Micellization-induced deprotonation of thermoresponsive surfactant CAE-85 — the telechelic carboxylic group derivative of Pluronic P85

Temperature-induced micellization of CAE-85, a carboxylic acid end-standing derivative of a triblock copolymer Pluronic P85, was studied by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FTIR) and density functional theory (DFT) model calculations. It was found that in polymer micelles carboxyl end groups dissociated and it was a two stage process. The first stage of deprotonation appeared with the onset of micellization and it was in agreement with predictions of existing models and theories for ionization processes in micellar corona. In micelles well above the critical micellization temperature, the degree of CAE-85 deprotonation increased further to values significantly higher compared to unimer solution. It is proposed that such deprotonation correlates with the formation of hydrogen bonded carboxyl end groups enabled by sufficient density of chains in the corona of developing micelles. It was demonstrated that the proton dissociation constant, pK_m, specific to the micellar form existed and was different from the proton dissociation constant of solution of unimers, pK_a.     

Bending Energetics of Tablet‐Shaped Micelles: A Novel Approach to Rationalize Micellar Systems

A novel approach to rationalize micellar systems is expounded in which the structural behavior of tablet‐shaped micelles is theoretically investigated as a function of the three bending elasticity constants: spontaneous curvature (H₀), bending rigidity (k_c), and saddle‐splay constant (k?_c). As a result, experimentally accessible micellar properties, such as aggregation number, length‐to‐width ratio, and polydispersity, may be related to the different bending elasticity constants. It is demonstrated that discrete micelles or connected cylinders form when H₀>1/4ξ, where ξ is the thickness of a surfactant monolayer, whereas various bilayer structures are expected to predominate when H₀<1/4ξ. Our theory predicts, in agreement with experiments, a transition from discrete globular (tablet‐shaped) micelles to a phase of ordered, or disordered, connected cylinders above a critical surfactant concentration. Moreover, a novel explanation for the mechanism of growth, from small globular to long rodlike or wormlike micelles, follows as a consequence from the theory. In accordance, polydisperse elongated micelles (large length‐to‐width ratio) form as the bending rigidity is lowered, approaching the critical point at k_c=0, whereas monodisperse globular micelles (small length‐to‐width ratio) are expected to be present at large k_c values. The spontaneous curvature mainly determines the width of tablet‐shaped or ribbonlike micelles, or the radius of disklike micelles, whereas the saddle‐splay constant primarily influences the size but not the shape of the micelles.     

Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol–water binary mixtures

The aggregation behaviour of tetradecyltrimethylammonium bromide in ethylene glycol–water mixtures across a range of temperatures has been investigated by electrical conductivity measurements. The critical micelle concentration (cmc) and the degree of counterion dissociation of micelles were obtained at each temperature from plots of differential conductivity, (κ/c) _{T , P} , versus the square root of the total concentration of the surfactant. This procedure not only enables us to determine the cmc values more precisely than the conventional method, based on plots of conductivity against total concentration of surfactant, but also allows straightforward determination of the limiting molar conductance and the molar conductance of micellar species. The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters of micellization. Only small differences have been observed in the standard molar Gibbs free energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, and it showed a strong dependence on temperature in the ethylene glycol poor solvent system. An enthalpy–entropy compensation effect was observed for all the systems, but whereas the micellization of the surfactant in the solvent system with 20 wt% ethylene glycol seems to occur under the same structural conditions as in pure water, in ethylene glycol rich mixtures the results suggest that the lower aggregation of the surfactant is due to the minor cohesive energy of the solvent system in relation to water. Received: 13 December 1998 Accepted in revised form: 25 February 1999     

Micellization through complexation of oppositely charged diblock copolymers: Effects of composition,polymer architecture,salt of different valency,and thermoresponsive block

