Adsorption of oppositely charged polyelectrolyte/gemini surfactant mixtures at the air/water interface and the effects of NaBr: a surface tension study

The adsorption of mixed solutions containing an anionic polyelectrolyte, carboxymethylchitosan (CMCH), and cationic gemini surfactants, alkanediyl-bis-(dimethyldodecyl-ammonium bromide) (C₁₂-s-C₁₂, s?=?2, 6, 12), has been investigated by surface tension method. The oppositely charged polyelectrolyte and the surfactants co-adsorb at the surface to form highly surface-active complexes. Combining the surface tension data with the Gibbs equation, it is referred that the surface layers of the mixed solutions have the multi-level structure, which includes the sublayers beneath an outermost layer. The gemini surfactant spacer with different length takes different conformations in the surface layers. The salt (NaBr) effects on the adsorption of the mixtures have also been studied. The spacer length of C₁₂-s-C₁₂ influences the responses of CMCH/C₁₂-s-C₁₂ mixtures to the salt effects. The comprehensive salt effects depend on the competition between the salt-enhancing effect and the salt-weakening effect.     

Adsorption and micellization of alkanediyl-bis(dimethyldodecylammonium bromide) in isopropyl myristate/aqueous system

The adsorption and the micellization of a series of cationic Gemini surfactants, alkanediyl-bis(dimethyldodecylammonium bromide), referred as 12-s-12, in the isopropyl myristate (IPM)/aqueous system have been investigated using interfacial tension measurements. These surfactants showed much stronger ability for their adsorption at the IPM/water interface and micellization in the water phase in comparison with the corresponding monomer, C₁₂TABr. The adsorption and micellization behavior of 12-s-12 was found to strongly depend on the length of spacer chain, which was comparable with those studied previously in the air/water system. Compared with the compounds with short spacer chain, the 12-s-12 having a long spacer chain (s?≥?12) showed complicated behavior for their adsorption and micellization due to the bending of the long spacer chain. Thus, the adsorption process could be divided into two stages. At the beginning stage, the adsorption was diffusion-only controlled and the long spacer chain had a free conformation on the interface. At the second stage, more 12-s-12 molecules crowd into the adsorption layer and forced the long spacer chain to bend strongly in order to make room for the new molecules. This resulted in more and more tight packing of the molecules on the interface and thus a quite large limiting dilational elasticity for 12-16-12 and 12-18-12. The addition of salt greatly promoted the adsorption and micellization of 12-s-12. The present results suggested that the 12-s-12 with s?≥?12 may be a good emulsifier to construct stable emulsion.     

Inhibition of Alkaline Hydrolysis of Ethyl p-Nitrophenyl (Chloromethyl)phosphonate in the System Cationic Surfactant-Water-Electrolyte

The rate of alkaline hydrolysis of ethyl p-nitrophenyl (chloromethyl)phosphonate in micellar solutions of cationic surfactants decreases with increasing concentration of chloride, bromide, and salicylate ions. In the presence of electrolytes, the critical micell concentration of surfactants and the surface potential of micelles also decrease.     

Catalytic effect of supramolecular system based on cationic surfactant and monopodands in nucleophilic substitution of phosphorus esters

The data are presented on the synthesis of podands with terminal quinoxaline fragments of rings and their influence on both the micellization properties of cetyltrimethylammonium bromide in a water—DMF solution and kinetics of basic hydrolysis of O-p-nitrophenyl O-ethyl chloromethylphosphonate and O-p-nitrophenyl O-hexyl chloromethylphosphonate in the absence and presence of surfactants. The mechanism of the podand effect on the reaction rate depends on the structures of phosphonate and podand. 1,8-Bis(3-ethyl-1,2-dihydro-2-oxoquinoxalin-1-yl)-3,6-dioxaoctane inhibits the basic hydrolysis of the substrates to 3—4 times. In a micellar solution of the surfactant, an approximately 20-fold acceleration of the reaction rate constant is observed. The observed rate constant decreases when podand is added to a micellar solution. The catalytic effect of the polycomponent system is due to concentrating of the reactants. The micellar microenvironment can exert both positive and negative effects on the reactivity of phosphonates.     

