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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A Adsorbate
An adsorbed molecule (see Adsorption).
Adsorbent
A solid or liquid surface capable of interacting with the Adsorbate.
Absorption
Incorporation of the sorbed molecule into the host material: For example, absorption of the hydrogen proton in a metal such as palladium after dissociation of H2 at the active surface, or absorption of a water molecule in a polymer. Absorption invariably causes swelling (dilation) of the host and can lead to embrittlement or rupture of rigid materials.
Adsorption
The accumulation of molecules at the surface of a solid or liquid. Broadly this has two classes, weak Physisorption (Physical Adsorption) predominantly due to attractive Van der Waals’ intramolecular forces, and Chemisorption (Chemical Adsorption) where a chemical bond is formed. The magnitude of adsorption can be expressed in terms of surface coverage: A Monolayer is an adsorbed surface film one molecule deep (i.e., a Langmuir Film) and further adsorbed layers are called Multilayers.
Adsorptive
A molecule capable of being adsorbed (see Adsorption). [return to top] B BET
Acronym for the Brunauer-Emmett -Teller equation which describes the isothermal formation of adsorbed multilayers as a function of Relative Pressure and from which the Monolayer coverage can be found. This equation is mostly widely applied for adsorption of nitrogen at LN2 temperature as the means to measure surface area in Volumetric sorption instruments.
Buoyancy Effect
In the Gravimetric method, the change in the measured weight due to the displacement of the gas, following Archimedes’ Principle. This is affected by the sample, the sample holder, and the balance counterweights and hangdowns, and is dependent on the volume and density of these components, and the density of the gas. The sample weight change with pressure in a non-adsorbing gas can also be used for the determination of the sample density. The dead space correction in the volumetric method is the equivalent of the Buoyancy Effect correction in Gravimetry. [return to top] C
Capillary Condensation
The process by which a liquid Adsorbate phase is drawn into pore structure most commonly applied to Mesoporous materials. This process exhibits irreversibility between adsorption and desorption generally assumed to be due to the difference in contact angle of the liquid phase with the pore wall. The Kelvin equation, which associates the vapor relative pressure with pore radius, is used to calculate pore size distribution from isothermal measurements of the desorption process.
Chemical Adsorption
See Chemisorption.
Chemisorption
Adsorption due to chemical as opposed to physical forces such as hydrogen chemisorption on the surface of a catalytically active surface. Chemisorption is usually an irreversible process. The magnitude of chemisorption measured from a chemisorption Isotherm is therefore proportional to the number of active sites (for example, a means of measuring the metal surface area in heterogeneous catalysts).
Chemical Potential (m)
The Gibbs Free Energy per mole of Sorbate that expresses the work required to bring a Sorbate molecule from a reference state (usually vacuum) to another state. For an ideal gas this is a function of both pressure and temperature and for a non-ideal gas this is a function of Fugacity and temperature.
Coadsorption Equilibria/Equilibrium
The equilibrium state of a system containing more than one adsorptive fluid. [return to top] D Desorption
Generic term for the reverse process of either Absorption or Adsorption induced by lowering the Chemical Potential. A reversible desorption process is where the sorption and desorption isotherms are identical, which is typical of surface adsorption.
Diffusion
The process by which the sorbate molecules move in the host material, due to random molecular motions. This is usually quantified by a Diffusion Coefficient to express how mobile the sorbed phase is. The diffusion coefficient is a function of temperature and concentration, and can be measured from sorption versus time curves in defined conditions during isothermal gravimetric experiments.
The chemical diffusion coefficient, DC, which is the magnitude of the flux of sorbed phase from one part of the material to another, is in the simplest model, described by Fick’s Law. The mobility of the sorbed phase at the microscopic level is described by the self-diffusion coefficient, DS.
N.B. The nomenclature for diffusivity varies between different application and science disciplines.
Diffusion coefficients are not the same as Permeation rates. Permeation is a function of both diffusivity and solubility. [return to top] E Equation of State
The relationship between the Sorbate pressure, temperature and volume.
Equilibration Time
The time required for equilibrium to be obtained within a specified criterion, for example, that the process is 99% complete.
Equilibrium
The state when the net rate of sorption is zero (the rates of sorption and desorption are equivalent). The approach to equilibrium is exponential (analogous to thermal equilibration) and therefore this state can never be measured but is extrapolated from the sorption trend.
