Defective Lamellar Phases and Micellar Polymorphism in Mixtures of Glycerol Monooleate and Cetyltrimethylammonium Bromide in Aqueous Solution

Abstract

The phase behavior of solvent-rich mixtures of glycerol monooleate and cetyltrimethylammonium bromide is reported for two solvents, water and aqueous solutions of 100 mM NaCl. The main concern is the properties of the lamellar phases and the formation of curvature defects within the bilayers. Defective lamellar phases are shown to form in both water and brine. The defective regions of lamellar phase are delineated by means of small-angle X-ray scattering (SAXS) studies. Direct visualization of the aggregate structure at high dilution is made by cryo-transmission electron microscopy (cryo-TEM). A rich micellar polymorphism is found, involving connected threadlike micelles as well as elongated micelles with noncircular cross sections. The correlation between the observed aggregate structures and the overall phase behavior is used to discuss possible defect geometries in the disrupted bilayers. We also report that the water-based lamellar phase made from the intact bilayers seems to contain a large fraction of curved multilamellar aggregates. Interestingly, the corresponding bilayers that show curvature defects appear to build lamellar phases free of such textural defects.

Phase Behavior and Aggregate Structure in Aqueous Mixtures of Sodium Cholate and Glycerol Monooleate

“…The swelling of the lamellar phase in the present systems, being less pronounced when brine is used instead of water, is most likely due to the negative charge acquired by the bilayers when cholate is incorporated. A similar swelling of a reversed bicontinuous cubic phase was observed recently on addition of the cationic surfactant cetyltrimethylammonium bromide (CTAB) to GMO (20), but otherwise few examples of such swelling have been reported in the literature (14,21,22).…”

“…It should be added in this connection that the uptake of solvent of the cubic phases is not only a function of interbilayer forces, but also depends on the spontaneous curvature of the mixed amphiphilic monolayers (20,21). It is important to note, therefore, that the influence of the accommodated sodium cholate in the lipid-based cubic phase is analogous to that of a normal ionic surfactant, which may be an indication that the bile salt is inserted in a similar way, perpendicular to the interface.…”

“…The (almost) ideal swelling in the tongue of the lamellar phase is a difference from the CTAB-GMO-water/brine and corresponding lecithin systems, where strong deviations from ideal swelling were ascribed to the formation of a perforated lamellar phase (20). The defective lamellar phase was replaced by L 1 without a marked two-phase region with increasing CTAB concentration.…”

“…The location of the micellar phase boundary shows that the lipid-saturated micelles contain fewer than 2 GMO molecules per cholate. For comparison we note that the formation of mixed CTAB-GMO micelles requires an excess of the cationic surfactant (20). Considering the average packing parameter (calculated from the sum of volumes divided by the sum of areas and by the longest length, i.e., from GMO) it is easily realized that the only way for the cholate to accomplish extremely effective solubilization of GMO is by adopting a flat configuration on the surface of the micelles, as discussed above.…”

Applications of Dissipative Particle Dynamics

“…7). Finally, addition of cationic surfactants to lipid membranes leads to hole formation [64,65]. It therefore seems reasonable to enquire whether the interaction of alcohol ethoxylates and phospholipids typical of bacterial membranes would naturally lead to such mesh phase which would make the bacterial cell leaky and leading to bacterial stasis and ultimately to cell death.…”