Cyclohexane self-diffusion

The conclusion of all these thermodynamic studies is the existence of thiazole-solvent and thiazole-thiazole associations. The most probable mode of association is of the n-rr type from the lone pair of the nitrogen of one molecule to the various other atoms of the other. These associations are confirmed by the results of viscosimetnc studies on thiazole and binary mixtures of thiazole and CCU or QHij. In the case of CCU, there is association of two thiazole molecules with one solvent molecule, whereas cyclohexane seems to destroy some thiazole self-associations (aggregates) existing in the pure liquid (312-314). The same conclusions are drawn from the study of the self-diffusion of thiazole (labeled with C) in thiazole-cyclohexane solutions (114). 

Fig.3.1.9 (a) The adsorption-desorption isotherm (circles, right axis) and the self-diffusion coefficients D (triangles, left axis) for cyclohexane in porous silicon with 3.6-nm pore diameter as a function of the relative vapor pressure z = P/PS1 where Ps is the saturated vapor pressure, (b) The self-diffusion coefficients D for acetone (squares) and cyclohexane (triangles) as a function of the concentration 0 of molecules in pores measured on the adsorption (open symbols) and the desorption (filled symbols) branches. 

The self-diffusion of benzene in PIB [36], cyclohexane in BR [37] and toluene in PIB [38-40] has been investigated by PFG NMR. In addition more recently Schlick and co-workers [41] have measured the self-diffusion of benzene and cyclohexane mixtures in polyisoprene. In the first reported study of this kind, Boss and co-workers [36] measured the self-diffusion coefficients of benzene in polyisoprene at 70.4 °C. The increase in Dself with increasing solvent volume fraction could be described by the Fujita-Doolittle theory which states that the rate of self-diffusion scales with the free volume which in turn increases linearly with temperature. At higher solvent volume fractions the rate of selfdiffusion deviates from the Fujita-Doolittle theory, as the entanglement density decreased below the critical value. 

Figure 13.2 Self-diffusion coefficients of cyclohexane in BR solutions as a function of temperature and solvent volume fraction (Vj = 0.05 (O), 0.19 ( ), 0.29 ( ), 0.375 ( ) and 0.45 (A) [37]. The solid and dashed lines are best fits to the WLF equation [37]...

Since the pioneering work of Stejskal, the pulse field gradient method is currently used to characterize the diffusion process of small molecules or of macromolecules in dilute or semi-dilute solutions [18-20]. In this Chapter, the NMR approach is illustrated from the self-diffiision of ( dohexane molecules through polybutadiene. Variations of the Ds self-diffusion coefficient of cyclohexane in polybutadiene have been reported as a temperature function considering several concentrations [21]. 

Fig. 11 The dynamic structure factor C(, r) of polybutadiene star 12880 (nominally f = 128, Ma = 80kgmol ) in cyclohexane at ci = 0.016gmL and q = 0.035nm , along with the fit (solid line) from the ILT analysis. The corresponding relaxation distribution function L(ln(T)) (shown here for f i and q = 0.023gmL ) embraces the cooperative diffusion (1), the collective apparent diffusion (2), and the self-diffusion (3). The slowing-down of the middle structural mode (2) and the increase of its intensity with q are shown in the upper inset whereas the lower cartoon illustrates the liquid-like ordering [43,189]. The core regions are drawn out of scale (larger) for clarity...

Figure 18 Illustration of the double-oil self-diffusion experiment with a cyclohexane-hexadecane mixture. K = D /D2, where D and Dj are the cyclohexane and hexadecane diffusion coefficients, respectively. In the pure oil mixture the ratio of the two diffusion coefficients is K = Kq— 1.69. For a water-in-oil droplet structure the two oil molecules have the same diffusion coefficient, that of the micelle, and the ratio A equals unity. In a bicontinuous structure, on the other hand, a molecular diffusion mechanism is dominating and the ratio K equals that of the pure oil mixture, Kq. By monitoring the diffusion coefficient ratio, the droplet-to-bicontinuous transition could be studied.

Self-diffusion measurements using the NMR technique have also elucidated the matter of water structure in the pool up to 10-15 water molecules per — 03 group (when AOT is the surfactant) were shown to be structurally perturbed [42]. From self-diffusion NMR studies of AOT-induced w/o microemulsions, Maitra proposed several regions of bound water in the water pool domain [43]. The state of water in the pool for cationic surfactant-containing microemulsions (water/dodecyidimethyl ammonium bromide or didodecyldimethyl ammonium bromide/chloroform or cyclohexane) was examined by NMR self-diffusion studies at different [Fl20]/[amphiphile] ratios or rvalues. Three types of water molecules— bound, aggregated, and free—in various proportions have been envisaged [44]. 

