F. G. Valeeva

Micellization and catalytic activity of the cetyltrimethylammonium bromide-Brij 97-water mixed micellar system.

Surface tension measurements and the kinetic study of the basic hydrolysis of ethyl p-nitrophenyl chloromethyl phosphonate were used to examine the structural behavior and catalytic activity of the cethyltrimethylammonium bromide (CTAB)-polyoxyethylene (10) oleyl ether, C(18)H(35)(OCH(2)CH(2))(10)OH (Brij 97)-water mixed micellar system. Application of the regular solution model to the experimental data yields the value of the interaction parameter beta as -4.6, which indicates an attractive interaction of the surfactants in the mixed micelle and reflects synergistic solution behavior of the mixture. The mixed micellar composition is found to be enriched in the surfactant with the lower critical micelle concentration (cmc). In the kinetic study a nonmonotonic change in the pseudo-first-order rate constant of basic hydrolysis of the substrate is observed with increasing mole fraction of nonionic surfactant. The pseudophase micellar model reveals that the concentration factor mainly contributes to the catalytic effect, while the microenvironmental factor plays a negative role.

Self-assembling systems based on amphiphilic alkyltriphenylphosphonium bromides: Elucidation of the role of head group

A systematic study of the aggregation behavior of alkyltriphenylphosphonium bromides (TPPB-n; n=8, 10, 12, 14, 16, 18; here n is the number of carbon atoms in alkyl groups) in aqueous solutions has been carried out and compared with trimethyl ammonium bromides (TMAB-n). Critical micelle concentrations (cmcs) of TPPB-n and TMAB-n decrease with the number of carbon atoms with the slope parameter of ca.0.3. The low cmcs and effective solubilization power toward Orange OT indicate high micellization capacity of phosphonium surfactants. The low counterion binding parameter β is revealed for TPPB-10 and TPPB-12, while high counterion binding of ≥80% is observed for high TPPB-n homologs. Values of the surface potential ψ calculated on the basis of pK(a) shifts of p-nitrophenols is similar for both series and monotonously increase with alkyl chain length. Several points indicate non-monotonic changes within TPPB-n series. There are peculiarities of the tensiometry and solubilization plots for high homologs and above mentioned increases in counterion binding on transiting from low to high molecular weight surfactants. Differences in aggregation behavior between TPPB and TMAB series and between low and high homologs can be due to the specific structural character of the TPP(+) cation, which is supported by X-ray data.

Supramolecular systems based on dicationic pyrimidine-containing surfactants and polyethyleneimine

Aggregates based on a novel pyrimidine-containing amphiphile containing three uracil fragments were quantitatively characterized. The critical micelle concentration (CMC) is 2 mmol L–1 and the degree of binding of counterions is β = 0.95, which is consistent with the low zeta potential of the micelles equal to +5 mV. Aggregates with a diameter of 45 nm are formed in the CMC region, and in the region of higher concentrations the aggregates are rearranged to micelle-like particles. In the presence of hydrophilic polymer polyethyleneimine, the CMC significantly decreases (to 0.2 mmol L–1), the degree of binding of counterions decreases, and the electrokinetic potential increases. The size of the aggregates in the binary system is 6—7 nm.

Novel bolaamphiphilic pyrimidinophane as building block for design of nanosized supramolecular systems with concentration-dependent structural behavior.

A new macrocyclic bolaamphiphile with thiocytosine fragments in the molecule (B1) has been synthesized and advanced as perspective platform for the design of soft supramolecular systems. Strong concentration-dependent structural behavior is observed in the water-DMF (20% vol) solution of B1 as revealed by methods of tensiometry, conductometry, dynamic light scattering, and atomic force microscopy. Two breakpoints are observed in the surface tension isotherms. The first one, around 0.002 M, is identified as a critical micelle concentration (cmc), whereas the second critical concentration of 0.01 M is a turning point between the two models of the association involved. Large aggregates of ca. 200 nm are mostly formed beyond the cmc, whereas small micelle-like aggregates exist above 0.01 M. The growth of aggregates between these critical points occurs, resulting in a gel-like behavior. An unusual decrease in the solution pH with concentration takes place, which is assumed to originate from the steric hindrance around the B1 head groups. Because of controllable structural behavior, B1 is assumed to be a candidate for the development of biomimetic catalysts, nanocontainers, drug and gene carriers, etc.

Solubilizing and catalytic properties of supramolecular systems based on gemini surfactants

Quantitative parameters characterizing the aggregation behavior of dicationic surfactants of the 12-s-12 type in aqueous solutions were determined: critical micelle concentrations, aggregation numbers, and surface potentials. The effects of the spacer length of the surfactants on their solubilizing effect with respect to hydrophobic (dye Orange OT) and water-soluble (p-nitrophenol) spectral probes and on their ability to shift acid-base equilibria were studied. The kinetics of alkaline hydrolysis of p-nitrophenyl acetate and p-nitrophenyl caprinate in 12-s-12 solutions was studied by spectrophotometry. A correlation between the micellar catalytic effect and the surface potential of micelles was established. The pronounced substrate specificity was revealed: the maximum acceleration of hydrolysis is observed in solutions of 12-6-12, attaining 1760 times for p-nitrophenyl caprinate.

