Premicellar Aggregates Research Articles

Molecular dynamics study of salt influence on transport and structural properties of SDS micellar solutions

The results of all-atom molecular dynamics simulations of micellization in the salt-free sodium dodecyl sulphate (SDS) aqueous solution and in the solution with added 1:1 salt (sodium chloride) are reported. Micellar systems of different size are considered in order to study size effects. The influence of added salt on transport and structural properties of the SDS micellar solutions is investigated. The formation of a larger aggregate in the presence of the salt, compared to the salt-free surfactant solution, is observed. With the increase of the salt concentration the diffusion coefficients of coions, counterions, and water molecules decrease. The diffusivity of a premicellar aggregate demonstrates the non-linear dependence on the salt concentration, while the size of the aggregate remains constant. Along with transport properties, we also studied structural properties of the SDS micellar solution: aggregation numbers and sizes of surfactant aggregates, radial distributions of ions and various atoms.

Steroidal Surfactants: Detection of Premicellar Aggregation, Secondary Aggregation Changes in Micelles, and Hosting of a Highly Charged Negative Substance.

CHAPSO and CHAPS are zwitterionic surfactants derived from bile salts which are usually employed in protein purification and for the preparation of liposomes and bicelles. Despite their spread use, there are significant discrepancies on the critical concentrations that determine their aggregation behavior. In this work, we study the interaction between these surfactants with the negative fluorescent dye pyranine (HPTS) by absorbance, fluorescence, and infrared spectrometry to establish their concentration-dependent aggregation. For the studied surfactants, we detect three critical concentrations showing their concentration-dependent presence as a monomeric form, premicellar aggregates, micelles, and a second type of micelle in aqueous medium. The nature of the interaction of HPTS with the surfactants was studied using analogues of their tails and the negative bile salt taurocholate (TC) as reference for the sterol ring. The results indicate that the chemical groups involved are the hydroxyl groups of the polar face of the sterol ring and the sulfonate groups of the dye. This interaction causes not only the incorporation of the negative dye in CHAPSO and CHAPS micelles but also its association with their premicellar aggregates. Surprisingly, this hosting behavior for a negative charged molecule was also detected for the negative bile salt TC, bypassing, in this way, the electrostatic repulsion between the guest and the host.

Static and dynamic surface tension behaviour of a triblock copolymer and a non ionic surfactant mixture

Abstract Static and dynamic surface tension measurements have been carried out on mixtures of a commonly used block copolymer Pluronics ® P123 and a non-ionic surfactant polyoxyethelene (9) lauryl ether (C 12 E 9 ) to understand the block polymer-surfactant interactions. The effects of the interaction on the adsorption and micellisation behaviour have also been studied. Three different compositions comprising 3:1, 1:1 and 1:3 molar ratios of Pluronic P123 and C 12 E 9 have been investigated and the results are compared with the data of pure P123 and C 12 E 9 solutions. It has been observed that the nature and magnitude of the polymer surfactant interactions depend on the composition of the mixtures. The mixture containing 3:1 molar ratio of P123 and C 12 E 9 shows synergistic effect with a negative value for the interaction parameter (β), as obtained from the analysis of surface tension data using Rubingh’s model. However, for the other two compositions (i.e., 1:1 and 1:3 molar ratios), the equilibrium surface tension data show premicellar adsorption and aggregation of the surfactant molecules on the polymer chain. The dynamic surface tension (DST) results indicate that the strong polymer surfactant interactions, as reflected in the equilibrium surface tension data, do not modify the DST behaviour significantly from that of pure components. This is attributed to the dominant effect of the polymer adsorption at the interface, during the short time scale studied.

Effects of added salts on adsorbed film and premicelle formation of crown ether surfactants

