Salvador Butí

Identification of reaction compounds in micrometric layers from gothic paintings using combined SR-XRD and SR-FTIR

Synchrotron radiation X-ray diffraction (micro-SR-XRD) and Fourier transform infrared spectroscopy (micro-SR-FTIR) are used in the non-destructive identification of reaction and aging compounds from micrometric ancient painting layers. The combination of the micrometer size and non-destructive nature of the techniques together with the high resolution and brilliance of the synchrotron radiation has proved to be a procedure most advantageous for the study of reaction, aging and degradation processes. Copper, lead and calcium carboxylates and oxalates are determined in the chromatic, preparation and alteration layers from 15th century egg tempera and oil paintings. Their nature and crystallinity have been assessed. Some hypothesis about the mechanisms of development of both carboxylates and oxalates are presented.

On the role of solvent in acid-base equilibria of diuretics in acetonitrile-water mixed solvents.

The dissociation constants of representative loop (furosemide), thiazide (chlorthiazide and trichlormethiazide) and potassium sparing (amiloride) diuretics in 10, 30, 40, 50 and 70% (w/w) acetonitrile-water mixtures at 298.15 K were determined, in accordance with IUPAC procedures. The variation in pK(a) values over the whole composition range can be explained by preferential solvation and the structural features in acetonitrile-water mixtures. Correlations between pK(a) values and various bulk and solvatochromic properties of the solvents were calculated. The linear solvation energy relationship (LSER) method was applied. The resulting equations allowed us to calculate pK(a) values for the diuretics studied in any acetonitrile-water mixture up to 70% (w/w) and should help to clarify the acid-base behaviour of diuretics in the widely-used acetonitrile-water mixed solvents.

Chromatographic behaviour of ionizable compounds in liquid chromatography. Part 1. pH scale, pKa and pHS values for standard buffers in tetrahydrofuran–water

Abstract The solvent characteristics of tetrahydrofuran (THF)–water mixtures that have an influence on the chromatographic behaviour of ionizable compounds in liquid chromatography (LC) were investigated (Debye–Huckel parameters, pH scale, LC-useful range, solvatochromic parameters). Solute properties, such as the p K a values of the acids used to prepare reference standard buffer solutions used in LC, depend on the composition of the mixture. The p K a values in several THF–water mixtures were subjected to factor analysis in order to determine how many factors affect the variation in the data sets, and then to target factor analysis to identify these factors. The influence of solvatochromic parameters on the interactions between acidic groups and the solvents studied was identified and quantified. Standard pH values, pH S , for the seven primary reference buffer solutions of the NIST scale and for acetate buffer in THF–water mixtures containing 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70% and 100% (w/w) THF were determined using IUPAC standardization rules. The quasi-lattice–quasi-chemical (QLQC) theory of preferential solvation was applied to obtain the degree of preferential solvation of hydrogen ions in these media in order to explain the variation in pH S values obtained over the range of mixtures studied. The relationships obtained allow calculation of the pH S values of the standard buffers in THF–water mixtures. Thus pH measurements in these media can be performed as in water. The chromatographic behaviour of the solute can then be related to the real pH value of the mobile phase used.

SR-XRD and SR-FTIR study of the alteration of silver foils in medieval paintings

AbstractAltarpieces and polychrome carved wood from the fifteenth century AD usually exhibit golden and silvery areas by the application of a very thin foil of metal. The metal foils were normally protected from the atmosphere by a varnish or resin which maybe either preserved or absent. Moreover, they were glued to the background surface by adhesive substances (egg yolk, drying oil or animal glue). The high proportion of the glueing substances often renders the development of reaction compounds. With time, silver alters blacken or simply disappear completely. In this paper, we study the alterations to metal foils from a selection of fifteenth century artworks showing different glueing agents, organic coatings and several degrees of conservation of the organic coatings and metal leafs. The submillimetric layered structure and the high variability and low amount of most of the compounds present in the different layers, as well as their differing nature (organic and inorganic) make the use of micron-sensitive high-resolution techniques essential for their study. In particular, the high resolution, high brilliance and small footprint renders synchrotron radiation most adequate for their study. SR-XRD was performed to identify the reaction compounds formed in the different layers; μFTIR was used at to identify the silver protecting organic coatings, the metal foil glueing layers and the corresponding reaction compounds. The results obtained suggest that atmospheric corrosion is the dominant mechanism, and therefore that the degree of corrosion of the metal foils is mainly related to the conservation state of the protecting coatings. FigureSampling point, MO image from surface of the sample and SEM image from polish cross-section of the sample

Preferential solvation in the THF–water mixtures. Dissociation constants of acid components of pH reference materials

Dissociation constants of acid components of pH reference materials in tetrahydrofuran–water mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) tetrahydrofuran, THF, were obtained. Dissociation constant values determined were: pK1 and pK2 for tartaric and phthalic acid; pK1, pK2 and pK3 for citric acid; pKa for acetic and boric acid and pK2 for phosphoric acid. These values are essential to the determination of the reference pHS values of the standard buffer solutions, which are needed for accurate pH measurements in THF–water mixtures. In order to explain the variation of the pKa values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. To identify the solvent characteristics affecting the pKa values, the results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pKa values with parameters of THF–water mixtures. The equations obtained allow calculation of the pKa values of substances in THF–water mixtures.

