Phase electric birefrigence measurements in a ternary microemulsion system I. Results for small volume fractions

Phase electric birefringence measurements (frequency spectra of the modulus βB(2ω)β and phase angle ϕ(2ω) of the alternating Kerr constant B (2ω) have been carried out, over the range, 0.1 kHz-10 MHz, on ternary water in oil microemulsion at small volume fraction ø (≤0.04). The measurements give evidence on the on the presence of droplets dimers (relaxation times 0.3-0.5 μs) easily observed at very low ø, and higher order anisotropic droplets aggregates. A semi-quantitative agreement with theory is obtained.     

Magnetic field dependence of spin-lattice relaxation enhancement using piperidinyl nitroxyl spin-labels

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.     

NMR study of phase transitions in new ferroelectric Crystal--(C5H5NH)5Bi2Br11

A T1 minimum at 216 K for Larmor frequency 90 MHz has been detected and for this minimum no analogous T, minimum according to the known quadratic dependence of the Larmor frequency 25 MHz is found. The analysis leads to the conclusion that this T1 minimum is a result of the relaxation of protons via quadrupole nuclei. The Kimmich theoretical treatment of 1H NMR experiments exhibiting the existence of this phenomenon in the case of relaxation of protons of piridinium cations in (C5H5NH)5Bi2Br11 and the estimated averaged quadrupole frequency of interacting quadrupole nuclei has been estimated to be around 71 MHz. Below the phase transition at 118 K a wide symmetric spin-lattice relaxation minimum at 25 MHz is detected and a model of small angle libration of the pyridinium cation has been applied to explain the observed T1 relaxation time minimum.     

On the relaxation of volume and shear viscosities in vegetable oils

Longitudinal and shear ultrasonic measurements were performed on castor, tung, soyabean and cottonseed oils in the frequency ranges 50 kHz–30 MHz and 10–150 MHz using reverberation, pulse and impedance techniques respectively. In castor and tung oils relaxation spectra for both volume and shear viscosities are broad and identical in shape. In soyabean and cottonseed oils both volume and shear viscosities undergo single relaxation but the volume relaxation time is about 10 times larger than the shear. It is suggested that shear relaxation results from molecular orientation and volume relaxation from rearrangement of molecular packing.     

Low relaxation rate in epitaxial vanadium-doped ultrathin iron films

The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate G. For many years, pure iron (Fe) has had the lowest known value of G = 57 MHz for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of G which are significantly reduced to 35 +/- 5 MHz at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number Z.     

Frequency (250 MHz to 9.2 GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin relaxation times for four triarylmethyl (trityl) radicals at room temperature were measured by long-pulse saturation recovery, inversion recovery, and electron spin echo at 250 MHz, 1.5, 3.1, and 9.2 GHz in mixtures of water and glycerol. At 250 MHz T(1) is shorter than at X-band and more strongly dependent on viscosity. The enhanced relaxation at 250 MHz is attributed to modulation of electron-proton dipolar coupling by tumbling of the trityl radicals at rates that are comparable to the reciprocal of the resonance frequency. Deuteration of the solvent was used to distinguish relaxation due to solvent protons from the relaxation due to intra-molecular electron-proton interactions at 250 MHz. For trityl-CD(3), which contains no protons, modulation of dipolar interaction with solvent protons dominates T(1). For proton-containing radicals the relative importance of modulation of intra- and inter-molecular proton interactions varies with solution viscosity. The viscosity and frequency dependence of T(1) was modeled based on dipolar interaction with a defined number of protons at specified distances from the unpaired electron. At each of the frequencies examined T(2) decreases with increasing viscosity consistent with contributions from T(1) and from incomplete motional averaging of anisotropic hyperfine interaction.     

Ultrasonic spectroscopy in bovine serum albumin solutions

Ultrasonic absorption and velocity spectra in bovine serum albumin (BSA) aqueous solutions have been measured at 20 degrees C over the broad frequency range 0.1-1600 MHz in the pH range 1.5-13.2. Five different techniques were used: the plano-concave resonator, plano-plano resonator, pulse-echo overlap, Bragg reflection, and high-resolution Bragg reflection methods. The absorption spectrum at neutral pH was well fitted to the relaxation curve assuming a distribution of relaxation frequency with a high-frequency cutoff and long low-frequency tail. The relaxation behavior was interpreted in terms of various degrees of hydration of BSA molecules. At acid pH's, excess absorption over that at pH 7 was explained by double relaxation. The pH dependences of the relaxation frequency and maximum absorption per wavelength showed that the relaxation at about 200 kHz was related to the expansion of molecules and that at 2 MHz resulted from the proton transfer reaction of carboxyl group. At alkaline pH's, the excess absorption was explained by triple relaxation. The relaxation at about 200 kHz was associated with a helix-coil transition, and the two relaxations at 2 and 15 MHz were attributed to the proton transfer reactions of phenolic and amino groups, respectively. The rate constants and volume changes associated with these processes were estimated.     

