Amphitropic liquid crystals. Two lamellar phases in a surfactant containing thermotropic and lyotropic mesogenic groups

Abstract

We have studied the lyotropic liquid-crystalline behaviour of cationic surfactants containing a potentially thermotropic moiety, a terminal cyanobiphenyloxy group. Both mono-alkyl and mid-chain substituted dialkyl surfactants have been examined using optical microscopy and NMR spectroscopy. Incorporation of the cyanobiphenyloxy group destabilizes the hexagonal and bicontinuous cubic phases, with only an extensive lamellar region being observed. For the dialkyl surfactant there is a range of compositions where two lamellar phases co-exist, one water-rich and the second surfactant-rich.     

Interactions between benzyl benzoate and single- and double-chain quaternary ammonium surfactants

Interactions between benzyl benzoate and two different twin-tailed cationic surfactants have been studied. NMR diffusometry measurements have shown that cationic micelles grow in one dimension when benzyl benzoate is added. The location of benzyl benzoate in the micelles was evaluated by NOESY, showing that benzyl benzoate gave cross-peaks both to the hydrophobic groups in the surfactant and to the surfactant head group. Additions of benzyl benzoate to a lamellar phase of double-tailed quaternary surfactants revealed differences in responses. With an increasing concentration of benzyl benzoate, the structure of the dialkyl quat aggregate goes from lamellar to cubic, while the dialkyl ester quat forms a lamellar structure for all benzyl benzoate concentrations.     

Thermotropic and lyotropic phase behaviour of monoalkyl glycosides

We have studied the thermotropic and lyotropic phase behaviour of seven monoalkyl glycosides using polarizing microscopy, differential scanning calorimetry and X-ray diffraction. The phase behaviour of these non-ionic surfactants is compared with that of two previously studied monoalkyl glycosides \[1]. We have found that small modifications in chemical structure can lead to large changes in the phase behaviour. The results reveal the effect of changes in alkyl glycoside chemical structure on the phase behaviour, in particular, on the melting point, the clearing point, the solubility in water, and the extent of the lamellar and curved phases. Seven out of the nine surfactants were found to exhibit thermotropic mesomorphism. Furthermore, in the presence of water, three formed lamellar (L ), type I cubic (Q ) and type I hexagonal (H ) phases, two formed lamellar and cubic phases, and two I I formed only lamellar phases. The cubic phase in each case was indexed, either from powderlike or monodomain samples, as spacegroup I a3 d . The lowest order low angle X-ray spacings were found to lie on single, continuous lines with varying water content across the L , Q and H phases, implying epitaxial relationships between these phases at the phase boundaries. I a a I     

Templating Polymerization of Dodecylammonium Surfactants with Polymerizable (Meth)acrylate Counter Ions

Both dodecylammonium acrylate (I) and dodecylammonium methacrylate (II) are reactive surfactants in which the polymerizable group is the organic counterion. Templating polymerization of I and II from the lyotropic liquid crystalline (LLC) lamellar phase was successfully performed with conservation of the supramolecular structure. A 20 wt% aqueous solution of II formed cubic bicontinuous phases, even upon addition of divinyl benzene (DVB). Polymerization from the cubic phases was attempted without and with DVB (3 wt%). Bicontinuous cubic phases always evolved toward lamellar structures upon polymerization.     

Synthesis and lyotropic phase behavior of novel nonionic surfactants for the crystallization of integral membrane proteins

A series of new sugar-based nonionic surfactants have been synthesized and their lyotropic liquid crystalline properties characterized. When in contact with water, these surfactants formed a range of lyotropic liquid crystalline phases, including cubic, hexagonal, and lamellar, as well as a separate micellar phase. These are features that have promise for the crystallization of integral membrane proteins.     

Water incorporation into a micellar-lamellar phase (dodecyl maltoside-water)

Many alkylated sugars are surfactants, and show lamellar phases which prove to be identical to those of more conventional surfactants. Most, if not all, previous investigations focus on the behaviour of these compounds in pure form. Dodecyl maltoside shows the same lamellar phase as when dry, but on the addition of water the layer spacing expands, and on addition of more water, a hexagonal phase forms. The area per head group increases substantially with water addition, indicating that the water is incorporated into the layers, possibly at the head groups. We suggest that the hexagonal phase is the system's answer to the frustration caused by a large head group area compared with that of the tails.     