Effect of surfactant micelles on solubility and spectral characteristics of 2,2’-bibenzimidazole

The solubilization capacity of surfactants of different types was quantitatively characterized by spectrophotometry toward 2,2’-bibenzimidazole (BBI), whose derivatives possess a wide range of pharmacological activity. 2,2’-Bibenzimidazole solubilized by sodium dodecyl sulfate micelles is characterized by a high molar absorption coefficient (9400 mol−1 L cm−1) at the absorbance maximum (342 nm at pH 4–10), which may serve as a basis for the development of procedures for analytical monitoring of the BBI content in aqueous solutions and biosystems. The ability of surfactants to enhance the solubility of BBI in water and, as a consequence, to increase its bioavailability in the series sodium dodecyl sulfate < cetyltrimethylammonium bromide < Tween-80.

Catalytic Effect of Multicomponent Supramolecular Systems in Phosphoryl-Group Transfer Reactions

The catalytic effect of the cetyltrimethylammonium bromide–poly(ethylenimine)–water system in the hydrolysis reactions of phosphonic acid esters (an increase in the rate constant by three orders of magnitude) is due to the concentration of reactants in polymer–colloid complexes and changes in their microenvironments. The catalyst efficiency depends on the structures of substrates, surfactants, and polymers.

Amphiphilic macrocycles bearing biofragment: Molecular design as factor controlling self-assembly

Two novel macrocyclic 6-methyluracilic amphiphiles (uracilophanes) with four (UP1) and two (UP2) uracil moieties and ammonium groups have been synthesized. Tetracationic multi-uracilophane is composed of two macrocyclic units bridged each other with an external methylene spacer, while in the cryptand-like dicationic uracilophane pyrimidinic moieties are connected with an internal methylene spacer. This internal spacer provided a conformational rigidity to the macrocycle. The self-assembly of the uracilophanes is studied and compared with a reference dicationic uracilophane (UP3) with no spacer fragment. Compounds UP1 and UP3 are capable of aggregating, which is characterized by the analogous critical micelle concentration of 1mM, although the former has four decyl tails versus two decyl tails in UP3 molecule. NMR self-diffusion, fluorimetry and DLS techniques revealed that bimodal size distribution occurs in the UP1 solution, with small (≤2nm) and large (ca. 30-50 nm) aggregates contributed. Unexpectedly, the cryptand-like uracilophane UP2 with the same hydrophobicity as UP3 does not form aggregates. The balance of the geometry and energetic factors was analyzed and compared with those contributing to the aggregation of the reference compound UP3. It was established that it is the geometry that controls the packing of the cryptand-like uracilophanes upon aggregation, while hydrophobic effect plays a minor role. In contrast, both factors control the aggregation of oligomeric macrocycle, with energetic factor prevailing. These findings are of importance for (i) the understanding the diverse structural behavior of bioamphiphiles that have very similar chemical structure, but different conformations; and (ii) the design of amphiphiles with controlled model of self-assembly. Supramolecular systems studied can be recommended for biotechnological applications.

Dicationic surfactant based catalytic systems for alkaline hydrolysis of phosphonic acid esters

Use of aqueous micellar solutions of dicationic surfactants with the general formula [R(CH3)2N(CH2)6N(CH3)2R]2+2Br− (R = n-C10H21 to n-C16H33) as the reaction medium for the alkaline hydrolysis of phosphonic acid esters has revealed a strong catalytic effect of the surfactants, which can increase the reaction rate by two orders of magnitude. This effect depends on the surfactant structure, shows itself at low surfactant concentrations, and is substrate-specific. The effect of the micelles on the phosphonate hydrolysis rate is largely determined by the hydrophobicity factor.

Supramolecular catalytic systems based on calix[4]resorcinarene for nucleophilic substitution in phosphorous acid esters

The aggregation and catalytic effect in the reaction of hydrolytic decomposition of phosphorous acid esters are studied for series of systems such as calixarene (CA)-sodium dodecyl sulfate (SDS), CA-poly(ethylene imine) (PEI), SDS-PEI, and CA-SDS-PEI. Using dynamic light scattering, sizes of aggregates are determined, cloud point is estimated, and catalytic effect is studied. Data obtained are consistent with the assumption of the joint aggregation in systems studied. The possibility of the passage from 20-fold inhibiting effect to the catalysis (higher than the order of magnitude) of studied reaction by the immobilization of anionic nanoaggregates (nanoreactors) on a catalytically active polymer matrix is demonstrated. The possibility of the passage from microheterogeneous to heterogeneous system and the separation of catalyst from the products are established.