Fiber-to-field angle dependence of proton nuclear magnetic relaxation in collagen

Longitudinal and transverse proton relaxation times were measured on pig tendon. For T₁, dispersion curves and more accurate measurements at 20 MHz are presented. Values of T₂ were obtained from CPMG pulse sequences, at 20 MHz. The dependence of relaxation times against the fiber-to-field angle was particularly investigated. Longitudinal relaxation rate was found to be almost orientation independent, and presented quadrupolar peaks between 1 and 4 MHz. On the contrary, transverse relaxation, that was well fitted by the sum of four exponentials, was highly orientation dependent. Deconvolution showed that the exponentials decaying most quickly are most orientation dependent. For those two fractions, a cross-relaxation model allowed explaining the fiber-to-field angle dependence, and the specially low rate corresponding to the magic angle of 55°. Finally, each decaying mode was assigned to a fraction of protons localized in the macromolecular structure and characterized by particular dynamics.     

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.     

Weakly bound dimers of fermionic atoms

We discuss the behavior of weakly bound bosonic dimers formed in a two-component cold Fermi gas at a large positive scattering length a for the interspecies interaction. We find the exact solution for the dimer-dimer elastic scattering and obtain a strong decrease of their collisional relaxation and decay with increasing a. The large ratio of the elastic to inelastic rate is promising for achieving Bose-Einstein condensation of the dimers and cooling the condensed gas to very low temperatures.     

Ultrasonic studies on lyotropic liquid crystal CTAB in heavy water

Ultrasonic absorption and velocity measurements have been made on cetyl trimethyl ammonium bromide (CTAB) solution in heavy water. Velocity measurements have been made of CTAB concentration at 25°C and a frequency of 2.0?MHz. Absorption measurements have been made in the frequency range of 5.0–55.0?MHz for various CTAB concentration. The results of these measurements indicate micelle formation. Absorption measurements show the existence of a single relaxation frequency. A detailed analysis of the data has been presented. Theoretical and experimental relaxation frequencies are approximately the same.     

Frequency-dependent spin-lattice relaxation study of transport processes in superionic conductors

The sensitivity of the¹⁹F spin-lattice relaxation dispersion, T₁,(ω), to motional disorder in crystalline superionic conductors of the type La_1?xSr_xF_3?x (x = 0; 0.03) is shown. T₁ times are measured in the frequency range from 90 kHz to 370 MHz using standard techniques in combination with field-cycling. The relaxation dispersion shows qualitative differences from the standard Bloembergen-Purcell-Pound behavior. At low frequencies a relaxation model using a distribution of correlation times for diffusing ions is found to be consistent with the experimental results. At frequencies higher than 50 MHz another process of the Debye type which is not induced by ionic hopping dominates the relaxation.     

Study of molecular dynamics and cross relaxation in tetramethylammonium hexafluorophosphate (CH(3))(4)NPF(6) by (1)H and (19)F NMR

(CH(3))(4)NPF(6) is studied by NMR measurements to understand the internal motions and cross relaxation mechanism between the heterogeneous nuclei. The spin lattice relaxation times (T(1)) are measured for (1)H and (19)F nuclei, at three (11.4, 16.1 and 21.34 MHz) Larmor frequencies in the temperature range 350-50K and (1)H NMR second moment measurements at 7 MHz in the temperature range 300-100K employing home made pulsed and wide-line NMR spectrometers. (1)H NMR results are attributed to the simultaneous reorientations of both methyl and tetramethylammonium groups and motional parameters are evaluated. (19)F NMR results are attributed to cross relaxation between proton and fluorine and motional parameters for the PF(6) group reorientation are evaluated.     

H,N,C-triple resonance NMR of very large systems at 900 MHz

We provide quantitative signal to noise data and feasibility study at 900 MHz for 1H-15N-13C triple resonance backbone assignment pulse sequences obtained from a medium sized 2H, 13C, 15N labeled protein slowed down in glycerol-water solution to mimic relaxation and spectroscopic properties of a much larger protein system with macromolecular tumbling correlation time of 52 and 80 ns, respectively, at 296 and 283 K (corresponding to molecular weights of 130 and 250 kDa). Comparisons of several different schemes for transferring magnetization from proton to nitrogen and back to proton confirms Yang and Kay's 1999 prediction that avoiding the unfavorable relaxation properties of 1H-15N multiple quantum coherence in the TROSY phase cycle of the final 15N-1H transfer before acquisition is crucial for maximal sensitivity from these very large molecular weight systems. We also show results which confirm some predictions regarding the superiority of TROSY at 900 MHz vs. 800 MHz especially as the molecular weights become very large.     