Standard pH values for standardization of potentiometric sensors in 10% (w/w) acetonitrile-water mixtures.

Reference value standards, pH(s) in 10% (w/w) acetonitrile-water solvent mixtures for 11 reference buffer solutions have been determined from reversible emf measurements of the cell Pt/Ag/AgCl/standard buffer + KCl, in acetonitrile-water/glass electrode, at 298.15 K. Values of ionization constants, required for above calculations, have been determined from reversible emf measurements of the cell Pt/Ag/AgCl/HA + A + KCl, in acetonitrile-water/glass electrode, in the same solvent composition and temperature.

Assignment of pHS values of reference buffer solutions for standardization of potentiometric sensors in THF–water

Abstract Standard pH S values for five reference buffer solutions of the NIST series: KH 2 citrate, KH-phthalate, acetate, tetraborate and carbonate buffers in tetrahydrofuran–water, THF–water, mixtures containing 0, 10, 20, 30, 40, 50, 60 and 70% (w/w) THF at 298.15 K, have been determined using IUPAC standardization rules. The variation in the pH S values over the whole composition range of THF–water mixtures studied has been explained by preferential solvation effects, and quantified by application of the quasi-lattice quasi-chemical theory (QLQC). In order to identify the solvent features affecting the variation of pH S values with the percentage of THF, the pH S values obtained were subjected to factor analysis (FA) and were correlated with macroscopic and solvatochromic parameters, π *, α , β and E T of the solvent mixtures. The equations obtained permit the standardization of potentiometric sensors in THF–water mixtures and thus allow pH measurements in the usual way. The correct pH values measured permit the study of acid–base behaviour of substances in methodologies using THF–water media.

Ageing of resin from Pinus species assessed by infrared spectroscopy

AbstractResins obtained from Pinus genus species have been widely used in very different fields throughout history. As soon as the resins are secreted, molecular changes start altering their chemical, mechanical and optical properties. The ageing processes are complex, and the chemical and structural changes associated with resin degradation are not yet fully known. Many questions still remain open, for instance changes happening in pimaranes, one of the two diterpenoid constituents of the resin. A systematic study of the ageing process of Pinus resins is done through Fourier transform infrared spectroscopy (FTIR) using chemical standards and complementing the obtained results with gas chromatography coupled to mass spectrometry (GC/MS) analysis when necessary. Moreover, long-term degradation processes are also investigated through the analysis of a selection of dated historical resins. This study overcomes the limitations of GC/MS and brings new information about the reactions and interactions between molecules during Pinus resin ageing processes. It also provides information about which bonds are affected and unaffected, and these can be used as specific markers of the degradation and of the resins themselves. Graphical AbstractChanges in the IR spectral features due to the Pinus resin ageing processes

New insights on blue pigments used in 15th century paintings by synchrotron radiation-based micro-FTIR and XRD

The blue pigments used on altarpieces in the 15th century in Catalonia and Crown of Aragon are principally composed of the azurite mineral. To a lesser extent, lapis lazuli, also of mineral origin, was used and occasionally in the background areas and outlining the principal figures, indigo (of vegetal origin) was used for the chromatic preparation layer. Data from several altarpieces belonging to well-known artists of that time are analysed by synchrotron radiation X-ray diffraction (SR-XRD), micro-infrared spectroscopy (μ-FTIR), synchrotron radiation micro-infrared spectroscopy (μSR-FTIR), Raman spectroscopy, and scanning electron microscopy with X-ray microanalysis (SEM-EDS). X-ray diffraction and infrared spectroscopy in association with synchrotron radiation have proven to be especially useful due to the micron-sized spot size, high brilliance and energy tunability, which help to obtain good separation of signals coming from different phases/substances and determine their localization in the various paint layers. The examples presented illustrate the potential of each analytical technique for the identification of the type of material present in the 15th century paintings. Moreover, the natural origin and composition of the pigments and their distribution in the paint layers are determined and some correlations with other contemporary paintings are proposed. Finally, the alteration compounds related to blue pigments are determined in each case.

Evaluation of acidity constants and preferential solvation in tetrahydrofuran–water mixtures

Abstract Dissociation constant values for reference materials were determined in tetrahydrofuran–water (THF–water) mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) THF. The values obtained were: p K 1 and p K 2 for phthalic acid; p K 1 , p K 2 and p K 3 for citric acid; p K a for acetic and boric acids. In order to explain the variation of the p K values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. The results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute in order to identify the solvent characteristics affecting the p K a values. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate p K a values with solvatochromic parameters of THF–water mixtures. The equations obtained allow calculation of the p K a values of substances in the widely used THF–water mixtures.