The reactivity of phosphonic esters in aqueous micellar solutions of cationic surfactants

The micellar effect of cationic surfactants in alkaline hydrolysis of O-alkyl-O-aryl-chloromethyl phosphonates involves a positive contribution of concentrating the reagents and a negative effect of the micellar environment due to a loss in the activation entropy. The reactivity of substrates in micelles depends on both electronic and hydrophobic characteristics of substituents in the aryl group.     

Aggregate formation in aqueous solutions of carboxymethylcellulose and cationic surfactants

The addition of cationic surfactants to an aqueous solution of an anionic polymer, carboxymethylcellulose (carboxyMC), causes the spontaneous formation of aggregates in a certain range of concentrations. Here we studied two surfactants, dodecyl and hexadecyl trimethylammonium bromide (DTAB and CTAB, respectively). Using different techniques (light scattering, potentiometry, viscosimetry, and zetametry), we found that a simple lengthening of the surfactant tail length by four CH2 groups drastically changes the aggregate morphology, size, and charge. We explored in detail how the surfactant and polymer concentrations act on these systems.     

Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions

The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases.     

芳香族小分子物质在表面活性剂溶液中的状态

阴、阳离子表面活性剂在水溶液中混合后很容易形成沉淀,因而以往在应用上将这两种表面活性剂视为配伍禁忌。但是最近几年,混合阴、阳离子表面活性剂水溶液理论性质的研究受到了一种程度的重视。研究发现,该混合体系具有与单纯的离子型表面活性剂或离子与非离子型表面活性剂混合物十分不同的性质,     

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.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

Aggregation properties of cationic gemini surfactants with partially fluorinated spacers in aqueous solution

The aggregation properties of cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide), [C(12)H(25)(CH(3))(2)N(CH(2))(m)(CF(2))(n)(CH(2))(m))N(CH(3))(2)C(12)H(25)]Br(2) [where 2m + n = 12 and n = 0, 4, and 6; designated as 12-12-12, 12-12(C(4)(F))-12, and 12-12(C(6)(F))-12, respectively] have been studied by microcalorimetry, time-resolved fluorescence quenching, and electrical conductivity. Compared with a fully hydrocarbon spacer of 12-12-12, the fluorinated spacer with a lower ratio of CF(2) to CH(2) in 12-12(C(4)(F))-12 tends to disfavor the aggregation, leading to larger critical micelle concentration (cmc), lower micelle aggregation number (N), and less negative Gibbs free energy of micellization (DeltaG(mic)). However, the fluorinated spacer with a higher ratio of CF(2) to CH(2) in 12-12(C(6)(F))-12 may prompt the aggregation, resulting in lower cmc, higher N, and more negative DeltaG(mic). It is also noted that enthalpy change of micellization (DeltaH(mic)) for 12-12(C(4)(F))-12 is the most exothermic, but the values of DeltaH(mic) for 12-12-12 and 12-12(C(6)(F))-12 are almost the same. These results are rationalized in terms of competition among the enhanced hydrophobicity and the rigidity of the fluorinated spacer, and the variation of immiscibility of the fluorinated spacer with the hydrocarbon side chains.     

Critical aggregation concentration in mixed solutions of anionic polyelectrolytes and cationic surfactants

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.     

Temperature-dependent vesicle formation of aqueous solutions of mixed cationic and anionic surfactants

The phase behavior of aqueous solutions of mixed cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) was examined at different temperatures (20, 30, 40, and 50 degrees C). While stable vesicles were formed in a narrow composition range on the SOS-rich side at 20 degrees C, the range widened remarkably when the temperature was raised to 30 degrees C. Thus, the vesicle region extended to cover almost the entire composition range, CTAB:SOS = 0.5:9.5-5.0:5.0, at the total surfactant concentrations of 50-70 mM on the SOS-rich side. To analyze the temperature dependence of this phase behavior of the mixed surfactant system, DSC and fluorescence polarization measurements were performed on the system. The experimental findings obtained revealed that pseudo-double-tailed CTAB/SOS complex, the major component of the bimolecular membrane formed by the surfactant mixture, undergoes a gel (Lbeta)-liquid crystal (Lalpha) phase transition at about 26 degrees C. This phenomenon was interpreted as showing that the bimolecular membrane has no curvature and is rigid and easy to precipitate at temperatures below the phase transition point, whereas it has a curvature and is loose enough to disperse in the solution as vesicles at temperatures above the phase transition point. Vesicles formed by the anionic/cationic surfactant complex were then stable at temperatures above the phase transition temperature of the complex.     

