Effects of polar group saturation on physical gelation of amphiphilic polymer solutions

Monte Carlo simulation on the basis of the comblike coarse grained nonpolar/polar (NP) model has been carried out to study the polar group saturation effect on physical gelation of amphiphilic polymer solutions. The effects of polar group saturation due to hydrogen bonding or ion bridging on the sol-gel phase diagram, microstructure of aggregates, and chain conformation of amphiphilic polymer solutions under four different solvent conditions to either the nonpolar backbone or the polar side chain in amphiphilic polymer chains have been investigated. It is found that an increase of polar group saturation results in a monotonically decreased critical concentration of gelation point, which can be qualitatively supported by the dynamic rheological measurements on pectin aqueous solutions. Furthermore, various solvent conditions to either the backbone or the side chain have significant impact on both chain conformation and microstructure of aggregates. When the solvent is repulsive to the nonpolar backbone but attractive to the polar side chain, the polymer chains are collapsed, and the gelation follows the mechanism of colloidal packing; at the other solvent conditions, the gelation follows the mechanism of random aggregation.     

On the theory of structuring magnetic suspensions

The results of the three-dimensional computer and analytical simulation are presented for the kinetics of chain-shaped aggregate growth in suspensions of magnetizable non-Brownian particles. The results of the computer experiment show that, when the volume fraction of particles is no larger than 2–3%, chain-shaped aggregates are formed in the suspensions under the action of a field. The dependence of average number <n> of particles in a chain on time t is adequately described by the power law <n> = Ct ^k . The experiment indicates that, in contract to the common power approximations, in which exponent k is considered to be a universal constant parameter, it depends on the concentration of particles and their interactions with walls bounding a suspension. At concentrations noticeably exceeding 2–3%, dense bulk aggregates are formed in suspensions. The kinetics of their growth depends on the sizes of a suspension-containing vessel.     

Kinetics of the alkaline hydrolysis of isatin and N-methylisatin in water and water-N,N-dimethylacetamide mixtures

The kinetics of amide bond cleavage of isatin and N-methylisatin in the presence of N,N-dimethylacetamide (DMA) was followed spectrophotometrically in the range of solvent composition (0–48.53 wt.%) and temperatures (40–70 °C) using piperidine as a nucleophile. The reaction was studied under pseudo-first-order kinetics. The rate of the reaction decreases largely with increasing organic solvent content. The thermodynamic activation parameters were calculated and discussed in terms of solvation of the activated complex. No linearity was observed between log rate constant and the reciprocal dielectric constant for the solvent used suggesting that there is a selective solvation by higher polar solvent (water). Finally, a mechanism for the ring opening for isatin and N-methylisatin was proposed.     

Solution properties of ionomers. I. Counterion effect

The effect of counterions on the solution properties of two types of ionomers, one based on sulfonated polystyrene and the other based on styrene–methacrylic acid copolymer, was studied by viscosity and light scattering measurements. It was found that the order of counterion binding of ionomers in a polar solvent and the order of aggregation of ionomers in a low-polarity solvent were the same for the same ionomer system. However, the order for the sulfonated ionomer was Li < Na < K < Cs, whereas that for the carboxylated ionomer was the opposite. This can be explained by a difference in desolvation during anion–cation interaction and by considering site-binding in a polar solvent and the association of ion pairs in a low-polarity solvent. These findings for ionomer systems are parallel to the association behavior of small ions in water, cation affinity in crosslinked resins, and counterion binding of polyelectrolytes in water.     

Copper-based reverse ATRP process of styrene in mixed solvents

Xylene/N,N-dimethylformamide (DMF) and xylene/ethanol were employed as mixed solvents, respectively, for the reverse atom transfer radical polymerization (R-ATRP) of styrene with the azobisisobutyronitrile (AIBN)/CuBr₂/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) initiating system. With a limited amount of DMF added in, CuBr₂/PMDETA complex could dissolve well in the reaction system, so the control of polymerization was enhanced compared with the one in which simplex xylene was used as solvent. But the polarity of DMF leaded kinetics to deviation from first order. Ethanol could also improve the solubility of catalyst and be scavenged quickly by argon at 110°, therefore the impact of polarity of solvent on kinetics was negligible. Induction periods were not observed here indicating rapidly establishment of equilibrium between Cu(I) and Cu(II). This method that adding a little amount of polar solvent with low boiling point into non-polar solvent gives a new way to solve the problem of poor solubility of the catalyst in R-ATRP.     

