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71.
72.
73.
The results of conductance measurements on pyridinium picrate, tetraphenylo-sonium picrate, potassium picrate, tetraphenylantimony picrate, tetrapropylam-monium, tetrafluoroborate, tetramethylammonium hexafluorophosphate ion association noncoulombic interaction in dimethyl sulfoxide (DMSO) at 25°C in the concentration range 1–15×10–4 M are reported. The data were analyzed by the Justice modification of the Fuoss–Hsia equation. Except for pyridinium picrate all salts studied were found to be associated.Application of the Justice Barthel–Bjerrum model of ion association permitted calculation of the noncoulombic portion of the potential of mean force, W
±. Ionic limiting conductances were calculated for six ions using known values of previously determined transport numbers. A table of most current limiting ionic conductances for a variety of ions in DMSO at 25°C has been established. 相似文献
74.
The eigenfunctions and energies of general dilated Hamiltonians are expanded in powers of the dilatation parameter. These expansions, augmented by stationarity and stability conditions, are used to derive exact sum rules for bound and resonance states. Particular attention is paid to Hamiltonians with potentials which depend on external parameters, such as the nuclear coordinates in molecules, and to self-consistent potentials. The sum rules can be employed in practical computations to improve the quality of the results and may also serve in analyzing the results from approximate calculations. 相似文献
75.
Christine Bonal Jean-Pierre Morel Nicole Morel-Desrosiers 《Journal of solution chemistry》1998,27(4):361-372
The apparent equilibrium constants and enthalpies of complexation of Nd3+, Sm3+, Eu3+, and Gd3+ by xylitol in aqueous solutions containing NaNO3 at an ionic strength of 2.0 mol-kg–1 have been determined by microcalorimetry at 25°C. Since nitrate anion weakly complexes the lanthanide cations, these values are analyzed in terms of competition between xylitol and NO
3
-
The method leads to the apparent equilibrium constants and enthalpies of complexation of the lanthanide cations by NO
3
-
at this particular ionic strength. Despite the difficulties encountered in characterizing rather weak associations, the results are, whenever comparison is possible, in good agreement with those obtained by direct microcalorimetry. The advantage of this competition method is that it can be used when the enthalpic effects are too weak and insufficiently concentration dependent for direct microcalorimetric determination. In the present case, it allows us to thermodynamically characterize the formation of SmNO
3
2+
and EuNO
3
2+
, processes we have not been able to study directly. 相似文献
76.
L. A. Errede E. B. Aus R. W. Duerst 《Journal of polymer science. Part A, Polymer chemistry》1992,30(6):1145-1153
The electron paramagnetic resonance (EPR) signal and the residual number (αt) of volatile molecules per phenyl group of poly(styrene-co-divinylbenzene) in samples that had been swelled to saturation in a dilute solution of a nitroxide spin-probe (TEMPO or 4-oxo-TEMPO), dissolved in a volatile liquid, were monitored simultaneously as the system containing excess liquid was allowed to evaporate to dryness. The results showed that the characteristic motionally narrowed three-line EPR spectrum began to change when αt became equal to αg (the number of sorbed molecules per phenyl group of polymer at liquid-saturation). The ratio of the intensity of the low-field and high-field hyperfine peaks relative to the middle peak decreased monotonically to an asymptotic limit that was attained when αt became equal to αg (the number of residual adsorbed molecules per phenyl group of polymer at completion of the transition from the rubbery state to the glassy state). The EPR hyperfine pattern, from which the rotational correlation times were estimated, changed most significantly as αt decreased from αG to αg while exhibiting inflections at about α′s and α′g the compositions that mark, respectively, incipient desorption of adsorbed molecules and incipient transition from the rubbery state to the glassy state. The pattern between these inflections points, however, varied with the affinity of the solvent for the polymer. 相似文献
77.
