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71.
As a solution theory, Raoult’s law is commonly used to estimate the activities of solutes and solvents of comparable molecular sizes while the Flory–Huggins (F–H) model is used for the activities of small liquids in high polymers. For a great many systems where the solute and solvent differ only moderately in molecular size (e.g., by 4–10 times), there has been no confirmed choice of a preferred model; examples of such systems are those of ordinary organic compounds in liquid triolein (MW = 885.4 g·mol?1) and poly(propylene glycol) (PPG) (MW = ~1,000 g·mol?1). The observed nearly athermal solubilities of many nonpolar organic solids in these solvents provide unique experimental data to examine the merit of a solution model. As found, Raoult’s law underestimates widely, and the F–H model underestimates slightly, the solid solubilities in triolein and PPG because these models underestimate the solution entropy for these solute–solvent pairs. To rectify this problem, the molecular segments of a large sized liquid solvent (e.g., triolein) are assumed to act as independent mixing units to increase the solute–solvent mixing entropy. This adjustment leads to a modified F–H model in which the “ideal” or “athermal” solubility of a solid in volume fraction, at a particular temperature, is equal to the solid’s activity at that temperature. Results from other studies give further support for the modified F–H model to interpret the partition data of compounds with organic solvents. 相似文献
72.
73.
Gregory S. Smith Cary B. Skidmore Philip M. Howe Jaroslaw Majewski 《Journal of polymer science. Part A, Polymer chemistry》2004,42(17):3258-3266
We present the first measurements of the simultaneous diffusion, surface enhancement, and evaporation of a plasticizer from a polymer, thin-film matrix using neutron reflection techniques. The reflectivity profiles as a function of the annealing time at an elevated temperature yield the time-dependent, plasticizer volume fraction profiles in a polyester–polyurethane (Estane) film. Thin, plasticizer-enriched layers form at both the polymer/substrate and polymer/air interfaces for annealed and unannealed samples. The diffusion equations for a material diffusing through a film and then evaporating into a vacuum at the free surface describe the loss of the plasticizer from the film for annealed samples. The loss of the plasticizer from the film is not limited by the movement of the plasticizer through the polymer matrix but is dominated by the plasticizer's rate of evaporation from the surface. The rate of evaporation and the volume fraction profiles for the plasticizer at the substrate interface are both consistent with surface attractions dominating over bulk attractions between the miscible plasticizer and the polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3258–3266, 2004 相似文献
74.
Cary L. Pint 《Surface science》2006,600(4):921-932
The melting transition in solid monolayers of a series of short-chained n-alkanes, n-octane (n-C8H18), n-decane (n-C10H22), and n-dodecane (n-C12H26) physisorbed onto the graphite basal plane are studied through use of molecular dynamics simulations. Utilizing previous experimental observations of the solid phase behavior of these monolayers, this study investigates the temperature dependence of the phases and phase transitions in these three monolayers during the solid-fluid phase transition, and compares the observed melting behavior to previous studies of hexane and butane monolayers. In particular, this study seems to indicate a greater dependence of the melting transition on the formation of gauche defects in the alkyl chains as the chain length is increased. In light of the previously proposed “footprint reduction” mechanism and variations where the formation of gauche defects are energetically negated, simulations seem to suggest that decane and dodecane monolayers are generally equally as dependent upon the formation of gauche defects for the melting transition to take place, whereas octane monolayers seem to have less dependence, but follow a trend that is established in previous studies of melting in butane and hexane monolayers. Also, the phase transition from a solid herringbone phase into an orientationally ordered “intermediate” phase is found to exhibit some differences as compared to a recent study of hexane monolayers, which may be interpreted as originating from the greater influence of gauche defects. Comparison to experimental melting temperatures is provided where possible, and applications involving thin film manipulation and lubrication is discussed. 相似文献
75.
G. Buogo G. Nadai A. Kern R. Scarpitti B. Rossi Mario Pittarelli M. F. Bengen E. Bohm Klostermann Schneck Cl. Zäch M. Paget Génel C. C. Proutv G. H. Wray G. P. Sanders E. B. Meigs N. R. Blatherwick C. A. Cary B. G. Hartmann F. Hillig W. Mohr W. Müller R. Barfuß-Knochendöppel Norabelle D. Weathers M. Fouassier W. Leithe G. Schwarz Edith Müller J. Tillmans R. Strohecker H. Hostettler S. Tanaka M. Endo H. Halstensen L. W. Brown W. V. Price G. Stamm J. Umbrecht Franz Maier F. E. Nottbohm O. Baumanna M. Siegfeld J. Pier O. Baumann J. Pien S. Herschdoerfer R. H. Shaw 《Analytical and bioanalytical chemistry》1934,99(11-12):439-452
76.
This paper describes the design, synthesis, and structural evaluation of a compound (4) comprising three molecular templates and a peptide strand that mimics a three-stranded protein beta-sheet. Two of the templates mimic the hydrogen-bonding functionality of peptide beta-strands and serve as the top and bottom strands by embracing the peptide strand, which is located in the middle of the sheet. The remaining template holds the three strands next to each other. The synthesis of artificial beta-sheet 4 begins with the bottom template and involves the sequential addition of the middle and top strands. (1)H NMR chemical shift and NOE studies establish that this compound folds to adopt a hydrogen-bonded beta-sheetlike structure in CDCl(3) solution. Chemical shift studies indicate that triply stranded artificial beta-sheet 4 is more tightly folded than its smaller doubly stranded homologue, artificial beta-sheet 1. 相似文献
77.
78.
Dichloroethylene (DCE), either cis or trans, was reacted with O3 at 23°C in both N2 and O2 buffered mixtures. Both reactant consumption and product formation were monitored by infrared spectroscopy and, in some cases, O3 consumption was monitored by ultraviolet absorption. For thoroughly dried mixtures, the initial products were only HCClO and O2, but geometrical isomerization also occurred. The stoichiometry of the overall reaction always was The HCClO was unstable and disappeared slowly in a first-order reaction which was, at least in part, heterogeneous. The products were CO and HCl so that the stoichiometric reaction was The rate law was complex. The rate was always faster in N2 than in O2. In the N2 buffered reaction, inhibition occurred as the reaction progressed and O2 was produced. From the reactant and product decay curves, the following rate behavior was established: where high and low concentrations are relative terms for the initial pressure ranges covered ([DCE]0 = 0.21?78.4 torr, [O3]0 = 0.30?6.76 torr). The rate coefficients k2, k3, and k4 were larger for the trans-DCE than the cis-DCE, and for each isomer they were larger in N2 than in O2 buffered reactions. The ozonolysis can be explained in terms of the mechanism where R2 is DCE, RO is HCClO, and RO2 is HCClO2. Rate ceofficients are computed. The isomerization is first order in [O3] and approximately first order in [DCE] for the limited kinetic data we were able to obtain. The isomerization does not appear to be explained by the reverse reactions of reactions (6), (7), and (9). Presumably isomerization occurs through some other route. 相似文献
79.
M. R. Davis C. E. Stronach R. S. Cary W. J. Kossler X. H. Yu H. E. Schone W. F. Lankford A. R. Moodenbaugh J. Oostens P. Boolchand 《Hyperfine Interactions》1991,63(1-4):305-309
The n-type cuprate superconductor and closely related samples of the general form Nd2−x
Ce
x
CuO4−y
have been studied with the muon spin relaxation technique. Non-super-conducting samples display antiferromagnetism (AFM)
withT
N below 150 K, the value being dependent upon both Ce concentration and oxygen depletion. The results are interpreted in terms
of carrier concentration. 相似文献
80.