Molecular dynamics and composition of crude oil by low‐field nuclear magnetic resonance

Nuclear magnetic resonance (NMR) techniques are widely used to identify pure substances and probe protein dynamics. Oil is a complex mixture composed of hydrocarbons, which have a wide range of molecular size distribution. Previous work show that empirical correlations of relaxation times and diffusion coefficients were found for simple alkane mixtures, and also the shape of the relaxation and diffusion distribution functions are related to the composition of the fluids. The 2D NMR is a promising qualitative evaluation method for oil composition. But uncertainty in the interpretation of crude oil indicated further study was required. In this research, the effect of each composition on relaxation distribution functions is analyzed in detail. We also suggest a new method for prediction of the rotational correlation time distribution of crude oil molecules using low field NMR (LF‐NMR) relaxation time distributions. A set of down‐hole NMR fluid analysis system is independently designed and developed for fluid measurement. We illustrate this with relaxation–relaxation correlation experiments and rotational correlation time distributions on a series of hydrocarbon mixtures that employ our laboratory‐designed downhole NMR fluid analyzer. The LF‐NMR is a useful tool for detecting oil composition and monitoring oil property changes. Copyright © 2016 John Wiley & Sons, Ltd.     

A nuclear magnetic resonance study of dynamics in toluene-polyisobutylene solutions: 1. Penetrant diffusion and Fujita theory

The self-diffusion coefficients of toluene in polyisobutylene (PIB) solutions were determined using the pulsed field gradient nuclear magnetic resonance technique. The volume fraction of toluene in the polymer was varied from 0.045 up to 0.712 and the temperature was varied from 225 K up to 368 K. The concentration dependence of the data was interpreted using the Fujita free volume theory and the temperature dependence was interpreted with the WLF equation. These models describe separately the concentration and temperature dependencies of the toluene self-diffusion coefficients very well and the resulting free volume parameters are in good agreement with the ones extracted from the analysis of viscosity data on the same system. ©1995 John Wiley & Sons, Inc.     

Self‐diffusion and interactions in mixtures of imidazolium bis(mandelato)borate ionic liquids with polyethylene glycol: 1H NMR study

We used ¹H nuclear magnetic resonance pulsed‐field gradient to study the self‐diffusion of polyethylene glycol (PEG) and ions in a mixture of PEG and imidazolium bis(mandelato)borate ionic liquids (ILs) at IL concentrations from 0 to 10 wt% and temperatures from 295 to 370 K. PEG behaves as a solvent for these ILs, allowing observation of separate lines in ¹H NMR spectra assigned to the cation and anion as well as to PEG. The diffusion coefficients of PEG, as well as the imidazolium cation and bis(mandelato)borate (BMB) anion, differ under all experimental conditions tested. This demonstrates that the IL in the mixture is present in at least a partially dissociated state, while the lifetimes of the associated states of the ions and ions with PEG are less than ~30 ms. Generally, increasing the concentration of the IL leads to a decrease in the diffusion coefficients of PEG and both ions. The diffusion coefficient of the anion is less than that of the cation; the molecular mass dependence of diffusion of ions can be described by the Stokes–Einstein model. NMR chemical shift alteration analysis showed that the presence of PEG changes mainly the chemical shifts of protons belonging to imidazole ring of the cation, while chemical shifts of protons of anions and PEG remain unchanged. This demonstrated that the imidazolium cation interacts mainly with PEG, which most probably occurs through the oxygen of PEG and the imidazole ring. The BMB anion does not strongly interact with PEG, but it may be indirectly affected by PEG through interaction with the cation, which directly interacts with PEG. Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetic resonance imaging of spiral patterns in crosslinked polymer gels produced via frontal polymerization

Frontal polymerization is a process in which a localized reaction zone propagates through a monomer reactant mixture, leaving a polymer product in its wake, and is the result of the coupling of the thermal transport and Arrhenius dependence of the exothermic polymerization. Under most conditions, a planar front is stable. However, for multifunctional acrylates at room temperature, fronts may propagate in a helical fashion along the axis of the reactor. This front propagation is typical of what is called a spin mode, in which the subsequent polymer sample has alternating spiral patterns of low and high monomer conversion evident on the sample surface. For the first time, we demonstrate that magnetic resonance imaging on a submillimeter scale can be used to show that the spiral patterns are not restricted to the sample surface but are distributed throughout the volume. Samples were soaked in water, and the transverse proton relaxation times were imaged. The results suggest proton mobility is smaller in the high‐conversion region in which the hot spot propagated than in the low‐conversion region. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1075–1080, 2001     

