Anionic Surfactant Systems Research Articles

ATMOSPHERIC OXIDATION OF VITAMIN C AND E IN THE SURFACTANT SYSTEM

The association of ascorbic acid (H2A, AA) with α-tocopherol (α-T) enhances the antioxidant capability of the two vitamins in a microemulsion formed by water, pentanol and sodium dodecyl sulphate. The comparison of vitamin C oxidation kinetics in the presence and absence of α-T in the system shows that there are no tangible differences in the time of the ascorbic acid decomposition. The main effect is the enhancement of the H2A oxidation with increased pentanol concentration in the system. Ascorbic anion interacts with surfactant association structures, changing W/O microemulsion towards O/W system. The polar part of H2A is exposed to aqueous solution and subjected to oxidation. For the vitamin C to vitamin E weight ratio equal 0.13, ascorbic acid stimulates α-T decomposition as long, as it does in the non-oxidised form. When not all amounts of both vitamins are decomposed their activity is similar to each other in the anionic surfactant system. It means they are both good antioxidants in W/O microemulsions. After some time, when vitamin C is completely decomposed, all these micellar systems become more stable with regard to vitamin E. Vitamin E occurs to be a better antioxidant in O/W microemulsion.

Anomalous phase behavior of water-soluble polyelectrolyte and oppositely charged surfactants

Aqueous mixtures of anionic surfactants with cationically substituted quaternary ammonium derivatives of hydroxyethylcellulose, JR and LR series, were investigated by several techniques. On adding sodium dodecyl sulfate (SDS) to a polyelectrolyte solution, phase separation with precipitation occurs in a co-operative way, and redissolution of precipitation is observed at the critical micelle concentration (CMC) of SDS. This is due to admicelle formation on the polyelectrolyte. The phase separation for the two-headed anionic surfactant systems is also seen, while the concentration where this takes place is near the CMC of the surfactant. This is remarkable in the case of the triethanolamine cocoyl glutamate (TCG)–JR 400 system, in which TCG has a CMC over 1 order of magnitude smaller CMC than that of SDS. Surface tension and the dynamic light scattering measurements show the existence of not only electrostatic interaction between the cationic polyelectrolyte and the two-headed anionic surfactant but also intraction between the adsorbed polymers. The scaling analysis of the precipitation line of the surfactant with polyelectrolyte concentration elucidates that one molecule of TCG can neutralize approximately two charges on JR 400.

Micellar thin-layer chromatographic separation of heavy metal cations: Effect of organic and inorganic additives on migration behavior

The migration behavior of heavy metal cations on cellulose layers using aqueous micellar, hydro-organic, and water-organic-surfactant mobile phases was investigated. Anionic, cationic, and nonionic surfactant systems were examined over a 0.001–5% concentration range. Brij-35, a nonionic surfactant capable of forming charged complexes with some metal ions, was identified as the best surfactant. The effect of the presence of organic additives, such as dimethylsulfoxide, dimethylformamide, methanol and acetone, on the mobility of metal ions was also studied. Acetone was found to be the most effective additive at 10% concentration with 3% Brij. Quantitative determination of UO2 2+ by spectrophotometry after preliminary thin-layer chromatographic (TLC) separation from Fe3+ and Hg2+ was also performed. A maximal recovery of 93% was obtained. This TLC method is rapid, with development times averaging 2 min.

Vesicles Formed from a Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Triblock Copolymer in Dilute Aqueous Solution

