Cyclohexane solution, effect

Cyclohexane. An excellent sohent for many determinations, particularly as, owing to the high value of K, a large fall in the freezing-point is obtained, and the accuracy of the determination is therefore correspondingly increased. Care should be taken to avoid super-cooling, however, as it has a marked effect on the true freezing-point of cyclohexane solutions. 

Reagents. Perylene was obtained from Sigma Chemical Company (St. Louis, Missouri). All other PAHs were supplied by Aldrich Chemical Company (Milwaukee, Wisconsin) and were reported to contain less that 3% impurities. All PAHs were used without further purification. Isopropyl ether (99%) for extraction work was also purchased from Aldrich. Hydroquinone, a fluorescent stabilizer present in the ether, was removed prior to solution preparation by rotary evaporation. Fluorometric-grade 1-butanol was supplied by Fisher Scientific Company (Fair Lawn, New Jersey). All solutions for extractions of PAHs were prepared by evaporating portions of a stock cyclohexane solution and diluting to the appropriate volume with isopropyl ether. Fluorescence measurements were performed on 1 10 dilutions of the stock and final organic phase solutions. The effect of dissolved CDx on the fluorescence intensity of the organic phase PAH was minimized by dilution with isopropyl ether. 

Conformational effects on 15N shifts in substituted cyclohexanes make an axial NH2 more shielded than an equatorial one. Also, 15N resonances are deshielded by ft substitution more extensively than are 13C resonances of cyclic hydrocarbons, but the magnitude of the effect depends on the degree of nitrogen substitution. Carbons in the y position shield the nitrogen in a manner analogous to 13C, but to a smaller extent in methanol than in cyclohexane solutions, and less for tertiary amines than for primary and secondary amines. These differences have been attributed in part to possible conformational influences on the stereoelectronic relationships between the lone pair and the C—C bonds. 

The effects of substitution and solvent polarity on the fluorescence properties of trans-9-styrylanthracenes 69a-k in terms of Stokes shift and fluorescence quantum yields have been summarized in Table 15. The fluorescence quantum yields in cyclohexane solution generally are about 0.5, exceptions with lower quantum yields (0.27) being the N,N-dimethylamino and nitro derivatives. For nonpolar substituted trons-9-styrylanthracenes in acetonitrile solution, the quantum yields are of the same order of magnitude as in cyclohexane. By contrast, the fluorescence quantum yields for trans-9-styrylanthracenes substituted by polar groups are drastically reduced in acetonitrile, as would be expected for bichromophoric excited state species of polar character (cf. Section III.B). 

Figure 15. Effect of segregation on polymerization of styrene in cyclohexane solution. Standard CSTR with h baffles and a 6-blade turbine, V = 670 cm, T = 75 °C. Dispersion Index DI vs. space time. Influence of agitation speed. Curves S (segregated flow) and M (well-micromixed flow) calculated from batch experiments. Initiator PERKAD0X l6, A = 0.033 mol L - -, kd = 5 x 10 5 s-1, f = 0.85 Mq = 6.65 mol L l, SQ = 2.22 mol IT1.

Dihydro-5,10-disilanthracene and AIBN can reduce xanthates and thiocarbonates in refluxing cyclohexane solution. Reduction of xanthates with monosilanes such as PhSiH3 and Ph2SiH2 initiated by AIBN does not work effectively because of their strong Si-H bond dissociation energies. However, the same reactions using either dibenzoyl peroxide or triethylborane as an initiator do induce the effective reduction of xanthates. 

Hourston142) has studied the effect of casting solvents on some physical properties of two SBS copolymers which seem to be Kraton 1101 and 1102. The properties of films cast from cyclohexane solution were found to be independent of the evaporation rate while those of films cast from toluene solution were found to be modified by the evaporation rate. 

As mentioned above, an important factor that controls the performance and especially the electrical properties of CNTs-reinforced composites is the state of dispersion of CNTs. Ultrasonication has been shown to be more effective in dispersing the nanotubes without the need for surfactants or other chemical treatments. Figure 12.5b presents electrical results of samples prepared by using a different composite processing. MWNTS were dispersed in this case in cyclohexane by ultrasonication and the MWNTs suspension was then mixed into a cyclohexane solution of SBR. Mixing was achieved by a further sonication for 30 minutes. Cyclohexane has been chosen in this case on account of the solubility of the rubbers in this solvent. As revealed in Figure 12.5b, the percolation threshold is shifted to a lower nanotube content and from this point of view, measurements of electrical resistivity appears as an indirect tool to evaluate the state of dispersion. 