Abstract Two crown ether surfactants, dodecanoyloxymethyl- (C11Φ5) and octanoyloxymethyl-15-crown-5 (C7Φ5), were synthesized and the concentration dependence of surface tension of their aqueous solutions was measured in the absence and the presence of various salts to study the effect of coexistent cation on adsorbed film and aggregation behavior of crown ether surfactants. In the absence of salt, the surface tension vs. concentration curves had two break points for both C11Φ5 and C7Φ5. The break point at the higher concentration (m0) was confirmed to be attributed to the micelle formation by the solubilization of water-insoluble dye and the concentration dependence of proton chemical shifts. Another break point at the lower concentration (mI) would be caused by the formation of premicellar aggregates. For C11Φ5 in the presence of KCl two break points were also observed. At about 0.05 mol kg−1 KCl, m0 and mI were minimized. The cause would be that the salting-out effect of KCl dominates below 0.05 mol kg−1 but above it the repulsive interaction between charged head groups of C11Φ5 complex with K+ becomes larger than it. Two break points were also observed for C11Φ5 in the presence of LiCl, NaCl, CsCl and NH4Cl of 0.22 mol kg−1. It has been found that the critical micelle concentration (m0) are the highest for Na+ having an ionic diameter comparable to the hole size of 15-crown-5 ring and decrease as an ionic diameter of added cation departs from the hole size. The aggregation number of premicelle, which was estimated from the surface excesses at mI and m0 on the assumption of two step micellization, has been about 4 in the absence of salt, about 3 for Li+ hydrated strongly and about 2 for all other cations.

Lysine-based oligomeric surfactants with cyanuric chloride: synthesis and micellization properties

Two lysine-based oligomeric surfactants 2C12LyNa2 and 3C12LyNa3 were synthesized using cyanuric chloride and Ne-lauroyl-l-lysine. Their physicochemical properties of aqueous solutions were determined by surface tension and conductivity as well as dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. From the results of surface tension and conductivity measurements, the critical micelle concentration decreases by one order of magnitude with increasing degree of oligomerization. 3C12LyNa3 and 2C12LyNa2 tend to form premicellar aggregates in solution at a sufficiently low concentration. In addition, their interface behaviors such as adsorption and micellization are reflected by parameters such as the minimum surface area per molecule (Amin), adsorption efficiency and effectiveness (pC20), maximum surface excess concentration (Γmax), and standard free energy for micellization and adsorption (ΔGmic∘, ΔGads°). The Amin value of 2C12LyNa2 and 3C12LyNa3 is two times that of the monomeric surfactant sodium Ne-lauroyl-l-lysine (C12LyNa). With increasing degree of oligomerization, pC20 increases, which follows the reverse order for ΔGmic° and ΔGads°. TEM results showed that C12LyNa forms slightly entangled rod-like micelles. 2C12LyNa2 and 3C12LyNa2 can self-assemble into regular spherical assemblies; DLS showed that with increasing degree of oligomerization for 2C12LyNa2 and 3C12LyNa3, the diameters of the spherical aggregates decrease and the hydrophobic interaction and hydrogen bonding become stronger.

Fluorescence of N-acylated dansylamide with a long hydrophobic tail: sensitive response to premicellar aggregation of sodium deoxycholate.

The present work describes the synthesis and photophysical studies of two fluorescent dansylamide derivatives, in which the amine group is acylated by a long hydrophobic chain (a part of a biologically relevant palmitic acid) and by a short hydrophobic tail (a part of acetic acid). The long chain tethered dansyl analogue is successfully utilized in estimating critical micellar concentration (CMC) of bile salts (NaDC, NaC) as well as anionic and cationic surfactants (SDS, CTAB) with the help of enhanced fluorescence intensity facilitated by better solubilization of the molecule in microheterogeneous media. The long chain tethered dansylamide derivative shows significant fluorescence solvatochromism with a red shift (ca. 4000 cm(-1)) from hexane to water. In contrast, the solvatochromism exhibited by the parent/short acyl chain analogue is much less (ca. 2230 cm(-1) from hexane to water) and the fluorescence is not sensitive to micellization. Interestingly, the long chain tethered fluorescent probe shows high sensitivity towards premicellar aggregation of sodium deoxycholate (NaDC) bile salt, through a clear blue shift of emission maxima and concomitant enhancement of fluorescent intensity. Such an observation of fluorescence sensing of premicellar aggregation is unusual.

Denaturant effects on HbGp hemoglobin as monitored by 8-anilino-1-naphtalene-sulfonic acid (ANS) probe

Glossoscolex paulistus extracellular hemoglobin (HbGp) stability has been monitored in the presence of denaturant agents. 8-Anilino-1-naphtalene-sulfonic acid (ANS) was used, and spectroscopic and hydrodynamic studies were developed. Dodecyltrimethylammonium bromide (DTAB) induces an increase in ANS fluorescence emission intensity, with maximum emission wavelength blue-shifted from 517 to 493nm. Two transitions are noticed, at 2.50 and 9.50mmol/L of DTAB, assigned to ANS interaction with pre-micellar aggregates and micelles, respectively. In oxy-HbGp, ANS binds to protein sites less exposed to solvent, as compared to DTAB micelles. In DTAB–HbGp–ANS ternary system, at pH 7.0, protein aggregation, oligomeric dissociation and unfolding were observed, while, at pH 5.0, aggregation is absent. DTAB induced unfolding process displays two transitions, one due to oligomeric dissociation and the second one, probably, to the denaturation of dissociated subunits. Moreover, guanidine hydrochloride and urea concentrations above 1.5 and 4.0mol/L, respectively, induce the full HbGp denaturation, with reduction of ANS-bound oxy-HbGp hydrophobic patches, as noticed by fluorescence quenching up to 1.0 and 5.0mol/L of denaturants. Our results show clearly the differences in probe sensitivity to the surfactant, in the presence and absence of protein, and new insights into the denaturant effects on HbGp unfolding.