Phase behaviour of n-alkyl- and bi-alkylbenzenesulphonates. Nematic lyotropics from double chain surfactants

The lyotropic mesophases in binary systems of surfactants in water: n-alkylbenzenesulphonates (C₈-C₁₂), two chain C₁₂-surfactants, and dodecyl-benzenesulphonic acid, were investigated. The micellar properties were examined by conductometry and viscosimetry. The phase diagrams were determined using crossed polarizers, ²H NMR spectroscopy and polarization microscopy. Besides lamellar and inverse cubic phases, new nematic lyotropic phases have been found, presenting precursors for the lamellar phases, and exhibiting very fast alignment in a magnetic field.     

Elongated silica nanoparticles with a mesh phase mesopore structure by fluorosurfactant templating

Mesoporous silica materials with pore structures such as 2D hexagonal close packed, bicontinuous cubic, lamellar, sponge, wormhole-like, and rectangular have been made by using surfactant templating sol-gel processes. However, there are still some "intermediate" phases, in particular mesh phases, that are formed by surfactants but which have not been made into analogous silica pore structures. Here, we describe the one-step synthesis of mesoporous silica with a mesh phase pore structure. The cationic fluorinated surfactant 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC) is used as the template. Like many fluorinated surfactants, HFDePC forms intermediate phases in water (including a mesh phase) over a wider range of compositions than do hydrocarbon surfactants. The materials produced by this technique are novel elongated particles in which the layers of the mesh phase are oriented orthogonal to the main axis of the particles.     

Shapes of Micelles and Molecular Geometry Synthesis and Studies on the Phase Behaviour, Surface Tension and Rheology of Rigid Rod-Like Surfactants in Aqueous Solutions

Amphiphiles with rigid rod-like hydrophobic moieties have been synthesized in order to investigate the effect of the packing restraints of such moieties on the micellar association behaviour of amphiphiles in aqueous solution. Investigations of the phase behaviour of amphiphile/water mixtures reveal that liquid-crystalline phases exist in defined temperature and concentration regimes and that they are all lamellar, regardless of the hydrophilic-hydrophobic balance of the amphiphile. For these lyotropic liquid-crystalline phases a polymorphism is observed which is similar to the polymorphism of thermotropic smectic liquid crystals. Surface tension measurements indicate critical micelle concentrations of the amphiphiles in dilute solutions which are similar to those of conventional surfactants. From rheological measurements it can be assumed that the variation of temperature and/or concentration of the solution does not influence the micellar shape. This is in contrast to the behaviour of non-ionic surfactants having a flexible hydrophobic group.     

Mixtures of branched non-ionic oligo-oxyethylene surfactants in aqueous solutions — the effect of molecular geometry on LC phase behaviour 4

The LC phase behavior of ternary mixtures of the two corresponding branched non-ionic surfactants 1,3-bis-(methoxy-tetraoxyethylene)-2-propoxy-tetradecane (Y-surfactant) and 1,3-bis-(heptyloxy)-2-propoxyoctaoxyethylene mono-methyl ether (V-surfactant) and water were studied by polarizing microscopy. The two branched surfactants, which have different molecular geometries but nearly the same hydrophilic-lipophilic volume ratio, exhibit extremely different phase behavior in binary surfactant/water systems. For the ternary mixtures of Y- and V-surfactant and water we found-according to established packing models-a continuous stabilization of the cubic and hexagonal phases and a destabilization of the lamellar phase with increasing amount of Y-surfactant. On the other hand, we observed a thermal stabilization of the lamellar phase. The maximal transition temperatures of the lamellar phase pass a maximum with increasing amount of Y-surfactant.     

Formation of reverse vesicles in silicone surfactant systems

This study deals with the formation of reverse vesicles based on the phase behavior of silicone surfactants. The surfactants, polyoxyethylene–polydimethylsiloxane and polyoxyethylene–polyoxypropylene–polydimethylsiloxane copolymer, were found to form lamellar liquid crystal phases in three different types of silicone oil upon the addition of a certain amount of water. A conventional method in which reverse vesicles are prepared by physically dispersing this lamellar liquid crystal phase in oil was employed in addition to a technique based on a temperature-induced phase transition. The particle sizes and stabilities of the resulting reverse vesicles were evaluated.     

