Study of host–guest interaction between ß-cyclodextrin and alkyltrimethylammonium bromides in water

Cyclodextrins were found to play important roles in self-assembly systems of surfactants. The interactions between host molecule ß-cyclodextrin (CD) and model cationic surfactants, alkyltrimethylammonium bromides with different alkyl chain length: dodecyl-(C₁₂TAB), tetradecyl-(C₁₄TAB) and hexadecyl-(C₁₆TAB) are studied by means of conductivity measurements at 313.2 K. The data obtained indicate that inclusion complexes (CD:S⁺) had formed, and apparent critical micelle concentration (CMC*) is equivalent to the combined concentrations of surfactant monomers complexed with the CD and that of a free dissolved monomer in equilibrium with the micellized surfactant without CD. Inclusion complexes were characterized by an equilibrium binding constant K ₁₁, which value increases as the length of alkyl chains, and consequently the hydrophobicity, increases. From mathematical model the concentrations of the uncomplexed cyclodextrin, uncomplexed surfactant ion, and inclusion complex in the submicellar, as well as in the micellar range were calculated. The competition between the micellization and complexation processes leads to the existence of a significant concentration of free CD in equilibrium with the micellar aggregates. The percentage of uncomplexed cyclodextrin in equilibrium with the micelles is independent on cyclodextrin concentration for a particular ternary system and is 31, 37, and 34 % for C₁₂TAB/water/ß-CD, C₁₄TAB/water/ß-CD and C₁₆TAB/water/ß-CD, respectively. By using standard Gibbs free energy for micellization and surfactant complexation by CD, we can explain the observed behavior.     

New insights in cyclodextrin: surfactant mixed systems from the use of neutral and anionic cyclodextrin derivatives

The kinetics of the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) have been studied in mixed systems made up of surfactant, sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (TTABr), and cyclodextrin, beta-CD or SBE-beta-CD(Captisol). The use of SBE-beta-CD instead of beta-CD allowed us to indicate certain characteristics of the mixed cyclodextrin-surfactant system: (a) The percentage of uncomplexed cyclodextrin is higher for SBE-beta-CD than for beta-CD when we use SDS, but the opposite effect was observed when we use TTABr. This behavior can be explained by taking into account the increase in salinity when we add SBE-beta-CD, and the electrostatic forces between the SBE-beta-CD and the surfactant that have influence on the complexation. (b) The presence or even the charge of cyclodextrin has no effect on the properties of surfactant micelles once they have been formed; in particular, it does not alter K(s)(m) or k(m), parameters very sensitive to the micellar system structure. Therefore, we can conclude that for surfactants concentrations lower than the micellization point, the charge of cyclodextrin modifies the cyclodextrin-surfactant interactions but once the micelles have been formed there is no interaction between them and the cyclodextrins.     

Basic hydrolysis of crystal violet in beta-cyclodextrin/surfactant mixed systems

The basic hydrolysis of crystal violet (CV) in mixed systems consisting of beta-cyclodextrin (beta-CD) and a micelle-forming surfactant, cetyltrimethylammonium chloride (CTACl), has been studied. beta-CD was found to catalyze the basic hydrolysis of CV through the interaction of its hydroxyl group, in its deprotonated form, with the carbocation in the complexed substrate. The addition of small amounts of CTACl, with [CTACl] below the critical micelle concentration, to beta-CD solutions does not have an effect upon the observed rate constant for the basic hydrolysis of CV. This behavior is different from that observed for the alkaline hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide and nitrophenyl acetates in mixed beta-CD/cationic surfactant systems. The proposed mechanism allows us to explain the experimental results on the basis of the high percentage of uncomplexed beta-CD in equilibrium with the micellar system, the low CV concentration, and the high value for the binding constant of CV by beta-CD.     