A new family of hydrophobically modified polyacrylamides was synthesized via copolymerization of acrylamide (AM) and anionic surface-active monomer of acrylamide-type, sodium 2-acrylamido-tetradecane sulfonate (NaAMC14S), in aqueous solution. In the copolymerization, by varying various factors, such as the feed ratio of NaAMC14S to AM and the amount of added electrolyte NaCl and initiator, we prepared copolymers NaAMC14S/AM with different block structures. The relationship between structures and hydrophobic association properties of copolymer chains was studied by using fluorescence probe and viscosimetry. Effects of the content and length of the hydrophobic blocks and the total molecular weight on hydrophobic association of the copolymers in pure water and in brine solution were examined, respectively. The results show that in pure water, hydrophobic association of the copolymers was enhanced as the content and length of the hydrophobic block increase. On the other hands, for a given content and length of the hydrophobic block, the hydrophobic association of the copolymers was enhanced as the total molecular weight increases. For all the copolymers studied, the apparent viscosity of their solutions in pure water has a limited value, but the apparent viscosities of the copolymer brine solutions are much higher than that of their corresponding water solutions, and show strong positive salinity sensitivity. Similarly, the hydrophobic association of the copolymer in brine solutions was enhanced as the content and length of the hydrophobic block increase. 相似文献
78.
Summary After a brief introduction of the subject, the paper focusses on the first step in any optimization procedure: the delineation
of the parameter space, wherein the global optimum is to be found. For organic modifier optimization in reversed-phase liquid
chromatography it is shown that the necessary information can be derived from a single water-methanol gradient. It first yields
an estimate of the total number of solutes in the sample, which is vital to define the peak capacity needed to achieve separation
at a certain confidence level. Next, the gradient allows the prediction of suitable isocratic methanol binary solvents, and
transfer rules formulate the iso-eluotropic composition of the common binary solvents (tetrahydrofuran and acetonitrile).
Because all predictions are based on a statistical analysis of a limited data base, attention is given to the practical situation
where an actual sample deviates from the average solute behaviour. Such deviations are revealed in the first isocratic run
and can be used to arrive at a better estimate of solute retention. 相似文献
79.
Paula Messina Marcela A. Morini Pablo C. Schulz Gerardo Ferrat 《Colloid and polymer science》2002,280(4):328-335
We used a battery of different methods to study the association in aqueous sodium dehydrocholate (NaDHC) solutions. This
salt associates by a stepwise mechanism. Below (9.6 ± 4.2) × 10−4 mol dm−3 there is a molecular solution with some strongly insoluble dehydrocholic acid produced by hydrolysis. Between (9.6 ± 4.2) × 10−4 and (5.2 ± 2.2) × 10−3 mol dm−3, an aggregate similar to acid soap (NaDHC.HDHC) appears and its amount and the aggregate's size increase with concentration.
At =(2.20 ± 0.85) × 10−2 mol dm−3 the aggregates formed have properties usually associated with true micelles, such as solubilisation of water-insoluble dyes.
These aggregates increase in size with concentration and change their shape at 8 × 10−2 mol dm−3, giving nonsymmetrical aggregates. The changes in the solution physicochemical properties at these concentrations may be
misinterpreted and this explains the different values of the critical micelle concentration reported in the literature for
substances with similar structure, such as bile salts.
Received: 14 May 2001 Accepted: 10 August 2001 相似文献
80.
John Ross George M. Whitesides Horia Metiu 《Angewandte Chemie (International ed. in English)》1979,18(5):377-392
The influence of molecular symmetry on reaction rates is examined with an approach in which reactions are viewed as electronic transitions between states of reacants and products (described, in turn by quasiadiabatic potential surface). The moleculer Hamiltonian is used to derive selection rules for these transitions. The complete Hamilatonian has no useful symmetery. Neglect of non-Born-Oppenheimer and spin-orbit terms (and of other angular momentum coupling terms) leads to an apporixmate Hamiltonian and to selection rules which from the basis of the Woodward-Hoffmann rules. This apporch provides an alternative to the adiabatic potantial surfaces, reaction coordinates, and transition state theory used in more familiar discussions of the Woodward-Hoffmann rules. Further, it provides a particulary clear method for discussing violations of these symmetry rules, and for differentiating concerted and nonconcerted reactions. 相似文献