A nuclear magnetic resonance study of toluene-polyisobutylene solutions. 2. Vrentas-Duda theory

The self-diffusion coefficients of toluene in polyisobutylene have been analyzed using the Vrentas-Duda free volume diffusion model. The diffusion coefficients were determined at different temperatures and concentrations, using the pulsed field gradient nuclear magnetic resonance technique. The data were satisfactorily described by the model and the size of the polymer jumping unit was extracted. Comparisons were made with the Fujita free volume theory and the Fujita free volume parameters were extracted from the Vrentas-Duda free volume parameters. From the diffusion data that now available, it can be concluded that for most polymers the jumping unit is about 1.5 times the polymer monomer molecular weight. The activation energy of the toluene diffusion in polyisobutylene is compared with the activation energies of other penetrants in the same polymer. The diffusion data presented in this work show that the energy per mole required to overcome the attractive forces which constrain a diffusing species to its neighbors should be considered to be zero, in order to be able to extract the free volume parameters (from viscosity and diffusion data) with an acceptable uncertainty. ©1995 John Wiley & Sons, Inc.     

Influence of local chain dynamics on diffusion of gases in polymers as determined by pulsed field gradient NMR

Diffusion of gases in polymers below the glass transition temperature, T_g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon‐13 PFG NMR measurements of [¹³C]O₂ diffusion in polymer films at 298 K are performed. Data obtained in polymers with T_g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T_g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T_g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010     

Mechanism of Water Dynamics in Hyaluronic Dermal Fillers Revealed by Nuclear Magnetic Resonance Relaxometry

¹H spin−lattice nuclear magnetic resonance relaxation experiments were performed for five kinds of dermal fillers based on hyaluronic acid. The relaxation data were collected over a broad frequency range between 4 kHz and 40 MHz, at body temperature. Thanks to the frequency range encompassing four orders of magnitude, the dynamics of water confined in the polymeric matrix was revealed. It is demonstrated that translation diffusion of the confined water molecules exhibits a two-dimensional character and the diffusion process is slower than diffusion in bulk water by 3–4 orders of magnitude. As far as rotational dynamics of the confined water is concerned, it is shown that in all cases there is a water pool characterized by a rotational correlation time of about 4×10⁻⁹ s. In some of the dermal fillers a fraction of the confined water (about 10 %) forms a pool that exhibits considerably slower (by an order of magnitude) rotational dynamics. In addition, the water binding capacity of the dermal fillers was quantitatively compared.     

An NMR investigation on the phase structure and molecular mobility of the novel exfoliated polyethylene/palygorskite nanocomposites

Novel exfoliated polyethylene (PE)/palygorskite nanocomposites prepared by in situ polymerization are characterized by solid‐state nuclear magnetic resonance (NMR). The phase structure and molecular mobility are investigated by a combination of proton and carbon NMR. The results showed that incorporation of small amounts of palygorskite had great influence on the phase structure and molecular mobility. The incorporated palygorskite hindered the crystallization process and introduced motion‐hindered chains in the NMR crystalline and amorphous phase. ¹³C cross‐polarization and magic‐angle spinning NMR revealed two orthorhombic crystalline phase with different line‐width. The chain mobility of orthorhombic crystalline phase with broad resonance line is obviously hindered compared with the phase with narrow resonance line when the filler is introduced. Additionally, the results of pulsed field gradient NMR technique show those the tortuosities in the nanocomposites are much higher than that in the bulk PE. The self‐diffusion process of probe molecules is also influenced by the palygorksite load. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1363–1371, 2010     

Nuclear magnetic resonance spectroscopic analysis of ethyl ester yield in the transesterification of vegetable oil: an accurate method for a truly quantitative analysis

Biodiesel has recently gained importance as an alternative to fossil diesel. However, the development of more efficient biodiesel formation processes still depends on the use of fast and accurate analytical techniques to evaluate the conversion degree of the transesterification reaction. Nuclear magnetic resonance (NMR) spectroscopy has been used for this purpose, but some experimental details still need to be addressed. Therefore, in this communication, the experimental conditions for a truly quantitative NMR analysis of biodiesel formation are presented. The longitudinal relaxation time (T₁), which is the determining factor for quantitative analysis, was measured using an inversion‐recovery method, and a maximum value of 2.35 s was obtained for a biodiesel sample. A linear determination coefficient of r² = 0.99 was obtained when a time delay between pulses longer than 5T₁ =15 s was used, whereas strong deviations were observed when using shorter delays. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamics of solutions of triblock copolymers in a selective solvent: Effect of varying copolymer concentration

