Necessity of aromatic carboxylate anions to be planar to induce growth of cationic micelles

It is well-known that some aromatic anions have the ability to induce viscoelastic transformation in aqueous solutions of cationic surfactants even at added salt concentrations as low as 10-20 mM. This behavior is associated with the formation of an entangled network of elongated micelles. However, the effect of aromatic ring substituents on the anion's ability to promote rapid micelle growth is not well-understood. We have performed ab initio calculations of the carbonyl group rotation barriers in a series of substituted benzoate and naphthoate anions at the MP2/STO-3G level of theory. It was found that aromatic carboxylates, known to be particularly effective in causing sphere-rod transition in cationic micelles, preferably adopt conformations with the COO(-) group in the same plane as the ring(s). This structural preference can be attributed to either intramolecular hydrogen bonding (o-hydroxyl derivatives) or pi-conjugation effects (m- and p-halogenated derivatives). In the former case the barrier to rotation is 40-50 kcal/mol, whereas in the latter case the threshold value is around 3.0 kcal/mol. Propensity for the planar conformation correlates with a greater depth of counterion penetration into the micelle surface, as inferred from NMR experiments, compared to the anions with less hindered carbonyl rotation. This points to favorable hydrophobic interactions between the surfactant methylene groups and the aromatic ring(s) of the anion as a possible explanation for the rapid growth of cationic micelles observed upon addition of certain aromatic carboxylates.     

Nature of cationic poly(propylenimine) dendrimers in aqueous solutions as studied using versatile indicator dyes

Aqueous solutions of four cationic poly(propylenimine) low-generation dendrimers of different architecture and hydrophobicity have been examined as media for acid-base reactions of indicator dyes. The cationic dendrimers in solution can be considered as oligomers of cationic polyelectrolytes, or surfactant-like species, able to form micelles through self-association or sometimes even as unimolecular micelles. The dendrimers influence the ionization constants, tautomeric equilibria, and absorption/emission/excitation spectra of indicator dyes. The p K a values of the majority of the indicator dyes decrease in dendrimer solutions, often by 1-2 p K a units, similar to effects registered in micellar solutions of cationic surfactants. Analogously, the shifts of absorption band maxima indicate that the microenvironments of the dyes bound to the dendrimers are less polar than in water. However, some spectral effects denote the specificity of the dendrimers. The greatest difference between the dendrimers and spherical surfactant micelles is revealed by kinetic processes, especially of bromophenol blue alkaline fading in a dendrimer solution but not in a micellar surfactant solution. Within the dendrimer series, the most significant differences were observed for substances possessing n-dodecyl tails on the one hand and those without such hydrophobic portions on the other. For the last-named, the decrease in p K a's of indicators, band shifts of their anions, and in particular displacement of tautomeric equilibria compared with aqueous solutions are much smaller than for more hydrophobic dendrimers.     

10-Methylacridinium ion as a fluorimetric probe measuring the activity of halide anions in aqueous solutions of cationic surfactants

The quenching of fluorescence of 10-methylacridinium ion by inorganic anions in aerated aqueous solutions was studied at room temperature. In the case of cationic surfactants, with chloride and bromide anions as counterions, characteristic breaks on the Stern-Volmer plots could be observed at concentrations corresponding to the critical micelle concentration of the surfactants. It is shown that the ratio of the slopes of the two linear fragments of the plots, in the micellar and premicellar concentration ranges, gives an estimate of the value of the ionization degree, alpha, of the micelles. This approach is applicable also in aqueous-alcohol systems.     

