Water−Ethylene Glycol Cationic Dimeric Micellar Solutions: Aggregation, Micellar Growth, and Characteristics As Reaction Media

Co-solvent Effect on Spontaneous Formation of Large Nanoscale Structures in Catanionic Mixtures in the Anionic-Rich Region

The aggregation behavior was investigated in mixtures of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) (anionic-rich catanionic) solutions. The study was conducted in solutions of water–ethylene glycol (EG) by means of surface tension, conductometry, cyclic voltammetry, zeta potential measurements, transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques. The degree of counterion dissociation (α), critical micelle concentration, aggregation numbers, interfacial properties, interparticle interaction parameters, and morphology of aggregates were determined. Based on regular solution theory, the cosolvent effects between SDS and CTAB as surfactants were also analyzed for both mixed monolayers at mixed micelles (βM) and the air/liquid interface (βσ). It was shown that the formation of large aggregates occurred in the presence of an excess of anionic surfactant. A phase transition from cylindrical micelles to spherical micelles in the anionic-rich regime was observed with an increase in the EG volume fraction. The inter particle interactions were assessed in terms of cosolvent effects on the micellar surface charge density and the cylindrical-to-spherical morphology change. Zeta potential and size of the aggregates were determined using dynamic light scattering and confirmed the models suggested for the processes taking place in each system.

Quantitative correlation between counterion-affinity to cationic micelles and counterion-induced micellar growth

The fascinating and serendipitous discovery, in 1976, of the characteristic viscoelastic behavior of wormlike micelles of cetyltrimethylammonium salicylate (CTASa) surfactant solution at ~2×10(-4) M CTASa became a catalyst for an increasing interest in both industrial application and mechanism of the origin of micellar growth of this and related wormlike micellar systems. It has been perceived for more than three decades, based upon qualitative evidence, that the extent of the strength of the counterion (X) binding to ionic micelles determines the counterion-induced micellar structural transition from spherical-to-small rodlike-to-linear long stiff/flexible rodlike/wormlike-to-entangled wormlike micelles. This perception predicts the presence of a possible quantitative correlation of counterionic micellar binding constants (KX) with counterion-induced micellar growth. The quantitative estimation of counterion binding affinity to cationic micelles, in terms of the values of the degree of counterion binding (βX), is concluded to be either inefficient or unreliable for moderately hydrophobic counterions (such as substituted benzoate ions). The values of KX, measured in terms of conventional ion exchange constants (KX(Y)), can provide a quantitative correlation between KX or KX(Y) (with a reference counterion Y=Cl(-) or Br(-)) and counterion-induced ionic micellar growth. A recent new semi-empirical kinetic (SEK) method provides the estimation of KX(Y) for Y=Br as well as ratio of counterionic micellar binding constants KX/KBr (= RX(Br)) where the values of KBr and KX have been derived from the kinetic parameters in the presence of cationic spherical and nonspherical micelles, respectively. The SEK method has been used to determine the values of KX(Br) or RX(Br) for X=2-, 3- and 4-ClC6H4CO2(-). Rheometric measurements on aqueous CTABr/MX (MX=2-,3- and 4-ClBzNa) solutions containing 0.015 M CTABr and varying values of [MX] reveal the presence of spherical micelles for MX=2-ClBzNa and long linear as well as entangled wormlike micelles for MX=3- and 4-ClBzNa. The respective values of KX(Br) or RX(Br) of 5.7, 50 and 48 for X=2-, 3- and 4-ClBz(-) give a quantitative correlation with the rheometric measurements of the structural features of micelles of 0.015 M CTABr solutions containing 2-, 3- and 4 ClBzNa.

