Henning Groenzin

Molecular size of asphaltene fractions obtained from residuum hydrotreatment

Previously, fluorescence depolarization techniques (FD) have been shown to measure asphaltene molecular size, thereby establishing the substantial difference between asphaltenes derived from crude oil vs from coal. Here, FD is used to track the changes of the asphaltenes from a petroleum atmospheric resid feedstock that has been subjected to increasing thermal severity of catalytic hydrothermal cracking. Changes in asphaltene properties with increasing cracking are readily observed and understood. In addition, asphaltene molecular size is measured for various asphaltene solubility fractions in binary solvent mixtures of toluene with either n-heptane or acetone; a strong dependence is found of asphaltene properties on the particular solvent mixtures in accord with recent publications.

Proton order in the ice crystal surface.

The physics of the ice crystal surface and its interaction with adsorbates are not only of fundamental interest but also of considerable importance to terrestrial and planetary chemistry. Yet the atomic-level structure of even the pristine ice surface at low temperature is still far from well understood. This computational study focuses on the pattern of dangling H and dangling O (lone pairs) atoms at the basal ice surface. Dangling atoms serve as binding sites for adsorbates capable of hydrogen- and electrostatic bonding. Extension of the well known orientational disorder (“proton disorder”) of bulk crystal ice to the surface would naturally suggest a disordered dangling atom pattern; however, extensive computer simulations employing two different empirical potentials indicate significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms, as suggested long ago by Fletcher [Fletcher NH (1992) Philos Mag 66:109–115]. The presence of striped phase domains within the basal surface is consistent with the hitherto unexplained minor fractional peaks in the helium diffraction pattern observed 10 years ago. Compared with the disordered model, the striped model yields improved agreement between computations and experimental ppp-polarized sum frequency generation spectra.

Gas-Oil Ratio of Live Crude Oils Determined by Near-Infrared Spectroscopy

The gas-oil ratio (GOR) of crude oil has a tremendous impact on the processes used for oil production from subsurface formations. Determination of GOR thus becomes of central importance to oil operating companies. Here, it is shown that near-infrared (NIR) spectral measurements on single-phase crude oils (with dissolved gas) can be used to obtain a reliable estimate of the GOR. A simple theoretical formulation accurately accounts for the data. Furthermore, the NIR measurement is shown to be compatible with process flow streams at elevated temperatures and pressures.

Asphaltene Molecular Size and Weight by Time-Resolved Fluorescence Depolarization

The most important attribute of any chemical compound is its elemental constituents. There is, fortunately, no uncertainty about the elemental composition of asphaltenes. The second most important attribute of any chemical compound is its molecular structure and, as a prerequisite to that information, molecular weight. Although the set of structures of individual chemical units constituting asphaltene, such as the number of fused aromatic rings, length of aliphatic chains, and common functional groups is mostly agreed upon, the asphaltene molecular weight has been the subject of a large and long-standing controversy. For the most part, literature reports differ by a factor of 10, but some reports differ by many orders of magnitude. The question is essentially if and how the chemical units are linked. These uncertainties are exacerbated by the corresponding possibilities that different asphaltenes are variable, thus prohibiting facile comparison of results across different laboratories on different asphaltenes. This controversy has retarded the development of asphaltene science in that knowledge of structure–function relations is precluded if the structure is unknown. Consequently, a phenomenological approach has been routine in asphaltene science. We employ time-resolved fluorescence depolarization (TRFD) to measure the molecular rotational correlation time of a large variety of asphaltenes. TRFD methods naturally allow interrogation of different chromophore classes in the asphaltenes enabling stringent predictions to be tested regarding molecular weight and molecular structure. n-Heptane asphaltenes from virgin crude oils are found to have a molecular weight distribution with a mean at ∼750 g/mol, and a FWHM at 500 g/mol and 1000 g/mol, with a rapidly diminishing tail at higher molecular weight. There is little variation of molecular weight among virgin crude

MOLECULAR SIZE AND STRUCTURE OF ASPHALTENES

Fluorescence depolarization measurements are used to determine the size of asphaltene molecules and of model compounds for comparison. Mean molecular weights of 750 amu have been found for petroleum asphaltenes. A strong correlation is established between the size of fused rings in asphaltene molecules and the overall size of these molecules, showing that asphaltenes have one or perhaps two fused ring systems per molecule. Coal asphaltene molecules are found to be much smaller than petroleum asphaltenes.