The structural and electrical characteristics of polyelectrolyte complex micelles (PCMs) formed by mixing of oppositely charged double hydrophilic copolymers are studied by means of molecular dynamics simulations. In mixtures of linear diblock copolymers we found that the preferential aggregation number N_p of PCMs is a universal function of the ratio γ_± of the total positive to total negative charges of the mixture. The addition of divalent salts ions induces a secondary micellization. In mixtures of copolymers bearing a common neutral thermoresponsive block, micelles with contracted corona consisting of thermoresponsive blocks and complex polyelectrolyte core are formed at low salt concentration and temperature far away the biphasic regime. At high salt concentration and temperature in the biphasic regime, reversed micelles are obtained. In equimolar mixtures of linear copolymers with miktoarm stars we found that N_p of PCMs decreases as the number of charged branches of miktoarm copolymer increases. The shape of micelles progressively changes from spherical to worm-like with the increase of number of branches of miktoarm copolymers. Our findings are in full agreement with existing experimental and theoretical predictions and provides new and additional insights.     

Oxidation-induced micellization of a diblock copolymer containing stable nitroxyl radicals

Oxidation-induced micellization was attained for a diblock copolymer containing 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed no self-assembly in carbon tetrachloride, a nonselective solvent. Dynamic light scattering demonstrated that the copolymer self-assembled into micelles of 49.5-nm hydrodynamic diameter when chlorine gas was added to the copolymer solution. The UV and electron spin resonance (ESR) analyses verified that as TEMPO was oxidized into the one-electron oxidant, that is, oxoaminium chloride (OAC) by the chlorine, the nonamphiphilic block copolymer became amphiphilic in nature, and thus, the polymers underwent micellization. An investigation of the relation between the micellization and the oxidation degree of the TEMPO into the OAC revealed that the micellization was induced by only 16% of the OAC. It was confirmed that the POAC-b-PSt micelles were spherical in shape by transmission electron microscopy observation. The micelles served as a two-electron oxidizing agent for benzyl alcohol to quantitatively give benzaldehyde. The micellar structure was maintained after the oxidation of benzyl alcohol without any dissociation into unimers because the OAC was converted into an insoluble hydroxylamine–hydrochloride salt. On the other hand, the micelles reacted with N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) to produce Wurster’s blue chloride by a one-electron transfer from TMPD to the OAC, converting themselves into PVTEMPO-b-PSt unimers.     

Water-induced micellar structure change in Pluronic P103/water/o-xylene ternary system

Both laser light scattering (LLS) and small-angle X-ray scattering (SAXS) were used to study the water-induced formation and structure of micelles and supramolecules of Pluronic P103 [(EO)₁₇(PO)₆₀(EO)₁₇] in o-xylene, a selective solvent for the long middle block. In pure o-xylene, P103 molecules exist as unimer coils with an equivalent hard-sphere radius of 1.6 nm even at fairly high concentrations. Micelles with a PEO/water core and a PPO dominated corona were formed in the presence of water when the P103 concentration ≥0.046 g/mL. The size and structure of micelles have been studied as a function of solubilized water content Z (the molar ratio of water to EO units) in micelles. The micelles change from a somewhat open structure with some EO units either dangling out of the micellar core or being incorporated into neighboring micellar cores at low Z values to a flower-like structure with relatively sharp interface at high Z values. At low Z values (< about 2.9), micelles tend to have a structure with part of the poorly solvated PEO blocks present in the corona. With more water added to the core, the PEO blocks in the corona gradually entered into the core, and the PPO blocks backfolded to form loops. With increasing Z, the micellar core radius, R_c, and the hard-sphere volume fraction, ϕ, of micelles increased; the aggregation number, N, kept nearly a constant; but the hydrodynamic radius, 〈R_h〉₀, and the corona thickness, R_s, decreased. At high Z values (> about 2.9), micelles have a flower-like structure with the two end PEO blocks belonging to the small micellar core. With increasing Z, the values of R_c, ϕ, and N increased, while R_s kept nearly a constant. In the concentrated regime (C > 0.30 g/mL), a stiff polymer network at a critical ϕ value of 0.49 was formed. The supramolecular structures with a face-center cubic packing, and a possible hexagonal packing at higher polymer concentrations (i.e. > 0.55 g/mL), were observed, respectively. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 889–900, 1998     