Micellar Characteristics and Surface Properties of Some Sulfobetaine Surfactants

The micellar and interfacial properties of three sulfobetaine surfactants of the type CnH2n+1N+Me2(CH2)3SO3 − (n = 10; SB3-10 n = 12; SB3-12, n = 16; SB3-16) have been studied by conductometry and surface tension measurements. The critical micellar concentration (CMC), surface excess concentration (Γmax), minimum surface area per molecule of surfactant (Amin), Gibbs free energy of micellization (ΔGo m), Gibbs energy of transfer (ΔGo trans), the surface pressure at CMC (πCMC) and the Gibbs energy of adsorption (ΔGo ads) of these surfactants have also been determined. Effect of formamide has been studied at higher temperature conductometrically. Results show that alkyl chain length and the amount of solvent composition influences micellization and surface properties of sulfobetaines. Increment in the CMC, ΔGo m and ΔGo trans, can be accounted for changes in the polarity of the bulk phase. Γmax, and πcmc decreases where as Amin increases with increase in the organic solvent. It was also observed that micellization in water-formamide mixture was even slower than in the case of ethylene glycol.

Self-assembly, surface activity and structure of n-octyl-β-D-thioglucopyranoside in ethylene glycol-water mixtures

The effect of the addition of ethylene glycol (EG) on the interfacial adsorption and micellar properties of the alkylglucoside surfactant n-octyl-β-d-thioglucopyranoside (OTG) has been investigated. Critical micelle concentrations (cmc) upon EG addition were obtained by both surface tension measurements and the pyrene 1:3 ratio method. A systematic increase in the cmc induced by the presence of the co-solvent was observed. This behavior was attributed to a reduction in the cohesive energy of the mixed solvent with respect to pure water, which favors an increase in the solubility of the surfactant with EG content. Static light scattering measurements revealed a decrease in the mean aggregation number of the OTG micelles with EG addition. Moreover, dynamic light scattering data showed that the effect of the surfactant concentration on micellar size is also controlled by the content of the co-solvent in the system. Finally, the effect of EG addition on the microstructure of OTG micelles was investigated using the hydrophobic probe Coumarin 153 (C153). Time-resolved fluorescence anisotropy decay curves of the probe solubilized in micelles were analyzed using the two-step model. The results indicate a slight reduction of the average reorientation time of the probe molecule with increasing EG in the mixed solvent system, thereby suggesting a lesser compactness induced by the presence of the co-solvent.

Effects of structure dissymmetry on aggregation behaviors of quaternary ammonium Gemini surfactants in a protic ionic liquid EAN

The aggregation behaviors of a series of dissymmetric cationic Gemini surfactants, [C(m)H(2m+1)(CH(3))(2)N(CH(2))(2)N(CH(3))(2)C(n)H(2n+1)]Br(2), designated as m-2-n (with a fixed m + n = 24, m = 16, 14, 12) have been investigated in a protic ionic liquid, ethylammonium nitrate (EAN). Surface tension, polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheological measurements are adopted to investigate the micellization and lyotropic liquid crystal (LLC) formation. The obtained results indicate that the structure dissymmetry plays an important role in aggregation process of m-2-n. With increasing degree of dissymmetry, the critical micellization concentration, the maximum reduction of solvent surface tension, and the minimum area occupied per surfactant molecule at the air/EAN interface all become smaller. The thermostability of formed LLCs is therefore improved because of the more compact molecules. These characteristics can be explained by the enhancement of solvophobic effect due to the increased structure dissymmetry of Gemini surfactants.

Study of the reaction 2-(p-nitrophenyl)ethyl bromide + OH⁻ in dimeric micellar solutions

The dehydrobromination reaction 2-(p-nitrophenyl)ethyl bromide + OH⁻was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br⁻ (with s = 2, 3, 4, 5, 6, 8, 10, 12) micellar solutions, in the presence of NaOH 5 × 10⁻³ M. The kinetic data were quantitatively rationalized within the whole surfactant concentration range by using an equation based on the pseudophase ion-exchange model and taking the variations in the micellar ionization degree caused by the morphological transitions into account. The agreement between the theoretical and the experimental data was good in all the dimeric micellar media studied, except for the 12-2-12,2Br⁻ micellar solutions. In this case, the strong tendency to micellar growth shown by the 12-2-12,2Br⁻ micelles could be responsible for the lack of accordance. Results showed that the dimeric micelles accelerate the reaction more than two orders of magnitude as compared to water.

