The porosity and pore-size distribution in the glass play an important role in glass corrosion; however, such information is difficult to be obtained non-destructively. Here we report the use of spectroscopic ellipsometry (SE) under controlled humidity environments to determine those structural parameters in the alteration layers formed on international simple glass (ISG), a model nuclear waste glass, in aqueous corrosion conditions. The SE results show that the ISG sample corroded for nearly 4.5 years develops pores larger than 2 nm in diameter, while the alteration layers formed initially in less than 1 month do not have such large pores. The development of larger pores over a long period of corrosion time, while the overall thickness remains relatively constant, implies structural rearrangements of the silicate network occurring within the alteration layer, which could eventually affect the transport of reactants and products as well as the structural stability.
Being a nonequilibrium material, the structure of glass varies with the sample history. Thus, the initial surface condition of a glass can vary with the preparation condition and have a large impact on its reactivity. This paper shows that the aqueous corrosion behavior of international simple glass (ISG) varies depending on the initial surface state. The ISG glass samples were prepared as‐polished‐only and polished‐then‐annealed and they were immersed in aqueous solution saturated with soluble SiO2 at 30°C (modeling a mild condition) and at 90°C (modeling a severe condition). Molecular dynamics simulations were performed to obtain coordination numbers of network formers of ISG to assist oxygen speciation calculations. The surface structures of as‐prepared and corroded ISG samples were analyzed using various imaging and spectroscopic techniques. Among these analyses, only the oxygen speciation with x‐ray photoelectron spectroscopy showed discernable differences between two uncorroded surfaces with different preparation histories; all other methods could not differentiate the surface preparation history before aqueous corrosion. Such minor difference in chemical structures was found to have a profound impact on corrosion behaviors in the mild condition. In the harsh condition, the surface history dependence was not as drastic as the corrosion in the mild condition. The analysis results of the corroded surfaces suggested that the thickness and structure of the alteration layer formed on ISG in aqueous corrosion can vary with the initial surface state.
Hydrogen bonding interactions play an important role in many chemical and physical processes occurring in bulk liquids and at interfaces. In this study, hydrous species (H 2 O and Si-OH) on nano-porous alteration layers (gels) formed on a boroaluminosilicate glass called International Simple Glass corroded in aqueous solutions at pH 7 and pH 9, and initially saturated with soluble siliconcontaining species were analyzed using linear and non-linear vibrational spectroscopy in combination with molecular dynamics simulations. The simulation results revealed various possible types of hydrogen bonds among these hydrous species in nanoconfinement environments with their populations depending on pore-size distribution. The nano-porous gels formed on corroded glass surfaces enhance hydrogen bond strength between hydrous species as revealed by attenuated total reflectance infrared spectroscopy. Sum frequency generation spectroscopy showed some significant differences in hydrogen bonding interactions on alteration layers formed at pH 7 and pH 9. The glass dissolution under the leaching conditions used in this study has been known to be ten times faster at pH 7 in comparison to that at pH 9 due to unknown reasons. The simulation and experimental results obtained in this study indicate that the water mobility in the gel formed at pH 9 could be slower than that in the gel formed at pH 7, and as a result, the leaching rate at pH 9 is slower than that at pH 7.npj Materials Degradation (2020) 4:1 ; https://doi.
Vibrational sum frequency
generation (SFG) spectroscopy was used
to study how hydrogen bonding interactions of physisorbed water layers
vary with the glass composition and surface condition. Three different
glass materials were used: amorphous silica (fused quartz), sodium
calcium silicate (also called soda lime silica), and calcium aluminosilicate
(boron-/fluorine-free E-glass). Two different surface conditions were
compared: (i) a mechanically polished, fire-polished, and then annealed
surface and (ii) an acid-leached surface (pH 1, 90 °C, 24 h).
Compositional and structural information obtained by X-ray photoelectron
spectroscopy, spectroscopic ellipsometry, and attenuated total reflection
infrared spectroscopy were correlated with SFG spectral features.
The temperature dependence of SFG spectral features revealed that
regardless of the glass composition, the glass surface is free of
physisorbed water at 250 °C, and during cooling in ambient air,
it starts absorbing water at a temperature corresponding to a relative
humidity of 1–2%. Humidity-controlled SFG measurements at room
temperature showed that the hydrogen bonding interactions of water
physisorbed on the glass surface cleaned with UV/O3 are
quite different depending on the glass composition and surface preparation
history. This finding will be useful in understanding how surface
properties of different glasses are altered by humidity in ambient
air.
Interactions
among antiwear additives (AWs), friction modifiers
(FMs), and dispersant in a lubricating oil are critical for tribological
performance. This study investigates compatibilities of three oil-soluble
ionic liquids (ILs, candidate AWs) with an FM, molybdenum dithiocarbamate
(MoDTC), and a dispersant, polyisobutene succinimide (PIBSI) under
boundary lubrication. Either synergistic or antagonistic effects were
observed depending on the IL’s chemistry. Adding an aprotic
phosphonium–alkylphosphate or phosphonium–alkylphosphinate
IL into the oil containing MoDTC and PIBSI had detrimental impact
on the friction and wear behavior. PIBSI was found to preferably interact/react
with the aprotic IL to lose its ability of suspending MoDTC and to
partially consume or even deplete the IL. In contrast, a protic ammonium–alkylphosphate
IL seemed to be able to coexist with PIBSI and work synergistically
with MoDTC, yielding a sustainable, ultralow boundary friction. A
three-stage tribochemical process is proposed to explain how this
IL + MoDTC pair interacts with the contact surface to form a chemically
reacted, wear-protective tribofilm supporting a physically adsorbed,
friction-reducing film on top. This study provides fundamental insights
of the compatibilities among three common lubricant components, antiwear,
friction modifier, and dispersant, which can be used to guide future
lubricant development.
Oil-soluble
ionic liquids (ILs) have been proved as effective additives
in lubricant oils through tribological experiments and post-test analytical
analyses. In this study, surface structures of lubricant base oil,
oil-soluble ILs, and their mixtures at the air/liquid and solid/liquid
interfaces have been studied using sum frequency generation (SFG)
vibrational spectroscopy. At the air/base oil and air/IL interfaces,
the alkyl chains of the studied compounds were shown to be conformationally
disordered and their terminal methyl groups point outward at the
liquid surface. The base oil dominates the air/(base oil + IL) interface
due to its higher surface excess propensity and larger bulk concentration.
At the solid (silica) surface, ILs adopt a structure with their charged
headgroups in contact with the silica surface, while their alkyl chains
are more conformationally ordered or packed compared to the air/IL
interface. At the interface between silica and (base oil + IL) mixtures,
ILs also preferentially adsorb to the silica surface with their layer
structures somewhat different from those of ILs alone. These results
showed that ILs can adsorb onto the solid surface even before tribological
contacts are made. The insights obtained from this SFG study provide
a better understanding of the role of ionic liquids in lubrication.
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