Natalia Lisitza - Academia.edu (original) (raw)

Papers by Natalia Lisitza

NMR spin-relaxation is directly sensitive to molecular dynamics and is therefore an excellent too... more NMR spin-relaxation is directly sensitive to molecular dynamics and is therefore an excellent tool for studying the formation of nano-aggregates. When aggregates form, the molecular rotations are slowed down, and the spin-relaxation rate is enhanced, which reduces the NMR signal. This gives rises to a remarkable kink in the NMR signal intensity at the Critical Nano-Aggregate Concentration (CNAC). In this talk, we use Debye's two-state model for micelle formation to describe asphaltene aggregation. We use the temperature dependence of the CNAC to determinate the enthalpy and entropy of aggregation. The enthalpy is negative, as expected, and indicates that the aggregate formation is energetically favorable due to π-stacking interactions. On the other hand, we find that the entropy is positive, which is quite a surprise. We propose that this increase comes from the depletion entropy of the solvent. As asphaltene molecules aggregate, the free volume available for the solvent molecules increases, giving rise to a depletion effect. An estimate of the depletion entropy from the sizes of asphaltene and toluene (solvent) is very close to the measured entropy gain. We will also discuss the applicability of Debye's two-state model to aggregate formation in asphaltene solutions.

The continuous rise of global demand for energy and the difficulty of significantly increasing pr... more The continuous rise of global demand for energy and the difficulty of significantly increasing production have driven the petroleum industry to develop much more difficult oil reservoirs, such as deep-water fields. Asphaltenes, naturally-occurring large aromatic molecules in crude oils, are known to be the ``cholesterol'' of petroleum because they self-associate to form solid aggregates and eventually clog the production pipes and the rocks. An extraordinary amount of work using many analytical techniques has been applied to elucidate the properties of asphaltenes. However, many fundamental issues, such as the molecular architecture and the aggregation mechanisms, are still in debate. Here we use NMR to detect asphaltene aggregation in toluene solutions and to obtain both the enthalpy and entropy of this process. We observe an abrupt drop of the asphaltene diffusion constant which is indicative of a molecular conformation (shape) change. This change is intimately related to or possibly a prerequisite of the aggregation. The entropy of aggregation was found to be positive due to the excluded volume effect for the solvent. This is reminiscent of the depletion entropy force, which is considered to be the driving force for the aggregation of much large artificial nanoparticles.

Aps Division of Atomic Molecular and Optical Physics Meeting Abstracts, May 1, 2005

Hyperpolarized noble gas MRI has recently emerged as a powerful diagnostic tool in medicine, as i... more Hyperpolarized noble gas MRI has recently emerged as a powerful diagnostic tool in medicine, as it allows researchers to obtain high-resolution lung images in real time. Yet perhaps more promising is the application of spin-polarized noble gas NMR to biological spectroscopy. Although both ^129Xe and ^3He benefit from the extremely high signal-to-noise ratios characteristic of polarized noble gas NMR, xenon is preferable for such studies because unlike helium it is highly lipophilic (thus readily absorbed by most tissues) and because its NMR chemical shift is much greater. Atherosclerosis is a good candidate for ^129Xe NMR study because xenon dissolved in blood will be absorbed by artery walls, exactly where the symptoms of the disease are most manifest. We have previously verified that ^129Xe spectroscopy can be used to detect the degree of atherosclerosis in human blood vessel samples by demonstrating that the dissolved xenon spectrum correlates with the apparent pathology and histology of the tissue. In this work we expand upon our earlier research by quantifying the features of the ^129Xe NMR spectrum in order to characterize the underlying physical effects of atherosclerosis. In addition to showing high-resolution NMR spectra of xenon dissolved in healthy and diseased artery tissue, we will also compare T1 and T2 data and diffusion measurements for the different samples.

Journal of Non-Crystalline Solids

Hyperpolarized ^129Xe has already found extensive applications in magnetic resonance imaging(M. S... more Hyperpolarized ^129Xe has already found extensive applications in magnetic resonance imaging(M. S. Albert et al., Nature 370, 199 (1994)) and nuclear magnetic resonance spectroscopy(J. P. Mugler, et al., Magn. Reson. Med. 37, 809 (1997)). Using hyperpolarized xenon in high-field NMR spectroscopy opens up new prospects for medical diagnostics of many pathologies and disorders(R. K. Mazitov, et al., Doklady Biophysics 364-366, 28-31 (1999)). Here we present a new set-up for production(N. N. Kuzma, et al., Phys. Rev. Lett. 88, 147602 (2002)), delivery, and temporary storage of pure hyperpolarized xenon gas for high-field (9.4T) NMR spectroscopy, with an emphasis on in-situ monitoring of ^129Xe polarization along the gas flow path and using a 1.1 T permanent magnet for cryogenic separation of hyperpolarized xenon from the buffer gases.