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1): kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1) decrease nonlinearly with increasing total concentration of nonionic surfactant C(m)E(n) (i.e. [C(m)E(n)](T) where m and n represent the respective number of methyl/methylene units in the tail and polyoxyethylene units in the headgroup of a surfactant molecule and m/n=16/20, 12/23 and 18/20) at constant 2% v/v CH(3)CN and 1.0 mM NaOH. The k(obs)vs. [C(m)E(n)](T) data follow the pseudophase micellar (PM) model at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 2.0 mM C(18)E(20) where rate of hydrolysis of 1 in micellar pseudophase could not be detected. The values of k(obs) fail to follow the PM model at > ∼50 mM C(16)E(20), > ∼1.4 mM C(12)E(23) and > ∼2.0 mM C(18)E(20) which has been attributed to a micellar structural transition from spherical to rodlike which in turn increases C(m)E(n) micellar binding constant (K(S)) of 1 with increasing values of [C(m)E(n)](T). Rheological measurements show the presence of spherical micelles at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 3.0 mM C(18)E(20). The presence of rodlike micelles is evident from rheological measurements at > ∼50 mM C(16)E(20), > ∼1.4 mM C(12)E(23) and > ∼3.0 mM C(18)E(20).

A new semi-empirical kinetic method for the determination of ion exchange constants for the counterions of cationic micelles

A new method, based upon semi-empirical kinetic approach, for the determination of ion exchange constant for ion exchange processes occurring between counterions at the cationic micellar surface is described in this review article. Basically, the method involves a reaction kinetic probe which gives observed pseudo-first-order rate constants (k(obs)) for a nucleophilic substitution reaction between the nonionic and anionic reactants (R and S) in the presence of a constant concentration of both reactants as well as cationic micelles and varying concentrations of an inert inorganic or organic salt (MX). The observed data (k(obs), versus [MX]) fit satisfactorily (in terms of residual errors) to an empirical equation which could be derived from an equation explaining the mechanism of the reaction of the kinetic probe in terms of pseudophase micellar (PM) model coupled with another empirical equation. This (another) empirical equation explains the effect of [MX] on cationic micellar binding constant (K(S)) of the anionic reactant (say S) and gives an empirical constant, K(X/S). The magnitude of K(X/S) is the measure of the ability of X(-) to expel S(-) from a cationic micellar pseudophase to the bulk aqueous phase through ion exchange X(-)/S(-). The values of K(X/S) and K(Y/S) (where Y(-) is another inert counterion) give the ion exchange constant, K(X)(Y) (=K(X)/K(Y) where K(X) and K(Y) represent cationic micellar binding constants of X(-) and Y(-), respectively). The suitability of this method is demonstrated by the use of three different reaction kinetic probes and various MX.

Effects of solvent media and temperature on the self-aggregation of cationic dimeric surfactant 14-6-14, 2Br- studied by conductometric and fluorescence techniques

To explore how the solvent characteristics influence the self-aggregation of the cationic dimeric surfactant hexamethylene-1,6-bis(tetradecyldimethylammonium bromide) (14-6-14, 2Br(-)) and to obtain various energetic parameters, conductometric experiments were performed on the binary mixtures of three organic solvents--viz., 2-methoxyethanol (ME)-, acetonitrile (AN)-, and formamide (FA)- water (WR) at different temperatures ranging from 298.15 to 323.15 K. The steady state fluorescence measurements allowed us to calculate the average aggregation number (N(agg)) and the air-bulk phase surface tension helped us to correlate the solvent cohesive energy density with Gibbs energy of micellization (DeltaG(0)(m)). The results showed that, although micellization process becomes less favorable with the increase in volume % of the ME, AN and FA in the binary mixed media, the increment in the cmc below 20% (v/v) of the organic solvents is comparatively less showing the predominance of WR character in the bulk phase. Because of the decrease in Gibbs interfacial energy contribution (DeltaG(0)(interf)) to DeltaG(0)(m), the average aggregation number (N(agg)) decreased with the increase in the volume % of the organic solvent in the mixed media. As the enthalpy of micellization became more negative, the corresponding entropy change became less positive and enthalpy-entropy compensation phenomenon was observed for the micellization of 14-6-14, 2Br(-) in all the mixed media in the studied temperature range.