Interactions of hydrophobically modified polyvinylamine with pluronic triblock copolymer micelles

The heats associated with the addition of triblock copolymers of ethylene oxide and propylene oxide (PEO-PPO-PEO pluronics) to solutions of linear polyvinylamine, with N-substituted pendant octyl groups (HMPVAm), were measured as functions of pH and pluronic structure. The interactions were exothermic with the overall enthalpy decreasing with increasing pH from 5 to 10. Surprisingly, the heat effect increased with increasing pluronic hydrophilicity; however, no enthalpy change was observed in the absence of micelles. The results were quantitatively modeled by assuming two competing processes-micellar dissolution and HMPVAm coating of micelles, preventing dissolution.     

Effects of a PPO-PEO-PPO triblock copolymer on micellization and gelation of a PEO-PPO-PEO triblock copolymer in aqueous solution

The effects of a PPO-PEO-PPO triblock copolymer (25R4, PO(19)-EO(33)-PO(19)) on thermoreversible micellization and gelation properties of a PEO-PPO-PEO triblock copolymer (F108, EO(133)-PO(50)-EO(133)) in water were studied by means of micro-DSC and rheology. A complete, mirror-image like thermoreversible behavior has been observed for all of the samples with various molar ratios of 25R4 to F108. At a given concentration of F108, the addition of 25R4 results in the salt-out like effect on the primary micellization of F108; that is, the critical micellization temperature (CMT) of F108 shifts to lower temperatures with increasing the content of 25R4. The enthalpy changes for micellization are a linear function of the 25R4/F108 molar ratio at a fixed F108 concentration. Beyond the primary peak for the micellization of F108, a secondary peak or shoulder is observed in the DSC curves for the samples with the higher 25R4/F108 molar ratios, due to the formation of the hydrophobic aggregates from both the PPO blocks of F108 and those (i.e., PPO blocks) of 25R4. Furthermore, as an example, the dynamic viscoelastic properties of 18 wt % F108 solutions with various contents of 25R4 have been examined. It is found that, when the 25R4/F108 molar ratio < or =1, 25R4 does not affect the gelation of F108 notably. When the ratio is greater than 1, however, the formation of the 25R4-bridged micellar aggregates delays the gelation of F108 significantly. A schematic model has been proposed to explain the mechanism for the 25R4-influenced micellization and gelation of F108.     

How critical micelle temperature influences rotational diffusion of hydrophobic probes solubilized in aqueous triblock copolymer solutions

Rotational diffusion of two structurally similar hydrophobic probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been examined in aqueous solutions of poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 triblock copolymer as a function of temperature. These studies have been carried out to explore the influence of critical micelle temperature (cmt) on probe dynamics. It has been observed that, below cmt, the anisotropy decays can be adequately described by single-exponential functions with one time constant each for DMDPP and DPP. However, above cmt, biexponential functions with two time constants are needed to satisfactorily fit the anisotropy decays. Another important observation is that both the probes rotate more rapidly below the critical micelle temperature. The dynamics of the probe molecules are akin to that in a homogeneous solution below cmt, whereas above cmt, the rotational diffusion of the probes has been accounted by the two-step model, which is usually employed to explain the results in micelles. A comparison between the microviscosities of these micelles with other nonionic micelles such as Triton X-100 and Brij-35 reveals that the internal environment of the micelles formed with the triblock copolymer is less fluid.     

Conformation studies on sol-gel transition in triblock copolymer solutions

The gelation of physically associating triblock copolymers in a good solvent was investigated by means of the Monte Carlo simulation and a gelation process based on the conformation transition of the copolymer that was described in detail. In our simulative system, it has been found that the gelation is closely related with chain conformations, and there exist four types of chains defined as free, dangling, loop, and bridge conformations. The copolymer chains with different conformations contribute to the formation of gel in different ways. We proposed a conformational transition model, by which we evaluated the role of these four types of chains in sol-gel transition. It was concluded that the free chains keeping the conformation transition equilibrium and the dangling conformation being the hinge of conformation transition, while the chain with loop conformation enlarges the size of the congeries and the chain with bridge conformations binds the congeries consisted of the copolymer chains. In addition, the effects of temperature and concentration on the physical gelation, the association of the copolymer congeries, and the copolymer chain conformations' distribution were discussed. Furthermore, we employed the structure factor analysis to study the association of copolymer conformations and long-range order of the simulation system and found our results are in agreement with the previous experimental conclusions.     

