Phase behaviour and lyotropic liquid crystals in cationic surfactant–water systems

Abstract

The sequence of lyotropic liquid-crystalline phases occurring in a range of cationic surfactants with water has been determined using the 'penetration' optical microscopy technique. Of the order of 50 different surfactants were examined. The variation of the polar group structure includes the series -NHl, -NMel, NEtl, NPrl and N B u~. In addition, the effect of altering counter-ion and alkyl-chain length are also examined. Other surfactants studied include 2-hydroxyethyl alkylammonium salts, nitroxide ring substituents, alkyl pyridinium salts and alkyl indole surfactants where the indole substituent is part of the hydrophobic chain.The mesophase sequences are similar to the patterns of behaviour already known for non-ionic and anionic surfactants, with hexagonal (H,), bicontinuous cubic (V,) and lamellar (La) being the most frequently observed phases (in addition to micellar solution). For long-chain surfactants, intermediate (Int) rather than V, phases are shown to form at temperatures just above the Krafft boundary. Qualitatively, much of the phase behaviour can be analysed in terms of alkyl chain packing constraints and intermicellar forces. For bulky alkyl headgroups mesophase formation is suppressed, but the hydroxyethyl substituent causes surprisingly little change from a trimethylammonium headgroup. The behaviour of the indole surfactants suggests that the indole group resides mainly in the micelle interior.The mesophase behaviour of monoalkyl anionic and non-ionic surfactants in water has been fairly thoroughly documented. 1-3 Cationic surfactants have received far less attention. Only a few descriptions of their phase behaviour have been reported, and these are limited to alkylammonium and alkylmethylammonium ~a l t s .~-'~ In this study we have undertaken a survey of the liquid crystals formed by a range of cationic surfactants having a variety of headgroup stuctures. The phase types have been monitored using the optical-microscope penetration technique. This enables a qualitative picture of the mesophase sequences as a function of surfactant concentration to be obtained rapidly. From this initial survey, several compounds with novel features in their phase properties have been selected for detailed study. These studies will be the subject of further papers. Surfactant-Water-Mesophase FormationThe principal phases formed in monoalkyl surfactant-water mixtures are lamellar (La), hexagonal ( H I ) and cubic ( I , , V,) liquid crystals,'73 in addition to the micellar solution (L,). Reversed phases (cubic V,, I, and hexagonal, H,) can occur, but these are more common with dialkyl surfactants or with ionic-non-ionic mixtures. The cubic phases consist either of ordered globular micelles (I,) or have a bicontinuous structure (V,). 'Intermediate' phases (Int) can also occur. These are birefringent and form at compositions between hexagonal and lamellar phases instead of the V, phase. The factors responsible for their occurrence are not understood, nor are their structures fully elucidated. To date, intermediat...

Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant

“…Water solutions of CAT16 were prepared at a concentration of 2 mM and diluted as necessary. CAT16 micellar solutions possess a Kraft point of 313 K. 23 However, EPR analysis (Vide infra) showed that the solubilization of CAT16 micelles was more conveniently achieved at 333 K. Therefore, all solutions of CAT16, with and without n.5-SBDs, were prepared and stored at 333 K. Since aging of the samples stored at 333 K showed a progressive decrease in the EPR signal intensity, presumably due to a thermal degradation process, freshly prepared CAT16 solutions were used for all the EPR measurements that were performed at 333 K.…”

Aqueous phase behaviour of choline carboxylate surfactants—exceptional variety and extent of cubic phases

“…Such phases are indeed known for divalent anionic, 29 zwitterionic, 30 non-ionic 31,32 and even mono-cationic surfactants with highly dissociated counterions (e.g. alkyltrimethylammonium chloride), 30 due to an increased effective area per headgroup a o .…”

“…alkyltrimethylammonium chloride), 30 due to an increased effective area per headgroup a o . Moreover, it should be noted that such cubic phases have also been identified in ternary or more complex mixtures of anionic surfactants.…”

In Situ Time-Resolved SAXS Study of the Formation of Mesostructured Organically Modified Silica through Modeling of Micelles Evolution during Surfactant-Templated Self-Assembly

“…59,60 The specific case of CTPA + , which is also precursor of a Pm n mesophase, will be discussed separately in a dedicated note in Sect. 3.4 because of its atypical micellar shape behaviour.…”