Study of the formation of dye-induced premicellar aggregates and its application to the determination of quaternary ammonium surfactants

Formation of dye-induced mixed premicellar aggregates from binary surfactant solutions is proposed for the determination of alkyltrimethylammonium surfactants at the muM level. The Coomassie Brilliant Blue G (CBBG) dye, negatively charged, induces the formation of cationic surfactant aggregates at concentrations far below the cmc. The role of CBBG in the formation of premicelles was studied by using pyrene as a fluorimetric probe. Formation of CBBG-cationic surfactant aggregates of well-defined stoichiometries that depend on the total surfactant concentration added is demonstrated. Also, the influence of analytical parameters affecting the concentration at which a given aggregate is formed was studied. Linear calibrations for alkyltrimethylammonium surfactants were obtained by using different cationic surfactants as titrants; therefore, the previously derived measurement parameter for mixed micelles is applicable to premicellar aggregates as well.     

Hydroxide Ion Initiated Reactions Under Phase Transfer Catalysis Conditions: Mechanism and Implications. New Synthetic Methods (62)

The development of Phase Transfer Catalysis (PTC) represents a major step forward in the employment of many organic reactions and renders them very convenient and useful processes. These reactions involve the application of nucleophiles in general, anions and bases in particular, in reactions carried out in a water-organic solvent system. They can be performed both in the laboratory and on an industrial scale. The ease of application of PTC processes is the main reason for their increasing utilization in industry. An outstanding achievement of this technique is the employment of aqueous bases in reactions which traditionally would otherwise require a strong base in a nonaqueous medium. The classical procedures that require severe anhydrous conditions, expensive solvents and dangerous bases such as metal hydrides and organometallic reagents are now replaced by aqueous solutions of, e.g., sodium or potassium hydroxides (PTC/OH processes). In contrast to the extensive synthetic applications of PTC/OH systems, the detailed mechanisms of these processes have been the subject of a great deal of controversy and various mechanisms have been suggested. However, it would seem that our knowledge concerning the mechanistic aspects of such reactions has now reached the stage where it can be used to advantage in synthesis planning. A better understanding of the various factors which influence the reaction would undoubtedly help to optimize PTC/OH processes such as to enable higher yields in shorter reaction times at lower temperatures. The importance of, inter alia, the catalyst will be pointed out and it is highly recommended that such catalysts be always available in the laboratory, for the range of organic reactions that they can efficiently, conveniently and safely catalyze is vast indeed.     

Threadlike micelle formation of anionic surfactants in aqueous solution

We have investigated the formation of threadlike micelles consisting of anionic surfactants and certain additives in aqueous solution. Threadlike micelles long enough to be entangled with each other were formed in a clear aqueous solution of two anionic surfactants, sodium hexadecyl sulfate and sodium tetradecyl sulfate. These solutions also contained pentylammonium bromides or p-toluidine halides and exhibited remarkable viscoelasticity. Because the molar ratio of surfactants to cationic additives in these micelles seemed close to unity, they formed 1:1 stoichiometric complexes between surfactant anions and additive cations, as previously found in systems of cationic surfactants such as hexadecyltrimethylammonium bromide and sodium salicylate. The viscoelastic behavior of these anionic threadlike micellar systems was adequately described by a simple Maxwell element with a single relaxation time and strength, as in many similar cationic systems.     

Formation of ionic associates by Phenol Red and their interaction with ionic surfactants in aqueous solutions

Formation of associates between single-or double-charged anions of sulfophthalein dye Phenol Red and single-charged cations of cyanines in aqueous solutions was considered. The association constants were estimated from the spectral data. Geometric and energy characteristics of the associates were calculated. The suitability of the associates formed by Phenol Red for quantitative determination of cationic surfactants in aqueous solutions was examined.     

STUDIES ON SELF-ASSOCIATIVE BEHAVIOR OF A NOVEL CATION AMPHIPHILIC POLYMER

A novel associating polymer P(AEBA) was synthesized by radical polymerization of the cationic amphiphilic monomer,4-(2-(acryloyloxy)ethoxy)benzyl tri-ethyl ammonium bromide(AEBA),in aqueous solutions.P(AEBA) displays a strong tendency for self-association in aqueous solutions and is sensitive to the external stimulation such as added salt.The associative properties and morphologies of P(AEBA) were studied by fluorescnece probe technique,viscometry and TEM.In dilute salt-free solutions P(AEBA) behaves as ...     