Wetting effect of aqueous binary mixed solutions of cationic and nonionic surfactants

Wetting of low-energy solid surfaces (polymers, hydrophobized glass) with aqueous solutions of binary mixtures of cationic and nonionic surfactants was investigated at molar fractions of the cationic surfactant of 0.2, 0.5, and 0.8. In a narrow concentration range, the non-additive effect of wetting was observed: wetting of the solid surfaces with solutions of the mixtures is better than that would be expected from the additive behavior of the components. The magnitude of the effect depends on the surface energy of the solid substrate, total surfactant concentration in a mixture, and molar fraction of the cationic component. The wetting effect of surfactant mixtures with respect to low-energy solid surfaces can be predicted using the surface tension isotherms.     

Ionic association with anions of alizarin red S in aqueous solutions with surfactants

The formation of associates of single (H₂An⁻) or doubly charged anions (HAn²⁻) of alizarin red S with cations (Ct⁺) of cyanine dye pinacyanol in aqueous solution is considered. Thermodynamic values of equilibrium association constants were determined according to spectrophotometric data. Values of enthalpy of formation for the associates of the composition Ct⁺ · H₂An⁻ and (Ct⁺)₂ · HAn²⁻ were calculated with the help of semi-empirical methods. It was determined that the addition of cationic or anionic surfactant results in the destruction of associates.     

Semi-quantitative determination of cationic surfactants in aqueous solutions using gold nanoparticles as reporter probes

Concentrations of cationic surfactants in aqueous solutions have been estimated on the basis of changes in the color of gold nanoparticles, used as reporter probes. We have shown that the colors of gold nanoparticles with anionic protective groups on their surfaces shift from red to indigo/purple and then back to red in a range of cationic surfactant solutions in which concentrations vary from very low to above the theoretical CMCs. The color changes occur near the theoretical CMCs, presumably because the presence of surfactant micelles in the solution prevents the gold nanoparticles from aggregating. We have used gold nanoparticles as reporter probes to determine the concentrations of cationic surfactants in products such as hair conditioners, which often contain large amounts of alkyltrimethylammonium halides. Although this approach can only provide an estimate, it can be performed simply by addition of a given amount of gold nanoparticles to a series of diluted solutions, without the need for instruments or labor-intensive procedures. Figure Photographs of a series of diluted hair conditioner solutions with added gold nanoparticles

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Synthesis and evaluation of cationic and cationogenic polymeric sorbents for removing surfactants from aqueous solutions

Polymers based on 2‐dimethylaminoethyl methacrylate (DMAEMA) crossslinked with ethylene glycol dimethacrylate (EGDMA) or N,N′‐methylenebisacrylamide (MBAA) have been prepared by solution‐suspension polymerization. Polymers containing ammonium groups were synthesized by quaternization of aminofunctional polymers obtained with ethyl or dodecyl bromide or by the polymerization of respective quaternized monomers. Influence of the content of DMAEMA in the polymers, nominal degree of crosslinking, length of alkyl substituent in quaternary ammonium groups as well as particle size on the sorption of anionic and nonionic surfactants from aqueous solutions has been studied.     

Adsorption of organic anions from aqueous solutions by layered silicates modified with cationic surfactant

The interaction of anionic organic dyes with layered silicates (kaolinite and hydromica) both natural and modified with a cationic surfactant (cetylpyridinium bromide) is studied by adsorption and spectral methods. The adsorption of organic anions by modified silicates is proved to proceed via the formation of ionic associates of these adsorbates with the modifier. It is found that the interaction of large organic anions with the modifier results in the desorption of the latter, followed by its secondary adsorption in the form of ionic associates with the adsorbates. The selectivity of layered silicates modified with the cationic surfactant to organic anionic dyes is determined by the stability of the formed dye/modifier ionic associates and their affinity to the surface. These factors depend on the sizes of the hydrocarbon moieties of both components of the associates. Therefore, the selection of a suitable modifier allows one to control the selectivity of modified minerals to different organic anions. Using long-chain organic cations as modifiers, organic anions of any sizes can be extracted from aqueous solutions.     