Study on cloud points of Triton X-100-cationic gemini surfactants mixtures: A spectroscopic approach

This study investigates the effects of various cationic surfactants on the cloud point (CP) of the nonionic surfactant Triton X-100 (TX-100) in aqueous solutions. Instead of visual observation, a spectrophotometer was used for measurement of the cloud point temperatures. The values of CPs for Triton X-100 can be measured directly because TX-100 has an average number of oxyethylene units per molecule of p ≈ 9.5 and a CP = 66.0 °C. Quaternary ammonium dimeric surfactants (m-s-m, m = 10, 12, and 16, and s = 2, 6, and 10) were synthesized and used. The melting temperature T_M and the Krafft temperature T_K were measured for 1 wt% aqueous solutions of these synthesized surfactants. The melting temperature of the solid gemini surfactants increased with the carbon number of the alkyl chain. The results showed that additions of the gemini surfactants (which are infinitely miscible with water) to Triton X-100 increased the cloud point of the TX-100 solutions. All salts tested in these studies had a large effect on the CPs of nonionic surfactants due to their effect on water structure and their hydrophilicity. The effect of the alkyl chain length of the gemini surfactant on the CP of Triton X-100 is therefore more important than the spacer chain length.     

Effect of cationic gemini surfactants on the hydrolysis of carboxylate and phosphate esters using hydroxamate ions

The kinetics of the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by hydroxamate ions mediated by gemini surfactants with quaternary ammonium bromide (16-n-16,2Br⁻, n = 3, 4, 6, 12) and pyridinium chloride (12py-n-py12,2Cl⁻, n = 3, 4) head group have been investigated at 27 °C. The gemini surfactant with the pyridinium head group, 12-py-4-py12,2Cl⁻ (tetramethylene-1,4 bis dodecylpyridinium chloride) shows a large rate acceleration effect than that with an ammonium head group, 16-12-16,2Br⁻, relative to those in water. The apparent pK _a of the hydroxamic acids have been determined in the presence of gemini surfactants. Catalytic system N-phenylbenzohydroxamate/12py-4-py12,2Cl⁻ demonstrated over ~1,590-fold and ~255-fold rate enhancement in the hydrolysis of PNPA and PNPDPP, respectively, for the identical reaction performed in buffer aqueous media at 27 °C. The second order rate constant and binding constants for reactions were determined employing pseudophase model for micellar catalysis.     

Systems based on nonionic amphiphilic compounds: aggregation and catalytic properties

The micellization properties, solubilization capability, and catalytic effect of conventional nonionic surfactants and amphiphilic compounds of oligomeric (Tyloxapol) and polymeric (Synperonic F-68, Pluronic F-127) structure were compared. The systems studied demonstrate a marked catalytic effect toward basic hydrolysis of p-nitrophenyl laurate, which exceeds the effect of aqueous alkali solutions by two orders of magnitude. Correlations between the solubilization capacity of aggregates and their catalytic effect were observed. The maximum efficiency was found for the Tyloxapol solution. The synergetic enhancement of the catalytic effect was observed for the mixed Tyloxapol-cetyltrimethylammonium bromide systems in the presence of small amounts of cationic surfactant     

Nucleophilic Substitution in Carboxylic Esters in Oil-In-Water Microemulsions

In hydrolysis and aminolysis of p-nitrophenyl esters of carboxylic acids in oil-in-water microemulsions on the basis of surfactants of various nature, a complex mechanism of the effect of the medium is operative, including shift of acid-base equilibria in the nucleophile. The rate constants of the processes studied are quantitatively related to the surface potential of the microdroplet. Varied hydrophobicity of the nucleophile and substrate changes the site of the reaction act and the relative contributions of aminolysis and hydrolysis.     