The results of TCE adsorption from cyclohexane solution showed that the effect of the surface chemistry was neghgible, indicating that TCE was not preferentially adsorbed on the surface oxygen complexes. However, these complexes reduced TCE adsorption from the aqueous solution with respect to the nonoxidized carbon, due to the formation of water clusters, as in MTBE adsorption. 

Polymerization of carbon suboxide in heterogeneous systems, i.e. polymer precipitating from the reaction solution, by various initiators was examined qualitatively by Hegar (9). It was found that the best catalysts in cyclohexane solution (at 0°C., then room temperature) were triethylamine and pyridine. Boron trifluoride was somewhat less effective, while sulfuric acid was inefficient and slow. With aluminum chloride the catalyst particles were rapidly coated with polymer, and no significant amount of polymer could be obtained. 

Tladiation chemists have been aware for about 15 years that the presence of dilute solutes in liquid hydrocarbons can change the course of radiation chemical reactions by other than the normally expected secondary radical reactions. For example, Manion and Burton (40) in early work on the radiolysis of benzene-cyclohexane solutions, drew attention to the possibility of energy transfer from solvent to solute. Furthermore, it is known that in hydrocarbon solvents certain solutes are capable of capturing electrons, thus interfering with the normal ion-recombination process (14, 20, 65, 72). Though ionic products can be observed readily in hydrocarbon glasses [e.g., (19, 21)] demonstration of effects which can be specifically ascribed to electron capture in the liquid state has been elusive until recently. Reaction of positive ions prior to neutralization can play an important role as demonstrated recently by studies on... 

One further comment can be made. The hydrogen yield observed for cyclohexane solutions 0.1 M in CH3I is 2.5 (65). This is lower (by 0.8) than the yield observed at similar concentrations of other electron scavengers (Figure 3). The total G(H2) + G(CH 3) 1 Gr(CH4)UnRcaveiiKeabl equals 6.0 which can be compared with the slightly lower totals indicated in Table IV for methyl chloride solutions. Thus, while studies of the effect of methyl iodide on the positive-ion reactions of cyclopropane indicate that the methyl iodide does undergo positive-ion reactions, these reactions do not seem to make more than a relatively minor contribution (possibly — 0.4) to the products under discussion. 

Radiolysis. The photochemical experiments suggest that in the radiolysis a reaction of nitrous oxide with excited molecules would be expected in cyclohexane but should be less important in 2,2,4-trimethylpentane. The radiolysis results (Figure 3 and Table III) show that at nitrous concentrations less than 10 mM, where reactions of excited molecules are unimportant, G(N2) is the same for cyclohexane and 2,2,4-trimethylpentane solutions. At concentrations of nitrous oxide from 20 to 160 mM, G(No) from cyclohexane solutions is greater than G(N2) from 2,2,4-trimethylpentane solutions, and the excess yield increases with the concentration of nitrous oxide. [The nitrogen yields reported here for the concentration range 5-200 mM are in good agreement with those reported by Sherman (20)] Nitrous oxide reduces G(H2) from cyclohexane (16, 17, 18, 20, and Table III), but it has little effect on G(H2) and G(CH4) from 2,2,4-trimethylpentane. 

Photochemical methods can also be used to effect cyclodehydrogenation under mild conditions. Thus, irradiating a cyclohexane solution of a stilbene at 22-23° in presence of oxygen and a little iodine, by means of a mercury-arc lamp... 

In an article which is critical of many generally accepted molecular fluorescence parameters of aromatic molecules (and by inference the parameters for other systems), Birks emphasizes the precautions necessary to eliminate errors due to self-absorption secondary fluorescence and/or self-quenching.1 The points are made that reliable data for rf and Of are available for only a few compounds, e.g. diphenylanthracene (DPA), perylene, quinine bisulphate, and acridone, and that these provide suitable standards. The value of Of (DPA) is now set at 0.83. The importance of solvent effects on Of and t( of DPA is stressed in a publication which reports Tf for DPA in cyclohexane and benzene.2 The value of 6.95 0.04 ns for benzene solution is in good agreement with the earlier work of Birks and Dyson3 and Ware and Baldwin 4 7 (7.35 0.05 ns). The value obtained for cyclohexane solution, 7.58 0.04 ns, although in poor agreement with earlier results, is probably the most acceptable. The absolute fluorescence quantum yield of quinine bisulphate has also been redetermined (Of = 0.56).8... 

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