Interaction between primary aliphatic amines and carboxylic acid esters in aqueous micellar solutions of cationic surfactants

The effects of cetylpyridinium bromide (CPB) on the acid-base equilibria of primary aliphatic amines and on the kinetics of reactions of the amines withp-nitrophenyl acetate (PNPA) andp-nitrophenyl caprylate (PNPC) were studied by potentiometric titration and UV spectroscopy. The values of apparent pK _a of the amines in the micellar phase, binding constants of their neutral forms, and the surface potentials of micelles were determined. Cetylpyridinium bromide accelerates the aminolysis of PNPA by factors of 3 to 8 by forming mixed micellar aggregates with the amines. The shift of pK _a values of the amines in micellar solutions is not the only factor that enhances their reactivity. The substrate specificity was found: in contrast to the reaction with PNPA, CPB accelerates (by factors of 15 to 65) or retards (by factors of 4 to 6) the aminolysis of PNPA depending on the hydrophobicity of the nucleophilic reagent. The binding constants of substrates, the rate constants in the micellar phase, and the critical concentrations of micellization were determined from the data obtained. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1333–1338, July, 1998.     

Aggregation behavior of naphthalimide fluorescent surfactants in aqueous solution

A group of novel fluorescent surfactants, N-n-alkyl-4-(1-methylpiperazine)-1,8-naphthalimide iodine [C_nndi]I (n?=?8, 10, and 12), have been synthesized and their aggregation behavior in aqueous solution have been explored by surface tension, electric conductivity, hydrogen-1 NMR spectra, absorption, and fluorescence spectra. Compared with traditional cationic surfactants, the [C_nndi]I have a rather lower critical micelle concentration and higher surface activity. Absorption and fluorescence spectra were proved to be facile method to monitor directly the aggregation states of fluorescent surfactant molecules in solution and revealed clearly the formation of face-to-face stacked structure of the [C_nndi]I molecules driven by the π–π interactions. The micelle formation process for [C_nndi]I was demonstrated to be enthalpy-driven in the temperature range investigated. Possible aggregation process was given based on the experimental results. The combination of dye and surfactant provides a way for monitoring the formation process of micelle directly by fluorescence spectra.     

Interaction of cationic surfactants with carboxymethylcellulose in aqueous media

We have examined the polymer-surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphosphonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with increasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hydration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.     

Aggregation of iron-containing fullerenols in aqueous solutions

Endometallofullerenols Fe@C₆₀(OH)₃₀ were synthesized by a new procedure using a special molecular precursor in the course of electric arc evaporation of a composite carbon electrode. Aqueous solutions of fullerenols Fe@C₆₀(OH)₃₀ were studied by small-angle neutron scattering (SANS) and viscometry under the conditions of transition from dilute to concentrated systems in two media: acidic (pH 3.7) and weakly alkaline (pH 7.8). In a weakly alkaline medium, fullerenols form relatively dense clusters (correlation radius R _C ~ 17 nm), and in acidic medium, less dense structures of size R _C ~ 18 nm in the concentration interval с = 0.5–1.4 wt %. The mechanisms of aggregation of fullerenol molecules in aqueous solutions are discussed. The water-soluble derivatives Fe@C₆₀(OH)₃₀ obtained show promise for biomedical applications in magnetic resonance imaging as contrasting agents that are potentially less toxic than presently used contrasts based on gadolinium chelate complexes.     

Aggregation and gelation in hydroxypropylmethyl cellulose aqueous solutions

In this work we present an analysis of the thermal behavior of hydroxypropylmethyl cellulose aqueous solutions, from room temperature to higher temperatures, above gelation. We focus on significant aspects, essentially overlooked in previous work, such as the correlation between polymer hydrophobicity and rheological behavior, and the shear effect on thermal gelation. Micropolarity and aggregation of the polymer chains were monitored by both UV/vis and fluorescence spectroscopic techniques, along with polarized light microscopy. Gel formation upon heating was investigated using rheological experiments, with both large strain (rotational) tests at different shear rates and small strain (oscillatory) tests. The present observations allow us to compose a picture of the evolution of the system upon heating: firstly, polymer reptation increases due to thermal motion, which leads to a weaker network. Secondly, above 55 degrees C, the polymer chains become more hydrophobic and polymer clusters start to form. Finally, the number of physical crosslinks between polymer clusters and the respective lifetimes increase and a three-dimensional network is formed. This network is drastically affected if higher shear rates, at non-Newtonian regimes, are applied to the system.