We have used static and dynamic light scattering and pulsed field gradient NMR to study the effect of varying concentration on the dynamics of the triblock copolymer, polystyrene block poly(ethylene, butylene) block polystyrene (PS‐PEB‐PS), dissolved in n‐heptane, a selective solvent for the middle block. The correlation function for a dilute solution with c = 0.49 % (w/v) corresponds to the translational diffusion of micelles. At intermediate concentrations [1.1 ≤ c ≤ 2.6 % (w/v)], the correlation functions can be fitted to the sum of a single exponential and stretched exponential functions. The slower mode is due to the diffusion of polydisperse clusters formed by random association of triblock copolymer molecules and the faster one again represents micelles. A complex behavior is observed in the semidilute region [4.0 ≤ c ≤ 6.9 % (w/v)]. Three dynamic processes can be extracted from the correlation function: (i) The fast diffusive mode is the collective diffusion mode in the physical gel, (ii) the middle, relaxational mode, is probably due to the local movement of insoluble domains trapped in the network of the physical gel, and (iii) the slow diffusive mode implies the existence of large‐scale inhomogeneities in the system. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1312–1322, 2000     

Colloidlike behaviour of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in n-decane investigated by pulsed field gradient nuclear magnetic resonance

The self-diffusion of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in a preferential solvent for the aliphatic block, n-decane, was investigated by pulsed field gradient NMR. The diblock copolymer forms micelles in solution, the structure of the solid polymer being preserved in the native solution because the polystyrene is in the glassy state. The equilibrium state is attained upon heating which again freezes in upon cooling to room temperature. The hydrodynamic radius of the micelles decreases by about 50% during this heating–cooling process. The concentration dependence of the self-diffusivity shows typical colloidlike behaviour, and it can be described by a Vogel–Fulcher–Tammann-like equation. No indications of crystallization at higher concentrations are observed in the micellar solution because the micellar sizes are slightly polydisperse. The self-diffusivity was measured up to the glasslike state, where in-cage- diffusion and dynamic heterogeneities could be detected. Received: 14 April 1999 Accepted in revised form: 14 June 1999     

Practical aspects of ROESY experiments for identification of bound waters in the cyclic tetrasaccharide

ROESY pulse sequences are presented and evaluated to identify bound waters in the cyclic tetrasaccharide. The first experiment incorporated the double-pulsed field gradient spin-echo (DPFGSE) for selective water excitation at the initial portion of the pulse sequence. Although long, shaped pulses were used in DPFGSE to achieve the highly selective excitation of water resonance that is very close to resonances of the cyclic tetrasaccharide, the approach was not effective because of the loss of sensitivity. Concomitant use of long delays and moderate length of shaped pulses in the portion of DPFGSE gained more sensitivity. A simple approach incorporating spin-echo with long delays instead of DPFGSE also afforded a sensitive spectrum. Practical aspects of these ROESY experiments are illustrated using the cyclic tetrasaccharide cyclo-{-->6}-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->6)-alpha-D-Glcp-(1-->3)-alpha-D-Glcp-(1-->).     

Pulsed field gradient NMR investigation of solubilization equilibria in amino acid and dipeptide terminated micellar and polymeric surfactant solutions

Pulsed field gradient NMR spectroscopy was used to investigate the association of toluene, chlorobenzene and benzyl alcohol with amino acid and dipeptide terminated polymerized surfactants (PS). The diffusion coefficient for each probe was measured in the presence and absence of the polymers and the mole fraction of bound probe molecules, f_b, was calculated. For all solutions investigated, the probes associated more strongly with unpolymerized surfactant micelles than with corresponding PS. For example, the toluene f_b values for association with sodium undecanoyl valinate micelles and the PS poly(sodium undecanoyl valinate) were 0.88 and 0.15, respectively. The relatively weak probe–polymer association was attributed to the polarity and fluidity of the polymers' hydrocarbon cores and to the fact that these PS have smaller aggregation numbers than the corresponding unpolymerized surfactant micelles. Copyright © 2002 John Wiley & Sons, Ltd.     

Identification of bound waters in the solution structure of ribonuclease T1 using the double pulsed field gradient spin‐echo NMR technique for selective water excitation

Novel pulse sequences incorporating the double pulsed field gradient spin‐echo technique are presented that have particular use in identifying macromolecular bound water. The use of these sequences is illustrated using ribonuclease T₁. Five amide protons cross‐relaxing with bound water protons were observed. Examination of the crystal structure revealed that all of these amide protons donate hydrogen bonds or are in close proximity to water molecules with very low temperature factors, indicating that these amide protons are highly correlated with the bound water molecules. This method rapidly provides reliable information for characterizing macromolecular bound water molecules. Copyright © 2002 John Wiley & Sons, Ltd.     

