The role of solvent structure in ligand substitution and solvent exchange at some divalent transition-metal cations

The role of the solvent in reactions involving ions is considered in relation to the structure of liquids. The rate constants and activation parameters for ligand substitutions at divalent transition metal cations in various solvents are compared with those for solvent exchanges. The differences are related to structural properties of the solvents, represented by their heats of evaporation and fluidities, and interpreted with the aid of a model developed from that of Frank and Wen. Water is not a typical solvent.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.     

Casein structure,self-assembly and gelation

The literature on recent studies of casein structure and function is reviewed. Where appropriate, we try to reconcile conflicting views on the issue of secondary structure in these proteins, steering a middle course where possible. A suggestion is put forward that a coarser view, treating the caseins as block copolymers may be sufficient to rationalise much of the behaviour of these proteins in self-association, adsorption and micellar assembly.     

Thermoreversible gelation of syndiotactic polystyrene: Effect of solvent type

The temperature-concentration phase diagrams of syndiotactic polystyrene (sPS)/benzene gels and sPS/toluene gels establish the existence of two polymer-solvent compounds in both systems. In particular it is found that the sto ichiometries of the compounds differ with the type of solvent: The stoichiometrics, expressed as solvent molecules/monomer are respectively for the benzene 4/1 and 1/1 and for the toluene 0.8/1 and 1/4. Neutron diffraction patterns allow us to confirm the existence of differents compounds in sPS/toluene gel and sPS/benzene gel.     

Effect of sucrose on molecular and interaction parameters of sodium caseinate in aqueous solution: relationship to protein gelation

The effect of sucrose on molecular and interaction parameters of sodium caseinate in aqueous medium has been investigated using static and dynamic multi-angle laser light scattering over a wide range of sucrose concentration (from 10 to 78 w/v%) and pH values (from 7.0 to 3.5). Measurements have been made of the molar mass, the radius of gyration, the hydrodynamic radius, and the second virial coefficient of sodium caseinate in aqueous solution. Pronounced dissociation of sodium caseinate sub-micelles¹ was found in the presence of sucrose at a pH above the protein's isoelectric point. The effect of sucrose at a pH near the isoelectric point is very different. This is reflected in the pronounced increase in molar mass, radius of gyration, and the difference between the radius of gyration and the hydrodynamic radius. It was found that the extent of the protein association, caused by the presence of sucrose, is a key factor contributing to the hydrophobic–hydrophilic balance of the protein surface, and hence to the thermodynamic affinity of the caseinate sub-micelles for the aqueous medium and for each other. Analysis of light-scattering data using structure-sensitive plots shows a clear transition from Gaussian to wormlike chain/rod behaviour for sodium caseinate on pH lowering. Apparent relationships between the effects of sucrose on the self-association of sodium caseinate and a marked enhancement of the viscoelasticity of acid-induced casein gels have been revealed. Moreover, the dissociation of sodium caseinate sub-micelles is in excellent agreement with the more homogeneous microstructure of acid-induced protein gels in the presence of sucrose as detected by confocal laser scanning microscopy. We discuss likely molecular mechanisms underlying the observed effects of sucrose on the interactions and rheology in acidified caseinate systems.     

Increasing molecular weight parameters of a helical polymer through polymerization in a chiral solvent

In this communication, we will demonstrate that polymerization in a chiral solvent can affect the molecular weight distribution of the product by perturbing the balance of the P and M helical screw senses of the growing chains. Specifically, for the Wurtz-type synthesis of polymethylphenylsilane (PMPS) in either (R) or (S)-limonene, the weight-average molecular weight of the products (average Mw = 80 000) was twice that of PMPS synthesized in (R/S)-limonene (average Mw = 39 200). Peturbation of the helical segmentation along the polymer chains leads to a reduction in the rate of occurrence of a key termination step. This the first time that a chiral solvent has been demonstrated to have such an effect on a polymerization process in affecting molecular weight parameters in contrast to affecting tacticity.     

Mesoscopic structure formation in colloidal aggregation and gelation

Recent small-angle light scattering experiments have revealed that diffusively aggregating spherical particles develop structure on a mesoscopic length scale (∼ tens of particles). The mesoscopic structural length scale persists even when the aggregation proceeds to the formation of a space-spanning network (a gel). We review the technique of small-angle light scattering, survey the experimental evidence for mesoscopic structure formation, discuss attempts at understanding these experimental observations by computer simulation of irreversible and reversible diffusion-limited cluster aggregation (DLCA), and propose a coherent picture for the understanding of non-equilibrium aggregation in the context of phase transitions.     

The gelation rule of the polyol acetal derivatives in binary solvent mixtures

The tendency of a gelator to gel in mixed solvents is strongly correlated with its gelation behaviors in the corresponding single solvents.     

Dissolution of cellulose in ethylene diamine/salt solvent systems

Investigation of the dissolution of cellulose in Ethylene Diamine (EDA)/Potassium thiocyanate (KSCN) solutions by infrared spectroscopy (FTIR) and thermal analysis (DSC) indicated that changes to the solvent during freeze thaw cycling of mixtures was consistent with increased interaction between cellulose and solvent. Thermal transitions in the system, however, occurred at temperatures outside the range used in thermal cycling to promote dissolution. Further exploration of the dissolution and mixing process indicated that mixing was the limiting step in solution formation. The dissolution of two types of cellulose with different molecular weights (Degree of Polymerization (DP)=210 and >1000) was studied using EDA/KSCN solution as the solvent. The solubility and the dissolution rate of cellulose depended on both the solvent composition and cellulose molecular weight. Cellulose could dissolve faster in the solvent with lower salt concentration but the highest cellulose concentration was obtained in the solvent with 30~35% KSCN. Rheological measurements showed that cellulose solutions exhibited viscous solution behavior at low KSCN concentration but primarily elastic behavior at high salt concentration.     