Critical Micellization Concentration of Ionic Surfactants: Comparison of Theoretical and Experimental Results

Theoretically calculated critical micellization concentration (CMC) of an ionic surfactant and the corresponding experimental value (CMC₀) are compared. The CMC values are calculated within the framework of the quasi-chemical model. The CMC₀ values are determined by the extrapolation of two linear parts of a dependence of a solution property on surfactant overall concentration. These parts of the aforementioned dependences correspond to the premicellar and micellar regions. The difference between CMC and CMC₀ is found to be negligible and comparable with the measurement error. Cases in which this difference must be taken into account when estimating the properties of micellar solutions are discussed. It is shown that the micellization constant K _c of surfactant molecules can be calculated from the data on CMC₀.     

The Self-Association and Mixed Micellization of an Anionic Surfactant,Sodium Dodecyl Sulfate,and a Cationic Surfactant,Cetyltrimethylammonium Bromide: Conductometric,Dye Solubilization,and Surface Tension Studies

In the present study, we investigate the self-association and mixed micellization of an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The critical micelle concentration (CMC) of SDS, CTAB, and mixed (SDS + CTAB) surfactants was measured by electrical conductivity, dye solubilization, and surface tension measurements. The surface properties (viz., C₂₀ (the surfactant concentration required to reduce the surface tension by 20 mN/m), Π_CMC (the surface pressure at the CMC), Γ_max (maximum surface excess concentration at the air/water interface), and A_min (the minimum area per surfactant molecule at the air/water interface)) of SDS, CTAB, and (SDS + CTAB) micellar/mixed micellar systems were evaluated. The thermodynamic parameters of the micellar (SDS and CTAB), and mixed micellar (SDS + CTAB) systems were evaluated.

A schematic representation of micelles and mixed micelles.     

Polycaprolactone-b-Poly(ethylene oxide) Biocompatible Micelles as Drug Delivery Nanocarriers: Dynamic Light Scattering and Fluorescence Experiments

Summary: Dynamic light scattering (DLS) and fluorescence experiments were carried out to study PCL₄₄-b-PEO₁₁₄ biocompatible micelles used as nanocarriers in drug delivery. Micelles prepared by a simple procedure (THF removal under nitrogen flow) exhibited a narrow size distribution with an average diameter of 100 nm. For micelles containing a hydrophobic model compound (pyrene) within the PCL core, a smaller average micellar size of 80 nm was observed, with a simultaneous broadening in the size distribution profile. In parallel to DLS results, fluorescence experiments showed evidence of pyrene encapsulation, and that the onset of the micellization process occurs at approximately 10/90 (v/v) THF/water mixtures in the case of PCL₄₄-b-PEO₁₁₄ polymer.     

Physicochemical studies on the micellization behavior of cetylpyridinium chloride and triton X-100 binary mixtures in aqueous medium

Physicochemistry of micellization of binary mixtures of cetylpyridinium chloride (CPC) and Triton X-100 (TX-100) have been investigated and the data collected have been analyzed and correlated. Tensiometric, conductometric, spectrophotometric, calorimetric and polarographic methods have been employed in the study. Parameters like critical micellar concentration (CMC), counter-ion binding, energetics of micellization, interfacial surfactant adsorption and minimum area of amphiphile head groups at CMC have been determined. The diffusion coefficients of pure and mixed micelles in solution have been determined by the polarographic method. The regular solution theory of Rubingh has been considered to get information on the micellar composition and their mutual interaction (synergistic for the studied system) in solution. The packing of the monomer in micelle has been estimated to witness spherical geometry for CPC and its mixtures with TX-100, whereas the later has been found to be spheroidal. Polarographic measurements have evidenced comparable diffusion coefficients of CPC and TX-100 micelles whereas their mixed micelles have shown lower values with a minimum, at equimolar composition.     