Measurement of aroma compound self-diffusion in food models by DOSY

Self-diffusion measurement of solutes in polymer gels has been investigated using pulsed gradient spin echo NMR spectroscopy. However, few data are available on the self-diffusion of small solutes in natural polysaccharide polymers used as thickeners in the food industry. Since aroma diffusion in food matrices could have an impact on flavor release, this is an interesting and economic challenge. Diffusion ordered spectroscopy (DOSY) resolves diffusion data for each component in complex mixtures. We used DOSY with the inverse Laplace transform approach with the maximum entropy algorithm to investigate diffusion of two aroma compounds, ethyl butanoate and linalool, in an iota-carrageenan matrix as the food model. We showed that the self-diffusion coefficient values of small molecules in a polysaccharide matrix could be easily extracted using this method. We then investigated the impact of the gelling state of iota-carrageenan matrices on the self-diffusion of ethyl butanoate.     

Direct NMR resonance assignments of the active site histidine residue in Escherichia coli thioesterase I/protease I/lysophospholipase L1

Owing to the hydrogen-bond interaction and rapid exchange rate with the bulk water, the transverse relaxation time for the N(delta1)-H proton of the catalytic histidine in Escherichia coli thioesterase I/protease I/lysophospholipase L1 (TEP-I) is rather short. Because of its catalytic importance, it is desirable to detect and assign this proton resonance. In this paper, we report the first direct NMR correlation between the short-lived N(delta1)-H proton and its covalently attached N(delta1)-nitrogen of the catalytic His157 residue in E. coli thioesterase/protease I. We have used gradient-enhanced jump-return spin-echo HMQC (GE-JR SE HMQC) to obtain a direct correlation between the short-lived N(delta1)-H proton and its covalently attached N(delta1)-nitrogen. The sensitivity of detection for the short-lived N(delta1)-H proton was enhanced substantially by improved water suppression, in particular, the suppression of radiation damping via pulsed field gradients.     

Reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in water

The reorientational dynamics of p-sulfonatocalix[4]arene and of its La(III) complex in deuterated water were studied by 1H NMR longitudinal relaxation rates. It is shown that the relaxation is purely dipolar in the non-extreme narrowing regime. The distance between the geminal protons could be determined from the NMR data, giving good agreement with the values generally used in correlation time calculations. The correlation times show an Arrhenius behaviour in good agreement with previously reported data from 13C measurements for a similar uncomplexed calixarene. The Arrhenius energies of activation are identical for the uncomplexed and the complexed calixarenes, suggesting a reorientational motion strongly dependent on the structure of the water cage around the complex. This is also in agreement with a complexation of the La(III) cation in the second sphere of solvation of the sulfonate groups, as shown by molecular dynamics simulations.     

Kinetics of Magnetic Field‐Induced Orientational Ordering in Block Copolymer/Superparamagnetic Nanoparticle Composites

The combination of external potential dynamics and Brownian dynamics is introduced to study the kinetics of orientational ordering in block copolymer/superparamagnetic nanoparticle composites where the particles are smaller than the domain spacing and preferentially segregate into one block of the copolymer. This simulation method accounts for both excluded volume interactions and dipolar interactions between particles to quantify alignment kinetics. Two‐dimensional simulations reveal that higher dipolar interaction strengths lead to faster alignment of the block copolymer, where the orientation kinetics obeys an exponential rate law. The observed rate of alignment increases with increasing dipolar interaction strength and is dependent on the initial state of the block copolymer. The primary mechanism of orientational ordering is found to be the redistribution of monomer segments leading to bridging and growth of the block copolymer domains around the nanoparticles.

     

Comparison of the accuracy of periodic reaction field methods in molecular dynamics simulations of a model liquid crystal system

A periodic reaction field (PRF) method is a technique to estimate long‐range interactions. The method has the potential to effectively reduce the computational cost while maintaining adequate accuracy. We performed molecular dynamics (MD) simulations of a model liquid‐crystal system to assess the accuracy of some variations of the PRF method in low‐charge‐density systems. All the methods had adequate accuracy compared with the results of the particle mesh Ewald (PME) method, except for a few simulation conditions. Furthermore, in all of the simulation conditions, one of the PRF methods had the same accuracy as the PME method. © 2015 Wiley Periodicals, Inc.     

Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy

 The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using ¹H and ²H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques. Received: 22 October 1999/Accepted in revised form: 19 November 1999