Micellar Shape Changes Research Articles

Supramolecular Packing Drives Morphological Transitions of Charged Surfactant Micelles

AbstractThe shape and size of self‐assembled structures upon local organization of their molecular building blocks are hard to predict in the presence of long‐range interactions. Combining small‐angle X‐ray/neutron scattering data, theoretical modelling, and computer simulations, sodium dodecyl sulfate (SDS), over a broad range of concentrations and ionic strengths, was investigated. Computer simulations indicate that micellar shape changes are associated with different binding of the counterions. By employing a toy model based on point charges on a surface, and comparing it to experiments and simulations, it is demonstrated that the observed morphological changes are caused by symmetry breaking of the irreducible building blocks, with the formation of transient surfactant dimers mediated by the counterions that promote the stabilization of cylindrical instead of spherical micelles. The present model is of general applicability and can be extended to all systems controlled by the presence of mobile charges.

Supramolecular Packing Drives Morphological Transitions of Charged Surfactant Micelles.

The shape and size of self‐assembled structures upon local organization of their molecular building blocks are hard to predict in the presence of long‐range interactions. Combining small‐angle X‐ray/neutron scattering data, theoretical modelling, and computer simulations, sodium dodecyl sulfate (SDS), over a broad range of concentrations and ionic strengths, was investigated. Computer simulations indicate that micellar shape changes are associated with different binding of the counterions. By employing a toy model based on point charges on a surface, and comparing it to experiments and simulations, it is demonstrated that the observed morphological changes are caused by symmetry breaking of the irreducible building blocks, with the formation of transient surfactant dimers mediated by the counterions that promote the stabilization of cylindrical instead of spherical micelles. The present model is of general applicability and can be extended to all systems controlled by the presence of mobile charges.

Interaction of hexadecylpyridinium bromide with glycine in aqueous medium using the Krafft temperature from conductivity measurement

Conductivity measurements were employed to investigate Kraft temperature (TK) of aqueous solution of hexadecylpyridinium bromide (HPyBr) and HPyBr+glycine with added counter-ions. The pseudophase ion exchange model was adopted for HPyBr in pure water, glycine (Gly) and potassium bromide (KBr). The TK values for HPyBr increase with the increase in concentration of Gly and KBr. However, the experimental results show that the degree of counter-ion dissociation or ionization (α) remains nearly constant as the total surfactant concentration or counter-ion concentrations of salt are varied at the slower rate of micellar growth. Although the (α) is dependent on micellar shapes and changes as the micellar shape changes. The TK measurements for ionic micelles can be used to estimate counter-ion dissociation (α). The TK increase with increase in the counter-ion concentration because the solubility of surfactant is reduced at faster rate than critical micellar concentration (CMC). Kraft temperature measurements were employed to find the interaction between HPyBr and Glycine.

Nonelectrolyte-Induced Micellar Shape Changes in Aqueous Solutions of Silicone Surfactants

Dynamic light scattering, cloud point (CP), and surface tension studies have been carried out to examine the influence of six (including two acetates, two alkoxyethanols, and two sugars) nonelectrolyte additives on two silicone surfactants based on poly (dimethyl siloxane)—graft—polyethers in aqueous solutions. The results indicate that the presence of alkoxyethanols induced the oblate ellipsoidal to spherical micellar transition, while sugars increase the size of the ellipsoidal micelles. The effect of cosolvent or additives on critical micelle concentration (CMC) and CP is discussed on the basis of water structure making and breaking effect. The thermodynamic and surface active parameters were calculated from the surface tension isotherm curves.

Molecular Dynamics Simulation of Micellar Shape Transition in Amphiphilic Solutions

The micellar shape transition in amphiphilic solutions is studied by coarse-grained molecular dynamics simulations of rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that the dominant micellar shape changes from disc to cylinder, and then to sphere as the hydrophilic interaction increases. We find that, as the hydrophilic interaction increases, the potential energy decreases monotonically even during the micellar shape transition, whereas the slope of the potential energy decreases in a stepwise manner in relation to the micellar shape transition. We also ascertained that there exists a wide coexistence region in the intensity of the hydrophilic interaction between a cylinder and a sphere, whereas the coexistence region between a cylinder and a disc is very narrow.