Fluorescence polarization anisotropy and kinetics of malachite green measured as a function of solvent viscosity

The fluorescence kinetics and polarization anisotropy of the triphenylmethane dye malachite green were measured as a function of solvent viscosity. The relationshp between the relaxation kinetics and the solvent viscosity was investigated in order to obtain information on the effect of enviromnental changes on the orientational order of dye molecules in solution. It was found that the fluorescence lifetimes follow an η^{$\text{2}\text{3}$} dependence for 1 < η < 60 P, η^{$\text{1}\text{2}$} dependence for 60 P < η < 1000 P and approach a constant for η > 1000 P. The dependence of the fluorescence decay rates on the solvent viscosity was fit to k = 5 × 10¹⁰η^{?$\text{2}\text{3}$} + 5 × 10⁸ s^?1. The fluorescence polarization anisotropy term, R(0), was also measured as a function of solvent viscosity. A marked decrease in R(0) was observed at a viscosity of 1000 P. For η < 1000 P, R(0) was found to be close to the expected value for a random distribution of molecules, 0.4; and for η > 3000 P, R(0) was measured to be ≈0.11. This small value of R(0) may indicate a nonrand The observed change of the polarization anisotropy with increasing viscosity indicates that the dye molecules become ordered at higher viscosities. This may arise through the formation of a long range order due to lack of rotational deexcitation of the malachite green dye molecules at high viscosities.     

Dimerization and association of an organophosphoric acid in various organic solvents

The association of bis (p-chlorophenyl) phosphoric acid with various organic solvents as well as its dimerization in these solvents was investigated. In inert solvents like kerosene, carbon tetrachloride and benzene, dimerization is very high and decreases with increasing polarity of the solvent. In polar solvents like alcohols and ketones, a strong association between HA and solvent molecules exists. The association constants were determined; the values may be regarded as a measure for the basicity of the solvents in question. The following order of the increasing basicity of the solvents was established: hydrocarbons and chlorinated hydrocarbons < ethers < ketones < alcohols.     

Experimental investigation of excited state electron transfer reactions between some bicyclic molecules and tetracyanoquinodimethane (TCNQ)

Investigations on photoinduced electron transfer (ET) reactions between excited (ground) bicyclic electron donors 5,6,7,8-tetrahydro-2-naphthol (TH2N), 2-methoxy-5,6,7,8-tetrahydro naphthalene (2MTHN) and ground state (excited) acceptor tetracyanoquinodimethane (TCNQ) in fluid solutions of different polarity at the ambient temperature (300 K) by electronic absorption, steady state fluorescence and time-resolved spectroscopic methods in the time domain of nanosecond order have been carried out. It is suggested that in highly polar solvent acetonitrile (ACN), a loosely-structured transient geminate ion-pair complex (GIP) in the excited singlet state (S₁) is formed due to the ET encounter between the present donor TH2N or 2MTHN and TCNQ and this GIP complex rapidly dissociates into stable excited radical ions, as evidenced from steady state spectra. In polar DMF solvents, TCNQ exhibits an electronic absorption band of its anion without the presence of donor molecules. Both steady state and time-resolved data indicate that ET reactions between the present donors and acceptor TCNQ are largely impeded in the less polar solvent tetrahydrofuran (THF). In the highly polar solvent ACN, ET reactions between the donors and acceptor TCNQ have been suggested to be of adiabatic or intermediate between adiabatic and non-adiabatic types, from the observation of radical ion species in the electronic excited state. For some bicyclic donors and TCNQ acceptor systems, large negative ΔG, which is a measure of the gap between locally excited and radical ion-pair states, shows reaction occurs in highly exothermic regions. Further observations of −ΔG>λ, nuclear reorganization energy parameters and the decrement of ET rate (k_ET) with increasing exothermicity (more negative ΔG values) suggest the ET reaction for the bicyclic donor—TCNQ acceptor systems studied in the present investigation might occur in the Marcus inverted region. The possibility of building up efficient photoconducting materials with the present donor acceptor systems is suggested.     