Spin-polarized ^129Xe has become an invaluable tool in nuclear magnetic resonance research, with ... more Spin-polarized ^129Xe has become an invaluable tool in nuclear magnetic resonance research, with applications ranging from medical imaging to high-resolution spectroscopy. High-field NMR studies using hyperpolarized xenon as a spectroscopic probe benefit from the high signal-to-noise ratios and large chemical shifts typical of optically-pumped noble gases. The experimental sensitivity is ultimately determined by the absolute polarization of the xenon in the sample, which can be substantially decreased during purification and transfer. NMR of xenon at high fields (9.4 Tesla) will be discussed, and potential mechanisms of spin relaxation during the distillation, storage(N. N. Kuzma, B. Patton, K. Raman, and W. Happer, Phys. Rev. Lett. 88), 147602 (2002)., and delivery of hyperpolarized xenon will be analyzed.

Asphaltenes, Heavy Oils, and Petroleomics, 2007

ABSTRACT We discuss examples and methods for using NMR diffusion measurements to obtain informati... more ABSTRACT We discuss examples and methods for using NMR diffusion measurements to obtain information about molecular sizes, their distributions, and dynamics. Scaling relationships between chain lengths and diffusion constants are derived and tested on diffusion measurements of many samples, including crude oils that are high in saturates. The diffusion constants of asphaltenes are also measured as a function of asphaltene concentration, indicating the formation of asphaltene aggregates at a concentration of approximately 0.2 g/L, and the sizes of the individual asphaltene molecules and aggregates are obtained. The examples and methods discussed in this paper can become the basis for in situ characterization of crude oils.

Physical Review B, 2002

The presence of pore structure affects diffusion in the pore-filling fluid. Such diffusion dynami... more The presence of pore structure affects diffusion in the pore-filling fluid. Such diffusion dynamics inside pores is often described mathematically by eigenmodes. A measurement of many eigenmodes is required for a detailed characterization of pore geometry. The short-wavelength modes are particularly important since they are capable of detecting the fine features. However, experimental excitation, detection, and manipulation of these eigenmodes are difficult for porous media. We report an experimental scheme to manipulate the eigenmode excitation spectrum by exploiting a unique symmetry of eigenmodes.

Applications and Science of Neural Networks, Fuzzy Systems, and Evolutionary Computation V, 2002

The joint action of two readily observed effects in solution magnetic resonance-radiation damping... more The joint action of two readily observed effects in solution magnetic resonance-radiation damping and the dipolar field-are shown to generate spatiotemporal chaos in routine experiments. The extreme sensitivity of the chaotic spin dynamics to experimental conditions during the initial evolution period can be used to construct a spin amplifier to enhance sensitivity and contrast in magnetic resonance spectroscopy and imaging.

STAPF: NMR IN CHEM. ENG. O-BK, 2005

Chemical Physics, 2002

Artifacts arising from aperiodic turbulent spin dynamics in gradient-based nuclear magnetic reson... more Artifacts arising from aperiodic turbulent spin dynamics in gradient-based nuclear magnetic resonance (NMR) applications are comprehensively surveyed and numerically simulated by a nonlinear Bloch equation. The unexpected dynamics, triggered by the joint action of radiation damping and the distant dipolar field, markedly deteriorate the performance of certain pulse sequences incorporating weak pulsed-field gradients and long evolution times. The effects are

Physical Chemistry Chemical Physics, 2010

The time-dependence of the NMR signal intensity of collagen type I is representative of protein a... more The time-dependence of the NMR signal intensity of collagen type I is representative of protein aggregation. It is pH sensitive and can be related to aggregation mechanism.

Magnetic Resonance Imaging, 2007

Natural fluids are often mixtures of many molecular species. Collective molecular dynamics of a s... more Natural fluids are often mixtures of many molecular species. Collective molecular dynamics of a subspecies can be critical for the system properties, such as in gels and micelles. However, the material heterogeneity often makes it difficult to study the moleculear dynamics directly. An example can be found in asphaltene-rich oils which exhibit a series of aggregation phenomena. Asphaltenes are naturally occurring large aromatic molecules in crude oils. They are known as the bcholesterolQ of petroleum because they self-associate and can eventually clog the rocks and production pipes. The asphaltenes are very heterogeneous, which makes understanding their aggregation processes difficult. We used spin-relaxation and diffusion to explore the initial stages of asphaltene aggregation in toluene solutions. We found that above a critical concentration (the CNAC), the spin-echo signal of the asphaltenes is substantially reduced as a result of fast spin relaxation within the aggregates. This fast relaxation appears to be due to the entanglement of the side chains when aggregates are formed, which results in hindered motion of the spins. The reduction in signal gives rise to a remarkable kink in the NMR proton signal intensity as a function of concentration . We used the temperature dependence of this kink to determine the enthalpy and entropy of aggregation. The enthalpy is negative (À15 kJ/mol), which indicates that the aggregation is energetically favorable. The entropy of aggregation was found to be positive (35 J/mol K). Although positive entropy changes are observed in aqueous solutions, due to the ordering of the solvent, and in mixtures with nano-particles, due to the excluded volume effect, they are unexpected for a nonpolar solvent. The diffusion measurements give the size of both the asphaltene molecules (r = 1.2 nm) and the aggregates (r = 3.6 nm). In addition, there is an abrupt drop in the asphaltene diffusion coefficient near the CNAC. This appears to reflect a conformational change of the asphaltene molecules and, because it is so rapid, is indicative of collective behavior which is surprising in such dilute solutions of heterogeneous molecules.

Journal of Non-Crystalline Solids, 1995