Micelle Formation Process Research Articles

CHANGES IN MEMBRANE FLUIDITY DURING THE MICELLE FORMATION DETERMINE THE EFFICIENCY OF THE SOLUBILIZATION OF MUSCARINIC RECEPTORS

The influence of various concentrations of 15 detergents at different temperatures on the fluidity of membranes was analysed by fluorescence anisotropy of inserted 1,6-diphenyl-1,3,5- hexatriene in rat cortical and Sf9 cell membranes. Comparison of the obtained data with the abilities of the detergents to solubilize muscarinic receptor subtypes indicated that not only the fluidity of the micelles formed but also the micelle formation process, the lipid/protein ratio, and presence of sterols have significant influence on the efficiency of the solubilization of mAChR. Digitonin seems to exploit efficiently all these factors by its biphasic modulatory effect on membrane fluidity during the receptor solubilization.

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.

Interpretation of the Differential Scanning Calorimetry Curves for Aqueous Solutions of Three Nonionic Surfactants

Differential scanning calorimetry curves were obtained with aqueous solutions of three alkyldimethylphosphine oxide samples, n-C8DPO, −C9DPO, and −C10DPO, from 293 to 383 K. The heat capacity curves exhibited minimums at 335 ± 0.5, 329 ± 1, and 321 ± 4 K, respectively, as do the curves that describe the temperature dependence of the critical micelle concentration (cmc) determined by an independent method. A simple two-state model was used to relate the observed specific heat to the change in heat capacity that accompanies the micelle formation process. The values obtained were used to describe the temperature dependence of the cmc (Desnoyers, J. E.; Caron, G. C.; DeLisi, R.; Roberts, D.; Roux, A.; Perron, G. J. Phys. Chem. 1983, 87, 1397). These results may provide an indication of what to expect for other self-associating systems that lead to micelle formation.

Behavior of aqueous solutions of poly(ethylene oxide-b-propylene oxide) copolymers containing a hydrotropic agent

The effect of the hydrotropic agent, sodium p-toluenesulfonate (NaPTS), was evaluated on the micelle formation process and on phase behavior of aqueous solutions containing poly(ethylene oxide-b-propylene oxide) (PEO–PPO) copolymers. We have studied monofunctional diblock copolymers coupled with hydrocarbons groups (R—PEO—PPO—OH and R—PPO—PEO—OH, where R length is linear C4 and C12–14). The critical micelle concentration (CMC) and critical micelle temperature (CMT) values of the aqueous copolymers solutions were obtained from both surface tension versus concentration plots and the dye solubilization method. The influence of the hydrocarbons groups length and PPO segment position in the structure of the copolymers were also analyzed. The same measures were obtained for the aqueous solutions of hydrotropic agent which, in turn, also presented molecular aggregation. The presence of the hydrotropic agent in the aqueous copolymers solutions altered the surface tension of these solutions and the occupied molecular area per copolymer molecule at air–water interface and CMC and CMT values of the copolymers. On the other hand, the aggregation points and the surface tension of the NaPTS solutions were dependent on the copolymer structure and composition. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2459–2468, 1998

Fluorescence Studies of Polymer Micelles: Intracoil Direct Energy Transfer

Doubly tagged polystyrene (PS) or polystyrene-poly(methacrylic acid) (PS-PMA) polymers have been prepared by anionic polymerization. These polymers may be represented by the structure N-PS n -A or N-Ps n -A-PMA where N and A represent a naphthalene or anthracene chromophore and n is the degree of polymerization (n = 40 or 283). Direct energy transfer (DET) from the N to the A group has been studied in micelles composed entirely of the tagged micelle or mixed micelles which contain ca. 1 wt % of tagged polymer. While DET clearly demonstrated the different end-to-end distances of the two PS segments, very strong environment effects on the fluorescence properties of both chromophores frustrated efforts to elucidate any modification in the end-to-end distribution in going from homogeneous solution to PS films to micelle structures. On the other hand, these complexities demonstrated that there is considerable heterogeneity present in an amphiphilic polymer micelle in mixed organic and aqueous solvents in which it equilibrates. These complexities are also a function of the details of the micelle preparation. Thus the use of fluorescently tagged polymers has permitted a much more molecular view of the process of polymer micelle formation.