Effect of "inverse melting transition" of aqueous triblock copolymer solutions on solute rotational dynamics

Rotational dynamics of two structurally similar hydrophobic solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in 30% wv aqueous solution of triblock copolymer, poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)(20) as a function of temperature. This study has been undertaken in an attempt to explore how the dynamics of a solute molecule solubilized in a copolymer solution is influenced when it undergoes sol-to-gel transition. It has been observed that the anisotropy decays of both DMDPP and DPP can be described by biexponential functions in the sol as well as in the gel phase. This observation has been rationalized on the basis of the probe molecule undergoing two different kinds of motion rather than being located in two different regions of the micelle. Even in the gel phase, which results as a consequence of micelle-micelle entanglement due to an increase in their volume fraction, the rotational relaxation of the solutes is similar to that observed in the micellar solution. The outcome of this work indicates that even though these gels have very high macroscopic viscosities and hence do not flow, the microenvironments experienced by the solutes are akin to that of a micellar solution.     

Temperature-induced self-assembly of triple-responsive triblock copolymers in aqueous solutions

A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was synthesized by sequential reversible addition-fragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20-25 °C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40-55 and 70-80 °C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 °C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.     

Interactions of counterions with dextran sulfate in aqueous solutions

Counterion activity coefficients in solutions of dextran sulfate with and without added salts were determined potentiometrically by using a cation-exchange membrane and a sodium glass electrode. Dextran sulfate was shown to interact with monovalent cations in the order of preference: K⁺ > Cs⁺ > Na⁺ > Li⁺, whereas no specificity was found for bivalent cations. On the basis of light-scattering measurements, the expansion of the dextran sulfate polyion in solutions of alkali metal salts was found to fall in the same order as the counterion activity coefficients in salt-free solutions. It was also shown that the dextran sulfate polyion assumes a more extended conformation in magnesium chloride solution than in calcium chloride. These results were substantiated by measurements of solution viscosities.     

Coarse-grained simulations of ABA amphiphilic triblock copolymer solutions in thin films

We study a coarse-grained model of A(10)-B(20)-A(10) amphiphilic triblock copolymers in aqueous solution under confinement. We focus on the influence of the wall interaction on the morphology of the ensuing self-assembled structures. We also study the dynamics of the polymers. All our simulations are confined between two walls. We study three different combinations of walls: hydrophobic and hydrophobic, hydrophobic and hydrophilic, hydrophilic and hydrophilic. We moreover elucidate the concentration influence. The conformation and behavior of the copolymer in strongly confined systems depend on the type of wall interaction and concentration.     

Modeling nonionic aqueous solutions: the acetone-water mixture

Several combinations of existing classical water and acetone models are studied by molecular dynamic simulations in order to sort out which models can reproduce available experimental data: enthalpies, pressure, densities, diffusion coefficients, and Kirkwood-Buff integrals. It turns out that all these properties, but the last, are rather well reproduced by all models, and with little numerical effort. By contrast, trials to measure by simulations the Kirkwood-Buff integrals lead to very long simulation times, thus revealing unexpected divergent behavior between the different models, such as phase separation, for example, and ultimately leading to a failure of any models combinations to reproduce these properties according to the experimental tendencies. It is argued herein that these deficiencies provide, in fact, an insightful picture of the microscopic structure of the solution, particularly into the relation between the hydrogen-bond network and the concentration fluctuations, as well as the role played by the solute in their spatial organization.     

Structure and ordering kinetics of micelles in triblock copolymer solutions in selective solvents

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane). Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus-Yevick interacting hard-sphere model. The temperature dependence of the core radius, the hard-sphere interaction radius and the volume fraction of hard spheres were obtained. We also used synchrotron-based time-resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus-Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented. The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl-Johnson-Avrami theory of nucleation.     

