Laura J. Bonales

Surface rheology: macro- and microrheology of poly(tert-butyl acrylate) monolayers

We have studied the surface shear viscoelasticity of poly(tert-butyl-acrylate) Langmuir monolayers spread at the air/water interface, by tracking the Brownian motion of tracer particles with different sizes and surface chemical nature, trapped at the same interface. Surface shear moduli have been extracted from the particles mean square displacements (MSD), using different approaches: hydrodynamic calculations of drag coefficients and direct inversion of the MSD by means of the generalized Stokes–Einstein equation. It has been found that these different theoretical approaches lead to comparable values of the shear interfacial viscosity independent of the polymer concentration and molecular weight. In addition, no effect of the size or chemical nature of the probe has been detected. The results have demonstrated the consistency of the microrheological techniques used, and confirm the existence of entanglements in PtBA monolayers, as recently deduced from dilational elasticity and viscosity measurements, [Maestro et al., Soft Matter, 2010, 6, 4407]. An unexpected result was that the interfacial viscosity values obtained from microrheology have been found to be several orders of magnitude lower than the ones obtained with macroscopic interfacial shear rheometers. At the moment there is no clear explanation for this disagreement, although it is not related to the probe size or their chemical nature. Furthermore, this discrepancy is not related to the analysis methodology used, including the calculation of the two-point correlation function used in 3D microrheology when there are heterogeneities present within the range of the probe size.

A detailed Raman and X-ray study of UO2+x oxides and related structure transitions

This work presents a detailed study of hyperstoichiometric UO2+x (0 < x < 0.25) oxides and an assessment of the structural evolution taking place as oxidation proceeds. For this purpose, different UO2+x powder samples with controlled degree of non-stoichiometry have been identified by thermogravimetric analysis and characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD analysis reflects that the commonly assumed Vegards law is not applicable over the whole hyperstoichiometry range, since a slight increase of the lattice constant is observed for 0.13 < x < 0.20. A quantitative Raman analysis of the UO2+x spectra as a function of the oxidation degree is also shown. A new method to characterize any UO2+x sample (for x < 0.20), based on the shift of the 630 cm-1 band observed in the Raman spectrum, is proposed here for the first time. Moreover, three structure transitions have been detected at x = 0.05, 0.11 and 0.20, giving rise to four distinct regions associated with consecutive structural rearrangements over the hyperstoichiometry range: x < 0.05, 0.05 < x < 0.11, 0.11 < x < 0.20 and 0.20 < x < 0.25.

pH and Salinity Evolution of Europa's Brines: Raman Spectroscopy Study of Fractional Precipitation at 1 and 300 Bar

Several lines of evidence indicate the existence of salty liquid water below the icy surface of the satellite Europa. Depending on the chemical composition of the original interior brines, minerals that precipitate will be varied as will be the resulting physicochemical parameters of the evolving solutions such as pH and salinity. These parameters are determinants apropos to the study of the possible habitability of the satellite. In this work, experiments of fractional precipitation by cooling of several brines with different chemical composition (acid, alkaline, and neutral) were performed at 1 and 300 bar. The gradual decrease in temperature leads to mineral precipitation and changes in salinity and pH values. During the experiment, Raman spectroscopy was used to analyze quantitatively the variation of the salt concentration in the aqueous solutions. The obtained laboratory data indicate the manner in which cryomagma differentiation might occur on Europa. These endogenous processes of differentiation require planetary energy, which seems to have been plentiful during Europas geological history. Ultimately, the dissipation of part of that energy is translated to a higher complexity of the cryopetrology in Europas crust. From the results, we conclude that fractional differentiation processes of briny cryomagmas produce several types of igneous salty mineral suites on icy moons.

Dynamics in Ultrathin Films: Particle Tracking Microrheology of Langmuir Monolayers

Particle tracking has been shown to be a powerful technique for measuring bulk and interfacial rheology of flu- ids. The Brownian motion of microparticles trapped at interfaces is very sensitive to the viscosity of the subphase, and to the contact angle of the particles. The Stokes-Einstein relation is fulfilled if the friction factor is properly taken into ac- count. The diffusion coefficient of the latex microparticles spread on surfactant monolayers allows one to calculate the shear viscosity of the monolayer using Danovs theory. Good agreement was found with previous results for monolayers of pentadecanoic acid. The method has also been used to study monolayers of n-dodecanol. Moreover, the shear viscosity of a polymer monolayer has been calculated by particle tracking, and the results show good agreement with data obtained by canal viscosimetry. The temperature dependence of the shear viscosity shows the existence of a glass transition for monolayers of poly(4-hydroxystyrene).

Quantitative Raman spectroscopy as a tool to study the kinetics and formation mechanism of carbonates.

We have carried out a systematic study of abiotic precipitation at different temperatures of several Mg and Ca carbonates (calcite, nesquehonite, hydrocalcite) present in carbonaceous chondrites. This study highlights the capability of Raman spectroscopy as a primary tool for performing full mineralogical analysis. The precipitation reaction and the structure of the resulting carbonates were monitored and identified with Raman spectroscopy. Raman spectroscopy enabled us to confirm that the precipitation reaction is very fast (minutes) when Ca(II) is present in the solution, whereas for Mg(II) such reactions developed at rather slow rates (weeks). We also observed that both the composition and the reaction mechanisms depended on temperature, which might help to clarify several issues in the fields of planetology and geology, because of the environmental implications of these carbonates on both terrestrial and extraterrestrial objects.