Formation of block copolymer micelles: a sedimentation study

The formation of block copolymer mixed micelles has been studied in solutions of polystyrene- block-polyisoprene and polystyrene- block-poly(ethylene- co-propylene) in decane, a selectively poor solvent for polystyrene blocks. The rate of mass exchange between two types of micelles, studied by the sedimentation velocity method, depends strongly on the rates of micelle formation and dissociation processes, which can be controlled by temperature.

Determination of critical micelle concentration in the thionin—sodium dodecyl sulphate micellar system

Critical micelle concentration data published for a given detergent show differences depending either on the nature of the organic molecule (dye) that was present during the determination, or on the determination method used. We studied the thionin monocation (ThH +) and the sodium dodecyl sulphate (SDS) system using spectrophotometric and spectrofluorimetric methods. It was established that the process of micelle formation could be followed by absorption spectra and the Rayleigh scattering of the system. Thionin in the micelle was stabilized in the monocationic form. Our results were checked by potentiometric measurements using a detergent-selective electrode developed in our laboratory.

Dynamic surface properties of solutions of colloidal surfactants

The dilatational dynamic properties of surfactant solutions are determined. In part 1 the boundary conditions at the free surface, corresponding to small perturbations, are formulated for the equations of motion of a viscous incompressible fluid. The exchange of surfactant between the surface layer and the volume phase and the kinetics of micelle formation in the volume phase are taken into account. In part 2 a solution of the boundary-value problem is proposed. The spectral density of the fluctuations of the interphase boundary displacement is found on the basis of the known static correlation function. Finally, part 3 gives expressions for the complex dynamic surface elasticity corresponding to the first and second stages of the process of micelle formation. The discussion is mainly confined to the particular case of a liquid-gas interphase boundary. Limiting relations for high and low frequencies are presented.

The effect of chemical structure upon the thermodynamics of micellization of model alkylarenesulfonates: II. Sodium p-(3-alkyl)benzenesulfonate homologs

Studies of the thermodynamics of micellization of sodium p-(3-alkyl)benzenesulfonates in water at temperatures from 15 to 70°C have shown that increasing the alkyl chain length from C 9 to C 12 decreases the critical micelle concentration (CMC) and increases the free energy change of micelle formation by 2.7 kJ/mole per CH 2 group. The process of micelle formation of these surfactants appears to be entropy driven at low temperatures, but energy driven at high temperatures. Below the CMC, the change in electrical conductance of these substances with temperature is primarily due to the change in viscosity of water; above the CMC, the fraction of counterions participating in the conductance process increases with temperature. This is in agreement with earlier results for sodium p-( x-decyl)benzenesulfonates.

Dynamic properties of micellar solutions: I. Effects of short-chain alcohols and polymers on micellar stability

The effect of n-alkanols from C1 to C5 on both equilibrium and dynamic properties of sodium dodecyl sulfate (SDS) micellar solutions has been investigated using electrical conductivity and pressure-jump methods. Methanol and ethanol were found to decrease the conductance of micellar solutions due to the lowering of dielectric constant of solvent. Higher alkanols from C3 to C5 may decrease or increase the conductance of micellar solutions depending on the concentration of SDS. At low surfactant concentration, a minimum of conductance was observed. This has been attributed to both effects of decrease in surfactant monomer concentration (CMC) and the increase in counterion dissociation of micelles upon the penetration of alcohols into micelles. At higher surfactant concentration, the conductance was found to increase directly due to the dominance of the latter effect. Unlike the complex change of conductance, the slow relaxation rate constant, 1/τ2, has been found to increase monotonically upon the addition of all alkanols except pentanol. The labilizing effect (i.e., decreasing τ2) of alkanols increases with increasing alkanol chain length from C1 to C3 and levels off at C4. The labilizing effect has also been found to vary with SDS concentration depending upon the mechanism of slow relaxation process. Based on the Aniansson and Wall theory, the labilizing effect of alkanols has been attributed to two factors: (1) the stabilization of micelle nuclei either due to a decrease of micelle nucleus size, or due to an increase of micelle nucleus population; (2) the decrease of activation energy for the formation of micelle nuclei. In view of the changes in both slow relaxation time and CMC upon the addition of alkanols, the concept of micelle stability has been discussed in terms of the “thermodynamic stability” and the “kinetic stability” of micelles. Finally, the effect of polymers on the slow relaxation process was also investigated. The slow relaxation rate constant 1/τ2 was found to increase by three orders of magnitude as the concentration of polyvinylpyrrolidone increased up to 8% (w/w). Based on the molecular model of polymer-micelle complex proposed by B. Cabane (J. Phys. Chem.81, 1639, 1977), the effect of polymer has been interpreted in terms of the polymer being the nucleating agent for micelle nuclei. The result verifies the concept that the rate-limiting step in micelle formation and dissolution process is the formation of micelle nuclei.

