Probe molecules ethane

The combined application of PFG NMR self-diffusion and tracer desorption experiments has thus proved to be an effective tool for studying the hydrothermal stability of A-type zeolites with respect to their transport properties [186]. It turns out that with commercial adsorbent samples there are considerable variations in hydrothermal stability between different batches of product and even between different pellets from the same batch. As an example. Fig. 24 shows the distribution curves [A(Tin,ra) versus Ti ,r.j] measured with ethane as a probe molecule at 293 K for two different samples of commercial 5A zeolites. Evidently batch 1 is more resistant to hydrothermal deterioration, because the lengthening of Tjn,ra is less dramatic than with batch 2. Since the intracrystalline diffusivity was the same for all samples, the deterioration can be attributed to the formation of a surface barrier. 

Figure 24 Distribution curves (A (Ti , ) versus Tj ,ra) measured with ethane as the probe molecule at 293 K for two different samples of a commercial 5 A zeolite, (a) The initial material (b) the same material after hydrothermal pretreatment. (From Ref. 186.)...

The differential molar adsorption heats of non-polar molecules such as ethane, n-butane, -pentane, and n-hexane on AIPO4-II, AIPO4-5, and VPl-5 activated at 670 K have been measured at a temperature of 303 K by Stach et al. [211]. The initial adsorption heats of n-paraffins increased with increasing density of the T-atoms (or decreasing pore diameter), and also increased step-by-step with increasing munber of CH2 groups of the probe molecules. 

The CH and CC vibrational modes of ethane were studied as a function temperature and pressure in the liquid, vapor, and SCF region. This system offers an opportunity to probe near critical solvation forces and their effects on different internal molecular coordinates within the same solute molecule. The room temperature frequency shifts values for the CC and symmetric CH stretch vibrations are shown in Figure 5. 

These lifetimes are far too short to support the hypothesis made at the beginning of this section. In our opinion the possibility that clusters formed by more than six molecules can attain lifetimes of several nanoseconds is rather unlikely anyway, work is in progress to probe the behavior of such larger cluster, whose simulation is more CPU-demanding. The present preliminary data seem to exclude the concerted diffusion of ethane cluster as a possible source of single file behavior over long time scales the clusters simply do not live enough to cover such times. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

In two-phase systems, however, where surfactant and water can partition between a fluid and a liquid phase, significant pressure effects occur. These effects were studied for AOT in ethane and propane by means of the absorption probe pyridine N-oxide and a fluorescence probe, ANS (8-anilino-l-naphthalenesulfonic acid) [20]. The UV absorbance of pyridine A-oxide is related to the interior polarity of reverse micelles, whereas the fluorescence behavior of ANS is an indicator of the freedom of motion of water molecules within reverse micelle water pools. In contrast to the blue-shift behavior of pyridine N-oxide, the emission maximum of ANS increases ( red shift ) as polarity and water motion around the molecule increase. At low pressures the interior polarity, degree of water motion, and absorbance intensity are all low for AOT reverse micelles in the fluid phase because only small amounts of surfactant and water are in solution. As pressure increases, polarity, intensity, and water motion all increase rapidly as large amounts of surfactant and water partition to the fluid phase. The data indicate that the surfactant partitions ahead of the water thus there is a constant increase in size and fluidity of the reverse micelle water pools up to the one-phase point. An example of such behavior is shown in Fig. 4 for AOT in propane with a total fVo of 40. The change in the ANS emission maximum suggests a continuous increase in water mobility, which is due to increasing fVo in the propane phase, up to the one-phase point at 200 bar. 

Another TICT molecule, ethyl p-(A,A -diethylamino)benzoate (DEAEB), was used to probe solute-solvent interactions in supercritical ethane, CO2, and fluoroform (3,50,56). Unlike DMABN and DMAEB, DEAEB forms a TICT state even in nonpolar solvents (Figure 13) (50), resulting in dual fluorescence emissions. Because of the excited-state thermodynamic equilibrium, the relative intensities (or fluorescence quantum yields) of the LE-state (xle) and TICT-state ( Ticr) emissions may be correlated with the enthalpy (AH) and entropy (A5) differences between the two excited states ... 

PBpT-012 in the nematic LC phase at 160°C is mechanically stretched to obtain highly-oriented fibers by cooling at room temperature. The nature of the channels with a diameter of about 3 nm in polyester fibers may be expected as one of materials with separation function. The nature of the inside of the fibers has been clarified through the observation of the diffusion coefficients of probe small-size molecules, methane and ethane, in direction parallel and perpendicular to the long channel by means of pulse field-gradient spin-echo(PFGSE) H NMR spectroscopy [34, 35],... 

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