Morphological evolution of aggregates induced by the hydrophobic effect and hydrogen bonding interaction

The aggregation behavior of poly(ethylene oxide)-block-poly(methylmethacrylate) (PEO-b-PMMA) in toluene without and with the addition of polar small-molecules has been characterized using transmission electron microscopy (TEM). The preparation method involved copolymer dissolution in toluene which is a selective solvent for polymethylmethacrylate at room temperature, followed by the addition of polar small-molecules. By increasing polarity of the small-molecules at the same copolymer concentrations, these aggregates were found to undergo morphological transformations from spherical aggregates to multiple morphologies, including bean-pod liked structures (the morphologies are biomimetic), spheres and vesicles. A possible mechanism for the formation of this aggregate is proposed.     

Substantial formation of hydrates and hemiacetals from pyridinium ketones

Pyridinium ketones have been found to exist as hydrates and hemiacetals in considerable amount in aqueous and alcoholic solutions, respectively. The relative position of the pyridinium positive charge has a large effect on the equilibrium constants. The polar substituent constants, σ∗, of the pyridinium group substituted at different positions can be estimated from the hydration constants.     

Counterintuitive solvation effect of ionic-liquid/DMSO solvents on acidic C–H dissociation and insight into respective solvation

How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL (the “apparent” or “measured” dielectric constant ε of the IL is around 10–15) into a strongly polar molecular solvent (e.g., ε of DMSO: 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard. Toward this, we, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic–molecular binary systems on self-dissociation of C–H acid phenylmalononitrile PhCH(CN)₂via pK_a determination. As disclosed, in this category of binary media, (1) no linear correspondence exists between pK_a and molar fractions of the respective solvent components; (2) only ∼1–2 mol% of weakly polar ILs in strongly polar DMSO make C–H bonds even more dissociative than in neat DMSO; (3) a small fraction of DMSO in ILs (<10 mol%) can dramatically ease acidic C–H-dissociation; and (4) while the DMSO fraction further increases, its acidifying effect becomes much attenuated. These findings, though maybe counterintuitive, have been rationalized on the basis of the precise pK_a measurement of this work in relation to the respective roles of each solvent component in solvation.

The dependence of PhCH(CN)₂ pK_a on the molar fraction of ionic liquids in ionic–molecular binary mixtures showed a nonlinear three-fragment plot, which was rationalized for the first time by the respective roles of each solvent component for solvation.     

Solvent isotope effects on the complexation and exchange kinetics of Tl(I), K and Na with 18-crown-6 by competitive NMR spectroscopy: Competition between homo- and heterobimolecular cations exchange

The thermodynamic properties of complexation and exchange kinetics of thallium by 18-crown-6 have been studied by thallium NMR spectroscopy. Effects of solvent isotope, counterion (ClO₄⁻ and NO₃⁻) and presence of competitive cations, such as Na⁺ and K⁺, on the exchange characteristics of the system have been considered. The obvious relationships between the effects of D₂O-H₂O solvent isotope on the thermodynamic properties and activation parameters of complexation have been investigated. In the absence of competitor cations, the mechanism of thallium exchange is unimolecular decomplexation and in the presence of competitor cations, homobimolecular cation exchange is the predominant mechanism at low concentrations of the ligand. At higher concentrations of the ligand, the measured rate constants show that the complexation/decomplexation process obeys a heterobimolecular cation interchange mechanism. The rate constants ratios (k_D2O/k_H2O < 1) for unimolecular mechanisms also show an inverse solvent isotope effect.     

A quantitative HPLC method for determining lactide content using hydrolytic kinetics

A new method for quantitative analysis of lactide has been developed by applying chemical kinetics to a HPLC system. The most important advance is its practical approach to the quantification of analytes that are unstable in the HPLC mobile phase. In HPLC analysis, anhydrous mobile phases cannot separate lactide from impurities, and only mixtures of water and organic solvent can achieve effective separation. By selecting conditions for testing and studying the kinetics of lactide hydrolysis, extensive experiments revealed that lactide degradation can be treated as a pseudo-first-order reaction under the given HPLC conditions, and lactide content or purity can be quantitatively determined. This method is practical for measuring the purity of the intermediate lactide in polylactic acid (PLA) production and the lactide content in PLA. When lactide content is high, the relative standard deviation (RSD) of the measurements is <2.0%, while RSD is <5.0% at low levels, which indicates that the method is suitably precise.     

