N I S T Center for Neutron Research
Accomplishments and Opportunities 2001
Pressure-Induced Phase Transition of C12E5 Micelles
Nonionic surfactants form a variety of microstructures in water, ranging from simple micelles at low surfactant concentrations to complex mesophases, such as hexagonal or lamellar phases at high concentrations. The effect of pressure on the structure of micelles and microemulsions has not been extensively studied. Small angle neutron scattering (S A N S) is particularly well suited for such measurements because the range of length scales probed includes both the particle size and the interparticle spacing.
Pressure effects observed in S A N S measurements of surfactant microstructure are typically interpreted in terms of geometric packing arguments that focus on the compressible hydrophobic tails of the surfactants. In this context, increasing pressure has the single effect of decreasing the surfactant tail volume, thereby increasing the curvature of the oil-water interface. Conversely, increasing temperature dehydrates the nonionic headgroups, decreasing the headgroup area and decreasing the film curvature. Thus, temperature and pressure can be viewed as thermodynamic variables with inherently different mechanisms and opposite effects for controlling microstructure in nonionic surfactant solutions.
Here we report the results of high-pressure S A N S experiments at 20 °C and pressures up to ≈ 300 M P a on a solution of pentaethylene glycol mono-n-dodecyl ether (C12E5) in D2O having a mass fraction of 1 %. The phase diagram for this system (Refer to Reference 1) at ambient pressure is shown in Figure 1. At this temperature and surfactant concentration, a single-phase micellar solution (L1 phase) forms at ambient pressure, well below the lower critical solution temperature (L C S T) for liquid-liquid equilibrium and far removed from the H1 hexagonal phase at much higher C12E5 concentrations. The microstructure of the L1 -phase at ambient pressure is known to be a network of branched semi-flexible, cylindrical micelles with the branch points comprised of three-armed junctions.
S A N S spectra were measured using the N I S T high-pressure cell and neutrons of wavelength λ = 6 Å, covering a q-range of 0.012 Å-1 < q < 0.22 Å-1. The scattering curves obtained at 3.4 MPa, 241 MPa, and 255 MPa are shown in Figure 2. The curves at 3.4 Mpa and 241 M P a are virtually identical, indicating no significant change in microstructure with increasing pressure up to 241 M P a. Fitting these curves using a form factor for cylindrical micelles gives a radius of (21.0 ± 0.2) Å and a length greater than 600 Å, independent of pressure. However, a small increase in pressure from 241 M P a to 255 M P a leads to the appearance of a peak in the scattering intensity at q approximately 0.130 Å-1, indicative of a locally ordered system. A similar transition has been reported in high-pressure S A N S studies of tetradecyldimethyl-aminoxide (T D M A O) micelles in D2O at pressures up to 300 M P a, but the high-pressure microstructure was never determined (Refer to Reference 2).
Graphics Caption FIGURE 1. Temperature-composition phase diagram for C12E5 / water solutions at ambient pressures (Refer to Reference 1).
Graphics Caption FIGURE 2. Measured S A N S spectra at 20 °C for 1 % mass fraction C12E5 in D2O as a function of pressure and 49 % mass fraction C12E5 in D2O at ambient pressure.
Shown in Figure 2 is the scattering curve for the H1 hexagonal phase at 49 % C12E5 mass fraction at 20 °C, and ambient pressure. The peak at q ≈ 0.120 Å-1, arising from the hexagonal lattice of cylindrical micelles, is similar to the peak for the 1 % C12E5 mass fraction solution at 255 M P a, suggesting that this new high-pressure phase may resemble a slightly compressed state of the H1 hexagonal phase at ambient pressure. The formation of a lamellar phase from cylindrical micelles is unlikely, since this corresponds to increasing the hydrophobic core volume-to-surface-area ratio per surfactant molecule, or equivalently, decreasing the spontaneous curvature of the surfactant film. The application of pressure would have the opposite effect. We conclude, therefore, that the observed change in microstructure corresponds to a pressure-induced L1-H1 phase transition from a network of branched semi-flexible, cylindrical micelles to hexagonally ordered bundles of cylindrical micelles.
To further understand the S A N S results, we have measured the temperature dependence of the L1-H1 transition pressure and find that the p - T curve follows the p - T freezing curves for liquid n - alkanes of comparable hydrocarbon chain length. N - decane solidifies at a pressure of ≈ 250 M P a at 20 °C, which is close to the pressure for the observed L1-H1 phase transition. We propose that the C12E5 micelle hydrophobic core, equivalent to n - decane, does solidify at these conditions, such that the micelles lose flexibility, and hence conformational entropy. An analysis of the geometric packing constraints for three - arm junctions coexisting with cylinders shows that when the surfactant tail volume decreases with increasing pressure, the fraction of surfactant forming junction points also decreases. This indicates that the formation of three - arm junctions becomes increasingly unfavorable at higher pressures due to the compression of the C12E5 micelle hydrophobic core. Our calculations predict that no junctions should be present at P > 275 M P a at 20 °C, which is in good agreement with our observation of a structural transition between 241 M P a and 255 M P a at this temperature. Consequently, the network of branched threadlike micelles becomes globally unstable. The formation of hexagonally ordered bundles of cylindrical micelles follows as the attractive van der Waals forces between the micelles are not offset by entropic repulsive undulation interactions that are not present in the now-solidified hydrophobic core.
The practical significance of these results is to show that pressure allows access to regions of the C12E5/water phase diagram that are virtually inaccessible to temperature. Thus, the use of pressure may offer unique approaches for directing and stabilizing certain surfactant microstructures that, in turn, could prove useful for creating novel soft materials.
 R. Strey, R. Schomäcker, D. Roux, F. Nallet, U. Olsson, J. Chem. Soc. Faraday Trans. 86, 2253 (1990).
 N. Gorski, J. Kalus, D. Schwahn, Langmuir 15, 8080 (1999).
D. P. Bossev and M. E. Paulaitis
Department of Chemical Engineering
Johns Hopkins University
Baltimore, Maryland 21228-2694
J. N. Israelachvili
Department of Chemical Engineering
University of California, Santa Barbara
Santa Barbara, CA 93106
S. R. Kline
N I S T Center for Neutron Research
National Institute of Standards and Technology
Gaithersburg, MD 20899-8562
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