Alkylation of carbonyl compounds in water: formation of C-C and C-O bonds in the presence of surfactants

The formation of C-C and C-O bonds by the reaction of enolate intermediates with electrophilic substrates commonly requires strong bases, aprotic solvents and very low temperatures. A way of performing the same reactions with sodium hydroxide at moderate temperatures in aqueous surfactant solutions is presented. Different halides, ketones and surfactants (cationic, zwitterionic and anionic) have been used. The results obtained show that the amount of ketone alkylation is much higher and that the reactions are faster in the presence than in the absence of surfactant aggregates. The hydrolysis of the halide is minimised in the presence of cationic or zwitterionic surfactants.     

Mixed Adsorption Layers of Nonionic and Cationic Surfactants on Quartz

Surfactant adsorption on quartz and wetting of glass by aqueous solutions of tetradecyltrimethylammonium bromide, Triton X-100, and their mixtures are studied. It is shown that synergistic adsorption of surfactants from mixed solutions occurs in the region of low concentrations. In the region of high concentrations, mixed molecular aggregates of the cationic and nonionic surfactants are formed on the surface. The structure of the mixed adsorption layers is discussed.     

Molecular dynamics simulations of the solution-air interface of aqueous sodium nitrate

Molecular dynamics simulations have been used to investigate the behavior of aqueous sodium nitrate in interfacial environments. Polarizable potentials for the water molecules and the nitrate ion in solution were employed. Calculated surface tension data at several concentrations are in good agreement with measured surface tension data. The surface potential of NaNO3 solutions at two concentrations also compare favorably with experimental measurements. Density profiles suggest that NO3- resides primarily below the surface of the solutions over a wide range of concentrations. When the nitrate anions approach the surface of the solution, they are significantly undercoordinated compared to in the bulk, and this may be important for reactions where solvent cage effects play a role such as photochemical processes. Surface water orientation is perturbed by the presence of nitrate ions, and this has implications for experimental studies that probe interfacial water orientation. Nitrate ions near the surface also have a preferred orientation that places the oxygen atoms in the plane of the interface.     

Synthesis of nanocrystalline calcium phosphate in microemulsion--effect of nature of surfactants

Nanosized calcium phosphate (CP) powders have been synthesized by an inverse microemulsion system using kerosene as the oil phase, a cationic surfactant Aliquat 336, a non-ionic surfactant Tween 20 and their mixture and aqueous solutions of calcium nitrate tetrahydrate and biammonium hydrogen phosphate as the water phase. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the calcium phosphate nanoparticles. The cationic surfactant Aliquat 336 has been found to regulate the nucleation and crystal growth. The synthesized powders have been comprehensively characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Our results show that the brushite (DCPD) is the major phase comprising the calcium phosphate nanoparticles. In mixed surfactants mediated system a morphological controlled highly crystalline particles have been synthesized. Further, the role of Aliquat 336 has been established and a plausible synthetic mechanism has been proposed.     

Effects of organic salt additives on the behavior of dimeric ("gemini") surfactants in aqueous solution

The effects of a series of aromatic anions, so-called hydrotropes, on characteristic solution properties of a family of ammonium gemini surfactants with dodecyl chains were explored. The stoichiometric addition of the organic salts to the geminis can result in clear solutions or in phase separation/precipitation, depending on the detailed nature of the added counterions and on the spacer group of the gemini surfactant. Many organic anions induce synergistic effects, strongly reducing the critical micellization concentration (cmc) and the surface tension at the cmc. Furthermore, a number of combinations of organic anions and geminis exhibit thickening of their aqueous solutions. The effects of the added salts are strongly enhanced for the gemini surfactants compared to the monomeric analogue N-dodecyl-N,N,N-trimethylammonium chloride. Even anions such as benzoate may be effective for thickening, and viscoelastic solutions can be obtained with salicylate despite the relatively short alkyl chains.     