Figure 17.8. The variation of the reduced self-diffusion coefficients, D/Dq, of water (A) and cyclohexane (O) with the total oil volume fraction within the microemulsion channel of the system displayed in Figure 17.6. (data taken from ref. (14))...

With nonionic surfactants, it is possible to generate an oil-in-water droplet to bicontinuous structural transition at constant composition by increasing the temperature. This transition can be studied in more detail by using self-diffusion experiments with mixed solvents. In a system where the oil was an equal weight mixture of cyclohexane (1) and hexadecane (2) the transition from normal oil-swollen micelles to a bicontinuous microstructure was studied by monitoring the variation of the self-diffusion coefficient ratio, K = as a... 

A similar series of samples as in the SANS experiments was studied in cooperation with the group of Prof. Wokaun by NMR self-diffusion experiments. Tbe pulsed field gradient spin echo (PGSE) method [67, 68] allows the determination of the self-diffusion coefficient of each of the individual constituent components in particular water, surfactant, and hydrocarbon. Here, in order to obtain simpler NMR spectra the hydrocarbon was cyclohexane. The molar ratio of C14DMAO cyclohexane was chosen to be 1 1.2, with three samples in the Lj phase and three samples in the cubic phase. 

Figure 11.24 Self-diffusion coefficient of surfactant and hydrocarbon in logarithmic representation as a function of the micellar volume fiaction for the system Ci4DMAO/cyclohexane/D20. The molar ratio of C14DMAO cyclohexane was always 1 1.2. a) of (N(CH3)2) b) of (CH2) c) of (CH2) for samples with deuterated cyclohexane d) of H determined by means of a different instrument [69] e) of H for samples with deuterated cyclohexane [69].

Figure 20.2 Influence of the hydrocarbon chain length m on the micellar self-diffusion coefficient D in ternary Ci4DMAO/CmH2ni+2/D20 surfactant systems. For m = 6, cyclohexane C6H12 has been chosen as the solute.

The exit rate constants for cyclohexane and p-xylene from the micelles present in the cubic phase of the sodium octanoate/octane/water system have been measured by combining TRLQ and NMR self-diffusion data on the system. The value found for p-xylene compares well with that determined on the basis of solubility measurements. ... 

The mass exponent value x decreases as the temperature increases to reach a value of 3-45 at 55 C when cyclohexane can be considered as a good solvent for polystyrene. One has to note that the exponent x is larger than the predicted va e x = 3. Self-diffusion coefficient measurements lead to Dg -- Af. Assuming that is the time required for the polymer chain to diffuse on a distance of order of its radius of gyration R - (R = Tffig) we should have Present results for relaxation time therefore disagree with self-diffusion coefficient measurements. From viscosity and relaxation time measurements, we can deduce the shear elastic modulus at short times G We find that G is mass-independent and increases with concentration ... 

C NMR spectra of CH4 in an AIPO4-II molecular sieve have revealed exchange effects between adsorbed and nonadsorbed methane gas. The diffusion of a mixture of methane and xenon in the zeoHte siUcahte has been studied by pulsed field gradient NMR spectroscopy. In situ H MAS NMR studies of the H/D exchange of deuterated propane adsorbed on H-ZSM-5 have been reported. C MAS NMR spectroscopy has been used to study the initial stages of propane activation over H-ZSM-5, and sorption properties of hnear alkanes in ferrierite. Protonated sites on sulfate-promoted zirconium oxide catalysts have been studied using H NMR spectroscopy. NMR relaxation and self-diffusion of pentane, neopentane, dodecane, benzene, cyclohexane, and... 

Fig. 3. Apparent self-diffusion coefficient of cyclohexane Cg Hi 2 adsorbed on silica, 90 A, 10p(gs) as a function of coverage.

Characterization of catalysts The zeolite structure was checked by X-ray diffraction patterns recorded on a CGR Theta 60 instrument using Cu Ka, filtered radiation. The chemical composition of the catalysts was determined by atomic absorption analysis after dissolution of the sample (SCA-CNRS, Solaize, France). Micropore volumes were measured by N2 adsorption at 77 K using a Micromeritics ASAP 2000 apparatus and by adsorption of cyclohexane (at P/Po=0.15) using a microbalance apparatus SET ARAM SF 85. Incorporation of tetrahedral cobalt (II) in the framework of Co-Al-BEA and Co-B-BEA was confirmed by electronic spectroscopy [18] using a Perkin Elmer Lambda 14 UV-visible diffuse reflectance spectrophotometer. Acidity measurements were performed by Fourier transform infrared spectroscopy (FT-IR, Nicolet FTIR 320) after pyridine adsorption. Self-supported wafer of pure zeolite (20 mg/cm ) was outgassed at 673 K for 6 hours at a pressure of lO Pa. After cooling at 423 K, the zeolite was saturated with pyridine vapour (30 kPa) for 5 min, evacuated at this temperature for 30 min and the IR spectrum was recorded. 

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