluids, changes in pressure induce large changes in fluid density, giving rise to changes in solvating strength of the fluid. As a result, one can control the interactions between microemulsion droplets and fluid environments by simply changing the fluid pressure. We also describe systems employing near-critical fluids. A near-critical fluid is herein defined as a liquid that is at a temperature above a reduced temperature (T =T/T ) of approximately 0.75 and below its critical temperature, T . Due to the proximity of the critical point, a near-critical fluid is still somewhat compressible in contrast to a normal liquid that has very low compressibility. An earlier small-angle neutron scattering (SANS) study (8) showed evidence of strong attractive interactions between AOT/water microemulsion droplets dispersed in liquid propane, and the magnitude of these interdroplet attractive interactions can be greatly increased by decreasing the pressure. In the present paper, we summarize our studies in three different surfactant-based near- or supercritical fluid systems. From both physical property measurements and spectroscopic information, we illustrate how the pressure-induced density change can affect the microstructure. r

c

c

Experimental The surfactant, didodecyldimethylammonium bromide (99%, DDAB) was purchased from Eastman Kodak Inc. and was used as received. Sodium dodecyl sulfate (>99%, SDS) was purchased from Fluka BioChemika and used without purification. L-a-phosphatidylchloline (soy-bean lecithin) was purchased from

Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Calbiochem with a reported purity >97%. This product contains about 62-65% of linoleic acid branches and 15-17% of palmitic acid esters branches. The remainder of the product is a mixture of shorter chain lipids. Its average molecular weight is approximately 796 dalton. Lecithin is a zwitterionic surfactant at pH=7.0. Formation of a lecithin gel is very dependent on the purity of lecithin (9). We have tested the gel formation in three liquid solvents (η-octane, cyclohexane, and ndodecane) and found that the critical water concentrations (W , water-to-surfactant molar ratio at which the gel is initially formed) for gelation are similar to those reported by others in these solvents (9). Propane, propylene and ethylene were all obtained from Scott Specialty Gases (c.p. grade). Various techniques were used for the solution preparation and property measurements. For the DDAB and lecithin systems, a pre-weighed amount of surfactant was loaded into a high pressure spectroscopy cell. Distilled-deionized water was directly injected into the cell using a microsyringe yielding a desired water-to-surfactant molar ratio. The system was then filled with a near-critical fluid and allowed to stir overnight for complete solubilization and equilibration. A l l experiments were performed in the one phase region. Experiments were done at 26.5°C for the DDAB/propane systems, 30°C for the lecithin/propylene systems, and 40°C for the SDS aqueous solutions. Measurements of the conductivity, viscosity, and spectroscopic properties were made by reducing the pressure through discharging small amounts of the microemulsion solutions (maintaining a constant mole fraction of surfactant and water). Details of the high-pressure conductivity (10) and viscosity techniques (3d) are described elsewhere. FT-IR experiments were performed after purging the instrument (Nicolet 740 FT-IR spectrometer) for at least 48 hours to eliminate interferences of air and moisture. Details about the high pressure FT-IR instrument, the cell design and solvent subtraction method are given in an earlier publication (11). For measurements of the SDS microemulsion, a different strategy was used. An SDS aqueous solution (170 mM) was loaded into a high pressure view cell for solubility measurements. Solubility of ethylene in pure water and 170 mM SDS aqueous solutions were both measured at 40°C. In contrast to the DDAB and lecithin systems, the SDS microemulsion is prepared as a two-phase system in the measurement cells: an upper supercritical ethylene phase is in contact with the lower aqueous microemulsion phase. All measurements of the SDS system were conducted on the lower microemulsion phase. The saturation concentration of the ethylene in the predominately-aqueous microemulsion was measured using a liquid/fluid saturation cell. Through vigorous stirring the aqueous phase would become saturated with ethylene in about 30 min. The ethylene-saturated microemulsion was discharged through a metering valve into a trap at ambient pressure. The quantity of evolved ethylene gas was measured volumetrically. For fluorescence lifetime measurements, pyrene (Molecular Probe) was used to study changes in the microemulsion environment as a function of pressure. Due to the dynamic quenching of pyrene fluorescence by oxygen, it is essential that oxygen be eliminated from the SDS aqueous solution by placing the solution under vacuum momentarily prior to measurement. Instrumentation for fluorescence measurements has been previously described (3c). c


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Results and Discussion Electrical Conductivity in a DDAB/Propane Microemulsion. Measurement of electrical conductivity is a simple and an effective method for studying certain aspects of microemulsion structure. Variables which induce conductivity changes in

Pressure-Induced, Microemulsion Droplet Clustering:

A t higher density, increased interactions between solvent and micelles reduce the exchange process