Hofmeister anion effects on surfactant self-assembly and the formation of mesoporous solids

Recent work on mesoporous silica formation using cationic and non-ionic templates has unveiled a large number of anion effects. Anions are seen to change the hydrolysis rates of the silicate precursors, affecting the surface properties and morphologies of the final products after calcination, and they often improve the hydrothermal stability of the silica materials. These advances are reviewed in connection with the Hofmeister series of anions and the known effects of anions on the self-assembly and phase behavior of cationic and non-ionic surfactants.     

Premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions: deprotonation of dye in ion pair micelles

The premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions monitored by surface tension and spectroscopic measurements has been described. The measurements have been made for three anionic sulfonephthalein dyes and cationic surfactants of different chain lengths, head groups, and counterions. The observations have been attributed to the formation of closely packed dye surfactant ion pairs which is similar to nonionic surfactants in very dilute concentrations of the surfactant. These ion pairs dominate in the monolayer at the air-water interface of the aqueous dye surfactant solutions below the CMC of the pure surfactant. It has been shown that the dye in the ion pair deprotonates on micelle formation by the ion pair surfactants at near CMC but submicellar surfactant concentrations. The results of an equilibrium study at varying pH agree with the model of deprotonated 1:1 dye-surfactant ion pair formation in the near CMC submicellar solutions. At concentrations above the CMC of the cationic surfactant the dye is solubilized in normal micelles and the monolayer at the air-water interface consists of the cationic surfactant alone even in the presence of the dyes.     

离子液体表面活性剂在水溶液中的自组装及其调控研究进展

离子液体表面活性剂在化学合成、材料制备和环境污染控制等方面的应用与它们在水溶液中的自组装及其微观结构密切相关。因此,研究离子液体表面活性剂在水溶液中的自组装行为具有重要的意义。本文重点综述了阳离子的结构、阴离子的类型、外加电解质、有机添加剂、环境因素(温度、溶液pH值和光)等对离子液体表面活性剂在水中的自组装行为以及对组装体结构影响的研究进展,总结了这些因素对离子液体表面活性剂在水中自组装的调控规律,展望了该领域的发展方向及面临的挑战。     

Orientation of a TICT probe trapped in the peripheral confined water created by ionic surfactant envelope around silver nanoparticles

Ionic surfactants are known to aggregate around the surface of a nanoparticle as a single layer in premicellar and a double layer in micellar concentrations. This motif of arrangement indicates the development of a layer of confined water of lower polarity than bulk water around the surface of the nanoparticle. We have demonstrated the behavior of a twisted intramolecular charge transfer (TICT) probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT), in the confined aqueous layer developed at the surface of spherical silver nanoparticles (Ag NPs) at and above the critical micellar concentrations (CMC) of a cationic and an anionic surfactant, namely, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). It is observed that the presence of charged surfactant head groups affects DMASBT differentially in the ground and the excited states. In presence of CTAB, DMASBT turns over in the excited state and interacts with the Ag NP surface, whereas in SDS the probe remains in its original orientation during the interaction. Steady-state and time-resolved fluorescence spectral studies provide enough evidence for orientation of the TICT probe in the peripheral water of Ag NP created by the surfactants. The results were confirmed by steady-state anisotropy measurements. The data show the difference between the properties of the confined peripheral water and the bulk aqueous environment. The TICT probe, DMASBT, is proved to be an excellent marker for the phenomenon.     