Study of the interaction between a nonyl phenyl ether and beta-cyclodextrin: declouding nonionic surfactant solutions by complexation

Absorption and fluorescence measurements for aqueous solutions at 298 K containing pentaoxyethylene nonyl phenyl ether (NPE5), in the absence and presence of beta-cyclodextrin (beta-CD), were analyzed to determine the effect of the complexation on the aggregation of the surfactant. For the binary system, the appearance of a new emission band and the presence of an isoemissive point in the emission spectra at the time and frequency domains indicate the formation of an excimer within the micellar core. The addition of beta-CD induces the formation of an inclusion complex strong enough to break the aggregates and avoid the excimer formation. For the ternary system, the increase in fluorescence has been used to assess the binding constants of 1:1 + 2:1 stoichiometries. Static light scattering, 1H NMR diffusion-ordered spectroscopy (DOSY), and two-dimensional rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments were used to characterize the cloud point of NPE5 at 298 K, and to ascertain the effects of complexation on the clouding process. In the presence of beta-CD, the analysis of the 1H NMR spectra and the self-diffusion coefficients reveal the existence of interactions between the beta-CD and the aggregates that increase the cloud-point concentration more than expected. Under conditions of excess of beta-CD, ROE enhancements point to a complex of dominant 2:1 stoichiometry (beta-CD:NPE5) in which the hydrophobic moiety of the surfactant threads two beta-CDs.     

Enantioseparation of phenothiazines in cyclodextrin-modified micellar electrokinetic chromatography

In this study, enantioseparations of five phenothiazines in cyclodextrin (CD)-modified micellar electrokinetic chromatography (MEKC) were investigated using a citrate buffer containing tetradecyltrimethylammonium bromide (TTAB) as a cationic surfactant at low pH. Beta-cyclodextrin (beta-CD) and hydroxylpropyl-beta-CD (HP-beta-CD) were selected as chiral selectors. The results indicate that the separation window is greatly enlarged by beta-CD concentration and that the separability and selectivity of phenothiazines are remarkably influenced by the concentrations of both beta-CD and TTAB, as well as buffer pH. The interaction of thioridazine with beta-CDs is considerably reduced in the presence of TTAB micelles due to competitive complexation of thioridazine with TTAB micelles, which is pH-dependent. As a result, effective enantioseparation of thioridazine is simultaneously achievable with that of trimeprazine and promethazine or ethopropazine in MEKC with addition of either beta-CD or HP-beta-CD, respectively, to a micellar citrate buffer containing TTAB at pH 3.5. Better enantioresolution of thioridazine in MEKC than in capillary zone electrophoresis can be obtained.     

Synthesis and characterization of an amphiphilic cyclodextrin, a micelle with two recognition sites

A cyclodextrin derivative (Mod-CD) was synthesized through the monoesterification of beta-cyclodextrin (beta-CD) with 3-((E)-dec-2-enyl)-dihydrofuran-2,5-dione. The compound is an interesting surfactant that can form large aggregates not only through the interaction of the hydrophobic tails as in common amphiphilic compounds but also through the inclusion of the alkenyl chain into the cavity of another Mod-CD molecule. The self-inclusion of the chain in the cavity of cyclodextrin as well as the intermolecular inclusion was demonstrated by 1H NMR measurements that were able to detect methyl groups in three different environments. Besides, in the aggregates of Mod-CD, the cavity is available to interact with external guests such as phenolphthalein, 1-amino adamantane, and Prodan. Phenolphthalein has the same binding constant with Mod-CD and beta-CD, but the equilibrium constant for the interaction with Prodan is about 2 times larger for Mod-CD than for beta-CD. The latter result is attributed to the fact that this probe interacts with the micelle in two binding sites: the cavity of the cyclodextrin and the apolar heart of the micelle as evidenced by the spectrofluorimetric behavior of Prodan in solutions containing different concentrations of Mod-CD.     

Interactions between a nonionic gemini surfactant and cyclodextrins investigated by small-angle neutron scattering

The microstructure of complexes between hydroxypropyl-cyclodextrins (HPCDs) (alpha, beta, and gamma) and a novel gemini surfactant has been investigated by small-angle neutron scattering (SANS). This nonionic hetero-gemini surfactant (denoted NIHG750) contains two hydrophobic groups and two hydrophilic groups. One is a methyl-capped polyoxyethylene chain with 16 oxyethylene units and the other is a secondary hydroxyl group. Various form factor models have been considered for fitting the SANS data. Spherical aggregates (25 to 40 A) with a size slightly larger than that of NIHG750 micelles (about 23 A) appear in mixed systems. These could be micellar aggregates partly covered with a few cyclodextrin molecules. In addition, the results indicate rod formation (r approximately 8 A, L approximately 70 A) for the NIHG-HPCD complexes. This result is consistent with the threading of HPCDs onto NIHG750 to such an extent that the surfactant molecule takes an extended conformation at high levels of HPCD. Also, the results indicate that HPCDs may interact with the oxyethylene groups of the spherical micellar aggregates leading to an increase in micelle size and a gradual transformation to rod-shaped aggregates. The tendency to form rods increases in the order gamma-CD相似文献   