Micelle formation of block copolymers in selective solvents (organic solvent or water) has been studied for many years.1-3 The morphology of polymeric micelles has been the subject of more recent investigations both theoretically and experimentally.4-8 Diblock copolymers with a long soluble block compared to the nonsoluble block may self-assemble into micelles of a starlike structure while copolymers with a short soluble block often form so-called crew-cut micelles.3,9 One class of water-soluble block copolymers that has attracted great interest in the literature is the triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), often abbreviated as PEOPPO-PEO or EOa-POb-EOa. These copolymers exist in a wide range of different compositions and display a rich phase behavior in water depending on their relative block lengths.10,11 It is well established that the more hydrophilic copolymers associate into micelles, composed of a PPO core and a PEO corona, with a spherical or rodlike shape depending on temperature.7,12-14 The present work focuses on the investigation of one of the most hydrophobic PEO-PPO-PEO copolymers (L121) in dilute water solutions. New interesting structures found in the L121/water system are presented and visualized by the cryogenic transmission electron microscopy (cryo-TEM) technique. For the first time, to our knowledge, unilamellar vesicles are shown to exist in a triblock copolymer system of this type. Vesicles of branched PEO-PPO block copolymers in water have only been predicted as a metastable state in earlier model calculations.15 There are only a few reports in the literature on vesicle formation of diblock and multiblock copolymers in dilute solutions of selective solvents.16-21 Vesicles were one among several different crew-cut aggregates found in polystyrene-poly(acrylic acid) (PS-PAA) and PS-PEO diblock copolymer aqueous systems.9 These vesicular aggregates were prepared by a dialysis method. Both copolymers form so-called glassy vesicles, which are preserved in water since the water-insoluble PS block is in a glassy state at ambient temperatures. However, a recent study on a polybutadiene-PAA (PBD-PAA) copolymer/water system has concluded that the vesicular morphology is not controlled by the glass temperature of the nonsoluble block.22 Other publications have reported the existence of multilamellar block copolymer vesicles in a homopolymer-rich matrix as a selective solvent and in a concentrated ternary block copolymer system exposed to a shear flow.23-27 This communication presents a different and new preparation procedure of block copolymer vesicles than described in earlier work. The method uses an extrusion method often utilized for the preparation of single-tailed and double-tailed surfactant (or lipid) vesicles, during which the dispersed bilayers close into a spherical structure with a water interior.28 The formation of unilamellar surfactant vesicles normally requires the input of energy by extrusion, ultrasonication, or dialysis, although it has been suggested that they may form spontaneously in some mixed cationic and anionic surfactant systems.29 The PEO-PPO-PEO triblock copolymer, denoted Pluronic L121, was a gift from BASF corporation and used without further purification. It has a nominal molecular weight of 4400 g/mol, and the average composition is EO5PO68EO5, which corresponds to 10 wt % of PEO. L121 copolymers do not form micelles. At low concentrations and temperatures, unimers (individual copolymer chains) coexist with larger unimer aggregates (an L1 phase). The hydrodynamic radii (RH) of the unimers and the aggregates, obtained using dynamic light scattering (DLS) at 10 °C, are 1.9 and 44 nm, respectively. Figure 1 displays one part of the binary phase diagram of L121 in water. Because of the short hydrophilic PEO chains, and therefore the small headgroup area, the copolymers can easily pack into a lamellar phase (LR) which appears at higher copolymer concentrations (>55 wt %). The lamellar phase boundaries, as preliminary determined by small-angle X-ray scattering (SAXS), are also indicated in the figure. At low concentrations and at ambient temperatures, the system phase separates, and the phase separation curve determined by visual inspection is shown in Figure 1. The two-phase region is an equilibrium between the lamellar phase and the L1 phase. The L121 block copolymer vesicles were prepared at 25 °C from dilute block copolymer solutions in this L1/LR two-phase regime by repeated extrusion of the * To whom correspondence should be addressed. E-mail address: Karin.Schillen@fkem1.lu.se. Figure 1. Schematic phase diagram of L121 in water. The vesicles were prepared in the two-phase region (2φ) at 25 °C from aqueous L121 solutions with concentrations of 0.2, 0.4, 0.6, and 1.2 wt %. 6885 Macromolecules 1999, 32, 6885-6888

The Surfactant Molecular Complex Formation(III): Systems of Anionic Surfactants and Various Phenols and Some Other Materials

In the system of a homologous series of sodium alkyl sulfates, including SLS (sodium lauryl sulfate) and other anionic surfactants with various phenols and other materials as additives, the surfactant molecular complex formation was obviously proved from results of X-ray diffraction experiment on their powdery specimens and from the elementary analysis such as UV spectroscopy. Especially, well-grown single crystals occurring in a SOS (sodium octyl sulfate)/2-naphthol system provided detailed structural knowledge of the anionic surfactant molecular complex to establish the unambiguous existence of those complex species. In comparison with previous results of the cationic surfactant molecular complexes, significant variation observed in the X-ray diffractograms of the anionic complex species deduced almost no change or only slight contraction of the unit cell of the anionic mother surfactant crystals along their longest lattice direction, while in all cationic complexes a clear expansion was involved associated with the additive intrusion into the surfactant crystal lattices. The remarkable features on the diffractograms of the anionic complex species also corresponded well to observations in which many sorts of anionic complex species were hydrated and the additive stability occluded in their mother surfactant crystal structure was very poor compared with the cationic complex species.

