H. E. King

Rotator phases of the normal alkanes: An x‐ray scattering study

We present results of a detailed x‐ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time‐space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26‐27 carbon vicinity.

Pressure and temperature dependent viscosity of two glass forming liquids: Glycerol and dibutyl phthalate

The pressure and temperature dependent viscosities of two glass forming liquids, glycerol and dibutyl phthalate (DBP), have been studied in the range P=0–3 GPa, T=0–125 °C, and η=101–1010 cP. These studies were made using a combination of a rolling‐ball and a centrifugal‐force diamond anvil cell viscometer. The majority of the results extend up to viscosities of 107 cP, with those at 22.5 °C going to 1010 cP. The overall precision of the data are approximately 10% or better throughout. This level of precision allows us to define a viscosity surface which can then be extrapolated to the glass transition along both temperature and pressure cuts. The T‐dependence of viscosity is larger for glycerol than DBP but the P‐dependence smaller for glycerol than for DBP, whereas the T‐dependence is much more pressure sensitive for DBP. These data provide an assessment of the T‐dependence of an isothermal model (free volume), the P‐dependence of an isobaric model (Vogel–Tammann–Fulcher) and by extension that for isoch...

Fragility under pressure: Diamond anvil cell viscometry of ortho-terphenyl and salol

Viscosities of the fragile glass-forming liquids ortho-terphenyl and salol have been measured in a diamond anvil cell for pressures up to 1 GPa and for temperatures 300 K<T<425 K. Utilizing a rolling ball technique viscosities up to 108 mPas could be determined. From these data the pressure dependence of the fragility index, corresponding to a reference viscosity of about 108 mPas, has been calculated. For ortho-terphenyl it is found that this “low-viscosity” fragility index is pressure invariant within experimental error. Salol exhibits a more complicated behavior.

The diamond‐anvil cell as a high‐pressure viscometer

A high‐pressure viscometer based on a diamond‐anvil pressure cell is described and its use demonstrated. A sphere (typical diameter 50 μm) rolls down a diamond surface under the force of gravity and its position is tracked electronically. The resulting velocity is converted to viscosity through a modified Stokes equation that includes a term describing the increased drag due to the close proximity of the walls within the pressure chamber. A model for this wall correction is described and that the drag coefficient is constant throughout the data collection is demonstrated. The precision of the resulting viscosity data averages 4%. The accessible viscosity range is 0.5–107 cP using a diamond cell that is capable of pressures exceeding 10 GPa. However, most fluids exceed 107 cP at far lower pressures. The accuracy of the device is demonstrated by comparing the presented data on the pressure dependence of the viscosity for five fluids [chloroform, water, octamethyltrisiloxane, 1‐pentanol, and bis(2‐ethylhexyl...

STRUCTURAL EFFECTS OF HIGH PRESSURE GAS ON THE ROTATOR PHASES OF NORMAL ALKANES

The effects of high pressure gases (P≤400 bar) on the RII, RI, and RV rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x‐ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters: layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases: helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.

Impurity mediated nucleation in hexadecane-in-water emulsions

We report detailed nucleation studies on the liquid-to-solid transition of hexadecane using nearly monodisperse hexadecane-in-water emulsions. A careful consideration of the kinetics of isothermal and nonisothermal freezing shows deviations from predictions of classical nucleation theory, if one assumes that the emulsion droplet population is homogeneous. Similar deviations have been observed previously (3). As an explanation, we propose an argument based on the dynamic generation of droplet heterogeneity mediated by mobile impurities. This proposal is in good agreement with existing data.

A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation

Alkanes exhibit intermediate plastic–crystalline rotator phases between the liquid and the low-temperature fully ordered crystal. Here we report measurements of the liquid-to-rotator and rotator-to-crystal homogeneous nucleation kinetics using x-ray scattering on emulsified samples. These are the first reported studies of rotator-to-crystal homogeneous nucleation, and they show an anomalous temperature dependence of nucleation. Through use of mixed alkanes, we probe this over a wide temperature range. For three compositions along the C19H40–C20H42 binary, we find that the liquid-to-rotator interfacial free energy, calculated from the nucleation barrier in classical nucleation theory, is the same for all samples. However, the rotator-to-crystal interfacial free energy is both considerably smaller and strongly dependent on composition and temperature. As the temperature range of the rotator phase widens due to chain mixing, both the thermodynamic transition temperature as well as the interfacial energy for ...

High pressure study of associated media: Raman scattering of pyridine complexes in aqueous solution

The ν1 and ν12 ring breathing vibrational modes of pyridine in aqueous solution were studied as functions of concentration (up to 12 M) and pressure (up to 35 kbar) in a diamond anvil cell. The pyridine isotopes ‐h5 and ‐d5 exhibited marked differences. The pressure dependence of ν1 is quite linear for pyr‐h5 but has pronounced curvature for pyr‐d5 and this curvature increases as the concentration decreases. This unusual isotope effect is attributed to pyridine–water complexes involving both O–H ⋅ ⋅ ⋅N and C–H ⋅ ⋅ ⋅O hydrogen bonds. For ν12, the pressure dependence does not change with concentration, but it is at least five times more steep for pyr‐h5 than pyr‐d5. This may occur because the mass at all six ring vertices becomes equal for the ‐d5 isotope, making the volume change during vibration very small. At high concentrations, ν1 appears as two distinct peaks with different pressure dependence, indicative of uncomplexed and complexed pyridine. For certain combinations of solvent and concentration, a s...

Compression of organic crystals

Abstract High-precision, isothermal compression data measured on several organic crystals at ambient temperature are collected from the literature and used to test two scaling models for the bulk moduli. Use of a simple inter-atomic potential energy expression is found to be inadequate, and this is attributed to the importance of thermal energy in these materials. This contrasts sharply with the situation for ionically bonded compounds. A second model, where space filling by incompressible molecules is assumed to control the bulk modulus, is found to explain the overall trends in bulk moduli variation, but substantial scatter in the data is found. Through effective medium theory one finds that the likely cause of this scatter is the neglect of molecule shape anisotropy.

Modeling Oil Recovery for Mixed Macro- and Micro-Pore Carbonate Grainstones

In general, modeling oil-recovery is a challenging problem involving detailed fluid flow calculations with required structural details that challenge current experimental resolution. Recent laboratory experiments on mixed micro- and macro-pore suggest that there is a systematic relationship between remaining oil saturation (ROS) and the fraction of micro-pores. Working with experimental measurements of the pores obtained from X-ray tomography and mercury intrusion capillary pressure porosimetry, we define a digital rock model exemplifying the key structural elements of these carbonate grainstones. We then test two fluid-flow models: invasion percolation model and effective medium model. Although invasion percolation identifies the important impact of macro-pore percolation on permeability, it does not describe the dependence of ROS on micro-pore percentage. We thus modified the effective medium model by introducing a single-parameter descriptor, reff. Oil from pores r ≥ reff is fully removed, while for the remaining pores with r < reff, their contribution is scaled by (r/reff)2. Applying this straightforward physics to pore size distributions for the mixed-pore grainstones reproduces the experimental ROS dependence.