Performance of solvent-resistant membranes for non-aqueous systems: solvent permeation results and modeling

Membrane processes like reverse osmosis (RO) and nanofiltration (NF) can be low energy consuming operations as compared to the traditional chemical engineering unit operations and have been widely used for aqueous systems. Since such membrane processes are low energy consuming operations, their use in non-aqueous systems would offer considerable energy savings. Thus, the study is directed towards development and experimental verification of membrane materials and transport models to explain permeation properties of non-aqueous solvent systems. The understanding of polymer–solvent interactions is critical towards the development of suitable materials and also the prediction of the transport mechanisms.Pure solvent permeation studies were conducted to understand the mechanism of solvent transport through polymeric membranes. Different membrane materials (hydrophilic and hydrophobic) as well as different solvents (polar and non-polar) were used for the study. Pure solvent fluxes for hydrophilic membranes used showed that polar solvents (methanol, ethanol, iso-propanol) had a significantly higher flux (8–10 times) than that of the non-polar solvents (pentane, hexane, octane). On the contrary, the non-polar solvent flux was two to four times that of the polar solvents for hydrophobic membranes. For example, hexane flux at ∼13 bar through a hydrophobic silicone based NF membrane was ∼0.6×10⁻⁴ cm³/cm² s. And that through a hydrophilic aromatic polyamide based NF membrane was ∼6×10⁻⁴ cm³/cm² s. A simple model based on a solution-diffusion approach is proposed for predicting the pure solvent permeation through hydrophobic polymeric membranes. The model uses molar volume and viscosity of the solvent as parameters for predicting the pure solvent permeability. The model reasonably predicts the pure solvent permeation (R²=0.89, S.E.∼4%) for hydrophobic membranes. The model has also been experimentally verified using high solution temperatures and also literature experimental data. To extend the predictions to different membranes (hydrophilic and hydrophobic), surface energy and sorption values have been used as a parameter along with the solvent physical properties.     

Low-temperature investigation of the growth mechanism of alkylsiloxane self-assembled monolayers

The growth behavior of self-assembled monolayer films strongly depends on parameters such as solvent, water concentration in the solvent, substrate type, and deposition method. A further parameter, the temperature, is of particular importance. It has been found that growth kinetics, size, and shape of the structures obtained strongly depend on the deposition temperature. Thus, exact adjustment and control of the solution temperature is of crucial importance for investigation of deposition mechanisms. The development of a temperature control unit has been the basis for a series of experiments on deposition of octadecyltrichlorosilane (OTS) on silicon wafers to study the influence of temperature on growth kinetics and film structure. Characterization of the films was performed with ellipsometry and atomic-force microscopy. It has been found that octadecylsiloxane (ODS) island sizes decrease with increasing temperature. Furthermore, a characteristic temperature exists above which increasingly disordered deposition occurs. At low temperatures (5–10 °C) smaller dot-like features are observed besides larger fractally shaped islands characteristic for self-assembly growth of ODS films. Our results indicate that these small dot-like features originate from ordered aggregates in the adsorption solution and that they are the precursors of the formation of larger islands. However, they can only be observed at low temperatures, because at room temperature they coalesce quickly to form larger units, due to the high surface mobility.     

Dynamics of solvent and rotational relaxation of coumarin 153 in room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate confined in Brij-35 micelles: a picosecond time-resolved fluorescence spectroscopic study

The dynamics of solvent and rotational relaxation of Coumarin 153 (C-153) in ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and in the ionic liquid confined in Brij-35 micellar aggregates have been investigated using steady-state and time-resolved fluorescence spectroscopy. We observed slower dynamics in the presence of micellar aggregates as compared to the pure IL. However, the slowing down in the solvation time on going from neat IL to IL-confined micelles is much smaller compared to that on going from water to water-confined micellar aggregates. The increase in solvation and rotational time in micelles is attributed to the increase in viscosity of the medium. The slow component is assumed to be dependent on the viscosity of the solution and involves large-scale rearrangement of the anions and cations while fast component is assumed to originate from the initial response of the anions during excitation. The slow component increases due to the increase in the viscosity of the medium and increase in fast component is probably due to the hydrogen bonding between the anions and polar headgroup of the surfactant. The dynamics of solvent relaxation was affected to a small extent due to the micelle formation.     