Lyotropic liquid crystalline phase behaviour of a monomeric and a polymeric monosaccharide amphiphile in aqueous solution

Monomeric and polymeric amphiphiles were synthesized which exhibit lyotropic liquid crystalline phases in aqueous solution. The hydrophobic group of the monomeric surfactant is a dodecane-group esterified with acrylic acid. The hydrophilic unit is a monosaccharide derivative. By radical polymerization the monomer (N-D(-)-gluco-N-methyl-(12-acryloyloxy)-dodecane-l-amide) is converted into the corresponding polymeric surfactant. The monomer as well as the polymer exhibit a lyotropic 1.c. phase of lamellar structure. Owing to the polymerization the regime of the lamellar phase is greatly enlarged for the polymer, compared to the monomeric sufactant. These results confirm earlier investigations on non-ionic ethylene-glycol surfactants.     

Estimation of the Interfacial Tension Between the Lamellar and the Sponge Phases of a Lyotropic System

In lyotropic systems, the sponge and the lamellar phases possess the same local structure: a membrane made of a bilayer of surfactants. In the quasiternary lyotropic system CPCl/brine/hexanol, the bilayer is continuous through the interface between the lamellar and the sponge phases. A model based on this phenomenon has predicted a very low value of the interfacial free energy. The study of hydrodynamic relaxation time of distorted spherical lamellar droplets gives an estimation of the interfacial tension value. Results confirm the validity of the model and the dependence on membrane volume fraction is explained by a simple scaling law.     

Effect of alkyl chain asymmetry on catanionic mixtures of hydrogenated and fluorinated surfactants

In this work we studied and compared the physicochemical properties of the catanionic mixtures cetyltrimethyl-ammonium bromide–sodium dodecanoate, cetyltrimethyl-ammonium bromide–sodium perfluorodacanoate, octyltrimethylammonium bromide–sodium perfluorodacanoate and cetyltrimethyl-ammonium bromide–sodium octanoate by a combination of rheological, transmission electron microscopy (TEM) and polarized optical microscopy measurements. The binary mixtures of the surfactants have been analyzed at different mixed ratios and total concentration of the mixture. Mixtures containing a perfluorinated surfactant are able to form lamellar liquid crystals and stable spontaneous vesicles. Meanwhile, system containing just hydrogenated surfactants form hexagonal phases or they are arranged in elongated aggregates.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The binary phase behaviour of two potentially polymerisable quaternary ammonium surfactants in water has been investigated. Allyldodecyldimethylammonium bromide (ADAB) a single-chain surfactant displays a conventional phase progression upon increasing concentration. Whereas the doublechain analogue allyldidodecylmethylammonium bromide (ADDAB) forms two lamellar liquid crystalline phases built from surfactant bilayers, which transform via a first order phase transition. The formation of two distinct lamellar phases and their coexistence has been evidenced by optical microscopy, small-angle x-ray scattering and D₂O deuterium quadrupolar nuclear magnetic resonance spectroscopy. The lamellar phase formed at higher surfactant compositions is a normal lamellar phase (typeL ) consisting of bilayers which are on average parallel and flat. The lower compositional lamellar phase (typeL ) in contrast may not be comprised of planar bilayers but rather aggregates having a high degree of curvature in comparison to those of theL phase. The presence of the allyl polymerisable moiety in the head group position of these surfactants has the effect of reducing the rigidity of the surfactant and increasing its solubility in comparison to nonpolymerisable analogues. Polymerisation of the surfactants was attempted by using thermal and photochemical initiation in isotropic and self-assembled systems. Polymerisation occurred to approximately 30% for DADB but did not occur for ADDAB. Where polymerisation did occur the polymer was incorporated into the monomer matrix by interweaving between the surfactant aggregates. The polymers had a molecular wieght not greater than 8000 Daltons, independent of the monomer concentration of the original solution and type of polymerisation.     