Specific interactions of anions and pre-micelles in the alkaline fading of crystal violet carbocation

Abstract The specific interactions of inorganic anions (Cl−, Br−, I− and SO42−) and pre-micelles in the alkaline fading of crystal violet carbocation (CV+) have been studied in the temperature range 303–318 K. The pseudo-first order rate constant decreases with increasing the concentration of anions and ultimately reaches a limiting value. Cl− and Br− ions appear to exert simple salt effects on the reaction rate, while larger ion I− and bivalent ion SO42− exert some specific electrostatic interaction effects on the CV carbocation. This salt effect on the reaction rate has been rationalized in terms of Olson–Simonson (O–S) model of ion pair formation. The same reaction has been studied in the presence of two anionic surfactants, namely, sodium dodecyl sulfate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at concentrations well below their CMC. A sharp decrease in the rate constant with increasing surfactant concentration suggests a pre-micellar aggregation between the CV+ ion and SDS/AOT monomers. The results have been explained using O–S model as well as Piszkiewicz cooperativity model. The single hydrocarbon tail in SDS binds favorably to the CV+ in the pre-micelle whereas in AOT the presence of two hydrocarbon tails makes the binding less favorable due to some steric crowding. The cooperativity index (n) values in the pre-micelle cluster suggest the formation of a 1:1 and 1:2 dye-surfactant ion pair. In AOT pre-micelle the nucleophile HO− gets easier access to the reaction center leading to a lower energy of activation and higher rate of reaction than those in case of SDS.

Spectroscopic investigation of the binding interactions of a membrane potential molecule in various supramolecular confined environments: contrasting behavior of surfactant molecules in relocation or release of the probe between nanocarriers and DNA surface.

The fluorescence and optical properties of membrane potential probes are widely used to measure cellular transmembrane potentials. Hemicyanine dyes are also able to bind to membranes. The spectral properties of these molecules depend upon the charge shift from the donor moiety to the acceptor moiety. Changes in their spectral properties, i.e. absorption and emission maxima or intensities, are helpful in characterizing model membranes, microheterogeneous media, etc. In this article, we have demonstrated the binding interaction of a membrane potential probe, 1-ethyl-2-(4-(p-dimethylaminophenyl)-1,3-butadienyl)-pyridinium perchlorate (LDS 698), with various supramolecular confined environments. The larger dipole moment in the ground state compared to the excited state is a unique feature of hemicyanine dyes. Due to this unique feature, red shifts in the absorption maxima are observed in hydrophobic environments, compared with bulk solvent. On addition of surfactants and CT DNA to an aqueous solution containing LDS 698, significant increase in the emission intensity along with the quantum yield and lifetime indicate partition of the probe molecules into organized assemblies. In the case of the sodium dodecyl sulfate (SDS)-water system, due to interactions between the cationic LDS 698 and the anionic dodecyl sulfate moiety, the fluorescence intensity at ∼666 nm decreases and an additional peak at ∼590 nm appears at premicellar concentration (∼0.20 mM-4.50 mM). But at ∼5.50 mM SDS concentration, the absorbance in the higher wavelength region increases again, indicating encapsulation of the probe in micellar aggregates. This observation indicates that the premicellar aggregation behavior of SDS can also be judged by observing the changes in the UV-vis and fluorescence spectral patterns. The temperature dependent study also indicates that non-radiative deactivation of the dye molecules is highly restricted in the DNA micro-environment, compared with micelles. Besides, we have also investigated the specific interaction of surfactant micelles with DNA. Our observations reveal that, in the presence of CT DNA, LDS 698 interacts exclusively with SDS micelles, but that it preferentially releases from micelles and relocates to DNA surfaces in solutions containing TX-100 micelles.

