The beam balance is one of the oldest known measuring instruments. Until the 20th century, balances had been the most sensitive and precise instruments used for scientific measurements. The original balances used a beam supported at the center with pans hung from cords on both ends. The modern electronic beam balances still resemble those original designs; however, the resolution, accuracy, and capabilities have been significantly improved. This review provides a short introduction to the history of beam balances followed by a detailed description of three gravimetric microbalances manufactured by Hiden Isochema for measuring gas and vapor sorption in a variety of materials.
The solubility, density (ρ), viscosity (η), and vapor pressure (P) of lithium bis(trifluoromethylsulfonyl)imide (LiTf 2 N) in water are evaluated. The maximum salt solublity for LiTf 2 N in water was determined to be between mass fractions (w s ) of 0.8065 and 0.8217 at 295.15 K. The density (ρ) and viscosity (η) were evaluated at temperatures ranging from 298.15 to 373.15 K and mass fractions of up to 0.8024. Least-squares regression is used to correlate the viscosity and density data over the entire range of temperatures and concentrations. The vapor pressure of LiTf 2 N in water is determined for temperatures of 274.15 to 471.15 K and mass fractions of up to 0.8065. In addition, this work describes the solvent activity (a s ), osmotic coefficient (Φ), molal activity (γ ± ), and enthalpy of vaporization (ΔH v ). The osmotic coefficients were calculated using the second virial coefficient of water, and Pitzer−Mayorga and Clausius−Clapeyron models are used to evaluate the vapor pressure data.
Molecular
level information about thermodynamic variations (enthalpy,
entropy, and free energy) of a gas molecule as it crosses a gas–liquid
interface is strongly lacking from an experimental perspective under
equilibrium conditions. Herein, we perform in situ measurements of water interacting with the ionic liquid (IL) 1-butyl-3-methylimidazolium
acetate, [C4mim][Ace], using ambient pressure X-ray photoelectron
spectroscopy in order to assess the interfacial uptake of water quantitatively
as a function of temperature, pressure, and water mole fraction (x
w). The surface spectroscopy results are compared
to existing bulk water absorption experiments, showing that the amount
of water in the interfacial region is consistently greater than that
in the bulk. The enthalpy and entropy of water sorption vary significantly
between the gas–liquid interface and the bulk as a function
of x
w, with a crossover that occurs near x
w = 0.6 where the water–IL mixture converts
from being homogeneous (x
w < 0.6) to
nanostructured (x
w > 0.6). Free energy
results reveal that water at the gas–IL interface is thermodynamically
more favorable than that in the bulk, consistent with the enhanced
water concentration in the interfacial region. The results herein
show that the efficacy for an ionic liquid to absorb a gas phase molecule
is not merely a function of bulk solvation parameters but also is
significantly influenced by the thermodynamics occurring across the
gas–IL interface during the mass transfer process.
The miscibility of ionic liquid (IL) pairs with a common cation (1-ethyl-3-methylimidazolium [C 2 C 1 im]) and different anions (bis(trifluoromethylsulfonyl)amide [TFSI], acetate [OAc], and chloride [Cl]) was investigated at a wide range of water concentrations at room temperature. Molecular simulations predicted that the addition of water to the [C 2 C 1 im][TFSI]:[C 2 C 1 im][OAc] and [C 2 C 1 im][TFSI]:[C 2 C 1 im][Cl] mixtures would induce a liquid-liquid phase separation and that water addition to the [C 2 C 1 im][OAc]:[C 2 C 1 im][Cl] mixture would not produce a phase separation. The effect of water on the phase behavior of the IL mixtures was verified experimentally, and the IL and water concentrations were determined in each phase. Of particular importance is the analytical methodology used to determine the species' concentration, where 1 H NMR and a combination of 19 F NMR, Karl Fischer titration, and ion chromatography techniques were applied.
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