Effect of hydrophobic chain length of the chiral surfactant on enantiomeric separations by electrokinetic chromatography: Comparison between micellar and vesicular pseudo-stationary phases

Two novel amino acid based surfactants sodium N-(4-n-decyloxybenzoyl)-l-valinate (SDeBV) and sodium N-(4-n-octyloxybenzoyl)-l-valinate (SOBV) have been synthesized and used as chiral selectors for enantiomeric separations by micellar electrokinetic chromatography (MEKC). The aggregation behavior of the surfactants was studied in buffered aqueous solution using surface tension and fluorescence probe techniques. The microenvironment of the aggregates was studied using pyrene, and 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe molecules. Results of these studies indicate that these two surfactants form micelles in buffered aqueous solution. Successful enentioseparation has been achieved for 1,1′-bi-2-naphthol (BOH), 1,1′-binaphthyl-2,2′-diylhydrogenphosphate (BNP), 2,8-dimethyl-6H-5,11-methanodibenzo[b,f] [1,5]diazocine (Tröger's base, TB), and benzoin (BZN) using the two chiral selectors SDeBV and SOBV. The separations were optimized with respect to surfactant concentration, pH, and buffer concentration. The results are discussed in light of the aggregation behavior of the surfactants. A comparison of the results of this study has been made with the data from literature to investigate the effect of self-assembly morphology on enantiomeric separations.     

Strong effect of NaBr on self-assembly of quaternary ammonium gemini surfactants at air/water interface and in aqueous solution studied by surface tension and fluorescence techniques

The effects of NaBr on the adsorption of alkanediyl-bis-(dimethyl dodecyl- ammonium bromide) (referred to as C₁₂-s-C₁₂ 2Br) at the air/water interface and on the micellization in the solution have been investigated by surface tension and fluorescence techniques. The results showed that the addition of NaBr greatly enhances their efficiency and effectiveness in surface tension reduction as well as the ability of micellization, even induces strong premicellar aggregation before the cmc. These were attributed to the unique molecular structure of gemini surfactant, where the flexible polymethylene chain was the spacer linking the two quaternary ammonium heads. By a short spacer, the charges of the two quaternary ammonium head groups are concentrated. Even for a long spacer (s = 12), since it is bent toward the alkyl tails, the similar effect is also produced. This results in the high sensitivity of their ionic head groups to salt. Besides, the addition of salt also effectively promotes the hydrophobic interaction between the alkyl tails of gemini surfactants. The addition of NaBr strongly promotes the adsorption of quaternary ammonium gemini surfactants C₁₂-s-C₁₂ 2Br at the air/water interface and the micellization in the solution.     

Micellar Exchange Kinetics of Quaternary Ammonium Type Gemini Surfactants Monitored by Nuclear Magnetic Resonance

The dynamic NMR (DNMR) method was used to detect kinetic parameters of the molecular exchange process between monomers in bulk solution and those in the micelle for Gemini surfactants, 12-s-12 and 14-s-14(s=2, 3 and 4).The escape rate constant, k^-, was derived based on the simplified equations of DNMR theory, and the apparent activation energy of escape, E_a^-, was obtained based on the Arrhenius equation through temperature variation experiments.Results show that the orders of magnitude of k^- for 14-s-14 and 12-s-12 are respectively 10 and 10³ s^-1, E_a^- of 14-s-14 and 12-s-12 are respectively 54.04-73.64 and 33.42-47.09 kJ/mol.Furthermore, increases and E_a^- decreases with the spacer length growing.In combination with the micro-polarity measurements, it was revealed that molecules of 14-s-14 and 12-s-12 have to experience conformation changes when escaping from the micelles.The two-step molecular exchange mechanism for Gemini surfactants was therefore supported.     