Self-modelling analysis applied to nanosecond transient absorption spectroscopy of flavone: an aid to elucidate and characterise reaction intermediates

A nanosecond transient absorption spectroscopy study of flavone performed with a 248 nm pump radiation has been investigated with the support of a chemometric treatment: SIMPLISMA. The experimental spectra obtained in various solvent with a pump-probe delay lower than about 2 micros are in quite good concordance with those already presented in the literature. Nevertheless after about 10 micros, the spectrum pattern significantly evolves as a function of time particularly for the methanolic solution. A qualitative analysis together with a SIMPLISMA chemometric treatment of the experimental data allowed to elucidate and characterise two interdependent transient species in the alcoholic medium: the lowest T1 triplet state of flavone and the ketyl radical forming by H-abstraction reaction from the solvent. In cyclohexane and acetonitrile, the same species seem to be produced in the studied time-scale but the radical form is generated with variable quantum yield depending on the solvent polarity. The pure spectrum and the photochemical kinetics of each reaction intermediate could have been determined with the help of the second derivative SIMPLISMA calculation procedure.     

Oxidation Kinetics of Simvastatin Using Cetyltrimethylammonium Dichromate

This article reports an attempt on the studies on resistance of oxidative stress by the prodrug, simvastatin (SV). Cetyltrimethylammonium dichromate has been used as a lipid compatible oxidant to study the oxidation kinetics of SV in organic media. The reaction undergoes via an ionic mechanism without any side product. The reaction is found to be acid catalyzed and sensitive to solvent polarity. The increase in the rate constant due to an increase in hydrophobicity (apolarity) of the solvent indicates the existence of a less polar transition state. Furthermore, the decrease in the rate constant due to an increase in [CTAB] suggests partitioning of the substrates and the oxidants into two different domains with different polar characteristics akin to a reversed micellar aggregates. Considering the above results and the thermodynamic parameters, a reaction mechanism has been proposed, wherein a complex formed at the interface of the two domains due to the reactant and the oxidant in a fast process decomposes to the products in a slow process in the nonpolar bulk.     

Influence of the solvents on the γ-ray polymerization of acrylic acid. I.

Bulk polymerization of acrylic acid is controlled by linear plurimolecular H-bonded aggregates of the monomer. It is proved that it is not the precipitating medium that is responsible for the accelerated rate of the polymerization, but the presence of the H-bonded plurimolecular aggregates. It has been shown that the presence of the previously formed polymer is important, as it gives a matrix effect which allows the monomer aggregated to be stabilized by associating with the polymer. In polymerizing acrylic acid solutions, two types of solvents have been characterized: first, the polar solvents which do not destroy the H-bonded aggregates up to high dilutions. Then, in the presence of hydrocarbons or chlorinated solvents, 10–20% of the solvents dissociate the aggregates. A very striking parallelism is observed between the polymerization kinetics and the associated form of the monomer.     

The effect of solvent composition on swelling and shrinking properties of poly(acrylamide-co-itaconic acid) hydrogels

The peculiarities of the equilibrium swelling ratio and swelling-shrinking kinetics of polyelectrolyte copolymeric hydrogels consisting of acrylamide and itaconic acid (AAm/IA) have been studied in water/nonsolvent (acetone, methanol, ethanol and 1-butanol) mixtures as a function of solvent composition and IA content in the hydrogel. The phase transition of these hydrogels was generated by changing the solvent composition by progressive substitution of water by the nonsolvent. For all P(AAm/IA) hydrogels, the form of the shrinking curves was determined to be strongly dependent on the type of the nonsolvent used. The rate of shrinking of these hydrogels increased in the order 1-butanol < ethanol < methanol < acetone.