Moira Ambrosi

Nanoparticles for Cultural Heritage Conservation: Calcium and Barium Hydroxide Nanoparticles for Wall Painting Consolidation

Nanotechnology provides new concepts and materials for the consolidation and protection of wall paintings. In particular, humble calcium and barium hydroxide nanoparticles offer a versatile and highly efficient tool to combat the main degradation processes altering wall paintings. Clear example of the efficacy and potentiality of nanotechnology is represented by the conservation in situ of Maya wall paintings in the archaeological area in Calakmul (Mexico).

Stable dispersions of Ca(0H)2 in aliphatic alcohols: properties and application in cultural heritage conservation

The kinetic stability of dispersions of Ca(OH)2 particles (1-2 µm) in short-chain aliphatic alcohols was investigated. The alcohols were shown to strongly enhance the kinetic stability with respect to water. Ca(OH)2 crystalline nanoparticles were also synthesised at 60 °C from aqueous supersaturated solutions. The nanoparticles were characterised by scanning electron microscopy and atomic force microscopy techniques. The kinetic stability of the Ca(OH)2 nanoparticle dispersions was higher than that of the micron-sized particles. Dispersions of the nanoparticles in 1-propanol were successfully tested as consolidating on aerial mortar and carbonatic stone specimens. Applications of these dispersions on carbonatic stones presenting flaking and powdering from architectonic sites in Rome and near Padua gave positive results.

Effect of Headgroup Chirality in Nanoassemblies. Part 1. Self-Assembly of D-Isoascorbic Acid Derivatives in Water

L-(+)-Ascorbic acid and D-(-)-isoascorbic acid are epimers, with an opposite configuration at the C5 stereogenic chiral center. Single-chained surfactants that carry a L-ascorbic or d-isoascorbic acid residue as hydrophilic headgroup and an alkanoate tail as hydrophobic chain were synthesized. We refer to these as L-ASCn and D-ASCn, with n=8, 10, or 12. The role of the headgroup configuration in determining the nature of both the pure compounds and their nano assembly in aqueous dispersions were studied. Surface tension, infrared spectroscopy, differential scanning calorimetry, conductivity, small-angle X-ray scattering, and wide-angle X-ray diffractometry were used to characterize surfactant properties as a function of temperature and concentration. The L and D headgroup forms have significantly different hydration. This greatly affects the structure and stability of the aggregates in the micellar and coagel states.

Bottom-up/top-down synthesis of stable zirconium hydroxide nanophases

It is well known that the structure of ZrO2 powder synthesized by low temperature calcination of hydrous zirconia is strongly affected by the nature and properties of precursors. In the present paper, we combined a precipitation/restructuring approach (bottom-up/top-down methodology) to produce zirconium hydroxide nanoparticles that transform into stable nanophasic tetragonal zirconia by low temperature calcination. The dimension and structure of precursors could be modulated by adjusting pH. The synthetic route was investigated by Raman spectroscopy, X-Ray Diffraction, Small-Angle X-Ray Scattering and High-Resolution Transmission Electron Microscopy. The zirconium hydroxide nanoparticles represent the final product of the overall process, which starts as a bottom-up synthesis of the hydroxide gel, followed by a top-down reorganization stage. During this rearrangement, the evolving structure passes through a mass fractal that arises from the clustering of the primary zirconium hydroxide nano-units.

Effect of Headgroup Chirality in Nanoassemblies. 2. Thermal Behavior of Vitamin C-Based Surfactants

l-Ascorbic and d-isoascorbic acids are epimers, and their alkanoyl-esters are surfactant-like in water. These ascorbic acid derivatives retain the same stereogenic configurations of the parent molecules. That circumstance strongly affects their physicochemical properties, both in the solid and in the dispersed states. The thermal behavior of 6-O-l-ascorbyl-dodecanoate (l-ASC12), of 6-O-d-isoascorbyl-dodecanoate (d-ASC12), and of their mixtures was investigated through DSC, XRD, and FTIR experiments. The results indicate that the mixtures of the two epimers produce two eutectics and a 1:1 molecular compound, both in the pure phase and in the coagel state. From the phase diagram data, the activity coefficients of the different species and the main excess thermodynamic functions can be calculated. The results provide insight into the effects of stereochemistry on their intermolecular interactions.

Water of hydration in coagelsPresented at the 17th Conference of the European Colloid & Interface Science Society, Firenze, Italy, September 21?26, 2003.