Shape transition of micelles in amphiphilic solution: A molecular dynamics study

The shape transition of micelles in an amphiphilic solution is studied by a molecular dynamics simulation of coarse-grained rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that the micellar shape changes from a disc into a cylinder, and then into a sphere as the hydrophilic interaction increases. We find that the potential energy decreases monotonically even during the micellar shape transition as the hydrophilic interaction increases. In contrast, it is ascertained that there exists a wide coexistence region in the intensity of the hydrophilic interaction between a cylinder and a sphere.

Molecular Dynamics Simulation of Micellar Shape Change in Amphiphilic Solution

Micellar shape change in an amphiphilic solution is investigated by a molecular dynamics simulation of coarse-grained semiflexible amphiphilic molecules with explicit solvent molecules. Our simulations show that a cylindrical micelle is obtained at small molecular rigidity while a disc-shaped micelle appears at large molecular rigidity. We find that most chains are in an extended conformation at large molecular rigidity whereas the fraction of the chains in a bent conformation becomes large at small molecular rigidity. It is also ascertained that the micellar shape starts to change immediately after sudden increase of the molecular rigidity while an induction time is needed to change the micellar shape after sudden decrease of the molecular rigidity. This result can be qualitatively explained by considering the bond-bending potential energy and the conformational entropy of the amphiphilic molecules.

Micellar Shape Driven Counterion Binding. Small-Angle Neutron Scattering Study of AOT Micelle

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior in aqueous electrolyte medium, viz., the counterion binding constant (β) abruptly increases by 2-fold at about 0.015 mol dm(-3) NaCl concentration (c*), but not in sodium salicylate (NaSa) solution. Since counterions affect the structure and performance of ionic surfactants, ascertaining the cause for the sudden shift in the β value of AOT micelle is of fundamental importance. In this study the special counterion binding behavior of AOT micelle has been ascertained at 40 °C by carrying out surface tension, zeta potential, and fluorescence emission (pyrene probe) measurements. The results of the small-angle neutron scattering experiment carried out at 40 °C showed that at c* the shape of AOT micelle changes from prolate spheroid to rodlike in NaCl solution, but not in NaSa solution, thus establishing micellar shape change as responsible for the abrupt change in β value. The absence of sudden shift in β of AOT micelle in NaSa solution is attributed to the binding of salicylate coanion to AOT micelle through hydrophobic interaction.

Characterization of the binding of multivalent ions to modified pluronic micelles by isothermal titration calorimetry and modified conductometry

CAE surfactants ( carboxylic acid end-standing triblock copolymers of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)) are amphiphiles that are able to bind multivalent cations thermoreversibly; a property that can be used to develop new environmentally friendly separation and ion exchange applications. The analysis of the binding of multivalent ions to CAE micelles is more difficult as compared to conventional surfactants because of its thermo-responsive micellization behavior and its low CMC at room temperature. Conventional analysis techniques like conductometry and surface tensiometry are highly sensitive to temperature and are therefore very difficult to apply in surfactant solutions with strong temperature dependent micellization behavior. For this reason, binding of multivalent ions to CAE micelles has been investigated using isothermal titration calorimetry (ITC) and an alternative conductometric analysis method. Conductometry was used to construct ‘Eisenberg’ plots, which show the binding and release behavior of monovalent ions in the presence of multivalent ions. Regarding the CAE-85 (modified Pluronic P85) micellar system, both methods provide information about the exchange of multivalent cations for monovalent ions above the critical micellization temperature (CMT) of CAE-85. ITC results confirm that dehydration of multivalent ions plays a dominant role in the observed heat effect during the titration of multivalent chloride salts to CAE-85 solutions. The multivalent ions investigated in this study are Ca 2+, Cu 2+, Zn 2+, Co 2+, Mg 2+, Ba 2+, Mn 2+ and La 3+. Additionally, using the alternative conductivity approach a change in micellar shape was observed upon the addition of multivalent ions when the molar amount of cationic charges balances the molar amount of anionic carboxylate end groups of the surfactant.