The single-crystal, basal face of ice Ih investigated with sum frequency generation

Sum frequency generation spectroscopy has been used to investigate the hydrogen-bonded region of single-crystal, hexagonal ice in the temperature range of 113-178 K. The temperature and polarization dependences of the signal are used in conjunction with a recent theoretical model to suggest an interpretation of the bluest and reddest of the hydrogen-bonded peaks. The reddest feature is associated with strong hydrogen bonding; the dynamic polarizability of this feature is primarily parallel to the surface. It is assigned to a cooperative motion among the companion to the free-OH and four-coordinate oscillators hydrogen bonded to dangling lone-pair molecules on the surface. The bluest hydrogen-bonded feature is similarly assigned to a cooperative motion of the OH stretch of dangling lone-pair molecules and of four-coordinate molecules in the lower half bilayer that are hydrogen bonded to free-OH molecules. Reconstruction induced strain is present at as low as 113 K. These results provide a richer picture of the ice surface than has heretofore been possible.

Linearity of Near-Infrared Spectra of Alkanes

Near-infrared (NIR) spectroscopy is used to monitor a large variety of process flow streams. Hydrocarbons with their strong and resolvable NIR spectral signatures are good candidate analytes. NIR has been exploited to monitor many chemical properties for optimal hydrocarbon utilization particularly for well-characterized flow streams of small variability for end users. The utility of NIR in the context of the production of hydrocarbon resources necessitates application over a much broader range of flow stream conditions. Here we examine the spectral impact of variable temperature, pressure, and composition to determine the robustness of NIR methods in upstream applications.

Multiplexed polarization spectroscopy: measuring surface hyperpolarizability orientation.

Infrared-visible sum frequency generation (SFG) has seen increasing usage as a surface probe, particularly for liquid interfaces since they are amenable to few alternate probes. Interpreting the SFG data to arrive at a molecular-level configuration on the surface, however, remains a challenge. This paper reports a technique for analyzing and interpreting SFG data--called polarization-angle null or PAN-SFG. PAN-SFG enables ready identification of the ratio of the surface tangential and longitudinal hyperpolarizabilities--the hyperpolarizability direction--as well as the phase relationship between these components separated from the optical factors due to the substrate and experimental geometry. Separation of the surface optical factors results in an immediate connection between the null angle and the surface species polarization. If the Raman polarizability is also known, then PAN-SFG analysis, like the previously reported null techniques, provides a very accurate orientation. In addition, the reported polarization-angle, phase-shift analysis enables facile separation of the nonresonant background polarization from that of the resonant signal. Beyond orientation, PAN-SFG can be used to deconvolute overlapping resonances and identify components beyond a dipole response. This paper reports PAN-SFG for two systems providing deeper insight into both. An acetonitrile-water mixture was previously reported to undergo a phase transition at 7 mol %, attributed to a sudden change in orientation. PAN-SFG demonstrates that acetonitrile generates a classic dipole response and provides compelling evidence that the acetonitrile configuration remains constant as a function of concentration. An alternate model for the phase transition is presented. Like many aqueous systems, the SFG spectrum of the hydrogen-bonded region of ice consists of broad and overlapping features; features previously identified with PAN-SFG. Here PAN-SFG analysis is used to show that the reddest of these, the feature at 3098 cm(-1), contains a significant quadrupole contribution that grows as the temperature is lowered. The quadrupole and its temperature dependence are used to assign the 3098 cm(-1) feature to bilayer-stitching-hydrogen bonds. This is the first definitive assignment in the hydrogen-bonded region of water.

Hydrogen Bonding in the Hexagonal Ice Surface

A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm(-1). On the basal face, the reddest resonance occurs at 3098 cm(-1); within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm(-1), occurs slightly blue-shifted at 3393 cm(-1) in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm(-1) mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm(-1) mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm(-1) mode is entirely longitudinal.

Sum-frequency generation: Polarization surface spectroscopy analysis of the vibrational surface modes on the basal face of ice Ih

In recent years, sum-frequency generation (SFG) has been used to investigate numerous interfaces including aqueous interfaces. A longstanding challenge to interpretation of the SFG results, along with the related aqueous-solution infrared and Raman spectra, is a lack of connection between features in the broad hydrogen-bonded region and molecular-level interactions or configurations. This paper reports results of a newly developed polarization analysis of the generated sum-frequency signal as a function of wavelength both to deconvolute spectral resonances and to characterize the dynamic polarization associated with the resonances. Operationally, the polarization angle of the generated sum frequency is determined by identifying the null angle. The technique is hence termed polarization-angle null analysis or PAN. PAN applied to ice is very powerful; it reveals that the hydrogen-bonded region of the basal face of ice I(h) contains at least five oscillators, each with a distinct polarization. The dynamic polarizability of the longest wavelength oscillator is nearly entirely transverse (perpendicular to the surface normal, i.e., in the surface plane); in contrast, the shortest wavelength oscillator is almost entirely longitudinal (along the surface normal).