Role of Surfactants in the Synthesis of Poly(p-phenylene) Film in Microemulsions

Poly(p-phenylene) (PPP), electrosynthesized in different microemulsions by the oxidation of benzene in the form of a uniform and adherent film on Pt surfaces is characterized with a view to understanding the role of different types (anionic, nonionic, cationic) of surfactants. The scanning electron microscopy results reveal distinct morphological changes in the films synthesized with cationic, nonionic, and anionic surfactant systems. The evidence from X-ray photoelectron spectroscopy investigations indicates the incorporation of the sulfur species in the film for the case of cationic surfactant while the anionic and nonionic surfactant systems do not show a sulfur signal. IR investigations of the polymer films show a small degree of cross-linked structure with a long backbone containing aromatic rings irrespective of the surfactant whereas 13C cross polarization magic angle spinning NMR (solid state) data support the 1,4 substitution (linkage) in the monomer unit in PPP. Differential thermal analysis/thermalgravimetric analysis indicates a higher thermal stability of PPP due to a longer polymer backbone deposited from the anionic surfactant system. The cyclic voltamograms of the films indicate a progressive change in the redox behavior, which correlates well with the relative stability of the intermediate species with a change in surfactant system.

Synthesis of Alkylphenyl Ethers in Aqueous Surfactant Solutions by Micellar Phase-Transfer Catalysis. 1. Single-Phase Systems

Phase-transfer catalysis and micellar catalysis are two conventional methods of promoting reactions between lipophilic and hydrophilic reactants. Phase-transfer catalysis employs organic solvents that may be undesirable for both economic and environmental reasons, while application of micellar catalysis is limited by the relatively low solubilization capacities of surfactant solutions. Micellar phase-transfer catalysis is a process that combines the best aspects of both conventional methods while avoiding some of the associated problems. Reaction systems consist of reactants, water, surfactant and a phase-transfer catalystno organic solvent is used. The surfactant acts to solubilize and emulsify the lipophilic reactant, while the role of phase-transfer catalyst is to shuttle the hydrophilic reactant from the aqueous phase into the micellar environment where the reaction primarily takes place. Alkylation of phenol with 1-bromobutane was studied under phase-transfer, micellar, and micellar phase-transfer conditions in single-phase solutions at relatively high reactant loadings. Cationic (dodecyltrimethylammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (dimethyldodecylamine oxide) surfactants were compared. Higher conversions with micellar phase-transfer catalysis over conventional micellar and phase-transfer catalysis were observed in nonionic and anionic surfactant systems. For cationic surfactant systems, no significant advantage was observed for micellar phase-transfer catalysis in comparison to conventional micellar catalysis. The effect of cationic surfactant concentration was studied and an optimum surfactant concentration was observed. Effects of initial reactant concentrations and two types of mixing were also studied. Mixing effects were significant, suggesting that mass transport rates of components between the aqueous and micellar pseudophases in these microheterogeneous systems at high reactant concentrations may affect reaction kinetics.

A CORRELATION FOR THREE-PHASE BEHAVIOR OF CATION1C SURFACTANT-OIL-WATER SYSTEMS

ABSTRACT The phase behavior of cationic surfactant/oil/water/alcohol systems was systematically scanned to correlate the attainment of three-phase microemulsion-oil-water systems with the following formulation variables: water phase salinity as sodium chloride concentration, oil alkane carbon number (CAN) or equivalent ACN when the oil phase is not an alkane, surfactant alkyl chain length, surfactant hydrophilic group (both amine salts at acid pH and quaternary ammonium compounds at neutral pH), alcohol and temperature effects. The numerical correlation is mathematically similar to the one found for anionic surfactant systems. Differences in coefficient and parameter values allow comparisons between both types of ionic surfactant systems. The linear expression corroborates the theoretical interpretation proposed in other publications.

Ordering of Interfacial Water Molecules at the Charged Air/Water Interface Observed by Vibrational Sum Frequency Generation

The molecular structure and orientation of interfacial water molecules at the air/water interface in the presence of soluble cationic and anionic surfactants has been characterized. We have employed vibrational sum frequency generation (VSFG) to probe the orientation of interfacial water molecules as a function of both bulk surfactant concentration and solution ion strength. The observed ordering of interfacial water molecules is manifested by an enhancement of the OH stretching modes in the VSFG spectra. We attribute the observed enhancement to an alignment of the interfacial water molecules induced by the large electrostatic field present at these charged interfaces. The nature of this enhancement is further explored by studying mixed (cationic and anionic) surfactant systems as well as surfactant systems at different ionic strengths. From these studies we find that the interfacial water molecules attain their highest degree of alignment at surface surfactant concentrations well below maximum surface co...

