Ana Perdigon

Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study

Abstract Changes produced after hydrothermal treatments at 400 °C and at different pressures in a set of Lu-saturated smectites have been compared with those occurring in a mixture of 2SiO2:1Lu2O3. The effect of the mineralogical composition of the smectite on the reactivity has been analyzed. The growth of a new crystalline phase, Lu2Si2O7 (detected by means of X-ray diffraction), at a different pressure for each smectite, has allowed us to establish a reactivity order among the samples. All the samples were much more reactive than the mixture of oxides. The reactivity of the smectites was modulated by their different mineralogical compositions. First, smectites having Al in the tetrahedral sheet were the most reactive. This finding is interpreted as due to a strong electrostatic interaction created by the Al that activates neighboring Si for the reaction. Second, a higher reactivity was observed for the smectites with total occupancy of the octahedral sheet, due to the simultaneous disruption of the tetrahedral-octahedral shared oxygen layer and the diffusion of octahedral cations into the interlayer space. Third, the layer charge of the smectite does not induce any variation in the reactivity for the set of samples analyzed.

The crystalline structures of the odd alkanes pentane, heptane, nonane, undecane, tridecane and pentadecane monolayers adsorbed on graphite at submonolayer coverages and from the liquid

A combination of neutron and X-ray diffraction has been used to structurally characterise the crystalline monolayer structures of all the alkanes with odd number of carbon atoms in their alkyl chains from pentane to pentadecane adsorbed on graphite. The structures of all the molecules investigated at submonolayer coverages are isomorphous with centred rectangular unit cells containing two molecules per cell in a parallel arrangement. This is a completely different structure from the ‘herringbone’ arrangement of molecules found for the shorter ‘even’ alkanes, such as hexane, octane and decane. The monolayers at sub-monolayer coverages are interpreted as uniaxial commensurate with the underlying graphite while those monolayers coexisting with the liquid, while structurally similar, are fully commensurate. The difference between the two structures is a uniaxial compression in the b-direction with the monolayers coexisting with the liquids significantly more dense than at submonolayer coverages. In the low coverage structures the ‘odd’ molecules have an all trans conformation with their extended axes parallel to the surface with the plane of the carbon skeleton also parallel to the graphite surface. At high coverages the carbon skeleton is no longer parallel to the graphite surface but significantly tilted. The longest alkanes, tridecane and pentadecane also show evidence of positional and/or rotational disorder at high coverages.

NMR study of n-dodecane adsorbed on graphite.

In this brief contribution we demonstrate that 1H and 2H NMR spectroscopy can be an effective method of investigating adsorption from liquids at the solid-liquid interface. The method is illustrated here with the adsorption of a simple alkane adsorbed on graphite, in particular the system n-dodecane and graphite at coverages of 1 and 5 monolayers. Static single-pulse proton nuclear magnetic resonance and static quadrupolar echo deuterium nuclear magnetic resonance spectra were recorded for both coverages. The experimental NMR results presented here show features clearly consistent with earlier calorimetric and neutron scattering work and demonstrate the formation of solid adsorbed layers that coexist with the bulk adsorbate with both isotopes. This ability to probe both deuterated and protonated materials simultaneously illustrates that this experimental approach can be readily extended to investigate the adsorption behaviour of multicomponent mixtures.

High-resolution 1H MAS NMR spectra of 2∶1 phyllosilicates

Solid-state 1H MAS NMR at high magnetic fields and high spinning speeds provides a powerful means of identifying the different proton sites in smectites and affects information on the octahedral nature, the octahedral layer charge deficit and the charges on the cations residing in the interlayer space of the silicate.

Layer-by-layer surface freezing of linear alcohols at the graphite/liquid interface

Differential scanning calorimetry (DSC), incoherent elastic neutron scattering, and neutron diffraction are used to demonstrate the presence of adsorbed solid multilayers of linear alcohols at the graphite-liquid alcohol interface. All alcohols studied (C(5)-C(18)) are found to form at least one monolayer. In addition all the even alcohols investigated (C(6)OH to C(18)OH) show multilayer formation. However, only the short odd alcohols (C(5)OH to C(11)OH) clearly exhibit additional features indicating multilayer formation.

Mixing behaviour of carboxylic acids adsorbed on graphite

Solid monolayer formation by all the linear carboxylic acids from C6 to C20 adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Δn = 2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Δn, and with more similar chain lengths (e.g. Δn = 1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.

Two-dimensional heteronuclear 1H ↔27Al-correlated MAS NMR spectra of layered silicates

The structural locations of aluminium in layered silicates, with different chemical environments but similar coordination polyhedra, can be established unequivocally from the correlation between the chemical shifts of protons and 27Al nuclei, via their heteronuclear dipole–dipole couplings.