Effect of Molecular Rigidity on Micelle Formation in Amphiphilic Solution

Micelle formation in an amphiphilic solution is investigated by a molecular dynamics simulation of coarse-grained semiflexible amphiphilic molecules with explicit solvent molecules. Our simulations show that the micellar shape changes from a cylinder to a disc as the intensity of the molecular rigidity increases. We find that the radius of gyration of the cylindrical micelle is larger than that of the disc-shaped micelle for small molecular rigidity, although the radius of gyration is almost steady even during the transition between a cylinder and a disc for large molecular rigidity. This indicates that a cylindrical micelle formed at small molecular rigidity is more anisotropic than the one obtained at large molecular rigidity. We also ascertained that a cylindrical micelle and a disc-shaped micelle coexist dynamically over a certain molecular rigidity range.

Molecular dynamics simulation of amphiphilic molecules in solution: Micelle formation and dynamic coexistence

The micelle formation and the dynamic coexistence in amphiphilic solution are investigated by molecular dynamics simulation of coarse-grained rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that three kinds of isolated micelles (disk, cylindrical, and spherical micelles) are observed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disk into a cylinder, and then into a sphere as the hydrophilic interaction increases whereas it is not so sensitive to the variation of the hydrophobic interaction. This fact indicates that the hydrophilic interaction plays an important role in determining the micellar shape in the range of the interaction parameters used. It is also found that in a certain interaction parameter range, two kinds of micellar shapes coexist dynamically. From the detailed analyses of the dynamic coexistence, it is ascertained that the dynamic coexistence of a cylindrical micelle and a spherical micelle accompanies the coalescence and fragmentation of micelles while that of a disk micelle and a cylindrical micelle does not, but exhibits the continuous change between them.

Molecular dynamics simulation of micelle formation in amphiphilic solution

The micelle formation in amphiphilic solution is investigated by molecular dynamics (MD) simulation of coarse-grained rigid amphiphiles with explicit solvent molecules. In our simulation model, the intensity of the hydrophilic interaction and the hydrophobic interaction can be varied independently. Our simulations show that various kinds of micellar structures are formed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disc (bilayer) into a cylinder and then into a sphere as the intensity of the hydrophilic interaction increases. It is also found that the micelle formation proceeds in a stepwise fashion through the coalescence of smaller micelles. From the analysis of the orientational order for the amphiphilic molecules, it is concluded that the orientational order parameters can be used to distinguish the micellar shapes clearly.

Sphere, Cylinder, and Vesicle Nanoaggregates in Poly(styrene-b-isoprene) Diblock Copolymer Solutions

An asymmetric poly(styrene-b-isoprene) diblock copolymer with block molecular weights of 13 000 and 71 000 g/mol, respectively, was dissolved at 1 vol % in a series of solvents with varying selectivity for styrene: dibuthyl phthalate (DBP), diethyl phthalate (DEP), and dimethyl phthalate (DMP). The degree of solvent selectivity was adjusted by mixing DBP/DEP and DEP/DMP in various proportions. With increasing solvent selectivity, the predominant micellar shape changes from spheres to cylinders to vesicles, reflecting the changing interfacial curvature. The detailed micellar morphologies were characterized by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). Recently developed form factors were used to characterize the micellar structures in detail, and a vesicle form factor was derived for this system. From the core dimensions, the packing properties, such as the interfacial area per chain and the core chain stretching, were determined. The cryo-TEM results de...

Quaternary salts as solubilization site modifiers of organic compounds in anionic micellar solutions

AbstractThe effects of the addition of different salts (quaternary bromides or NaBr) on the viscosity (measured under Newtonian flow conditions) of sodium dodecylbenzenesulfonate (SDBS) micellar solutions were studied at 30°C. Tetra‐n‐butylammonium bromide (Bu4NBr) was found most effective in increasing the viscosity (due to possible change in micellar shape) at fairly low SDBS concentration (50 mM). The effectiveness of Bu4NBr may be due to the presence of four butyl chains and to the positive charge on its counterion (Bu4N+) which can subsequently interact with anionic SDBS micelles electrostatically as well as hydrophobically; owing to solubility/steric problems, other salts were ineffective. Organic additives such as cyclohexylamine and cyclohexanol had marginal effects on viscosity when added to 50 mM SDBS solutions having no Bu4NBr. However, in the presence of Bu4NBr, the effect was dependent on salt concentration and the nature of the additive. The overall effect is discussed on the basis of change in the solubilization site of the additive in the presence of Bu4NBr.

Micellar shape change and internal segregation induced by chemical modification of a tryptych block copolymer surfactant.