1H,15N,13C-triple resonance NMR of very large systems at 900 MHz

We provide quantitative signal to noise data and feasibility study at 900 MHz for 1H-15N-13C triple resonance backbone assignment pulse sequences obtained from a medium sized 2H, 13C, 15N labeled protein slowed down in glycerol-water solution to mimic relaxation and spectroscopic properties of a much larger protein system with macromolecular tumbling correlation time of 52 and 80 ns, respectively, at 296 and 283 K (corresponding to molecular weights of 130 and 250 kDa). Comparisons of several different schemes for transferring magnetization from proton to nitrogen and back to proton confirms Yang and Kay's 1999 prediction that avoiding the unfavorable relaxation properties of 1H-15N multiple quantum coherence in the TROSY phase cycle of the final 15N-1H transfer before acquisition is crucial for maximal sensitivity from these very large molecular weight systems. We also show results which confirm some predictions regarding the superiority of TROSY at 900 MHz vs. 800 MHz especially as the molecular weights become very large.     

T1 as a function of larmor frequency in solid CH4 at 4.2 K: An experiment on recent theories

The spin-lattice relaxation time T₁ has been measured in partially converted samples at NMR frequencies ranging from 4 to 55 MHz. Results allow to check in detail the existing models for relaxation in solid CH₄.     

Electric Modulus Spectroscopic Studies of the Dielectric Properties of Carbon Nanotubes/Epoxy Polymer Composite Materials

The electrical properties of epoxy polymer/carbon nanotubes composites were characterized using impedance spectroscopy in the frequency range between 1 Hz and 10 MHz and temperature range between 25°C and 105°C. We report the analysis of the experimental data using the electric modulus formalisms to understand the dielectric relaxation mechanisms. The variation of the real and imaginary parts of the electric modulus versus frequency and temperature were suggestive of two relaxation processes, associated with dipolar relaxation and CNT-polymer interfaces. The Havriliak-Negami model of dielectric relaxation was used for modelling the relaxation processes, extracting the relaxation parameters.     

Metal-nitroxyl interactions. 18. Spin-labeled copper carboxylate dimers and monomers

EPR spectra of spin-labeled copper carboxylate dimers and monomers were obtained in room-temperature solutions and in frozen glasses at about −180°C. At room temperature the copper-nitroxyl interaction in the dimer results in severe broadening of the nitroxyl signal and a decrease in nitroxyl relaxation time. In the frozen-glass spectrum of the dimer the nitroxyl relaxation time is decreased by interaction with the copper but the lineshape changes are dominated by nitroxyl-nitroxyl interactions. In 1:1 pyridine: toluene solution the dimers dissociate into monomers. The spectra of the monomers in solution and in frozen glasses have g and A values which are averages of those expected for the copper and nitroxyl electrons, indicating that the electron-electron exchange interaction is large relative to the g-value differences and the nuclear hyperfine interactions.     

Tunable feedback loop for suppression of relaxation oscillations in a diode-pumped Nd: laser

An optoelectronic noise suppression circuit connected to the RF input of a commercial diode laser mount is presented. Adjustable phase shift and amplitude control enable efficient suppression of the relaxation oscillations of a diode laser-pumped Nd:YVO₄ laser at variable frequencies between a few hundred kHz and 1.8 MHz. At least 25 dB reduction of the relaxation oscillations at 500 kHz and 1.5 MHz noise peaks is demonstrated.     

The effect of dexamethasone on tissue water distribution and proton relaxation in Panc02 tumors

The present experiments were conducted to determine the effects of dexamethasone mediated changes in tumor water distribution on proton relaxation times (T1, T2) in a murine pancreatic adenocarcinoma (Panc02). Spin lattice (T1) and spin-spin (T2) relaxation times were determined by ex vivo methods (10MHz) and by in vivo imaging techniques (6.25 MHz) at various intervals after single or multiple dexamethasone treatments. In complementary studies, dexamethasone mediated changes in tumor capillary permeability, tumor water distribution, relative tumor blood flow and tumor cell proliferation were also determined.

Proton spin lattice (T1) and spin-spin (T2 relaxation times for Panc02 tumors shortened within two hours of a single dexamethasone treatment. The time course and magnitude of this response was dexamethasone dose dependent. The time dependent changes in T1 and T2 after dexamethasone were similar at 10 MHz (ex vivo) and 6.25 MHz (in vivo imaging). Although dexamethasone produced little or no change in total tumor water content and tumor cell proliferation, transient changes in the physiologic distribution of tumor water were clearly demonstrated.

The data supports the idea that dexamethasone induced changes in the distribution of tumor water were mediated by changes in capillary permeability and tumor blood flow. These physiologic responses produced serial changes in tumor extracellular extravascular water content that were consistent with the observed changes in tumor T1 and T2. The results from these experiments might imply that therapy associated changes in tumor proton relaxation times may not only reflect changes in tissue water content, but may also reflect physiologic responses which alter the distribution of tissue water and solute.