A capillary electrophoresis study on the influence of beta-cyclodextrin on the critical micelle concentration of sodium dodecyl sulfate

The influence of beta-cyclodextrin (beta-CD) on the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) was investigated by capillary electrophoresis using anionic chlorophenols as probe molecules at pH 7.0. The variations of the electrophoretic mobility of probe molecules as a function of surfactant concentration in both premicellar and micellar regions in the absence and presence of beta-CD was analyzed. The results indicate that, as a consequence of a strong inclusion complexation between beta-CD and SDS, the encapsulation of beta-CD with probe molecules is greatly diminished, or even vanished, in the presence of SDS. The complexes formed between beta-CD and SDS monomers exist predominantly in the form of a 1:1 stoichiometry, while the complexes with a 2:1 stoichiometry reported previously in the literature as a minor component may exist by less than 10%. The elevation of the CMC value of SDS depends not only on the concentration of beta-CD in the buffer electrolyte but also on methanol content in the sample solution. The binding constants of probe molecules to beta-CD, to surfactant molecules, and to the complexes formed between beta-CD and SDS are reported.     

Micellization kinetics with allowance for fussion and fission of spherical and cylindrical micelles: 1. Set of nonlinear equations describing slow relaxation

Based on the general kinetic equation that describes the aggregation and fragmentation of surfactant molecular aggregates, a closed set of nonlinear equations is derived for the slow relaxation of surfactant monomer concentration and the total concentrations of coexisting spherical and cylindrical micelles to the equilibrium state of a micellar solution. Both the transitions accompanied by the emission and capture of surfactant monomers by micelles and the transitions resulting from the fussion and fission of micelles, are taken into account. The derived set of equations describes all stages of the slow relaxation from the initial perturbance to the final equilibrium state of a micellar solution.     

Solubilization mechanism of vesicles by surfactants: effect of hydrophobicity

Simulations based on dissipative particle dynamics are performed to investigate the solubilization mechanism of vesicles by surfactants. Surfactants tend to partition themselves between vesicle and the bulk solution. It is found that only surfactants with suitable hydrophobicity are able to solubilize vesicles by forming small mixed micelles. Surfactants with inadequate hydrophobicity tend to stay in the bulk solution and only a few of them enter into the vesicle. Consequently, the vesicle structure remains intact for all surfactant concentrations studied. On the contrary, surfactants with excessive hydrophobicity are inclined to incorporate with the vesicle and thus the vesicle size continues to grow as the surfactant concentration increases. Instead of forming discrete mixed micelles, lipid and surfactant are associated into large aggregates taking the shapes of cylinders, donuts, bilayers, etc. For addition of surfactant with moderate hydrophobicity, perforated vesicles are observed before the formation of mixed micelles and thus the solubilization mechanism is more intricate than the well-known three-stage hypothesis. As the apparent critical micellar concentration (φ(s,v)(a,CMC)) is attained, pure surfactant micelles form and the vesicle deforms because the distribution of surfactant within the bilayer is no longer uniform. When the surfactant concentration reaches φ(s,v)(p), the vesicle perforates. The extent of perforation grows with increasing surfactant concentration. The solubilization process begins at φ(s,v) (sol), and lipids leave the vesicle and join surfactant micelles to form mixed micelles. Eventually, total collapse of the vesicle is observed. In general, one has φ(s,v)(a,CMC)≤φ(s,v)(p)≤φ(s,v)(sol).     

Power-law stage of slow relaxation in solutions with spherical micelles

General (independent of models selected for surfactant molecular aggregates) analytical relations are derived to describe the initial stage of slow relaxation in micellar solutions with spherical micelles. This stage precedes the final stage of the relaxation occurring via an exponential decay of disturbances with time. The relations obtained are applicable throughout the interval of micellar solution concentrations from the first to the second critical micellization concentration. It is shown that the initial stage is characterized by power laws of variations in the concentrations of monomers and micelles with time, these laws being different for the relaxation processes proceeding from above and below toward equilibrium values of micellar solution parameters. Relations are derived for the duration of this stage, and the effect of initial conditions is studied. Characteristic times of the power-law stage are determined and compared with the characteristic time of the final exponent-law relaxation stage. The behavior of these times is investigated at surfactant solution concentrations in the vicinity of, and noticeably above, the first critical micellization concentration. On the basis of the droplet and quasi-droplet thermodynamic models of surfactant molecular aggregates, numerical solutions are found for nonlinearized equations of slow relaxation for the time dependence of surfactant monomer concentrations at all stages of the slow relaxation. Numerical results obtained from the models are compared with the results of a general analytical study.     