Effect of organized anionic surfactant system on the kinetics of V(V)-organic substrate-initiated polymerization of acrylonitrile

The kinetics of polymerization of acyrlonitrile (AN) initiated by a V(V)-cyclohexanone redox system in the presence of a surfactant was studied over a temperature range of 30–50°C in acidic medium. The anionic surfactant sodium dodecyl sulfate (SDS) enhances the rate of polymerization (Rp) as well as rate of V(V) consumption (−Rv). The cationic surfactant, cetyltrimethylammonium bromide (CTAB), decreases both Rp and −Rv, while the nonionic surfactant, Triton-X-100, has no effect on the rate. The change of concentration of the surfactant, monomer, acid, and cosolvent on the rate of polymerization were examined. Both Rp and −Rv are computed in the presence of different organic substrates, i.e., cyclohexanone, cyclopentanone, glycerol, and citric acid. The viscosity-average molecular weight (Mv) of the polycrylonitrile, obtained at different [SDS], was determined using the Mark-Howink relationship. A suitable mechanistic scheme was proposed for the process. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1825–1833, 1997

Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective quenching rule at different ‘effective’ micellar surface charge densities

Applicability of nitromethane as a selective quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for select solutes dissolved in micellar sodium dodecylsulfate (SDS) + 1-pentanol, sodium dodecylsulfate + 1-hexanol and cetyltrimethylammonium bromide (CTAB) + 1-pentanol solvent media. Results of measurements show that nitromethane quenched fluorescence emission of only the 6 alternant PAHs in the cationic CTAB + 1-pentanol solvent media as expected. Emission intensities of nonalternant PAHs, except for two exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar anionic SDS + 1-pentanol and micellar anionic SDS + 1-hexanol. Nitromethane quenched fluorescence emission of all nonalternant PAHs studied in these two anionic surfactant systems, which is contrary to the selective quenching rule. The reduced emission quenching that is observed at increasing alcohol cosolvent concentration is rationalized in terms of the decreased micellar surface charge density that occurs when more alcohol molecules are incorporated into the anionic micelle.

Surface viscoelasticity in mixed polycation anionic surfactant systems studied by a simple test

The talc particle test, performed on mixed solutions of polyelectrolyte and oppositely charged surfactant, was used to examine the changes in rheology of the air/water interface of these solutions over a large range of compositions. The systems scrutinized consisted of sodium dodecyl sulfate (SDS, concentration ranging from 0.005 to 0.5 g L −1) and either the cationic cellulosic polymer, Polymer JR400, or the corresponding hydrophobically modified polymer, Quatrisoft ™ LM200, of concentration ranging from 0.001 to 1 g L −1. Clear indications of viscoelasticity at the air/water interface were obtained over a wide composition domain in both instances. Viscoelasticity provides evidence of a synergistic adsorption of the two components at the air/water interface. When the SDS was increased sufficiently with respect to the polyelectrolyte concentration, the viscoelasticity vanished, and the interface became fluid; the fluidity recovery is an indication that the interface is denuded of polyelectrolytes by growing competition from micellar interfaces in solution.

Effect of organized anionic surfactant system on kinetics of polymerization of acrylonitrile initiated by Ce(IV): Citric acid and other organic substrates

The polymerization of acrylonitrile (AN) using the Ce(IV)-citric acid (CA) redox system as an initiator in aqueous nitric acid solution, in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS), has been kinetically studied at a temperature range of 25-45°C. The rate of polymerization (R p ) and disappearance of Ce(IV) (-R Ce ) increase with increasing concentration of SDS, above its critical micelle concentration (cmc), when the surfactant molecules are organized. R p was found to be proportional to [AN] 1.5 and [CA] 0.5 . With other organic substrates, R p follows the increasing order of sorbitol ≥ mannitol > glycerol > CA. But it was found to decrease considerably in the presence of cationic surfactant (CTAB), and nonionic surfactant (Triton-X-100) had no effect on the rate. -R Ce varies linearly with [Ce(IV)] and [CA]. Both R p and -R Ce increase with increasing temperature. The overall activation energy was found to be 18.31 and 13.72 kcal/mol in the absence and presence of 0.015M SDS, respectively. The chain length of the polyacrylonitrile has also increased with increasing SDS concentration