Micellization properties and the catalytic effect of the aqueous cetyltrimethylammonium bromide-Triton X-100 binary system in nucleophilic substitution in esters of phosphorus acids

The aggregation properties and catalytic effect of the cetyltrimethylammonium bromide-Triton X-100 binary system in nucleophilic substitution in esters of phosphorus acids are studied in the region of normal micelles and oil/water microemulsions. In the presence of phenol, the dissociation of O-alkyl-O-para-nitrophenyl chloromethyl phosphonates is up to 200 times more rapid than the same reaction in water. The efficiency of catalysis depends on the molar ratio of the surfactants and is determined by several factors: concentration, changes in the microenvironment of the reactants, and the shift of the pK _a of phenol.     

Effect of Chain Length on Enzymatic Hydrolysis of p-Nitrophenyl Esters in Supercritical Carbon Dioxide

The effect of chain length on the enzymatic hydrolysis of various p-nitrophenyl esters was investigated. Specifically, the hydrolysis of various esters p-nitrophenyl butyrate (PNPB), p-nitrophenyl caprylate (PNPC), p-nitrophenyl laurate (PNPL), p-nitrophenyl myristate (PNPM) and p-nitrophenyl palmitate (PNPP) was studied in supercritical carbon dioxide (ScCO₂) with lipase (Novozym 435). This indicates that the conversion of nitrophenyl esters decreases with increasing chain length. The effect of various parameters such as amount of water added, temperature, and enzyme loading was studied. The optimum temperature for the hydrolysis of PNPB and PNPC was 50°C but was 55°C for PNPL, PNPM, and PNPP in ScCO₂. The reactions were also conducted in acetonitrile as the solvent, and it was found that the reactions reach equilibrium much faster in ScCO₂ than in acetonitrile. The kinetics of the hydrolysis reactions were modeled using a Ping Pong Bi Bi model.     

Radiation chemistry of Cu(II) tetrasulfophthalocyanine in water-methanol system

The effect of ionising radiation on copper(II) phthalocyanine 3,4′,4′′,4′′′ tetrasulfonic acid, tetrasodium salt (Cu(II)tspc^4-) in aqueous as well as in water-methanol solutions has been studied. The determined yields of complex decomposition (measured on the basis of absorption decay) depend on the composition of matrices and the dose applied. The rate constant of electron scavenging by Cu(II)tspc^4-, k = (1.3 ± 0.1) × 10¹⁰ M^-1 s^-1 in methanol-water (2:8 v/v) solutions has been determined using the pulse radiolysis technique. The rate constant k = (1.16 ± 0.1) × 10¹⁰ M^-1 s^-1 of scavenging of OH radicals applying the competion method with p-nitrosodimethylaniline (pNDA) has been determined in aqueous solutions.     

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.     

Supramolecular systems based on alkylated <Emphasis Type="Italic">p</Emphasis>-sulfonatocalix[<Emphasis Type="Italic">n</Emphasis>]arenes: aggregation and catalytic and biological activity

Aggregation, catalytic activity, and the influence of alkylated p-sulfonatocalix[n]arenes (SCA; n = 4, 6, 8) on the energy exchange in cells of wheat roots were studied. In water SCA are surfactants, and their efficiency changes with an increase in the number of aromatic rings (n) and the length of hydrocarbon substituents (R) in the molecule. In the range of critical micelle concentration, SCA form micelles with an effective hydrodynamic diameter of ∼200 nm. The catalytic activity of micellar solutions of SCA (n = 4, 8; R = C₁₂H₂₅) in hydrolysis of ethyl 4-nitrophenyl chloromethylphosphonate (ENCP) decreases with an increase in n, whereas the binding constant of ENCP increases. The modifying effect of SCA on the membranes and the energy exchange in cells of wheat roots was revealed: the membranotropic activity of SCA increases with an increase in the hydrophobicity of substituents R and the macrocycle size.     