We report the detailed characterization of micelles formed by two nonionic, amphiphilic ABC triblock copolymers. Poly(ethylene oxide)-b-poly(styrene)-b-1,2-poly(butadiene) (PEO-b-PS-b-PB) triblock copolymer "OSB" forms core-corona spherical micelles in aqueous solution, and the two hydrophobic blocks S and B are mixed homogeneously within the micelle core. PEO-b-PS-b-PB:C6F13I triblock copolymer "OSF" was prepared by selective fluorination of the B block in OSB with n-perfluorohexyl iodide. Fluorination of the B block induces internal segregation into an inner F core and an intermediate S shell. Furthermore, the strong incompatibility that results from fluorination drives a shape change into an oblate ellipsoid. These micellar morphologies are confirmed by combined light, neutron, and X-ray scattering measurements, as well as TEM imaging.

Lecithin Microemulsions in Dimethyl Ether and Propane for the Generation of Pharmaceutical Aerosols Containing Polar Solutes

Water soluble compounds have been incorporated into solution phase metered dose inhalers (MDIs) utilizing lecithin inverse microemulsions in dimethyl ether (DME) and propane. DME and propane acted as both solvent and propellant. Experiments utilizing model propellants (dimethylethyleneglycol (DMEG) and hexane) were used to investigate microemulsion physicochemical phenomena, and the results were used to design and interpret the technically more challenging MDI experiments. NMR and viscosity experiments with model propellants were consistent with a “sphere-to-string” micellar shape change as the solvent was varied from pure DMEG to pure hexane. Water soluble solutes, including selected peptides and fluorescently labeled poly-α,β-[N-(2-hydroxyethyl) d,l-aspartamide] (fPHEAs), dissolved in DME/propane dependent on lecithin and water content. MDIs containing microemulsions generated aerosols with mass median aerodynamic values ranging from 2.7 to 3.1 μm, within the range of commercially available formulations. Fine particle fraction values (50–70%) exceeded those of commercial formulations. fPHEA up to 18 kDa did not adversely affect the aerosol characteristics. Deposition of the aerosol onto a water surface resulted in the formation of liposomes with partially entrapped solute.

Dielectric Relaxation Behavior of Aqueous Dodecyldimethylamineoxide Solutions

Dielectric relaxation behavior was examined for aqueous solutions of dodecyldimethylamineoxide (DDAO) in the range of concentration (cD) from 100 to 500 mM varying the average degree of protonation (〈α〉) by adding HBr. DDAO has an electric dipole moment due to an O←N coordination chemical bond on its headgroup, and the fraction of O←N is reduced by adding HBr, while that of HO−N+ increases. This process was detected as a change in dielectric spectra. Dielectric spectra for aqueous DDAO solutions had four types of relaxation modes. A fast relaxation time (τ1 ∼ 10-10 s) was essentially independent of cD, and the relaxation strength (Δε1) was proportional to cD at 〈α〉 = 0. Moreover, the magnitude of Δε1 decreased with 〈α〉; therefore, it was attributed to the rotational relaxation mode of O←N type headgroups in spherical micelles. A relaxation mode with cD-independent τ2 (∼10-9 s) possessed relaxation strength (Δε2) proportional to cD. Because Δε2 increased with 〈α〉 by adding HBr and the number of Br- entrapped in micelles increased with 〈α〉, the mode was assigned to the rotational relaxation mode of an ion pair formed between HO−N+ and Br- on the micellar surface. A relaxation mode with τ3 (∼10-8 s) at 〈α〉 > 0 was attributed to the rotational relaxation mode of intermolecular association particles formed between protonated and unprotonated DDAO molecules due to intermolecular forces such as hydrogen bonding. A slow mode with τ4 (∼10-7 s at 〈α〉 = 0) was assigned to the relaxation of a counterion distribution around spherical DDAO micelles. When 〈α〉 was higher than 0.5, dielectric behavior of aqueous DDAO solutions was essentially the same as that of aqueous solutions of cationic detergents. The addition of NaBr to aqueous DDAO solutions at 〈α〉 greater than 0.5 made a spherical micellar shape change into a spheroidal shape. This change in micellar shapes made dielectric relaxation spectra much broader.