Phase science of surfactants

Recent major developments that have taken place in surfactant phase science may be summarized in five points. First, the equilibrium vesicle phase formed by catanionic surfactant mixtures in the very dilute region has been investigated for several systems which also form several mesophases in the concentrated region. Coexistence of two reversed hexagonal phases and first order vesicle→micelle phase transition have been demonstrated. Second, by manipulating the hydrophilic-lipophilic balance and molecular geometry of many surfactant systems, the changes in observed phase behaviour serve to increase our knowledge of aggregation processes in general and the structures of the biréfringent intermediate phases, sponge phase and dilute lamellar phase in particular. Third, the lyotropic nematic and defective mesh-type lamellar phases are readily formed by fluorinated surfactants; the structures of the two phases, which are still debatable, have been studied by several techniques. The first observation of a lower consolute loop along the dilute lamellar phase was reported for the fluorinated anionic surfactant systems. Fourth, the valency of counterions has been shown to have dramatic effects on the aggregation and phase behaviour of ionic surfactants. Finally, surfactant-protein systems form novel gel phases and locations of phases provide information concerning the contribution of electrostatic and hydrophobic effects to surfactant-protein interactions.

Alkyl Chain Symmetry Effects in Mixed Cationic–Anionic Surfactant Systems

We study symmetry effects of hydrophobic tails in mixed oppositely charged surfactant systems. Equal hydrophobicity of the participating surfactants is obtained either (1) by equal hydrocarbon surfactants chain length or (2) by unequal surfactants chain length when one chain is perfluorinated. Three systems were studied: symmetric system of type (1) or (2), and an asymmetric system. Some unusual structural changes are observed in a hydrocarbon–fluorocarbon mixed single-chain oppositely charged surfactant system in the dilute region. Both the cationic hydrocarbon surfactant, dodecytrimethylammoniumchloride (DoTAC), and the anionic fluorocarbon surfactant, sodium perfluorononanoate (SPFN), are separately soluble in water, forming micellar solutions at high water concentration followed by the formation of liquid crystal phases at low water content. Upon mixing the two surfactants in an equimolar ratio at low total surfactant concentration (C< 5 wt%), a stable, dense, flow-birefringent phase is formed at the bottom of the test tube, and a clear phase is formed at the top. Cryo-transmission electron microscopy (cryo-TEM) micrographs of the dense phase show dispersed lamellar droplets having a very smalldspacing, and fairly monodispersed ∼100 nm single-walled vesicles in the supernatant. The latter is supported by NMR self-diffusion measurements. This structural behavior is studied at different molar ratios of the two surfactants. A micelle–vesicle transformation is indicated by NMR self-diffusion measurements. The same behavior is found for symmetry of type (1). The asymmetric system DoTAC–sodium nonanoate (SN) shows an isotropic phase in all molar ratios up to 40 wt% of surfactant, where both imaging and rheological measurements indicate the existence of elongated micelles in the equimolar solution. It was found that the micellar size distribution is coupled with the molar ratio only in the SN-rich part.

Derivative spectrophotometric resolution of mixtures of the food colourants Tartrazine, Amaranth and Curcumin in a micellar medium

A study of the absorption spectra of the food colourants Tartrazine (E-102), Amaranth (E-123) and Curcumin (E-100) in various organic solvents and pH media, have been carried out. In addition, the spectral behaviour in aqueous micellar systems of anionic, cationic and non-ionic surfactants, as well as β-cyclodextrin, was investigated. Analytical methods are proposed for the determination of the three dyes in mixtures without any prior separation step, by the application of the derivative technique to the normal absorption spectra, based on the use of cetyltrimethylammonium bromide (CTAB) in 0.1 M NaOH, over the concentration range 2–50 μg ml −1. The methods are applied for determining the three dyes in commercial food products.