Fluorometric analysis of the micelle formation process of surfactants in aqueous solution. II. Interaction of cholate molecules during premicelle formation.

The nature of the premicellar state of sodium cholate formed at the early stages of the micelle formation process has been analyzed using pyrene as a fluorescent probe. Formation of the premicellar aggregates of cholate is dependent on a diffusion-controlled process, and stabilization of the premicellar aggregates of cholate molecules appears to require both hydrophobic and electrostatic interactions. In addition, it has been demonstrated that there is a close relationship between the efficiency of the premicellar aggregate formation and the value of the critical micelle concentration of cholate. From these observations, it is suggested that the premicellar aggregates serve as nuclei in the micelle formation process.

Fluorometric analysis of the micelle formation process of surfactants in aqueous solution. I. Utility of pyrene in determination of the critical micelle concentration.

Pyrene, a fluorescent hydrocarbon, is a useful, sensitive probe for the determination of the critical micelle concentration of all types of detergents, non-ionic, anionic and cationic. In addition, this reagent has proved useful in research on the micelle formation process in aqueous solution and on differences in the rearrangement of the hydrocarbon chains of surfactants during micelle formation, based on measurements of the excimer formation efficiency.

Effects of chaotropic and antichaotropic agents on elution of poliovirus adsorbed on membrane filters.

The association of poliovirus with membrane filters results from both electrostatic and hydrophobic interactions. At low pH, electrostatic interactions appear to dominate. However, at high pH, hydrophobic interactions appear to dominate with both Millipore and Zeta plus filters. With both filters, viral elution was prevented at high pH by the presence of antichaotropic salts, which strengthen hydrophobic associations. This effect was antagonized by detergents and by chaotropic salts, which weaken hydrophobic associations. Excellent correlation was observed between viral elution, the solubilization of adenine, and the micelle-formation process in the presence of chaotropic and antichaotropic agents. Such simple measurements of the relative ability of solvents to accommodate hydrophobic groups may be of predictive value in designing methods for concentration and purification of viruses. The hypothesis that hydrophobic interactions are the primary force involved in the stabilization of virus attachment at high pH is consistent with observations that cannot be explained by consideration of electrostatic interactions as the major stabilizing factor.

Micelle formation by 31P NMR

An investigation of the process of micelle formation of amphiphilic molecules in dilute solution is described. This report shows that nuclear magnetic resonance using plutonium-31 is effective. Plutonium-31 measurements were made with a spectrometer under ambient conditions. The spectra were measured with proton decoupling, and the shifts are reported with respect to an internal capillary containing 85% H/sub 3/PO/sub 4/. The number of transients accumulated varied from 100 to 10,000. The critical micelle concentration in water is consistent with that previously determined. The spectra of the sample were compatible with the trialkylphosphine oxide structure reported previously. 17 references.

Effect of temperature and counterion on the micelle formation process studied by positron annihilation techniques

The positron annihilation technique was applied to the study of the micelle formation process in micellar systems Aerosol OT and dodecylammonium propionate (DAP) in apolar solvents, such as benzene and isooctane. At certain surfactant concentrations abrupt changes in the number of thermal o-Ps atoms formed can be observed. The surfactant concentration at which this effect can be seen is widely temperature independent in Aerosol OT solutions while a significant shift to higher surfactant concentrations can be found with increasing temperature in DAP solutions. The effect of the nature of the counterion on the mechanism of micelle formation is further investigated by substituting the counterion Na/sup +/ in Aerosol OT by tetraalkylammonium ions, N/sup +/H/sub 4/, N/sup +/(CH/sub 3/)/sub 4/, and N/sup +/(C/sub 2/H/sub 5/)/sub 4/. The results are discussed in terms of the pseudophase model and multiple equilibrium model for micelle formation.