Evidence of co-operativity in the pre-micellar region in the hydrolytic cleavage of phenyl salicylate in the presence of cationic surfactants of CTAB, TTAB and CPC

The effects of cationic surfactants of cetyl trimethyl ammonium bromide (CTAB), tetradecyl trimethyl ammonium bromide (TTAB) and cetyl pyridinium chloride (CPC) on the kinetics of intramolecular general base catalyzed hydrolysis ([OH−] range 0.05–0.1molL−1) of phenyl salicylate have been studied at different temperatures. The rate is independent of [OH−] in the studied range. The anionic surfactant sodium dodecyl sulphate (SDS) has no effect on the rate. The presence of small amount of any of these cationic surfactants well below its critical micelle concentration markedly inhibits the rate of reaction suggesting a pre-micellar aggregation between the substrate and surfactant monomers. The kinetic data have been analyzed in terms of earlier reported models (Piszkiewicz's co-operativity model and Raghavan and Srinivasan's model) for micellar catalysis. The binding constants between the substrate and the surfactants evaluated from the two models are in good agreement. Three dimensional structure of the pre-micellar aggregate controls the approach of the nucleophile water molecule to the reaction center. The planar structure of the pyridinium head group of CPC provides less steric hindrance to the attacking water molecule that leads to the least enthalpy of activation for CPC among the three surfactants. The association between the negatively charged substrate and the cationic surfactant is favored owing to electrostatic as well as hydrophobic interactions. The binding between the substrate and pre-micelles follows the order: CPC>TTAB>CTAB.

Adsorption and Premicellar Aggregation of CTAB Molecules and Fabrication of Nanosized Platinum Lattice on the Glass Surface

Premicellar aggregation processes were investigated in a wide range of concentrations and temperatures of cetyltrimethylammonium bromide (CTAB) aqueous solutions. Two independent techniques were involved to study adsorption and aggregation of CTAB molecules at the glass/solution interface. Electronic spin resonance (ESR) was used to estimate microviscosity properties through the reorientation correlation time τc of (2,2,6,6-tetramethylpiperidin-1-oxyl), while atomic force microscopy (AFM) was involved to evaluate the CTAB molecule morphology at the glass/solution interface. In the dependence of τc vs the CTAB concentration three discontinuities were revealed within 0.2–0.5, 0.5–1.02, and 1.02–1.1 mM narrow concentration ranges, which are probably connected with the formation of bilayer and hemispherical, hemicylindrical, cylindrical, and spherical admicelles. The images of some of them at the glass surface have been independently obtained by AFM. One-dimensional thin layer (2 nm) of Pt parallel strips on a glass surface have been synthesized by chemical vapor deposition of the Pt on the surface micellar CTAB linear templates followed by washing of the latter.

Surfactant selection principle for reducing critical micelle concentration in mixtures of oppositely charged gemini surfactants.

Cationic quaternary ammonium gemini surfactants C(n)H(2n+1)(CH3)2N(+)CH2CHCHCH2(CH3)2N(+)C(n)H(2n+1)2Br(-) (C(n)C4C(n), n = 12, 8, 6) with alkyl spacers, C(n)H(2n+1)(CH3)2N(+)CH2CHOHCHOHCH2(CH3)2N(+)C(n)H(2n+1)2Br(-) (C(n)C4(OH)2C(n), n = 12, 8, 6, 4) with two hydroxyl groups in alkyl spacers, and cationic ammonium single-chain surfactants C(n)H(2n+1)(CH3)2N(+)Br(-) (C(n)TAB, n = 12, 8, 6) have been chosen to fabricate oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12H25N(CH2CH2CH2SO3(-))CH2CH2CH2(CH3)2N(CH2CH2CH2SO3(-))C12H252Na (C12C3C12(SO3)2). Surface tension, electrical conductivity, and isothermal titration microcalorimetry (ITC) were used to study their surface properties, aggregation behaviors, and intermolecular interactions. The mixtures of C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 12, 8) and C12C3C12(SO3)2/C12C4C12 show anomalous larger critical micelle concentration (CMC) than C12C3C12(SO3)2, while the mixtures of C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 6, 4), C12C3C12(SO3)2/C(n)C4(OH)2C(n) (n = 6, 4), and C12C3C12(SO3)2/C(n)TAB (n = 12, 8, 6) exhibit much lower CMC than C12C3C12(SO3)2. The results indicate that strong hydrophobic interactions between the alkyl chains assisted by strong electrostatic attractions between the headgroups and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution, resulting in the increase of CMC. If these interactions are weakened or inhibited, less surface active premicellar aggregates are no longer formed in the mixtures, and thus the CMC values are reduced. The work reveals that the combination of two surfactants with great self-assembling ability separately may have strong intermolecular binding interactions; however, their mixtures do not always generate superior synergism properties. Only moderate intermolecular interaction can generate the strongest synergism in CMC reduction.