Self-organization of double-chained and pseudodouble-chained surfactants: counterion and geometry effects

Self-organization in aqueous systems based on ionic surfactants, and their mixtures, can be broadly understood by a balance between the packing properties of the surfactants and double-layer electrostatic interactions. While the equilibrium properties of micellar systems have been extensively studied and are understood, those of bilayer systems are less well characterized. Double-chained and pseudodouble-chained (or catanionic) surfactants are among the amphiphiles which typically form bilayer structures, such as lamellar liquid–crystalline phases and vesicles. In the past 10–15 years, an experimental effort has been made to get deeper insight into their aggregation patterns. With the double-chained amphiphiles, by changing counterion, adding salt or adding anionic surfactant, there are possibilities to depart from the bilayer aggregate in a controlled manner. This is demonstrated by several studies on the didodecyldimethylammonium bromide surfactant. Mixtures of cationic and anionic surfactants yield the catanionics, surfactants of the swelling type, and also show a rich phase behavior per se. A variety of liquid–crystalline phases and, in dilute regimes, equilibrium vesicles and different micellar shapes are often encountered. Phase diagrams and detailed structural studies, based on several techniques (NMR, microscopy and scattering methods), have been reported, as well as theoretical studies. The main features and conclusions emerging from such investigations are presented.     

Phase behavior of concentrated aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium hydroxy naphthoate (SHN)

We have characterized the phase behavior of mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN) over a wide range of surfactant concentrations using polarizing optical microscopy and X-ray diffraction. A variety of liquid crystalline phases, such as hexagonal, lamellar with and without curvature defects, and nematic, are observed in these mixtures. At high temperatures the curvature defects in the lamellar phase are annealed gradually on decreasing the water content. However, at lower temperatures these two lamellar structures are separated by an intermediate phase, where the bilayer defects appear to order into a lattice. The ternary phase diagram shows a high degree of symmetry about the line corresponding to equimolar CTAB/SHN composition, as in the case of mixtures of cationic and anionic surfactants.     

Lyotropic and interfacial behaviour of an anionic gemini surfactant

The sodium salt of N,N'-hexane-bis (1-dodecen-1-ylsuccinamic acid) is an anionic dimeric (gemini) surfactant. A flooding penetration scan of this surfactant in water demonstrates a sequence of lyotropic phases at room temperature (20 degrees C). Preparation of surfactant-water mixtures has resulted in a phase diagram which shows that the same sequence of phases exists up to 100 degrees C. These phases are tentatively assigned to the sequence: micellar to normal hexagonal (H1) to cubic (V1) to lamellar (Lalpha). The interfacial tension at the n-heptane/water interface has been determined in the presence of this surfactant. The surfactant head group area at the interface is large (2.8+/-0.3 nm2 at 298 K) and the interfacial tension above the critical micelle concentration is low (7 mN m(-1)), but considerably higher than the ultra-low values that have been reported for cationic dimeric surfactants at various hydrocarbon-water interfaces.     

Lyotropic mesomorphism of alkyl esters of acyl-L-carnitines

The lyotropic polymorphism of a series of alkyl esters of acyl-L-carnitine has been studied by optical polarizing microscopy and X-ray diffraction. The different structures observed as a function of concentration and temperature have been characterized and their topology determined. As a result, two different phase sequence patterns have been detected: esters of normal alcohols bearing an alkyl chain of 6 or more carbon atoms in the acyl substituent display only a lamellar phase, while compounds which bear a relatively short alkyl chain (4 or less carbon atoms) show in addition non-lamellar type I hexagonal and cubic Q²³⁰ phases. From the analysis of the areas-per-molecule at the polar/apolar interface, the ability of the compounds investigated to form not only non-lamellar phases, but also direct micelles in isotropic solution has been related to the structural characteristics of the molecules. Curved, convex interfaces (in micelles and in non-lamellar phases) are possible only for the most polar acylcarnitines which have a relatively short alkyl chain, so that they behave like single chain surfactants; the most paraffinic derivatives, which have a relatively long alkyl chain, are effective double chain surfactants and then generate only quasi-planar interfaces.     

Phase studies of surfactant–water systems

Significant advances have been made in establishing phase behavior of a number of nonionic, cationic, anionic, catanionic and fluorinated surfactants in water. An interest in phase equilibria existing at sub-ambient temperatures is developing. The study of cubic, intermediate, defective lamellar and sponge phases is an active field of research at present. Further work is needed in exploring thermodynamic stability of rigid nanodisks and densely packed vesicles. Colloidal aspects, thermodynamic and volumetric properties of the surfactant-containing systems deserve special attention.