Soft fluctuating surfactant membranes in supercritical CO₂-microemulsions

Abstract

The bending rigidity of surfactant membranes in novel bicontinuous CO 2 -microemulsions of the type H 2 O/NaCl-scCO 2 -Zonyl FSH/Zonyl FSN 100 was determined using both high pressure small angle neutron scattering and neutron-spin echo spectroscopy. As an important result it was found, that the stiffness of the membrane increases solely by an increase of the pressure.The waste of chemical solvents from chemical processing and related industries represent a huge environmental concern. During the last decades supercritical fluids have attracted much attention as potential replacements for these conventional organic solvents. Supercritical CO 2 (T c = 31.1 1C, p c = 72.8 bar) is seen as the most promising candidate because it is cheap, abundant, incombustible, non-toxic, bio-and foodcompatible. Unfortunately scCO 2 is generally a very poor solvent, in particular for polar and/or high molecular weight solutes. Thus, in applications where both polar and nonpolar components needed to be dissolved, emulsions and microemulsions were used to overcome this severe limitation. 1 Furthermore, scCO 2 microemulsions are also of great interest from a theoretical point of view because, contrary to classical ''state of the art'' microemulsions, the properties of scCO 2 microemulsions can be strongly influenced just by varying the pressure, i.e. the solvent properties of CO 2 . So far studies on these novel microemulsion systems concentrate on the phase behavior and the microstructure of CO 2 -rich microemulsions. 1-3Recently, we were able to formulate balanced supercritical CO 2 -microemulsions containing equal volumes of water and CO 2 using technical grade n-alkyl-polyglycolether-and perfluoroalkyl-polyglycolether-surfactants. 4,5 The phase behavior of such a microemulsion system H 2 O/NaCl-scCO 2 -Zonyl FSH/Zonyl FSN 100 is shown in Fig. 1 for pressures of p = 160 (top), 220 (middle) and 300 bar (bottom) as a function of the overall surfactant mass fraction g and the temperature T.5 Thereby the mass fraction a of CO 2 in the mixture of water and CO 2 was adjusted to a = 0.40 and the surfactant mixture consists of equal amounts of Zonyl FSH and Zonyl FSN 100 (d = 0.50). NaCl (1 wt% in the mixture of water and NaCl; e = 0.01) was added to screen possible electrostatic interactions induced by ionic impurities.As can be seen in Fig. 1 the phase behavior of this system resemble the phase behavior of well-known non-ionic microemulsions at ambient pressure.6 Thus, at low temperatures a CO 2 -in-water microemulsion coexists with a CO 2 -excess phase ( À 2), while at high temperatures a coexistence of a water-in-CO 2 microemulsion with a water-excess phase ( 2) is found. This phase inversion manifests itself in an extended three phase region which can be observed at intermediate temperatures and low surfactant weight fractions g. Increasing g the three phase region meets the one phase region at the so-called optimum point (g,T˜). It defines the minimum amount of surfactant needed to solubilize water and CO 2 at the phase inversion temp...

Properties of surfactant films in water-in-CO2 microemulsions obtained by small-angle neutron scattering

“…At first sight, there is little apparent correlation between the film rigidity and water content, however, the results also show that the film rigidities with a longer chain surfactants are generally higher, as on average, 2 + ̅ = 1.7 for 6FS(EO)2, and 1.2 for 4FS(EO)2. In previous studies by Klostermann et al 34,35 , the film bending rigidity was also determined for bi-continuous w/c microemulsions using SANS and NSE (Neutron Spin Echo), and owing to the different topology of bi-continuous microemulsions, a different methodology was used, representing a so called 'bare bending rigidity' 0~0 .9 for two different polydisperse polymeric F-carbon surfactant films 34,35 . Comparing with the 4-and 6FS(EO)2 stabilized systems, this value appears to be lower, which is possibly owing to the significantly reduced contribution from ̅ for the saddle-shaped curvature in bi-continuous microemulsions.…”

Experimental determination of bending rigidity and saddle splay modulus in bicontinuous microemulsions

“…Recently, rst NSE experiments on bicontinuous microemulsions containing supercritical CO 2 as the oil phase and a uorosurfactant have allowed us to obtain the bare bending rigidity of these systems. 21,22 These systems allow us to access different points in the phase diagram simply by changing the applied pressure. Similar to the addition of homopolymers or diblock copolymers, the X-point shis towards a lower surfactant concentration with increasing pressure.…”

Two-dimensional lattice liquid models

“…Two-dimensionally extended materials, or membranes, have attracted much attention in various fields of study, such as lipid bilayers [1][2][3] and surfactants [4] in soft matter and biophysics [5]. In addition, many theoretical studies on those membrane-type objects have been done toward a general understanding of the physical properties of such materials [1,3,[5][6][7].…”