Structural Isomerization and Molecular Motions of Liquidn-Alkanes. Ultrasonic and High-Frequency Shear Viscosity Relaxation

Abstract

Between 20 and 120 MHz, the complex shear viscosity of n-tetradecane, n-pentadecane, n-hexadecane, and a n-eicosane/n-tetradecane mixture has been measured using a shear impedance spectrometer. The data are compared to ultrasonic absorption spectra of the liquids as measured between 1 MHz and 2 GHz. Relaxational behavior has been found. The extrapolated high-frequency shear viscosity ηs(∞) is distinctly smaller than the static shear viscosity ηs(0). The shear viscosity relaxation time is discussed in light of literature data of the orientational correlation time from depolarized Rayleigh scattering and also of the collision time as resulting from dielectric spectrometry. The assumption appears to be likely that the shear viscosity relaxation is due to rotational isomerizations of the chain molecules. A damped torsional oscillator model has been used to evaluate the spectra. The reasonable number of three carbon atoms per oscillator unit results. The activation energies and enthalpies are derived from measurements at different temperatures.

Critical Fluctuations of Membranes: Spectroscopy Meets Thermodynamics

“…6 Relaxation times of Debye-term "1" in the ultrasonic spectra (closed symbols [29,31]) and dielectric relaxation times of the phospholipid head group reorientations (open symbols [69] Relaxation times τ 2 of the high-frequency process reveal a distinct step-like change at T m . The τ 2 values between roughly 100 ps and 1 ns agree with ultrasonic relaxation times of n-alkanes with corresponding lengths [70]. In conformity with results from NMR studies of deuterium-labeled membrane systems [5], they have been assigned to a collective mode of alkyl chain conformational isomerization of the lipids [29] and have been discussed in terms of a torsional oscillator model [71,72].…”

Hydrogen Network Fluctuations of Alcohols. Evidence from Ultrasonic, Dielectric, and Shear Impedance Spectrometry

“…10, ν t0 is the characteristic frequency for the rotation of an individual C-C bond, n t0 is the number of torsional oscillator par alkyl chain, and R is the universal gas constant. Using ν t0 = 8.5 THz as reported for ethane [27] and n t0 = n/3, where n is the number of methyl groups per alkyl chain, reasonable values between 15.9 kJ · mol −1 (n-hexanol) and 13.8 kJ · mol −1 (n-dodecanol) follow [14], in fair agreement with Gibbs free energies of activation for n-alkanes (e.g., G # = 14.1 kJ·mol −1 , n-dodecane [24]). The chain isomerization is associated with a high-frequency shear viscosity relaxation [24].…”

“…yields shear viscosities η s (∞) -η s2 on the order of 1 mPa·s [24] at frequencies well above the relaxation region (ν > 5 GHz).…”

“…As the concentration of single-Hbonded OH-groups is significantly smaller than that of the double-bonded hydroxy groups, term "d2" with amplitude A d2 smaller than A d1 [12] is assumed to be due to the ; = 6, 7, ; = 7, 5, •; = 12, 7, ; [12]). The inset shows ultrasonic relaxation times τ α2 of the high-frequency term of alcohol spectra [14] and relaxation times τ α of the Debye-type relaxation of alkane spectra at 25 • C as a function of number of methyl groups [24]. Lines are drawn to guide the eye terminal OH-groups and term "d1" is assigned to the multiply H-bonded OH-groups within clusters.…”

Volumetric and acoustical study of ammonium ceric (IV) sulphate in aqueous medium at T=(283.15,293.15,303.15 and 313.15)K

“…[1][2][3][4][5] . Acoustical study provides an excellent tool for solute-solvent interaction, solvent-solvent interaction study which are used to elucidate structural isomerism and molecular motion in n-alkanes 6 , aqueous fluids 7 ,desorption of metal ions from activated carbon 8 etc. So volumetric and acoustical study in aqueous and non-aqueous solvent media predict a basic idea about solution theory and molecular dynamics.…”