Study of the micelle formation and the effect of additives on this process in reversed micellar systems by positron annihilation techniques

The positron annihilation technique was applied to the study of the micelle formation process in reversed micellar systems, Aerosol OT and dodecylammonium propionate in apolar solvents, such as benzene, isooctane, and cyclohexane. The results indicate that the positronium formation probability responds very sensitively to microphase changes in reversed micellar solutions. The abrupt changes in positronium formation probability observed at certain surfactant concentrations appear to coincide with variations in the aggregation state of the surfactant molecules in solutions, as postulated by the modified pseudophase model which considers the possibility of conformational changes between premicellar aggregates, and the surfactant concentrations at which they occur may be interpreted as operational critical micelle concentrations. Additives or probe molecules can affect these changes and shift them to lower surfactant concentrations. The additions of H/sub 2/O and its solubilization in form of clusters inside the reverse micelle leads to microphase changes also detectable by the positron annihilation technique. From a comparison of the rate constants between positronium and probe molecules observed in the neat solvents and in the corresponding micellar solutions, it was concluded that the probe molecules are attracted to various degrees by the reverse micelles, the nature of the surfactant and solvent determining themore » relative distribution of the probe molecule in the outer hydrocarbon layer and in the bulk apolar solvent of the reversed micelles.« less

Kinetics of conformational change during micelle formation in apolar media

AbstractAccording to previously developed ideas on the nucleation process of micelle formation the kinetics of the micellization of an ionic surfactant (Aerosol OT) was studied in various apolar solvents. The applied kinetic method utilizes the dielectric field effect due to a considerable variation of the reaction moment produced by the extensive compensation of the effective dipole moment of the monomers in the micelle. Rotational relaxations of about 120, 23 and 20 MHz in dioxane, benzene and cyclohexane solutions, respectively, were assigned to the expected premicellar aggregates (nuclei) and the micelles; in the latter solutions, however, orientational and chemical relaxation processes are superimposed. The analysis of the corresponding data yields a concentration dependence of the reciprocal relaxation times of the chemical process which conforms very satisfactorily to the corresponding theoretical plot derived from a model which assumes a conformational change between two premicellar states during the formation of the micelle.

Nucleation process of micelle formation in apolar solvents

Vapor pressure osmometric, dipole moment and dielectric dispersion measurements of aerosol OT, AY, di-2-ethylhexyl sodium phosphate, and lecithin in apolar solvents give strong indications as to the existence of nuclei below the cmc. Based on these observations, a model has been proposed to interpret the micellization process as a phase change. According to this model, explicit calculations of the enthalpy and entropy changes for the micelle formation lead to an estimation of the size of the micelle, of the micellar nucleus, and in addition, to a determination of the free energy of formation of the nucleus. The dependence of these quantities on the selfenergy of the micelle, the temperature, and the cmc is discussed.

Electron-donor-acceptor properties and the mode of action of some additives to oils

1. It has been shown that the polarity and electron-donor-acceptor properties of alkylsalicylate, sulfonate, and succinimide additives affect their detergent, stabilizing, and solubilizing actions. 2. The high polarity and weak polarizability of alkylsalicylate additives determine their activity on a Metal. interface, good detergency, and low stabilizing action, 3. The polarity and polarizability of sulfonate additives, which have an effect on the micelle formation process and on the properties of the micelles in the oil Volume, explain the solubilizing and stabilizing properties of these products and their low detergent action. 4. The least polar and most polarizable are the succinimide additives; this determines their high additivity in the oil Vol.ume and on the Metal. interface, as well as their low CMC, and is the cause of their high solubilizing and stabilizing action.

Aggregation of Certain Medicinal Amines in Aqueous Solutions of Their Salts

The osmotic coefficient and the equivalent conductance of aqueous solutions of certain medicinal amine salts were determined in order to examine their aggregating properties. Procaine HCl and tripelennamine HCl formed small ionic micelles. Partially ionized micelles of approximately constant size were the main constituent of solutions of dibucaine HCl and bromodiphenhydramine HCl. Tetracaine HCl and diphenhydramine HCl formed aggregates of different sizes in their solutions. No definite conclusion could be drawn about the types of aggregates that were present in solutions of pyrilamine maleate. An apparent critical micelle concentration (CMC) was obtained only for dibucaine HCl and bromodiphenhydramine HCl. From the CMC values at 0° and 25° it was concluded that the standard enthalpy change associated with formation of micelles in solutions of these two salts was practically zero. This finding together with a negative change in the standard free energy of micellization indicated that the process of micelle formation was mainly an entropy effect.