Rheological properties of multisticker associative polyelectrolytes in semidilute aqueous solutions

Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (N_H). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (N_H = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the N_H value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (N_H = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004     

Fitting complex potential energy surfaces to simple model potentials: application of the simplex-annealing method

A stochastic method of optimization, which combines simulated annealing with simplex, is implemented to fit the parameters of a simple model potential. The main characteristic of the method is that it explores the whole space of the parameters of the model potential, and therefore it is very efficient in locating the global minimum of the cost function, in addition to being independent of the initial guess of the parameters. The method is employed to fit the complex intermolecular potential energy surface of the dimer of water, using as a reference the spectroscopic quality anisotropic site-site potential of Feller et al. The simple model potential chosen for its reparameterization is the MCY model potential of Clementi et al. The quality of the fit is assessed by comparing the geometry of the minimum, the harmonic frequencies, and the second virial coefficients of the parameterized potential with the reference one. Finally, to prove more rigorously the robustness of this method, it is compared with standard nonstochastic methods of optimization.     

有机溶剂性质及其水含量对酶催化合成含D－氨基酸残基的二肽衍生物的影响

有机溶剂性质及其水含量对酶催化合成保护的寡肽及其衍生物有显著影响。系 统地研究了有机溶剂性质及在不同有机溶剂中水含量对α－胰凝乳蛋白酶催化合成 含D－氨基酸残基的二肽衍生物产率的影响。用α－胰凝乳蛋白酶、枯草杆菌蛋白 酶和嗜热杆菌蛋白酶在有机溶剂中催化合成了一系列保护的二肽及其衍生物，并研 究了水含量对反应产率的影响，得到了十分有意义的结果。     

The preparation of a cabbage reference material for environmental monitoring and food analysis

 Cabbage is frequently used in environmental monitoring and food control, and, hence, cabbage reference materials (RMs) are required for ensuring quality assurance. A cabbage RM was prepared in view of certification of specific elements from the "black list" of high toxicological interest and nutritive importance. All tasks of the RM production (production of the plant material, cutting and freeze-drying, determination of the residual water content, preparation of the RM, homogeneity testing, stability testing, certification measurements) are described in detail. Received: 12 November 1999 / Accepted: 29 January 2000     

Crystal hydrates: an investigation of the use of crystal hydrates as stationary phases in gas chromatography

Summary The use of crystallohydrates, crystallohydrate solutions and melts as stationary phases in gas chromatography has been proposed. Crystallohydrates have been shown to display high selectivity in the separation of polar organic compounds when use is made of water vapours as the mobile phase. Some aspects of gas-liquid chromatography in water vapours and with the stationary phase comprising crystallohydrates have been studied, and the preliminary results obtained point to the desirability of further progress in this field.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Method Development for the Speciation of Chromium in River and Industrial Wastewater Using GFAAS

A simple, sensitive and inexpensive method for the speciation of Cr(VI) and Cr(III) in river and industrial wastewater was developed, optimized and validated. Samples were collected, filtered and then chromium species were determined in the filtrate. In the filtrate, total Cr was determined directly by Graphite Furnace Atomic Absorption Spectrometry (GFAAS). A portion of the filtered sample was buffered to pH 6.4, extracted with Amberlite LA-2/MIBK and finally Cr(VI) was determined in the organic extract and Cr(III) in the aqueous phase using GFAAS. The method was evaluated by analysis of the certified reference material, CRM 544, and good accuracy was obtained. The limit of detection for Cr(VI) and Cr(tot)/Cr(III) was found to be 0.30 and 0.08 µg L⁻¹, respectively, and a precision expressed as relative standard deviation of better than 11% was achieved for nine replicates. A number of water samples from the Tinishu Akaki River, Ethiopia, and wastewater samples from a chemical plant in Germany, were analysed. In addition to the GFAAS method, Cr(tot) was also determined using ICP-MS (in CRM 544, lyophilized water, and Tinishu Akaki River water samples) and Cr(tot) and Cr(VI) using UV-VIS spectrophotometry (in CRM 544 and industrial wastewater samples). Good agreement between the different methods was found.     

ViscosityB-coefficients,structural entropies and heat capacities,and the effects of ions on the structure of water

The water-structural contributions to the entropies and heat capacities of hydration of over 120 ions and the viscosity B-coefficients of nearly 80 aqueous ions are tabulated and correlated. B-coefficients for many more ions are predicted from this relationship and from their dependence on ionic size and charge. The structural entropies determine a unique scale of water structure making and breaking by the ions.     

Heats of solution of gaseous hydrocarbons in water at 25°C

The heats of solution at 25°C for a number of hydrocarbon gases are reported as measured by a calorimetric method. There is excellent agreement between the standard enthalpy changes of solution measured calorimetrically and those derived from high precision temperature dependent solubility measurements. However the calorimetrically determined standard enthalpies of solution of a number of gases are greatly improved over values obtained from low precision temperature dependent solubility measurements. A method is presented to readily estimate the standard errors in the standard enthalpy change for any process derived from the temperature dependence of the equilibrium constant for the process. Comparison of the standard enthalpies and entropies of solution of hydrocarbon gases in water shows that the standard free energies of solution for all hydrocarbon gases investigated are dominated by unfavorable entropy contributions. A strong linear correlation between the standard entropy of solution and the number of hydrogens in the hydrocarbon molecule is found. This correlation suggests that the hydrocarbon hydrophobic effect is regulated by the number of allowable configurations of a water molecule in contact with each C–H group.     

Chemical conversion of various celluloses to glucose and its derivatives in supercritical water

The supercritical water biomass conversion system was designed and developed in our laboratory. The reaction vessel with cellulose sample was treated with this system at supercritical state of water for a designated period (3–105s) under the conditions of a tin bath temperature of 500°C and pressure of 35MPa. The recovered products of hydrolysates were then analyzed by high performance liquid chromatography. The obtained results indicated that a high amount of glucose and levoglucosan can be achieved from both celluloses I and II for 5–10s supercritical treatment, while that from starch for 3–5s treatment. Although this difference could be due to a difference in the molecular structure between cellulose and starch, a difference between celluloses I and II was not significant. Instead, an accessibility of the water towards cellulose molecules seemed to be significant for their chemical conversion. With the longer treatment, amounts of these compounds observed were decreased due to decomposition. Therefore, it may be concluded that, compared with acid hydrolysis or enzymatic saccharification, cellulose may be hydrolyzed to glucose and its derivatives more or less to the same degree as in corn starch under supercritical state. This finding suggests that the supercritical treatment can overcome the difficulties in hydrolyzing cellulose to glucose, found in the acid hydrolysis or enzymatic saccharification techniques.     

Phenol O–H bond dissociation energy in water clusters

We are reporting ab initio and density functional theory (DFT) calculations for the phenol O–H bond dissociation energy in the gas phase and in phenol–water clusters. We have tested a series of recently proposed functionals and verified that DFT systematically underestimates the O–H bond dissociation energy of phenol. However, O–H bond dissociation energies in water clusters are in reasonable agreement with experimental data for phenol in solution. We have evaluated electronic difference densities in phenol–water, phenoxy–water, and water, and we are suggesting that the representation of this quantity gives an interesting picture of the electronic density rearrangement induced by hydrogen bond interactions in phenol–water clusters. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001