Preparation of micelles with azobenzene at their coronas or cores from ‘nonamphiphilic’ diblock copolymers

Micelles with azobenzene at the coronas or the cores were prepared by the micellization of nonamphiphilic diblock copolymers through hydrogen bond cross-linking. We used 4-(phenylazophenoxymethyl)styrene (AS) as the azobenzene. A poly(vinylphenol)-block-poly(AS-co-styrene) diblock copolymer (PVPh-b-P(AS-co-St)) was prepared by combination of the nitroxide-mediated living radical polymerization and the hydrolysis. The copolymer contained ca. 1 mol% of the azobenzene units in the P(AS-co-St) blocks on the basis of ¹H NMR analysis. The PVPh-b-P(AS-co-St) copolymer showed no micellization in 1,4-dioxane, the nonselective solvent. Dynamic light scattering demonstrated that the copolymer formed micelles in the presence of 1,4-butanediamine (BDA) in this solvent. ¹H NMR analysis revealed that the azobenzene moieties were located at the coronas of the micelles, because the signals of the aromatic protons originating from the azobenzene had no changes in the shape and the intensity by the micellization. UV analysis supported the presence of the azobenzene at the micellar coronas. The size of the PVPh-b-P(AS-co-St) micelles was independent of the copolymer concentration. On the other hand, the aggregation number of the micelles was dependent not only on the copolymer concentration but also on the kind of the diamine. A poly(AS-co-vinylphenol)-block-polystyrene diblock copolymer (P(AS-co-VPh)-b-PSt) formed the micelles with the azobenzene at the cores of the micelles by BDA. UV analysis demonstrated that the azobenzene at the micellar cores still had the potential to function as photorefractive switching.     

Non-ionic amphiphilic block copolymers by RAFT-polymerization and their self-organization

Water-soluble, amphiphilic diblock copolymers were synthesized by reversible addition fragmentation chain transfer polymerization. They consist of poly(butyl acrylate) as hydrophobic block with a low glass transition temperature and three different nonionic water-soluble blocks, namely, the classical hydrophilic block poly(dimethylacrylamide), the strongly hydrophilic poly(acryloyloxyethyl methylsulfoxide), and the thermally sensitive poly(N-acryloylpyrrolidine). Aqueous micellar solutions of the block copolymers were prepared and characterized by static and dynamic light scattering analysis (DLS and SLS). No critical micelle concentration could be detected. The micellization was thermodynamically favored, although kinetically slow, exhibiting a marked dependence on the preparation conditions. The polymers formed micelles with a hydrodynamic diameter from 20 to 100 nm, which were stable upon dilution. The micellar size was correlated with the composition of the block copolymers and their overall molar mass. The micelles formed with the two most hydrophilic blocks were particularly stable upon temperature cycles, whereas the thermally sensitive poly(N-acryloylpyrrolidine) block showed a temperature-induced precipitation. According to combined SLS and DLS analysis, the micelles exhibited an elongated shape such as rods or worms. It should be noted that the block copolymers with the most hydrophilic poly(sulfoxide) block formed inverse micelles in certain organic solvents.     

