N I S T Center for Neutron Research

Accomplishments and Opportunities 2001

Pressure-Induced Phase Transition of C_{12}E_{5} 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 (C_{12}E_{5}) in D_{2}O 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 (L_{1} 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 H_{1} hexagonal phase at much
higher C_{12}E_{5} concentrations. The microstructure of the L_{1} -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 D_{2}O 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 C_{12}E_{5} / 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
C_{12}E_{5} in D_{2}O as a function of pressure and
49 % mass fraction C_{12}E_{5} in D_{2}O at ambient pressure.

Shown in Figure 2 is the scattering curve for the H_{1} hexagonal phase at
49 % C_{12}E_{5} 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 % C_{12}E_{5} mass
fraction solution at 255 M P a, suggesting that this new high-pressure phase may resemble a
slightly compressed state of the H_{1} 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 L_{1}-H_{1} 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 L_{1}-H_{1} 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 L_{1}-H_{1} phase transition. We propose that
the C_{12}E_{5} 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 C_{12}E_{5} 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 C_{12}E_{5}/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.

References

[1] R. Strey, R. Schomäcker, D. Roux, F. Nallet, U. Olsson, J. Chem. Soc. Faraday Trans. 86, 2253 (1990).

[2] N. Gorski, J. Kalus, D. Schwahn, Langmuir 15, 8080 (1999).

Authors

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