Effect of phenylazoaniline to cetyltrimethyl-ammonium bromide ratio on micellar shape

The purpose of this study was to determine the influence of phenylazoaniline (PAA) on the shape of cetyltrimethylammonium bromide (CTAB) micelles. Change in micellar shape may have an impact on PAA transport characteristics. In a previous publication (Yoon, K.A. and Burgess, D.J., Pharm. Res. 13 (1996) 433), it was reported that the concentration of CTAB is related to the permeability coefficient of PAA in a triphasic system containing mineral oil, water and micellar phases. The diffusion coefficients of PAA in CTAB solutions were calculated from viscosity, dynamic light scattering and dialysis studies, (using U-tube viscometry, a Nicomp particle sizer and side-by-side diffusion cells, respectively) to determine any change in micellar shape with change in CTAB concentration. Diffusion coefficients were estimated from intrinsic viscosity using Simha's relationship (Tanford, C., Physical Chemistry of Macromolecules. Wiley, New Yoirk, 1961). According to the viscosity and dynamic light scattering studies, CTAB micelles change from spheres to rods at a PAA: CTAB molar ratio of 0.468±0.02, whereas this transition occurred at 0.32±0.02, according to the dialysis data. The difference between these data can be explained by the effects of aqueous boundary layer present on either side of the permeating membrane and the membrane adsorption in the dialysis studies. Aqueous boundary layer effects were determined by changing the stirring rate and the membrane adsorption effect was determined by presaturating the membrane. These studies indicate that change in the permeability coefficient of PAA with increase in CTAB concentration in the triphasic system may be due to change in micellar shape.

Electrolyte-Induced Structural Evolution of Triton X-100 Micelles

The long-time self-diffusion coefficients of Triton X-100 micelles have been determined as a function of surfactant and electrolyte concentrations using rotating disk electrode voltammetry in conjunction with ferrocene acting as an electroactive probe. Micellar diffusion conforms to the linear interaction theory and yields micellar hydrodynamic radii values and interaction parameters as a function of electrolyte concentration. The interaction parameters varied from 3.4 to 3.6 over the electrolyte concentration range 0.01−0.8 mol dm-3 KCl, which is consistent with excluded volume interactions and indicated the absence of significant micellar shape changes with the addition of electrolyte. Micellar hydrodynamic radii varied linearly with respect to [KCl] from 4.22 to 6.21 nm, indicating the progressive evolutionary growth of the oblate micelles due to increasing aggregation number and “hydration”. The semimajor to semiminor axial ratio (a/b) increased from 2.0 to 3.5, indicating evolution to a more asymmetric structure. Micellar molecular weights increased from 77 700 to 326 000 over the electrolyte concentration range studied, which is predominantly due to water entrapment at the periphery of the self-assembled structures. Intrinsic viscosities were found to decrease from 6.09 to 4.61 cm3 g-1, indicating a decrease in specific micellar volume, which may be attributed to ethylene oxide chain collapse due to dehydration. The decrease in micellar specific volume was found to correlate precisely with the electrolyte induced decrease of the cloud point.

A novel in vitro release method for submicron sized dispersed systems.

Sink conditions are often violated when using conventional release methods for dispersed systems. A novel reverse dialysis bag method was designed to overcome this problem. Model drug transport rates from submicron emulsions obtained using the conventional diffusion cell method and this novel method were compared. In the side-by-side diffusion cell method, emulsions were placed in the donor chamber and surfactant/buffer solutions in the receiver chamber. In the novel dialysis bag method, emulsions were diluted infinitely in the donor phase and surfactant/buffer solutions were placed in the receiver phase (dialysis bags). Slow release rates and linear release profiles were obtained using the side-by-side diffusion cell method apparently due to limited model drug solubility in the donor chamber resulting in violation of sink conditions. Biphasic release profiles were obtained using the dialysis bag method apparently due to an initial rapid release of free and micellar solubilized model drug from the donor to the receiver chambers followed by slow release from the oil droplets. Using both release methods, an initial increase and latter decrease in release rates were observed with increase in surfactant concentration. The initial increase was considered to be due to a decrease in the model drug oil-in-water partition coefficients and the subsequent decrease in release rates was due to micellar shape change (spheres to rods) causing a decrease in diffusion rates. Sink conditions were violated using the side-by-side diffusion cell method but were maintained in the dialysis bag method since emulsions were diluted infinitely in the donor phase.