Conductivity of lithium perfluorononanoate in water-poly(vinyl pyrrolidone) solutions

Conductivity measurements, focused on the counterion binding of lithium perfluorononanoate (LiPFN) micelles in pure water and in the presence of poly(vinyl pyrrolidone) (PVP), have been carried out. An abrupt decrease of the conductance of the LiPFN in pure water, due to the self aggregation of anions and to dynamic linkage of cations on the micellar surface, has been found. Analysis of the conductometric data indicates that about 50% of the stoichiometric concentration of Li⁺ interacts with the head groups of the perfluorinated anions involved in micellar assembly. Conductometric data of LiPFN-water-PVP systems reveals a favorable influence of the PVP on the micellization process modulated by the concentration and by the molecular weight of the polymer. Analysis of these data shows that in presence of PVP the degree of binding of lithium ion to the micellar assemblies linked to the polymer is smaller than in pure water. By increasing the amount of surfactant in solution up to the concentration where the polymer becomes saturated by LiPFN micelles, the binding of lithium ion in the system becomes slightly greater than that observed in LiPFN-water system. This finding can be interpreted in terms of additional binding of lithium ion to the polymer chains. Conductivity measurements carried out on LiClO₄ and KClO₄ in water + PVP support this interpretation.     

Structural Behavior of Aggregate Assemblies of Cationic Surfactants and Their Mixtures with Triblock Polymers

Small angle neutron scattering (SANS) technique has been employed to study the structural aspects of micellar system of cationic surfactants viz. alkyltriphenylphosphonium bromide (C₁₂-, C₁₄-, C₁₆TPB) and hexadecylpyridinium chloride (C₁₆PyCl) with triblock polymers (L64, F68, and F127). SANS data analysis reveals the prolate ellipsoidal shape of mixed micelles and increase in the micellar size upon addition of triblock polymers (L64, F68, and F127). The influence of effective size of the head group segment on the growth of micelles of HTPB (larger head group) has also been compared with that of HPyCl (smaller head group). A proportionate micellar growth of cationic surfactants has been found with increase in the length of tail segment of cationic surfactants. The observed mixed micellar growth in mixed systems is also accounted on the basis of simultaneous increase in the hydrophobicity of both the components in the mixed system. Results from the present study enlightened the effect of variation in head group segment and hydrophobicity on the structural aspects of mixed micellar system.     

The effect of electrolytes on the reaction rates and acid-base equilibria in ionic micelles

The influence of a number of electrolytes on the micellar effect of cetyltrimethylammoniurn bromide (CTAB) in the hydrolysis ofp-nitrophenyl acetate (1) and bis(p-nitrophenyl) methylphosphonate (2) and in the course of the acid-base dissociation of thep-nitroanilide of bis(chloromethyl)phosphinic acid (3) has been examined. The activity of the salts studied increases in the following order: MeCOOK2CO₃3<p-MeC₆H₄SO₃K. It has been found that in the presence of electrolytes the catalytic effect of CTAB micelles in the hydrolysis reactions of esters1 and2 decreases, and the pK _a value of anilide3 increases. The results obtained are interpreted in terms of the pseudophase model of micellar catalysis. The analysis of the experimental data carried out using logarithmic coordinates revealed a relationship between phase transitions in micellar catalysis and in micellization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1396–1400, August, 1993.     

Thermodynamic and Dynamic Properties of Micellar Aggregates of Nonionic Surfactants with Short Hydrophobic Tails

The densities and viscosities of binary aqueous mixtures of poly(ethylenoxide)hexanols [C₆H₁₃(OCH₂CH₂)_mOH, C₆E_m] (m= 3, 4, and 5) have been studied in the micellar composition range. For the same surfactants the self-diffusion coefficients in mixtures with heavy water have been determined by the spin-echo pulsed field gradient method. The volumetric data are interpreted by means of the phase separation model, and values of the CMC, volume change, and standard free energy change of micellization are obtained. From the viscosity data the hydration numbers of the surfactant hydrophilic head in the micellar state are computed; they are in agreement with those obtained from HDO self-diffusion data. The surfactant self-diffusion data are used to calculate the apparent micelle radius and the aggregation number. The micellization parameters obtained for the different surfactants are compared and discussed.     