Phase separation and heat-induced gelation characteristics of cellulose acetate in a mixed solvent system

The addition of cellulose acetate (CA) to a mixed solvent of N,N dimethylacetamide (DMA) and water is examined at different CA concentrations and water contents. Unlike the addition of water to a CA/DMA solution, the addition of CA to a DMA/water solution leads to a transition from a homogeneous solution at low water content to a two-phase system at higher water contents. The two-phase system has an upper liquid layer with an almost clear “solution-like” appearance and a lower gel-like layer. Treating the two-phase system at 100 °C for 30 min results in the formation of a uniform gel structure, similar to that produced by directly adding water to a CA/DMA solution. Although the phase behavior of these systems is similar, their viscoelastic properties are not. The moduli of the heat treated gels do not significantly vary from that of the bottom gel-like portion of the original two-phase samples. This may indicate that thermal treatment at 100 °C is not able to disrupt the pre-established CA intra- and intermolecular interactions and form the stronger CA-DMA interactions.     

The role of polymer-solvent compounds in thermoreversible gelation

The definition of a thermoreversible gel is first discussed through the use of two criteria dealing with topology and with thermodynamics. According to this definition only fibrillar systems, as opposed to spherulitic packings, ought to be considered gels. A simple mechanical test is presented that helps recognize such gels. The gelation mechanisms are briefly reviewed and the importance of the role of the polymer-solvent compound in preventing chain from folding, which favours fibrillar structures, is stressed. Also, the role of such compounds in gel ageing, particularly in the case of PVC, is highlighted.     

Investigation of the LCST of polyacrylamides as a function of molecular parameters and the solvent composition

The paper investigates the thermoprecipitation of two macromolecule structures, poly(N-isopropylacrylamide) (poly-NIPA) and poly(N,N-diethylacrylamide) (poly-DEA) from aqueous solution. The majority of the data are collected for small (M_w < 5000 g/mol) homogeneous (D < 1.3) molecules of the indicated type synthesized by anionic, group transfer, and radical polymerization in the presence of a chain transfer agent. Conventional radical polymers (M_w < 200,000 g/mol) are also synthesized and used for comparison. Turbidity curves (photometry) and transition enthalpies (high sensitivity differential scanning calorimetry) are measured to investigate the phase transition as a function of the molecular size and the tacticity as well as the concentration of certain solution additives (simple salts, glucose, and the surfactant tetrabutylammonium acetate) and mixtures thereof. Where applicable, the results are interpreted on the basis of a two-state model to gain insight in the cooperativity of the transition. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2977–2989, 1999     

Effects of molecular structure parameters on ring-opening reaction of benzoxazines

A colorless monoclinic crystal of dichloro-benzoxazine was obtained and examined by single crystal X-ray diffraction analysis. At the same time, molecular modeling analysis of nine benzoxazine compounds with different substituting groups was performed. Both single crystal X-ray diffraction analysis and molecular modeling analysis provide a detailed picture of molecular structure on molecular level and show good consistence with each other. On the basis of structural analysis, the effects of molecular structure parameters on ring-opening polymerization of ben-zoxazines have been explored.     

Polypeptide multilayer films: role of molecular structure and charge

The role of molecular structure, charge, and hydrophobicity in polyelectrolyte layer-by-layer assembly (LbL) of thin films has been studied using the model polypeptides poly-L-glutamatic acid (PLGA) and poly-L-lysine (PLL), quartz crystal microbalance (QCM), and circular dichroism spectroscopy (CD). The adsorption behavior of PLGA and PLL has been compared with the structure of these molecules in aqueous solution under the same conditions. The data show that the deposition of polypeptide per adsorption step scales with average secondary structure content, whether alpha helix or beta sheet. This is contrary to the expectation based on the view that hydrogen bonds are crucial to polypeptide film assembly, because secondary structure formation in a polypeptide reduces its intermolecular hydrogen-bonding potential. The data also show that polypeptide adsorption scales with ionic strength and chain length. Taken together, the results increase knowledge of polypeptide-based LbL thin film fabrication and will help to provide a firmer foundation for the use of natural or designed polypeptides in LbL.     

Effect of solvent molecular structure on vibration—rotation spectra of DCl and CO solutes in cycloalkanes

Infrared vibration—rotation spectra have been obtained for DCl and CO in a variety of cyclic and bicyclic alkanes and in alkyl-substituted cyclopentanes and cyclohexanes. The band shapes for CO depend on solvent free volume. For DCl they are sensitive to solvent structure, the rotation being perturbed by surface features (“roughness”) of the surrounding molecules. Molecular roughness is assessed qualitatively either through inspection of molecular models or through _V*, the solvent volume at close-packing, obtained from equation of state data.     

Ultrafast photoinduced intramolecular charge transfer in push-pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ～200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.     

溶剂条件下苯并咪唑及其衍生物缓蚀性能的DFT研究

通过量子化学密度泛函理论中的B3LYP方法，在6-31G＊基组水平上，并考虑到溶剂的影响，应用PCM模型，对苯并咪唑及其衍生物缓蚀性能与分子结构关系进行了研究，用Fukui指数分析了分子中反应活性位点，结果表明5种分子均为存在共轭体系的平面分子，2-SH-BI易于垂直吸附在金属表面，缓蚀效率与分子最高占有轨道能量EHOMO、能隙△E、分子化学硬度η偶极距μ有良好的线性关系．