Factors determining the catalytic activity of the mixed micellar system cetyltrimethylammonium bromide-Brij-35 in the hydrolysis of a phosphonic acid ester

The micellization and surface properties, Stern layer micropolarity, and catalytic action of the cetyltrimethylammonium bromide-Brij-35 system in the alkaline hydrolysis of O-ethyl O-p-nitrophenyl chloromethylphosphonate have been investigated at different proportions of the constituent surfactants. This system has unique, invertible catalytic properties ranging from acceleration of the hydrolysis reaction by more than one order of magnitude to strong inhibition of this reaction and even termination of the reaction at certain proportions of the surfactants. The factors in the changes of the catalytic effect under variation of the surfactant ratio are micropolarity in the reaction zone and surface potential. A likely cause of the termination of the process is the shift of the reaction zone from the surface layer to the micelle core, where the environment is less polar.     

Surface and Solution Properties of Cationic Gemini Surfactants with Primary Linear Alkanols

Using surface tension and fluorescence methods, the surface and solution properties of two cationic gemini surfactants {pentanediyl-1,5-bis(dimethylcetylammonium bromide) and hexanediyl-1,6-bis(dimethylcetylammonium bromide)} (referred to as 16-5-16 and 16-6-16) have been studied in the presence and absence of primary linear alkanols. Parameters studied include the critical micelle concentration (CMC), C ₂₀ (the surfactant concentration required to reduce the surface tension of the solvent by 20 mN·m^?1), Г _max (maximum surface excess), and A _min (minimum surface area per molecule). These parameters indicate mixed micelle formation and, therefore, surfactant-additive interaction parameters in mixed micelles and mixed monolayers, as well as activity coefficients, were calculated. A synergistic effect was observed in all instances and was found to be correlated with the chain length of the alkanols. The CMC values of 16-s-16 (s = 5, 6) decrease with increasing alkanol concentration and the extent of this effect follows the sequence: 1-octanol (C₈OH) > 1-heptanol (C₇OH) > hexan-1-ol (C₆OH) > 1-pentanol (C₅OH) > butanol (C₄OH). The micelle aggregation number (N _agg) of mixed micelles has been obtained using the steady state fluorescence quenching method. The micropolarity of gemini/alkanol systems has been evaluated from the ratio of intensity of peaks (I ₁/I ₃) of the pyrene fluorescence emission spectra. Results are interpreted on the basis of the structure of mixed micelles and monolayers.     

Solid-phase synthesis of fused [2,1-b]quinazolinone alkaloids

Solid-phase synthesis of fused [2,1-b]quinazolinone alkaloids has been developed for the preparation of vasicinone and deoxyvasicinone by two approaches. The derivative of polymer-supported p-nitrophenyl carbonate was attached to anthranilic acid and then coupled with various bromo-lactams. This resin-linked bromo intermediate upon acetylation, hydrolysis and resin cleavage gave the cyclized [2,1-b]quinazolinones (vasicinone). Alternatively, resin-linked azido-benzoic acids were coupled with bromo-substituted lactams followed by cyclization in an aza-Wittig reductive cyclization process giving the bromo-substituted quinazolinone intermediates, with subsequent acetylation, hydrolysis and resin cleavage affording the fused [2,1-b]quinazolinones.     

Phosphorylation of Pyrimidinophane in the Reversed Micellar Solutions of Cationic and Nonionogenic Surfactants

Reactions of a macrocycle containing three pyrimidine fragments and N,N′-bis(2-methylthio-6-methylpyrimidin-4-yl)-hexamethylenediamine with p-nitrophenyl bis(chloromethyl)phosphinate in the micellar solutions of cetyltrimethylammonium bromide, polyethyleneglycol-600 laurate and their mixture in chloroform was investigated spectrophotometrically. Catalytic activity of micelles depends on their structure, concentration of surfactants, molar fraction of the latter in solution, and structure of the nucleophilic reagent.