Infinite Elution Range in Micellar Electrokinetic Capillary Chromatography Using a Nonionic/Anionic Mixed Micellar System

The feasibility of increasing the elution range in micellar electrokinetic capillary chromatography (MEKC) was investigated using a nonionic/anionic mixed micellar system. The micellar “pseudostationary” phase was comprised of a mixture of nonionic (poly(oxyethylene)(23) dodecanol) and anionic (sodium dodecyl sulfate) surfactants. Variation in poly(oxyethylene)(23) dodecanol (Brij 35, Aldrich) concentration affected the electroosmotic and micellar electrophoretic mobilities differently. Equal but opposite electroosmotic and micellar electrophoretic velocities were obtained at a specitk Brij 35 concentration, thereby producing a “stationary” micellar phase. Retention characteristics of a series of n-alkylphenone homologues suggested a nearly Mite elution range. Inhite elution ranges are possible at neutral pH (7.0) using deactivated fused-silica capillaries and at a pH of 6.2 using untreated fused-silica capillaries. Micellar electrokinetic capillary chromatography (MEKC) has grown significantly since its inception due to its ability to provide high-efficiency separations for a wide variety of neutral solute systems. The mechanism behind separations in MEKC is the differential partitioning of solutes between an aqueous phase and a micellar “pseudostationary” phase. Anionic surfactant systems are typically used in MEKC since the resultant micelles electrophoretically migrate opposing electroosmotic flow and do not interact appreciably with the negatively charged walls of the fused-silica capillary columns. Sodium dodecyl sulfate (SDS) is by far the most popular micellar system in MEKC due to its low cost, availability in high purity, W absorption characteristics, and

Foam enhancement by short‐chain hydrophobe alcohol ethoxylates in light‐duty liquids

AbstractThis study examined linear alcohol ethoxylates as foam‐enhancing agents in light‐duty liquids. The ethoxylates were formulated as supplementary surfactants to an anionic main surfactant system. The relationship between ethoxylate structure and performance was elucidated, comparisons were made between ethoxylates and other recognized foam‐enhancing agents, and a representative light‐duty liquid formula was optimized. Performance evaluation focused on manual plate washing capacity but also included agitation foam volume, surface activity, cloud and clear temperatures, viscosity and grease cutting. Shortchain alcohol ethoxylates with a high degree of ethoxylation (such as C8 with 70% or 7 moles of ethylene oxide) gave optimum performance that was comparable to or better than acknowledged foam‐enhancing agents. The substitution of an alcohol ethoxylate for a fatty acid ethanolamide in the test formulation resulted in substantially lower formulation viscosity.

Application of the Retention Index Concept in Micellar Electrokinetic Capillary Chromatography

The application of the retention index concept in micellar electrokinetic capillary chromatog. (MECC) was evaluated for the identification of neutral species. Homologous series of alkylbenzenes and alkylaryl ketones were applied as retention index stds. and also for the calcn. of the micelle migration time by an iteration procedure. The relation between retention indexes, I, and octanol-H2O partition coeffs. is discussed, and DI values were calcd. from retention indexes, obtained with both anionic and cationic surfactant systems, to study the sepn. mechanism for solutes with different functionalities in MECC. The influence of the phase ratio is treated theor., and the capacity factor is linearly related to the surfactant concn., whereas the retention index is independent of the surfactant concn. The temp. dependence of the retention index was studied and the std. enthalpy, entropy, and Gibbs free energy for micellar solubilization of the sample compds. were detd., showing that the hydrophobic interaction plays a significant role in MECC anal

Photochemically Triggered Isothermal Phase Transitions in Lyotropic Liquid Crystalline Systems of Anionic and Cationic Surfactants

Transition temperatures in lyotropic liquid crystalline aqueous potassium myristate (KMy), potassium oleate (KOl), and cetyltrimethylammonium bromide (CTAB) were measured in the presence of small amounts of two photoisomerizable solubilizates: a crown ether bearing azobenzene (AB15C5) and 4-hydroxystilbene (4HS). Transition temperatures T HI (hexagonal → isotropic) in the anionic surfactant systems of KMy and KOl rose by up to 15°C with increasing concentration (<1% w/w) of trans-AB15C5 or trans -4HS. In situ trans → cis photoisomerization of 4HS in KMy leads to a further increase of T HI by up to 3.5°C. Similarly, in cationic CTAB systems the presence of 4HS caused an increase of T NI (nematic → isotropic), cis-4HS being more efficient than trans-4HS. In the two latter systems isothermal phase transitions can be brought about by exposing samples to light. The origin of the observed effects is connected with a variation of the shape of the solubilizates. This was indicated by measurements of T NI and T HI in CTAB and KMy, respectively, in the presence of model compounds differing in shape only. Rate constants of thermal cis → trans isomerizations in the surfactant systems were 1.6 × 10 -4 s -1 for AB15C5 at 35°C and 6.1 × 10 -5 s -1 for 4HS at 30°C. An activation energy of 89.9 kJ/mol was determined for the thermal cis → trans isomerization of 4HS.