Study of the thermal stability of studtite by in situ Raman spectroscopy and DFT calculations

The design of a safe spent nuclear fuel repository requires the knowledge of the stability of the secondary phases which precipitate when water reaches the fuel surface. Studtite is recognized as one of the secondary phases that play a key-role in the mobilization of the radionuclides contained in the spent fuel. Thereby, it has been identified as a product formed under oxidation conditions at the surface of the fuel, and recently found as a corrosion product in the Fukushima-Daiichi nuclear plant accident. Thermal stability is one of the properties that should be determined due to the high temperature of the fuel. In this work we report a detailed analysis of the structure and thermal stability of studtite. The structure has been studied both by experimental techniques (SEM, TGA, XRD and Raman spectroscopy) and theoretical DFT electronic structure and spectroscopic calculations. The comparison of the results allows us to perform for the first time the Raman bands assignment of the whole spectrum. The thermal stability of studtite has been analyzed by in situ Raman spectroscopy, with the aim of studying the effect of the heating rate and the presence of water. For this purpose, a new cell has been designed. The results show that studtite is stable under dry conditions only at temperatures below 30°C, in contrast with the higher temperatures published up to date (~130°C). Opposite behaviour has been found when studtite is in contact with water; under these conditions studtite is stable up to 90°C, what is consistent with the encounter of this phase after the Fukushima-Daiichi accident.

Local, solvation pressures and conformational changes in ethylenediamine aqueous solutions probed using Raman spectroscopy

Raman spectra of 1,2-ethylenediamine (EDA) in aqueous solutions are used to demonstrate that EDA molecules experience an anti-gauche conformational change resulting from the interactions with water. The observed Raman shift reveals a compressive (hydrophobic) effect of water on both methylene and amino groups of EDA. Raman spectra of EDA at high pressures are used as reference to quantify the intermolecular EDA-H2O interactions in terms of local pressures. These results are compared with macroscopic solvation pressures calculated from the cohesive energy parameter. We compare and discuss all our observations with available computational and experimental studies.

Raman spectroscopy as a tool to study the solubility of CO2 in magnesium sulphate brines: application to the fluids of Europa’s cryomagmatic reservoirs

The solubility of CO 2 in water at different temperature and pressure conditions has been studied for more than 150 years because of the common presence of this system in the Earth, and its importance for engineering and in several biological and geological processes. However, there are many contradictions in the literature with regard to the solubility of CO 2 in salt-rich aqueous solutions, and associated effects such as the salting out have been studied in detail for only few salts such as NaCl or Na 2 SO 4 . In this study we introduce Raman spectroscopy as a useful tool to measure gas solubility in brines. We have obtained the solubility of CO 2 in aqueous solution of MgSO 4 at different concentrations (5 and 17 wt %), in the range of temperatures from 5 to 20 °C and pressures up to 50 bar. The results allow one to approach different aqueous reactions and processes that are relevant in geochemical phenomena of some planetary objects. Thus, we have modelled the degassing processes that could occur through the crust of Europa, a satellite of Jupiter.

Characterization of Salting-Out Processes during CO2-Clathrate Formation Using Raman Spectroscopy: Planetological Application

ABSTRACT Clathrate hydrates are particular solids that planetologists study in detail because those solids may be present in several bodies of the solar system, such as Mars, comets, and the icy satellites. The solids are formed by solid H2O, like common water ice, but adopt open structures with cavities containing gas molecules. Clathrate hydrates are usually stable at relatively low temperature and high pressure, which are the typical conditions present inside these planetary objects. Their interest for astrobiology is that they represent potential sources of liquid water and gases when they decompose. The present work is focused on the crystallization of clathrates in Europas (icy satellite of Jupiter) interior conditions. We postulate that clathrate hydrates may play an important role in its crust mineralogy and that it can explain some features of the satellites surface due to their formation/destabilization. An in situ kinetic study by Raman Spectroscopy of the clathrate formation from salty solutions was performed in our laboratory. The chemical composition that we used mimics those obtained from Europas surface during the Galileo mission. An effect of the salting-out process in the solution was monitored through the clathrate formational path. Our results demonstrate that this process may have geological consequences on Europa and confirm the suitability of Raman spectroscopy for planetary detection of clathrate hydrates and other ices.

Raman Spectroscopy, a Useful Tool to Study Nuclear Materials

The use of the Raman technique is nowadays being widely spread in many scientific and industrial disciplines. The rise of this spectroscopy is due to the technology developed in some of its main components (the laser, charge-coupled device (CCD) sensors, gratings, filters, etc.), what reduces the cost of the equipment. Characterization by Raman spectroscopy has long and well-established tradition in fields such as condensed matter physics and chemistry. In nuclear sciences, by contrast, it is far from being extensively applied, even though this technique can be especially useful. It is a fact that only a scarce number of Raman laboratories dealing with nuclear materials exist, and therefore a limited database related to these materials. In such a context, this chapter is devoted to the practical use of Raman spectroscopy for nuclear materials characterization.