Mutual and self-diffusion of charged porphyrines in aqueous solutions

We have investigated the diffusion properties for an ionic porphyrin in water. Specifically, for the {tetrasodium tetraphenylporphyrintetrasulfonate (Na₄TPPS) + water} binary system, the self-diffusion coefficients of TPPS⁴⁻ and Na⁺, and the mutual diffusion coefficients were experimentally determined as a function of Na₄TPPS concentration from (0 to 4) · 10⁻³ mol · dm⁻³ at T = 298.15 K. Absorption spectra for this system were obtained over the same concentration range. Molecular mechanics were used to compute size and shape of the TPPS⁴⁻ porphyrin. We have found that, at low solute concentrations (<0.5 · 10⁻³ mol · dm⁻³), the mutual diffusion coefficient sharply decreases as the concentration increases. This can be related to both the ionic nature of the porphyrin and complex associative processes in solution. Our experimental results are discussed on the basis of the Nernst equation, Onsager–Fuoss theory and porphyrin metal ion association. In addition, self-diffusion of TPPS⁴⁻ was used, together with the Stokes–Einstein equation, to determine the equivalent hydrodynamic radius of TPPS⁴⁻. By approximating this porphyrin to a disk, we have estimated structural parameters of TPPS⁴⁻. These were found to be in good agreement with those obtained using molecular mechanics. Our work shows how the self-diffusion coefficient of an ionic porphyrin in water is substantially different from the corresponding mutual-diffusion coefficient in both magnitude and concentration dependence. This aspect should be taken into account when diffusion-based transport is modelled for in vitro and in vivo applications of pharmaceutical relevance.     

Solubilization of polyelectrolytic hairy-rod polyfluorene in aqueous solutions of nonionic surfactant

We report on the solubilization, phase behavior, and self-organized colloidal structure of a ternary water-polyfluorene-surfactant (amphiphile) system comprised of polyelectrolytic poly{1,4-phenylene[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) in nonionic pentaethylene glycol monododecyl ether (C12E5) at 20 degrees C. We show in particular how a high amount (milligrams per milliliter) of polyfluorene can be solubilized by aqueous C12E5 via aggregate formation. The PBS-PFP and C12E5 concentrations of 0.31 x 10(-4)-5 x 10(-4) M and 2.5 x 10(-4)-75 x 10(-4) M, respectively, were used. Under the studied conditions, the photoluminescence (PL), surface tension, static contact angle, and (pi-A) isotherm measurements imply that D2O-PBS-PFP(C12E5)x realizes three phase regimes with an increasing molar ratio of surfactant over monomer unit (x). First, for x < or = 0.5, the mixture is cloudy. In this regime polymer is only partially dissolved. Second, for 1 < or = x < or = 2, the solution is homogeneous. In this regime polymer is dissolved down to the colloidal level. Small-angle neutron scattering (SANS) patterns indicate rigid elongated (polymer-surfactant) aggregates with a diameter of 30 A and mean length of approximately 900 A. The ratio between contour length and persistence length is less than 3. Third, for x > or = 4, the solution is homogeneous and there is cooperative binding between polymer and surfactant. Surface tension, contact angle, and surface pressure remain essentially constant with increasing x. A PL spectrum characteristic of single separated polyfluorene molecules is observed. SANS curves show an interference maximum at q approximately 0.015 A(-1), indicating an ordered phase. This ordering is suggested to be due to the electrostatic repulsion between polymer molecules adsorbed on or incorporated into the C12E5 aggregates (micelles). On dilution the distance between micelles increases via 3-dimensional packing. In this regime the polymer is potentially dissolved down to the molecular level. We show further that the aggregates (x = 2) form a floating layer at the air-water interface and can be transferred onto hydrophilic substrates.     

Interactions of quinine with polyacrylic and poly-L-glutamic acids in aqueous solutions

Drug-loaded polymers provide an attractive form for controlled drug delivery systems. A proper knowledge of polymer-drug interactions can aid in the designing of polymers for various drug-delivery applications. In this paper we have investigated the interaction of a drug such as quinine, with synthetic macromolecules such as poly(acrylic acid), PAA, and poly(L-glutamic acid), PGA, at pH 7 and 37 °C by fluorescence spectroscopy and viscometry. The analysis of the binding isotherms revealed that the association process is positive cooperative up to a threshold concentration and then it is negative. In addition, the thermodynamic parameters vary along the isotherm. Results also suggest that there is an optimum polymer to quinine ratio. Based on the viscometry results a mechanism of the interaction in which the polymer conformation plays a determinant role is proposed. Since the conformation depends on the molecular size, the architecture of the macromolecule, the effective charge and ergo the ionic strength, all these variables have been taken into account and their effect on the binding discussed.     