Kinetics of micellisation and relaxation of cylindrical micelles described by the difference Becker–Döring equation

A numerical description of micellisation and relaxation to an aggregate equilibrium in a nonionic surfactant solution with spherical premicellar aggregates and stable polydisperse cylindrical micelles is presented for a wide interval of total surfactant concentrations and initial conditions. The Smoluchowsky-type model for the attachment-detachment rates of surfactant monomers to and from surfactant aggregates with matching rates for small spherical premicellar aggregates and the rates for larger cylindrical micelles have been used. The full discrete spectrum of characteristic times of micellar relaxation and the first three relaxation modes in their dependence on the equilibrium monomer concentration have been computed using the linearized form of the Becker-Döring difference equations. The overall time behavior of the surfactant monomer and aggregate concentrations in micellisation and relaxation at large initial deviations from the final equilibrium has been studied with the help of nonlinearized discrete Becker-Döring kinetic equations. The studies demonstrate a possibility for non-monotonic evolution of the monomer concentration at the initial stages. A comparison of the computed results with the analytical ones known from solutions of the linearized and nonlinearized differential Becker-Döring kinetic equations demonstrates a general agreement at higher concentrations of the surfactant above the critical micellar concentration.

Tailoring strained oxanorbornane headgroups to dimensionally controlled nanostructures through hydrogen bonding

An amphiphilic oxanorbornane skeletal frame has been efficiently organized to direct the formation of dimensionally controllable, geometry-specific nanostructures as diverse as 0-D surface/reverse micelles, 1-D nanofibers, 2-D rectangular/square sheets, and 3-D flowers. The racemate with its precise stereo-projection of the –OH groups and the ring oxygen could be used for Li+ ion sensing and aided in the formation of pre-micellar aggregates of conventional surfactants.

Probing the micellar solubilisation and inter-micellar exchange of polyphenols using the DPPH[rad] free radical

Encapsulation of polyphenols can be used for improving their stability and targeting. We present here a spectrophotometric method to probe the micellar solubilisation and inter-micellar exchange of polyphenols using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical as a visible probe. Our method relies on the partitioning of DPPH into micelles, on the reduction of DPPH by polyphenols, and on the change in absorbance of DPPH when reduced/oxidised. Hence, an absorbance drop at 528nm gives evidence of the co-localisation of polyphenols and DPPH in micelles. Using catechin and sodium dodecyl sulfate (SDS) as model molecules, we have shown that the reduction stoichiometry increases up to the critical micelle concentration (CMC) of SDS, where it reaches a plateau: this is due to the solubilisation of catechin in pre-micellar aggregates and then in micelles. The initial rate of reduction increases with increasing SDS concentration up to the CMC and then decreases due to a dilution effect.

Stabilization of diketo tautomer of curcumin by premicellar cationic surfactants: A spectroscopic, tensiometric and TD-DFT study

The interaction of aqueous curcumin with cationic surfactants of varying chain lengths and head group, stabilizing its β-diketo tautomer and exhibiting an UV band around 355nm, has been studied in buffered aqueous solutions in the pH range of 2.00–7.00 by UV–visible and fluorescence spectroscopy, and surface tensiometry under optimal experimental conditions of submicellar concentration of the surfactants. Time Dependent Density Functional Theory (TD-DFT) has also been used to predict the molecular structure and excitation energies of the interaction product in the ground state and the nature of the interaction. As the surfactant concentration increases, the curcumin–cationic complex forms premicellar aggregates where the curcumin exists in a highly polar microenvironment before shifting its location to the nonpolar core of the surfactant micelles. The strength of the interaction increases with the chain length of the surfactant and also changes on changing the head group of the surfactant. An observed secondary salt effect on the interaction indicates an unusual involvement of a proton in the interaction. Based on the experimental and theoretical evidences the following mechanism has been proposed for the stabilization of the β-diketo form: on the approach of the cationic surfactant, a protonation of the methylenic central carbon atom of curcumin takes place simultaneously breaking the π-conjugation and facilitating the detachment of the enolic proton paving the way for an ion dipole binding between the cationic head group and the electron-rich β-diketo oxygen atoms. Hydrophobic interaction between the tail of the surfactant and the nonionic curcumin molecule assists the electrostatic interaction.

