Vapours

About: Vapours is a research topic. Over the lifetime, 1153 publications have been published within this topic receiving 15022 citations.

Cholesteric liquid crystals for detection of organic vapours

Abstract: Studies on the use of cholesteric liquid crystals (CLCs) as sensing phase for detection of organic vapours in air are described. Stock solutions of 1.0% (w/v) cholesteryl nonanoate (CN) and cholesteryl chloride (CC) were prepared in tetrahydrofuran. Binary mixtures, with compositions ranging from 0.18 to 0.25% of CC and 0.82–0.75% of CN, respectively, were prepared by appropriate mixing of the stock solutions. Films were cast by pipetting three 10 μl aliquots of the CLC solution mixture onto a glass disk, whose reverse side was made black to absorb unscattered light. The glass disk was adapted to the common end of a bifurcated optical fibre bundle and placed in a glass vial, which provided a headspace of organic vapours. Measurements were carried out at 27±1 °C, a temperature in which the CLC mixtures maintain their liquid crystalline properties. The responses of the CLC mixtures to vapours of ethanol, acetone, benzene, pyridine and hexane were investigated. The colour of the sensing phases depended on their compositions and exposure to organic vapours gives rise to a change in the optical characteristics of liquid crystals. It was found that the CLC layers containing 0.23–0.25% of CC had no significant change in optical properties when exposed to organic vapours and that ethanol did not cause any optical changes in the liquid crystal layers. Benzene as well as hexane always turned all the coloured liquid crystalline layers to colourless. The CLC layers exhibited different behaviours to vapours of acetone and pyridine. For example, the wavelengths of maximum scattering for the 0.19% CC layer were 530 nm in air, 545 nm in pyridine and 580 nm in acetone. The CLC layers showed reversibility. The lifetimes of these layers (interval of time in which the liquid crystalline phase exists, before crystallisation) were investigated by employing acetone and n -hexane vapours. Average lifetimes of 14–15 min were found for films in contact with these vapours, while a lifetime of 205 min was possible when the CLC film was exposed to air.

The Existence of Intermetallic Compounds in the Vapour State. The Spectra of the Alkali Metals, and of their Alloys with Each Other

Abstract: It has been uncertain until recently, whether any of the many intermetallic compounds which have been established in liquid and solid alloys can persist into the vapour state, or whether they invariably dissociate into mere mixtures of the individual metal vapours. Volatile compounds have occasionally been suspected ( e. g ., between magnesium and zinc) on account of the deposition of typical crystals of compounds during the distillation of an alloy of the two metals, but such evidence is so indirect that it carries little weight. The most promising way of approaching this question is by a study of the absorption spectra of mixtures of metallic vapours. The metals that are known to possess a measurable proportion of diatomic or polyatomic molecules in the vapour state invariably indicate this property by the appearance of bands in their absorption spectra. In fact, since it has been accepted that band spectra cannot originate from single atoms, the observation of such bands has become the most delicate test for the existence of association in the vapours. For example, the usual methods of molecular weight determination have led to the belief that the alkali metal vapours are solely monatomic. This cannot, however, be true, as each of these metals possesses an extensive band spectrum, which appears at quite low vapour pressures. There can be no doubt, on the basis of this spectroscopic evidence, that an appreciable fraction of the molecules in the vapour of boiling sodium, etc., are di- (or poly-) atomic. This point will be discussed fully below.

Application of metal organic framework crystals for sensing of volatile organic gases

Abstract: In this work, metal organic frameworks are used for the first time to detect volatile organic compounds. A sensitive nanoporous metal organic framework layer was deposited using a custom electrospraying system onto the surface of quartz resonators to build a gravimetric sensor. The response of sensors made using the electrosprayed film is compared against those prepared with the more common technique of drop-casting. It is demonstrated that our electrospraying technique resulted in a significantly higher quality factor for the quartz resonators which leads to improved sensor resolution and stability. The experimental set-up is presented and the response of the system to various concentrations of the vapours of three ubiquitous solvents, i.e., acetone, tetrahydrofuran, and isopropyl alcohol, is investigated. The minimum detectable concentrations of these organic vapours are 50 ppm, 10 ppm, and 2 ppm, respectively.