Effect of aqueous electrolyte concentration and valency on contact angle on flat glass surfaces and inside capillary glass tubes

Effects of chemical solution components on the contact angle of typical minerals in soil: quartz, orthoclase and plagioclase

Different chemical solutions can significantly change the contact angle (CA) of soil, but few studies have studied the change rule and action mechanism of the CA from the mineral composition of soil essence. In unsaturated soil mechanics, the CA is an important parameter to calculate the wet suction between soil particles in unsaturated soil. When the chemical composition of the soil pore liquid changes, the CA will also change, which will affect the wet suction and other parameters, thus changing the macroscopic mechanical properties of the soil. In this study, the CA of air-solution-mineral phases with different solution components (pH, type and concentration of salt solution) of different minerals (quartz, orthoclase and plagioclase) were measured. The results show that the CAs of quartz, orthoclase and plagioclase all rise first and then drop with the rise of pH. The peak CAs are pH = 5, pH = 4 and pH = 3, respectively. Quartz, orthoclase and plagioclase all have valley values in different concentrations of NaCl and KCl solutions. In CaCl2 solution, only quartz has valley value, while orthoclase and plagioclase increase monotonously. Quartz in soil plays a main role in the influence of soil CA, followed by orthoclase and plagioclase. The CA of different minerals in different chemical solutions is mainly controlled by electric double layer interaction and functional groups interaction. In different salt solution environment, in addition to the above two effects, the mineral CA is also affected by the surface tension.

Harvesting of aerial humidity with natural hygroscopic salt excretions

Plants and animals that thrive in arid regions utilize the diurnal changes in environmental temperature and humidity to optimize their water budget by combining water-harvesting mechanisms and morphophysiological traits. The Athel tamarisk (Tamarix aphylla) is a halophytic desert shrub that survives in arid, hypersaline conditions by excreting concentrated solutions of ions as droplets on its surface that crystallize into salt crystals and fall off the branches. Here, we describe the crystallization on the surface of the plant and explore the effects of external conditions such as diurnal changes in humidity and temperature. The salt mixtures contain at least ten common minerals, with NaCl and CaSO4·2H2O being the major products, SiO2 and CaCO3 main sand contaminants, and Li2SO4, CaSO4, KCl, K2Ca(SO4)2·H2O, CaMg(CO3)2 and AlNaSi3O8 present in smaller amounts. In natural conditions, the hanging or sitting droplets remain firmly attached to the surface, with an average adhesion force of 275 ± 3.5 µN measured for pure water. Rather than using morphological features of the surface, the droplets adhere by chemical interactions, predominantly by hydrogen bonding. Increasing ion concentration slightly increases the contact angle on the hydrophobic cuticle, thereby lowering surface wettability. Small amounts of lithium sulfate and possibly other hygroscopic salts result in strong hygroscopicity and propensity for deliquescence of the salt mixture overnight. Within a broader context, this natural mechanism for humidity harvesting that uses environmentally benign salts as moisture adsorbents could provide a bioinspired approach that complements the currently available water collection or cloud-seeding technologies.

Estimating Internal Stress of an Alteration Layer Formed on Corroded Boroaluminosilicate Glass through Spectroscopic Ellipsometry Analysis

Aqueous corrosion of glass may result in the formation of an alteration layer in the glass surface of which chemical composition and network structure are different from those of the bulk glass. Since corrosion occurs far below the glass-transition temperature, the alteration layer cannot fully relax to the new structure with the lowest possible energy. Molecular dynamics simulations suggested that such a network will contain highly strained chemical bonds, which can be manifested as a stress in the alteration layer. Common techniques to measure stress in thin films or surface layers were found inadequate for thick monolithic glass samples corroded in water. Here, we explored the use of spectroscopic ellipsometry to test the presence of internal stress in the alteration layer formed by aqueous corrosion of glass. A procedure for analyses of spectroscopic ellipsometry data to determine birefringence in the alteration layer was developed. Findings with the established fitting procedure suggested that a stress builds up in the corroded surface layer of a boroaluminosilicate glass if there is a change in relative humidity, pH, or electrolyte concentration of the environment to which the glass surface is exposed. A similar process may occur in other types of glass, and it may affect the surface properties of corroded glass objects.

Foam Silica Films Synthesized by Calcium Chloride-Assisted Emulsification

The development of porous films with an accessible high specific surface area is important for designing new adsorbents, sensors, or catalyst supports. Here, we describe a simple method to prepare a silica foam coating using a calcium chloride-assisted evaporation-induced emulsification method. An alcoholic silica sol containing calcium chloride and a poly(ethylene oxide)-based polymer is deposited on a substrate by dipping. The evaporation of the alcohol induces a phase separation between the silica-rich phase and the calcium-rich one. The size of the droplets increases via a coalescence process until the gelation of the sol, which determines the final pore size between 100 nm and 3 μm. Thermal analysis and monitoring of droplet evaporation confirm that the departure of the solvent is delayed by the presence of calcium chloride in the sol. The influence of the nature of the polymer on the porosity is discussed. The use of a block copolymer such as the Pluronic F-127, which strongly stabilizes the emulsion, allows to reach a low pore size (400 nm), while on the contrary, we propose to use a short poly(ethylene glycol) (PEG) such as PEG-400, which weakly stabilizes it, leading to larger pores (2-3 μm). Furthermore, we show that the addition of a zirconium salt (ZrOCl₂·8H₂O) to the silica sol accelerates the condensation step of the silica and leads to the decrease in the pore size.