A t lower density, strong micelle clustering enhances the exchange process

\

Propane

DDAB Micelle

Scheme I


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microemulsions include volume fraction (12,13), temperature (14), and pressure (10). In the systems that are predominately oil (φοί^Ο-70), a remarkable increase in conductivity is sometimes observed corresponding to what has been called a percolation threshold. However, the precise percolation mechanism is not always clear and this behavior has been described by both static and dynamic percolation models. The static percolation model is consistent with micelle coalescence in a precursor to a bicontinuous phase, whilst percolation clusters from the dynamic percolation model are associated with "sticky encounters" due to attractive interactions. Figure 1 shows the conductivity of a propane/DDAB (0.326M)/water microemulsion (s/o=0.42) as a function of pressure. The conductivity of the singlephase system is reduced by almost three orders of magnitude when the pressure is increased from 100 to 450 bar. The systems become two-phase below 100 bar, so that reducing the pressure has the effect of bringing the system closer to a phase boundary. The increase in the electrical conductivity as a function of pressure was attributed to either a structural transition to a bicontinuous phase or micelle clustering due to attractive micelle interactions. The latter mechanism was favored in a detailed study (3c), applying fluorescence quenching techniques to this high pressure microemulsion solution. In that study, we clearly showed that the sharp decrease in conductivity with increasing pressure is attributed to the disappearance of percolative conduction. The mechanism of conduction above the percolation threshold is thought to be one of the surfactant ions hopping from droplet to droplet within the droplet clusters but no micelle coalescence occurring because the DDAB micelle size is uniform over the entire pressure range (100-450 bar) from our fluorescence quenching measurements. This supports the dynamic percolation model that is represented by Scheme I. That is, the overall shape of the micelles in the clusters is little changed from the micelles that are individually dispersed. However, strong interdroplet attractive interactions will enhance the solute exchange rates. The correlation between electrical conductivity and solute exchange rates between micelle droplets is in a good agreement with literature reports in the liquid microemulsion systems (15-18). Viscosity Measurements and FT-IR Spectroscopy of the L-oc-Lecithin Microemulsion. For a lecithin/propylene/water system, we observed a dramatic transition from a solid-like gel to a low viscosity fluid as the pressure of the system is increased. Similar to the electrical conductivity changes observed in a DDAB microemulsion, the viscosity change in the lecithin system with changing pressure is another indication that the extent of interactions between aggregates is changed as a function of pressure. Unlike the DDAB/propane microemulsion system, soy-bean lecithin is a zwitterionic surfactant. In the lecithin/propylene microemulsion, the very small changes in the electrical conductivity observed with changing pressure are ascribed to a lack of appreciable amounts of ions in the aqueous microphase. It is known that addition of water to a liquid organic solvent containing the lecithin surfactant induces large viscosity changes (9). This phenomenon was also observed in the lecithin/near-critical propylene microemulsion at 30°C. In addition,


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T=26.5°C [DDAB]=0.326 M

100 Figure 1.

200

400

300

Pressure (bar)

500

Conductivity of a DDAB/propane (W=24) microemulsion at 26.5°C.

[Lecithin]=33mM W=14 T=30.0°C

\

2Φ

1Φ

Ο Gel χ \

150

200

250

300

Ο

350

400

Pressure (bar)

Figure 2.

Dynamic viscosity (log-scale) of the lecithin/propylene microemulsion (W=14) as a function of pressure at 30°C.


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we found that a phase transition from non-gel to gel occurs when pressure is varied from high to low at a constant water concentration. Figure 2 shows the effect of pressure on the dynamic viscosity (log-scale) for the lecithin/propylene microemulsion with W=14 at 30°C. The system viscosity at W=14 undergoes an increase of almost three orders-of-magnitude as the pressure is reduced to 225 bar. However, over this pressure range, propylene density only drops about 4% from 0.56 to 0.54 g/mL. The viscosity appears to reach a maximum and than decreases slightly just before reaching the phase boundary of the system. In order to get a clear understanding of the mechanisms for both water- and pressure-induced lecithin gel formations, we applied FT-IR spectroscopy to look at the phosphate (P=0) vibrational band. Earlier NMR studies (19,20) showed that the water- induced gel formation in a conventional liquid was attributed to a strong stiffening near the phosphorus atom and on the adjacent triglyceride. Figure 3 shows plots of the P=0 band frequency vs. W-value at 210 bar (the bottom axis) and vs. pressure at W=14 (the top axis), both at 30°C. We observed a continuous red shift in the P=0 stretching frequency and band broadening as W values were increased from 0 to about 8 but little change in the vibrational mode for W values above 8. In comparison, we observed no pressure effect on the P=0 vibrational frequency although the system viscosity changes by almost three orders-of-magnitude over this pressure range. The water-induced P=0 band shift and broadening are due to hydration or/and Η-bonding of the phosphate group. The Η-bonding causes changes in the electron distribution on the phosphate group, affecting the P=0 force constant thus lowering the vibrational frequency, as well as broadening the band. Changing the system pressure does not affect the local solvent environment of the interfacial regions since no change in the P=0 stretching frequency is observed. Rather, varying the system pressure changes the lecithin structure through the changes in the interrod interactions. Scheme II depicts the two mechanisms for the gel formation induced by either water content (scheme II (A)) or pressure (scheme II (B)). Addition of water into the lecithin microemulsion induces one-dimensional growth of the aggregates into long tubular rods. By continuously increasing the water content at or above a threshold lecithin volume fraction (0.014

Innovations in Supercritical Fluids - ACS Publications - American

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