Temperature and Pressure Dependence of Self Diffusion in Long Liquid n‐Alkanes

Abstract

A titanium autoclave for the determination of self diffusion coefficients by the pulsed field gradient spin‐echo method is described, that has been used in the temperature range between 200 and 500 K at pressures up to 600 MPa.—In a first series of experiments, melts of n‐alkanes with chain lengths between 16 and 154 carbon atoms were studied. The data are analyzed with modifications of the hard sphere model and tested against the Rouse‐ and reptation model.

Scaling Laws for Diffusion Coefficients in Mixtures of Alkanes

“…However, such scaling laws often fail when the chain length distribution is broad and the contribution from monomer-sized molecules is significant [2]. Pure alkanes also show a similar scaling relation with 2 [7][8][9][10]. However, alkane mixtures such as crude oils exhibit a very broad chain length distribution; i.e., the distribution width is often comparable to the average chain length.…”

Density dependence of rotational and translational molecular dynamics in liquids studied by high pressure NMR

“…kT kT l+f =~kT/6nrlr , fl~oo Ds=¢t(o~=0 )=47tt/r 1 +(3/2)f (kT/4ntlr~ fl--, 0 (21) kT kT l+f=~kT/87Ol r3, fl ~ oo Dr = {r(tO = 0) = 8nnr 3 f (0o, fl ---~ 0 (22) f = flr/ 3rl.…”

“…When fl becomes large the liquid sticks at the surface of the sphere and eqns (21) and (22) reduce to the well-known Stokes-Einstein and Debye-Stokes relations, respectively. For vanishingly small fl the friction coefficient ~t(co=0)=4nr/r corresponds to perfect slip conditions.…”

“…Thus self-diffusion starts to decrease even at pressures where rotation is still enhanced by further compression. Furthermore, these pressure effects upon orientational and positional molecular displacements are the most direct indication of the failure of simple hydrodynamic mode-coupling relations (eqns (21,22)) to account for density effects on transport coefficients in cold water. Proton relaxation rate measurements in H-bonded liquids are especially difficult to interpret as a separation of inter-and intramolecular dipolar contributions to the relaxation rate cannot be achieved by the usual isotopic dilution technique because of rapid proton exchange.…”

“…Since quadrupolar relaxation is the dominant relaxation mechanism for the deuteron relaxation in both D20 and dioxane ds, 2H-T1 can provide direct information about the dynamic structure of these solutions. Lee and Jonas (Sts) discussed their relaxation time measurements mainly in terms of hydrodynamic mode-coupling relations (eqns (21,136)) and the effect of pressure on the reorientation of water and dioxane molecules in a strongly disrupted H-bond network. Figure 96 compares the pressure dependence of the deuteron T 1 of D20 (Fig.…”