Interaction of Bulk Nanobubbles (Ultrafine Bubbles) with a Solid Surface

The experimental results [Kanematsu, W. Chem. Eng. Sci. 2020, 219, 115594] on the temporal variations of number concentrations of bulk nanobubbles (ultrafine bubbles) in contact with polymer materials are theoretically analyzed based on the dynamic equilibrium model of bulk nanobubbles partly covered with hydrophobic materials (impurities). It is suggested that bulk nanobubbles are adsorbed on a polymer surface by attractive hydrophobic interaction between a polymer surface and a hydrophobic material partly covering the bubble surface, overcoming the repulsive double-layer interaction. There are two mysteries. One is that the maximum surface number concentration of bulk nanobubbles of about 70 nm in diameter adsorbed on a hydrophobic polymer surface is more than an order of magnitude lower than the typical value for colloid particles of a similar or larger size. The other is that the experimental adsorption rate of bulk nanobubbles on hydrophobic polymer surface is several orders of magnitude lower than the theoretically estimated one. The mysteries are resolved if many of the bulk nanobubbles adsorbed on a hydrophobic polymer surface change to surface nanobubbles with a footprint diameter of about 1 μm.

New details of assembling bioactive films from dispersions of amphiphilic molecules on titania surfaces

Tailoring the surface properties of materials for biomedical applications is important to avoid clinical complications. Forming thin layers of amphiphilic molecules with apolar regions that facilitate attractive intermolecular interactions, can be a suitable and versatile approach to achieve hydrophobic surface modification and provide functional antibacterial properties. Aiming to correlate layer structure and properties starting from film formation, octadecylphosphonic acid (ODPA) and dimethyloctadecyl (3-trimethoxysilylpropyl) ammonium chloride (DMOAP) layers were adsorbed onto smooth titania surfaces. Then the films were studied by atomic force microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS), and their interactions with aqueous environments were characterized by contact angle and zeta potential measurements. In addition, antibacterial assays were performed using E. coli and S. mutants to reveal the antibacterial properties effected by the surface modification. Immediately after sputter deposition, titania was hydrophilic; however, after air storage and adsorption of DMOAP or ODPA, an increase in the water contact angle was observed. XPS investigations after layer formation and after antibacterial tests revealed that the attachment of layers assembled from ODPA on titania substrates is considerably stronger and more stable than that observed for DMOAP films. Heat treatment strongly affects DMOAP layers. Furthermore, DMOAP layers are not stable under biological conditions.

Structure–property relationship of amphiphilic molecules on smooth substrates was explored through a multi-step approach and its influence on biological activity.

The effect of sodium chloride concentration on the mutarotation and structure of d-xylose in water: Experimental and theoretical investigation

The effect of NaCl concentration on the structure of d-xylose in H₂O was studied. It was found that NaCl could prolong the equilibrium time between the two main configurations, α-xylopyranose and β-xylopyranose. The proportion of α-xylopyranose was slightly increased in NaCl-H₂O solution than that in H₂O, and the alteration of NaCl on α-xylopyranose and β-xylopyranose was different. Theoretical calculations demonstrated that NaCl was more favorable to stabilize the structure of α-xylopyranose. Na⁺ had attraction with O atoms (α: O₆; β: O₆ and O₁), with the outflow of electron from C atom to O atom on the C₁-O₆ bond, which was beneficial to the transformation between chain form and pyran forms. Cl^- had interaction with the hydroxyl groups of xylose. The interaction between xylose and NaCl, was also evidenced by the variation of ³⁵Cl and ²³Na NMR spectra. The findings could provide guidance for understanding the conformational change and design of xylose conversion ways. It also provided valuable information for making efficient use of hemicellulose.

Simultaneous measurement of liquid surface tension and contact angle by light reflection

We present an optical method of simultaneous measurement of liquid surface tension, contact angle, and the curved liquid surface shape, which uses the light reflection from this liquid surface due to the wettability. When an expanded and collimated laser beam is incident upon the curved liquid surfaces vertically, the special light reflection pattern, which includes a dark central region and a bright field outside, was observed. A critical spot on the curved liquid surface was found, and the dark field distribution is related to both the width of incidence beam and this critical spot. In our experiment, the different dark field distribution patterns were recorded when the width of the incidence beam changed. The liquid surface tension, contact angle, and the liquid surface shape were measured simultaneously. The proposed method is a new effective tool for present wetting characterization methods.