Gelation of micellar solutions of diblock-copoly (oxyethylene/oxybutylene) in aqueous K2SO4. An investigation of excluded volume effects

The micellization and gelation properties of oxyethylene/oxybutylene diblock copolymers E₄₀B₁₀ and E₄₁B₈ in aqueous K₂SO₄ solutions were investigated. The thermodynamic and hydrodynamic volumes of the micelles in salt solutions of various concentrations up to 0.4 mol dm^?3 were determined by static and dynamic light scattering, respectively. The related changes in the gelation behavior of concentrated micellar solutions of the copolymers were explained as thermodynamic (excluded) volume effects. The thermodynamic volumes of micelles measured in moderately concentrated solution were used successfully to predict the critical gel concentrations of concentrated solutions. ©1995 John Wiley & Sons, Inc.     

Direct preparation of core cross-linked micelles in a nonselective solvent

This is the first light scattering study demonstrating that the size of micelles, the aggregation number, and the mobility of the core blocks of the micelles could be controlled by the length of the cross-linker in the micellar cores. The core cross-linked micelles were prepared using a poly[(4-pyridinemethoxy-methyl)styrene]-block-polystyrene (PPySt-b-PSt) diblock copolymer and perfluoroalkyl dicarboxylic acid. The PPySt-b-PSt copolymer formed the micelles in THF, a nonselective solvent, in the presence of the perfluoroalkyl dicarboxylic acid. The light scattering studies demonstrated that the micellar size and aggregation number were dependent on the chain length of the perfluoroalkyl dicarboxylic acid. Perfluoroazelaic acid produced micelles with a larger hydrodynamic radius and higher aggregation number than tetrafluorosuccinic acid. The micellization proceeded through the formation of the pyridinium carboxylate and the cross-linkage between the PPySt blocks via the dicarboxylic acid. The core cross-linked micelles were thermally stable and maintained its structure with changes in the temperature. A ¹H NMR analysis revealed that the micelles prepared by perfluoroazelaic acid had more mobility of the core blocks than those by tetrafluorosuccinic acid.     

The thermodynamics of micelles in surfactant solution: Lattice mean field theory

The lattice self-consistent mean field theory is applied to study the thermodynamics of micelles in surfactant solution. The model surfactants used are H ₄ T ₄ and H ₂ T ₄. The formation of spherical micelles is considered. The effect of the head length on the thermodynamic stability of the micellar solution is examined. The critical micelle concentration is studied at different lengths of head, fractional charge, solvent quality, and univalent salt concentration. A lower critical micelle concentration is associated with a larger aggregation number, while the smallest micelles are found at the lowest univalent salt concentration. The text was submitted by the authors in English.     

Self-assembled and Nanostructured Copolymer Aggregations of the Tertiary Amine Methacrylate Based Triblock Copolymers

The micellization behavior of novel tertiary amine methacrylate-based ABA type triblock copolymers formed by poly[2-(dimethylamino)ethyl methacrylate] [PDMA] middle block and poly[2-(diethylamino)ethyl methacrylate] [PDEA] or poly[2-(diisopropylamino)ethyl methacrylate] [PDPA] side blocks, PDPA_m-b-PDMA_n-b-PDPA_l, and PDEA_m-b-PDMA_n-b-PDEA_l was investigated. Both types of triblock copolymers were water-soluble and had potential for various applications due to their self-assembled and the bottom-up nanoscale micellar construction. The micellar aggregations of the triblock copolymers in aqueous solutions with varying comonomer ratios, molecular weights, temperatures, and pH values were investigated by small-angle X-ray scattering and dynamic light scattering. Compact micellar aggregations were obtained as 0.5 weight percent solutions at 20–21°C and pH 8.67 to 9.05, and characterized as polydispersed spherical core-shells. One group of triblock copolymer micelles had PDPA-cores with radii from 18 to 21 Å and PDMA-shell thicknesses of 89–105 Å, whereas the other group had PDEA-core spherical micelles with core radii of 60–62 Å and a PDMA-shell thicknesses of 64–66 Å.