Thermodynamic and Kinetic Foundations of the Micellization Theory: 5. Hierarchy of Kinetic Times

The characteristic kinetic times of micellization in the solution of a nonionic surfactant: the times of establishment of quasi-equilibrium concentrations of molecular aggregates in micellar, subcritical, and overcritical regions, times of establishment of quasi-equilibrium concentrations of molecular aggregates in the near-critical region of their sizes, the average time between two successive acts of emission of surfactant monomers by a micelle, the average value of micelle lifetime, the time of establishment of quasi-stationary mode of matter exchange between the solution and molecular aggregate, as well as the times of fast and slow relaxation in a solution were analyzed. The hierarchy of these times disclosing complex multistage kinetic process of micelle formation and decomposition and the establishment of equilibrium in the micellar solution was revealed. It was shown that this hierarchy is provided by the small parameters of the kinetic theory. The inverse problem of micellization kinetics was discussed; this problem allows us to find the characteristics of the formation work for micellar aggregate from the experimental data on the relaxation time of micellar solution.     

Aqueous sodium dehydrocholate-sodium deoxycholate mixtures at low concentration

The behavior of the sodium dehydrocholate (NaDHC)-sodium deoxycholate (NaDC) mixed system was studied by a battery of methods that examine effects caused by the different components of the system: monomers, micelles, and both components. The behavior of the mixed micellar system was studied by the application of Rubingh's model. The obtained results show that micellar interaction was repulsive when the aggregates were rich in NaDHC. The gradual inclusion of NaDC in micelles led to a structural transformation in the aggregates and the interaction became attractive. The bile salts' behavior in mixed monolayers at the air-solution interface was also investigated. Mixed monolayers are monotonically rich in NaDC, giving a stable and compact adsorbed layer. Results have shown that the interaction in both micelles and monolayer is not ideal and such behavior is assumed to be due to a structural factor in their hydrocarbon backbone.     

Inhibition of the beta-cyclodextrin catalyzed dediazoniation of 4-nitrobenzenediazonium tetrafluoroborate. blocking effect of sodium dodecyl sulfate

We have investigated the effects of sodium dodecyl sulfate, SDS, on the reaction between 4-nitrobenzenediazonium, 4NBD, ions and beta-cyclodextrin, beta-CD, under acidic conditions at T = 60 degrees C by employing a combination of spectrophotometric, chromatographic, and conductometric techniques. Previous studies under acidic conditions indicate that the secondary -OH groups of beta-CD solvate 4NBD ions, which are included in the beta-CD cavity, leading to the formation of a highly unstable transient diazo ether complex that undergoes homolytic fragmentation with an observed rate constant about 1700 times higher than that in pure aqueous acid solution (t(1/2) = 6 h at T = 60 degrees C) when [beta-CD]/[4NBD] = 40. Addition of SDS to a 4NBD/beta-CD system makes the k(obs) values decrease up to its value in a SDS micellar solution, which is similar to that in aqueous acid solution. Dediazoniation product distribution is significantly affected; the reaction between 4NBD and beta-CD ([beta-CD]/[4NBD] = 40), in the absence of SDS, proceeds exclusively through a homolytic mechanism leading to the quantitative formation of nitrobenzene, ArH, but addition of SDS turns over the mechanism by promoting the heterolytic mechanism. In addition, mixtures of 4-nitrophenol, ArOH, and ArH dediazoniation products are formed; their relative yields depend on the amount of added SDS so that at very high [SDS(T)], the heterolytic mechanism becomes the predominant one. Results are consistent with conductometric measurements showing that addition of beta-CD to an aqueous surfactant solution inhibits micelle formation and elevates CMC(app) values because CD encapsulation of surfactant monomers competes with the micellization process and are interpreted in terms of SDS monomers blocking the beta-CD cavity by forming a nonreactive complex, releasing 4NBD to the bulk solution.     