Vitamin C-based alkanoyl-6-O-ascorbic acid esters, ASCn, are a class of surfactants, interesting both on account of their phase behaviour, and of the properties of the supramolecular assemblies they form. When dispersed in water at room, or lower, temperatures above ca. 5% w/w concentration, they form coagels. At higher temperatures, the microstructure changes to micellar solutions for surfactants of low hydrocarbon chain length n. The longer chained systems form gel phases. The transition enthalpy change is dominated by rearrangements in hydrophobic chain packing. On the other hand the Krafft point seems instead to be dictated mainly by interactions between the polar headgroups and the solvent. The coagel phase is usually thought of as formed of surfactant lamellae separated by thin interlayers of strongly bound, essentially “frozen”, water molecules. In this work, DSC measurements were performed to explore the interactions between water and the surfactant molecules. Two kinds of water were detected: interlayer hydration water and bulk water. The number of hydration water molecules per polar headgroup was inferred from the experimental results. Further insights into the coagel structure were gained from X-ray diffraction and optical microscopy. The effects of glycerin (GLY), propylene glycol (PG), and poly(ethylene glycol) (PEG), as co-solvents were investigated by conductivity and DSC experiments. Glycerin seems to stabilize the coagel, probably through the formation of hydrogen bonds that compete for the polar headgroups with the strongly bound water molecules. By contrast, PG and PEG decrease the compactness of the lamellar structure. This is most likely because these compounds can penetrate into the lipid portion of the lamellae, and reduce the hydrophobic interactions that hold the compact assemblies together.

Effect of the Alkyl Chains and of the Headgroups on the Thermal Behavior of Ascorbic Acid Surfactants Mixtures

The role of the alkyl chain length and of the headgroup on the thermal behavior of mixtures of ASC8 (ascorbyl octanoate) and ASC16 (ascorbyl hexadecanoate) was investigated through differential scanning calorimetry, small- and wide-angle X-ray scattering, and Fourier transform infrared spectroscopy experiments. The formation of two eutectics and of a peritectic point was found from the phase diagram, and their structural properties were studied. The results were compared by investigating the thermal behavior of mixtures of octanoic acid and hexadecanoic acid. The findings provide insights into the role of the ascorbyl headgroups on the intermolecular interactions that determine the phase behavior of the two ascorbic acid based surfactants in the solid state.

Organogels from Double-Chained Vitamin C Amphiphilic Derivatives

The syntheses and physicochemical characterization of double-chained amphiphilic compounds obtained from vitamin C are reported: dialkanoyl-5,6-O-ascorbic acid esters (Di-ASCn, n=8, 10, and 12). The acetyl-5-dodecanoyl-6-ascorbic acid ester is synthesized and investigated for comparison. These products are quite insoluble in water and in polar solvents, although they form homogeneous dispersions in cyclohexane. Upon cooling, these dispersions turn into a gel-like phase. Differential scanning calorimetry, FTIR spectroscopy, and small- and wide-angle X-ray scattering experiments are performed to investigate the properties of pure solids and their liquid dispersions. Di-ASCn retain the same redox properties of the parent molecule and represent a valid candidate for the production of nanosized protective carriers for valuable guests that are sensitive to oxidative radical attack. Moreover, the contribution of the vitamin C hydroxyl group in position 5 to the overall hydration properties of single- and double-chained amphiphilic derivatives is discussed.

Specific ion effects in polysaccharide dispersions

The specific effects induced by some strong electrolytes or neutral co-solutes on aqueous mixtures of guar gum (GG), sodium alginate (SA) and sodium hyaluronate (SH) were studied through rheology and DSC experiments. The results are discussed in terms of changes in the polymer conformation, structure of the network and hydration properties. This study is also aimed at controlling the viscosity of the aqueous mixtures for application in green formulations to be used as fracturing fluids for shale gas extraction plants.

High-performance and anti-stain coating for porcelain stoneware tiles based on nanostructured zirconium compounds

The technological characteristics of porcelain stoneware tiles make them suitable for a wide range of applications spanning far beyond traditional uses. Due to the high density, porcelain stoneware tiles show high bending strength, wear resistance, surface hardness, and high fracture toughness. Nevertheless, despite being usually claimed as stain resistant, the surface porosity renders porcelain stoneware tiles vulnerable to dirt penetration with the formation of stains that can be very difficult to remove. In the present work, we report an innovative and versatile method to realize stain resistant porcelain stoneware tiles. The tile surface is treated by mixtures of nanosized zirconium hydroxide and nano- and micron-sized glass frits that thanks to the low particle dimension are able to penetrate inside the surface pores. The firing step leads to the formation of a glass matrix that can partially or totally close the surface porosity. As a result, the fired tiles become permanently stain resistant still preserving the original esthetical qualities of the original material. Treated tiles also show a remarkably enhanced hardness due to the inclusion of zirconium compounds in the glass coating.