Positively charged colloidal species in aqueous anionic surfactant solutions

The solution chemistry of fatty acids was reassessed. Transmittance measurements confirmed the existence of a colloidal precipitate in accordance with the constructed thermodynamic diagram. The electrokinetic potential of the precipitate and its dependence on pH was examined and an i.e.p. was found around pH 3. It results then that positively charged species exist below pH 3 in the broad range of fatty acid aqueous solution concentrations.     

Interactions of KLVFF-PEG peptide conjugate with fibrinogen in neutral aqueous solutions

In this work we report on the interaction of KLVFF-PEG with fibrinogen (Fbg) in neutral aqueous solutions at 20 degrees C, for particular ratios of KLVFF-PEG to Fbg concentration, Delta = c(KLVFF-PEG)/c(Fbg). Our results show the formation of Fbg/KLVFF-PEG complexes for Delta > 0, such that there is not an extended network of complexes throughout the solution. In addition, cleaved protein and Fbg dimers are identified in the solution for Delta >or= 0. There is a dramatic change in the tertiary structure of the Fbg upon KLVFF-PEG binding, although the KLVFF-PEG binds to the Fbg without affecting the secondary structure elements of the glycoprotein.     

Interactions of xanthine and its derivatives with cyclodextrins in aqueous solutions

Using calorimetry, ¹H NMR, UV spectroscopy, and solubility methods, the interactions of natural and hydroxypropylated 6h-, β-, and γ-cyclodextrins with xanthine and its methylated derivatives (theophylline, theobromine, and caffeine) were studied in aqueous solutions at 298.15 K. Cyclodextrins revealed low complexation ability toward xanthine and its methylated derivatives. Hydroxypropyl-γ-cyclodextrin with the largest internal cavity is the most effective solubilizing agent for this type of compounds. The calculated thermodynamic parameters are discussed in terms of structural effects of cyclodextrins and purine alkaloids on the character of their intermolecular interactions in aqueous medium.     

Nonlinear rheology of aqueous solutions of hydrophobically modified hydroxyethyl cellulose with nonionic surfactant

Shear thickening and strain hardening behavior of hydrophobically modified hydroxyethyl cellulose (HMHEC) aqueous solutions was experimentally examined. We focused on the effects of polymer concentration, temperature, and addition of nonionic surfactant. It is found that HMHEC shows stronger shear thickening at intermediate shear rates in a certain concentration range. In this range, the zero-shear viscosity scales with polymer concentration as eta(0) approximately c(5.7), showing a stronger concentration dependence than for more concentrated solutions. The critical shear stress for complete disruption of the transient network follows tau(c) approximately c(1.62) in the concentrated regime. Dynamic tests of the transient network on addition of surfactants show that the enhanced zero-shear viscosity is due to an increase in network junction strength, rather than their number, which in fact decreases. The reduction in the junction number could partly explain the weak variation of strain hardening extent for low surfactant concentrations, because of longer and looser bridging chain segments, and hence lesser nonlinear chain stretching.     

Interactions between nonpolar surfaces in mixed solutions of cationic and nonionic surfactants

The interaction energy between hydrophobic SiO₂ particles in aqueous solutions of a cationic surfactant (dodecylpyridinium bromide, DDPB), a nonionic surfactant (Triton X-100, TX-100), and their mixed solutions was measured as a function of concentration. Synergism has been observed in mixed surfactant solutions: the surfactant concentration required for achieving the set interaction energy in the mixed solutions was lower than in the solutions of the individual surfactants. The molecular interaction parameters in surfactant mixtures were calculated using the Rosen model. Chain-chain interactions between nonionic and cationic surfactants were suggested as the main reason for the synergism.