Polymeric Premicelles as Efficient Lipophilic Nanocarriers: Extending Drug Uptake to the Submicellar Regime

A multitechnique investigation on the self-assembly behavior of a biocompatible polymer in the high dilution regime is reported herein. The obtained results unambiguously reveal the existence of premicellar structures that may further extend the efficiency of traditional polymeric micelles as drug-delivery vehicles. Such an expansion in the excipient capacity arises from (i) the increased drug retention of submicellar assemblies due to their higher resistance to dilution and therefore to their improved circulation time and (ii) the superior carrier permeability of these premicellar aggregates as a result of their smaller size, which makes these drug vehicles more effectively targeted to the tumors through the so-called enhanced permeability and retention effect. The uptake ability of the polymeric premicelles described in this work has been tested through the use of Nile Red as drug model given its intermediate lipophilicity (log P ≈ 3-5) similar to that of potent chemotherapy agents and its microenvironment-sensitive fluorescence properties relevant for localization purposes. Thus, it has been found that an efficient drug encapsulation can be achieved under conditions well below the normally required critical micelle concentration. These results may constitute a promising strategy in order to develop new and more efficient polymeric formulations in drug delivery technology.

Thermodynamic properties of new gluconamide-based cationic surfactants in aqueous solution: Experimental and modeling approaches

The micellization processes of 2-(alkyldimethylammonio)ethylgluconamide bromides (CnGAB), dodecyltrimethylammonium bromide (DTAB) and dodecylethyldimethylammonium bromide (DEDAB) were studied using solution conductivity and isothermal titration calorimetry (ITC). The critical micelle concentrations (CMC), the degree of counterion binding (β), the enthalpies (ΔHm) and entropies (ΔSm) of micellization as well as the contributions of the headgroups to Gibbs free energies (ΔGm0(hy)) were calculated. Approximate relative partial enthalpies (L2) were evaluated as functions of concentration from the calorimetric curves. In comparison with DTAB, the hydrophobicity of C12GAB is reduced by an extent corresponding to the shortening of DTAB’s alkyl chain by one methylene group. The micelles of C12GAB are hydrated by one methylene group deeper than those of DTAB. The free energy of the transfer of the headgroup from the bulk solution to the micelle is more favorable for DTAB than C12GAB. This effect is approximately equal to the ΔGm0(CH2) contribution. In contrast to the usual ionic surfactant behavior, the degree of counterions binding for CnGABs decreases with increasing alkyl chain length. This is supposed to be a consequence of screening of the charged layer of the micelle by the large, hydrated and hydrogen-bonded headgroups from the bromide anions in bulk solution. The computational modeling made it possible to relate the surfactants’ molecular properties to the interactions the surfactants enter into, suggesting that, due to the strongly polar character of their headgroup, CnGABs form small premicellar aggregates.

Micellar and pre-micellar aggregates of oxyethylated calixarenes studied by ESR of spin probes and cyclic voltammetry

The critical micelle formation concentrations of oxyethylated calixarenes 9CO8 and 9CO16 were determined by the spin probe ESR method. A decrease in the current passing through the system was revealed during the electrochemical oxidation of the probe in the presence of 9CO8. The current decrease depends on the amphiphile concentration and is due to the fact that the surface of micellar and pre-micellar aggregates impedes the access of charged particles to the probe. The pre-micellar aggregation phase was found by ESR and electrochemistry of spin probes. The concentrations of amphiphilic substances, at which structural rearrangements for pre-micellar aggregation are observed, were elucidated.

Fluctuations Near the Critical Micelle Concentration. I. Premicellar Aggregation, Relaxation Rate, and Isentropic Compressibility

A thermodynamic model including fluctuations of micelle sizes has been derived to describe solution properties of amphiphile systems close to the critical micelle concentration. Owing to the consideration of an affinity field in the free energy of the system, the model is capable of featuring experimental findings that are incorrectly reflected by established theories of the micelle formation and disintegration kinetics. In conformity with experiments, the thermodynamic theory predicts the onset of micellar structure formation already at amphiphile concentrations below the critical micelle concentration. It also applies well for the distinctive concentration dependency of the relaxation rate of monomer exchange and likewise for the sound velocity and thus the isentropic compressibility variations with concentration. Comparison of the theoretical predictions is made with special emphasis to short-chain surfactant systems, which allowed for reliable measurements in the range below the critical micelle concentration.