Structure and composition of cationic-nonionic surfactant mixed adsorbed layers on mica

The composition and morphology of mixed adsorbed layers comprising one of several poly(oxyethylene) alkyl ether nonionic surfactants, C(i)E(j), and two cationic surfactants-dodecyltrimethylammonium bromide (DTAB) and tetradecyltriethylammonium bromide (TTeAB)-at the mica/solution interface have been studied using depletion adsorption and atomic force microscopy. The nonionic surfactants do not themselves adsorb onto mica, but can coadsorb with a cationic surfactant. The extent of their hydrophobic association with the adsorbed cationic surfactant depends on alkyl chain length, while the adsorbed layer morphologies are sensitive to the number of ethoxy groups. Nonionic surfactants with headgroups containing less than eight ethylene oxide units decrease the adsorbed aggregate curvature, gradually transforming globular TTeAB or cylindrical DTAB adsorbed aggregates into a rod, mesh, or bilayer structure. Those with larger headgroups favor globular aggregates. The mechanism by which the nonionic surfactant modifies the adsorbed morphology is the formation of defects in the form of cylinder end-caps or branch-points, leading to adsorbed layer compositions that differ from ideal mixing predictions. All mixed adsorbed films become saturated with the nonionic component when the capacity of the aqueous side of the adsorbed layer is reached.     

Competition between aggregation and hydrolysis in the reaction of arylperfluorooctanoates in micellar solutions

The rate of hydrolysis of phenyl and p-nitrophenyl perfluorooctanoate (2a and 2b) was measured in water and in the presence of different cationic (dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, cetyltrimethylammonium bromide), anionic (sodium dodecyl sulfate (SDS) and perfluorooctanoate (PFO)) and neutral (Brij-35) surfactants. In water solution, the formation of phenol from 2a and p-nitro phenol from 2b takes place through two kinetic processes, both of which are much slower than the expected rate of hydrolysis for the monomeric compounds in water. The two kinetic processes are attributed to a coupling of the rates of hydrolysis and aggregation of the substrates. In the presence of charged surfactants at concentrations below the respective critical micellar concentration (cmc), two relaxation times are also observed. These are of the same order of magnitude as the substrates alone in the case of SDS, but faster for the cationic surfactants. At some concentration above the cmc, all the surfactants, except for PFO, showed a clean pseudo-first-order behavior attributed to the hydrolysis of the substrate incorporated into the micellar phase. In cationic micelles, the rates for 2a are slower and those for 2b are faster than the value expected for the monomer in water. The difference in behavior is attributed to the location of the substrates in the micellar phase and to the charge distribution in the transition state of the reactions. It is shown that the reactions in the micellar phase are catalyzed by the buffer PO4H(2-)/PO4H2(-). The reactions in SDS micelles are faster than those in water but slower than the estimated value for the monomer in water. The rate of the reactions in the presence of nonionic surfactant has values between those in cationic and anionic surfactants, that is, the rates are k(cationic) > k(nonionic) > k(anionic.) The behavior of 2a and 2b in water and in micellar solutions indicates that the substrates form aggregates in water at a rate that competes with the rate of hydrolysis.     

Fluorescence delineation of the surfactant microstructures in the CTAB-sOS-H2O catanionic system

A key feature of amphiphilic molecules is their ability to undergo self-assembly, a process in which a complex hierarchical structure is established without external intervention. Ternary systems consisting of aqueous mixtures of cationic and anionic surfactants exhibit a rich array of self-assembled microstructures such as spherical and rodlike micelles, unilamellar and multilamellar vesicles, planar bilayers, and bicontinuous structures. In general, multiple complementary techniques are required to explore the phase behavior and morphology of aqueous systems of oppositely charged surfactants. As a novel and effective alternative approach, we use fluorescence spectroscopic measurements to examine the microstructures of aqueous cationic/anionic surfactant systems in the dilute surfactant region. In particular, we demonstrate that the polarity-sensitive fluorophore prodan can be used to demarcate the surfactant microstructures of the ternary system of cetyltrimethylammonium bromide, sodium octyl sulfate, and water. As the fluorescence signature of this probe is dependent on the nature of the surfactant aggregates present, our method is a promising new approach to effectively map complex surfactant phase diagrams.