Kinetic study for the inclusion complex of carboxylic acids with cyclodextrin by the ultrasonic relaxation method

Ultrasonic absorption coefficients in the frequency range of 0.8-95 MHz were measured in aqueous solutions containing both beta-cyclodextrin (beta-CD) (host) and butanoic acid (in its dissociated form and undissociated one) (guest). A single relaxational phenomenon was observed only when the solutes were coexisting, although no relaxation was found in the beta-CD solution or in the acid solutions. The absorption was also measured in a solution of pentanoic acid (dissociated form) with beta-CD, and single relaxation was detected. The ultrasonic relaxation observed in these solutions was due to a perturbation of a chemical equilibrium related to a reaction of an inclusion complex formed by the host and guest. The equilibrium constant was obtained from the dependence of the maximum absorption per wavelength on the guest concentration. The rate constant for the inclusion process of the guest into a cavity of beta-CD and that for the leaving process from the cavity were determined from the obtained relaxation frequency and the equilibrium constant. The standard volume change of the reaction was also computed from the maximum absorption per wavelength. These results were compared with those in solutions containing both beta-CD and different guest molecules. It was found that the hydrophobicity of guest molecules played an important role in the formation of the inclusion complex and also that the charge on the carboxylic group had a considerable effect on the kinetic characteristics of the complexation reaction.     

Thermodynamic and Kinetic Foundations of the Micellization Theory: 4. Kinetics of Establishment of Equilibrium in a Micellar Solution

A system of the kinetic equations of the material balance for the concentrations of surfactant monomers and micelles in a micellar nonionic surfactant solution was formulated. The equilibrium state of a materially isolated micellar solution was analyzed. The system of the kinetic equations of the material balance of a micellar solution was solved. The total time of the establishment of equilibrium in a micellar solution was determined. It was shown that this time increases or (typically) decreases with an increase in micelle concentration, depending on the degree of micellization.     

Micellization of Sodium Dodecyl Sulfate in Glycerol Aqueous Mixtures

The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles.     

Breakdown kinetics of aggregates from poly(ethylene glycol‐bl‐propylene sulfide) di‐ and triblock copolymers induced by a non‐ionic surfactant

We explored the effects of addition of the nonionic surfactant Triton X‐100 on the stability of aggregates of poly(ethylene glycol‐bl‐propylene sulfide) di‐ and triblock copolymers. Fluorescence spectra of pyrene, used as a probe molecule, elucidated the various stages of transformation from pure copolymeric micelles to surfactant‐rich micelles. Turbidity measurements yielded insight into the mechanism of the interaction, the hydrophobicity of the copolymer driving the process. Triton X‐100 tends to strongly interact with highly hydrophobic copolymers by inserting into the core of the micellar aggregates. On the other hand, Triton X‐100 tends to interact with the corona of micelles formed by less hydrophobic copolymers which, for this reason, are more stable upon addition of this destabilizing agent. Kinetic data give evidence that only monomers, not micelles of surfactant, interact with the copolymer micelles. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2477–2487, 2008     

Assembly behavior of inclusion complexes of beta-cyclodextrin with 4-hydroxyazobenzene and 4-aminoazobenzene

To further reveal the factors governing the supramolecular assembly of beta-cyclodextrin (beta-CD) inclusion complexes, two aggregates (1 and 2) were prepared from the inclusion complexes of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene, respectively, and their binding behavior were investigated by means of X-ray analysis, UV-vis, NMR, and circular dichroism spectra in both solution and the solid state. The obtained results indicated that the beta-CD/4-hydroxyazobenzene complex 1 could form head-to-head dimers (triclinic system, space group P1) in the solid state, which were further self-assembled to a linear supramolecular architecture by the intra- and interdimer hydrogen bond interactions as well as the intradimer pi-pi interactions. However, when the included guest 4-hydroxyazobenzene was switched to a 4-aminoazobenzene, the resultant beta-CD/4-aminoazobenzene complex 2 (monoclinic system, space group P2(1)) could be self-assembled to a wave-type supramolecular aggregate under similar conditions. Furthermore, the combination of crystallographic and spectral investigations jointly revealed the inclusion complexation geometry of beta-CD with 4-hydroxyazobenzene and 4-aminoazobenzene in both solution and the solid state, which demonstrated that the disparity of substituents in the azobenzenes played an important role in the inclusion complexation and molecular assembly, affecting not only the structural features of aggregates but also the binding abilities of azobenzenes with beta-CD.