Raman spectroscopic studies of the phase transitions in hexane at high pressure

Pressure and Composition Effects on a Common Nanoparticle Ligand-Solvent Pair

The effect of pressure on the properties of nanoparticles is a growing area of investigation. These measurements are typically performed in a colloidal suspension; however, pressure-induced changes in the interactions between the nanoparticle surface and the solvent are often neglected. Here, we report vibrational spectroscopy of a common nanoparticle ligand, 1-dodecanethiol, and a common solvent, toluene, under pressure. We find that the pressure-induced phase change of the 1-dodecanethiol is altered by the presence of toluene and that change depends on the concentration of the free ligand in the solution. At near-equal concentrations, phase segregation is observed and the dodecanethiol crystallizes independently from the toluene. On the other hand, at unequal concentrations, concerted phase transitions are observed in the dodecanethiol and toluene, and a disordered conformation of dodecanethiol is maintained under much higher pressures. These results shed light on the pressure-induced changes in intermolecular interactions between nanoparticle ligands and solvents, which must be considered in the design of high-pressure investigations of colloidal nanoparticles.

An experimental investigation of n-hexane at high temperature and pressure

At present, no high temperature experiments on phase change are reported. In this study, we have measured the Raman bands ν_s(CH₃), ν_s(CH₂), ν_as(CH₃), and ν_as(CH₂) of n-hexane in a hydrothermal diamond cell up to 588 K. We determined that the liquid-solid phase transition pressure of n-hexane is 1.17 GPa, and we also gave a number of high temperatures and pressures data on phase change which are not reported previously. In addition, we defined the solidus of n-hexane which can be represented by the equation P = 8.581T-1550.16, and the relation dP/dT = 8.581 which can be used to calculate the thermodynamic parameters for n-hexane in the liquid-solid phase transition. For all we know, the above two equations are presented here for the first time. Furthermore, it is the first report here in a graphic way on high-temperature phase change in n-hexane, and it is also the first to be shown in the 3-D figure.

An in-situ Raman study on pristane at high pressure and ambient temperature

The CH Raman spectroscopic band (2800-3000cm^-1) of pristane was measured in a diamond anvil cell at 1.1-1532MPa and ambient temperature. Three models are used for the peak-fitting of this CH Raman band, and the linear correlations between pressure and corresponding peak positions are calculated as well. The results demonstrate that 1) the number of peaks that one chooses to fit the spectrum affects the results, which indicates that the application of the spectroscopic barometry with a function group of organic matters suffers significant limitations; and 2) the linear correlation between pressure and fitted peak positions from one-peak model is more superior than that from multiple-peak model, meanwhile the standard error of the latter is much higher than that of the former. It indicates that the Raman shift of CH band fitted with one-peak model, which could be treated as a spectroscopic barometry, is more realistic in mixture systems than the traditional strategy which uses the Raman characteristic shift of one function group.

The influence of ionic strength on carbonate-based spectroscopic barometry for aqueous fluids: an in-situ Raman study on Na2CO3-NaCl solutions

The Raman wavenumber of the symmetric stretching vibration of carbonate ion (ν₁-CO₃²⁻) was measured in three aqueous solutions containing 2.0 mol·L⁻¹ Na₂CO₃ and 0.20, 0.42, or 0.92 mol·L⁻¹ NaCl, respectively, from 122 to 1538 MPa at 22 °C using a moissanite anvil cell. The ν₁ Raman signal linearly shifted to higher wavenumbers with increasing pressure. Most importantly, the slope of ν₁-CO₃²⁻ Raman frequency shift (∂ν₁/∂P)_I was independent of NaCl concentration. Moreover, elevated ionic strength was found to shift the apparent outline of the carbonate peak toward low wavenumbers, possibly by increasing the proportion of the contact ion pair NaCO₃⁻. Further investigations revealed no cross-interaction between the pressure effect and the ionic strength effect on the Raman spectra, possibly because the distribution of different ion-pair species in the carbonate equilibrium was largely pressure-independent. These results suggested that the ionic strength should be incorporated as an additional constraint for measuring the internal pressure of various solution-based systems. Combining the ν₁-CO₃²⁻ Raman frequency slope with the pressure herein with the values for the temperature or the ionic strength dependencies determined from previous studies, we developed an empirical equation that can be used to estimate the pressure of carbonate-bearing aqueous solutions.

Effect of Pressure and Solvent on Raman Spectra of All-trans-β-Carotene

The ground state Raman spectra of all-trans-beta-carotene in n-hexane and CS2 solutions are measured by simultaneously changing the solvent environment and molecular structure under high hydrostatic pressure. The diverse pressure dependencies of several representative Raman bands are explained using a competitive mechanism involving bond length changes and vibronic coupling. It is therefore concluded that (a) the in-phase C=C stretching mode plays an essential role in the conversion of energy from S1 to S0 states in carotenoids, (b) internal conversion and intramolecular vibrational redistribution can be accelerated by high pressure, and (c) the environmental effect, but not the structural distortion or pi-electron delocalization, is responsible for the spectral properties of a given carotenoid species. These findings revealed the potential of high pressure in exploring the nature of the biological functions of carotenoids.

Pressure-Induced Structural Transition in n-Pentane: A Raman Study

Pressure-induced Raman spectroscopy studies on n-pentane have been carried out up to 17 GPa at ambient temperature. n-Pentane undergoes a liquid-solid transition around 3.0 GPa and a solid-solid transition around 12.3 GPa. The intensity ratio of the Raman modes related to all-trans conformation (1130 cm-1 and 2850 cm-1) to that of gauche conformation (1090 cm-1 and 2922 cm-1) suggests an increase in the gauche population conformers above 12.3 GPa. This is accompanied with broadening of Raman modes above 12.3 GPa. The high-pressure phase of n-pentane above 12.3 GPa is a disordered phase where the carbon chains are kinked. The pressure-induced order-disorder phase transition is different from the behavior of higher hydrocarbon like n-heptane.