Excess Adsorption
The quantity determined in a gravimetric or volumetric adsorption experiment. Also Gibb’s Excess or Gibbsian Surface Excess. [return to top] F Fick’s Law
A relationship where the flux of diffusing sorbate is proportional to the sorbate concentration gradient. The constant of proportionality is the chemical diffusion coefficient (see Diffusion).
Fickian Diffusion
Diffusion that obey’s Fick’s Law.
Fugacity
The tendency of the Sorbate to change phase or to expand at constant temperature. For an ideal gas this is equivalent to the gas pressure and is otherwise dependent on the Equation of State. [return to top] G
GAB
Acronym for the Guggenheim-Anderson-de Boer isotherm model. This is most commonly used in the analysis of water sorption on food samples and is an extension of the BET equation and similarly yields an estimate for the Monolayer coverage and thereby the ideal storage conditions.
Gas Sorption
The adsorption or absorption of a fluid in its gas phase. For example, the sorption of an inert probe gas such as nitrogen near its boiling temperature.
Gravimetric
Pertaining to the measurement of weight or weight change. The first gravimetric sorption analysers were called McBain spring balances and the weight change due to sorption was measured manually from the extension of a quartz spring. Modern automatic analysers like the IGA system use ultra-sensitive electronic microbalance to directly measure weight change due to Sorption and this is corrected for Buoyancy to give the mass uptake isotherm. In the absence of sorption, a gravimetric analyser can also be used as a densitometer to measure the Equation of State of the sorbate or the density of the sample.
[return to top] H Helium Density
The chemical or skeletal density of a porous material determined with Helium (using pycnometry).
Hydride
A chemical compound formed between hydrogen and an element such as palladium or a compound such as LaNi5. Some hydrides such as LaNi5-H are reversible with hydrogen pressure at near ambient conditions and are cited as potential hydrogen storage materials.
Hydrogenation
The process of forming a metal hydride. [return to top] I
Isosteric Enthalpy of Adsorption
The standard enthalpy of adsorption at a fixed surface coverage. Also known as the Isosteric Heat or the Isosteric Heat of Adsorption.
Isotherm
Measurement of the amount sorbed as a function of Sorbate pressure at constant temperature. The pressure may be the absolute pressure referenced to vacuum (IGA-001 ), the relative pressure referenced to saturation vapor pressure P0 (IGA-002 ), the partial pressure (IGA-003 ) or relative density (e.g. humidity) for dynamic water sorption experiments using the IGAsorp.
Isotherms may be measured at different temperatures to map phase diagrams or to enable calculation of the differential enthalpy and entropy of sorption using the Clausius-Clapeyron equation. The amount sorbed is expressed in a range of units such as mmol/g and percentage uptake. It is also referred to as uptake, coverage, composition and concentration. [return to top] K Kinetic Selectivity
The property shown by molecular sieve materials in which the difference in the sorption kinetics of molecules of a different size or mass is used to select one species over another. The kinetic selectivity of carbon molecular sieves (CMS) is exploited in Pressure Swing Adsorption (PSA) for gas separation applications. [return to top] L Langmuir Isotherm The isotherm, introduced by Langmuir in 1916, that describes the sorption of an adsorbate monolayer on a uniform surface in the absence of adsorbate-adsorbate interactions. [return to top] M
Mesopore
A pore of diameter between 2nm and 50nm; for example, those found in silica gel desiccants. Mesopores which are many times the diameter of adsorbate molecules will exhibit Capillary Condensation.
Micropore
A pore of diameter below 2nm for example those found in activated carbons and zeolites which exhibit molecular sieving properties.
Moisture Sorption
The sorption of water in the liquid or vapour phase.
Monolayer
An adsorbed film completely covering the surface that is exactly one molecule thick (a Langmuir film). This state is rarely observed experimentally but rather the monolayer coverage is interpolated from the Multilayer behaviour using, for example, the BET and GAB equations. The surface area of the material can then be calculated if the molecular packing arrangement is known.
Multilayer
Adsorption on the Monolayer.
MVTR
See Permeation. [return to top] P
Permeation
The steady state transfer rate of sorbate through a host material in defined conditions, which is a function of both solubility and diffusivity. This is often expressed as a permeability constant or transmission rate. For example, Moisture Vapour Transmission Rate (MVTR) is used to describe the rate at which water molecules move through packaging films.
PCI
Acronym for Pressure-Composition Isotherm. A series of PCIs form a PCT diagram (a phase diagram in terms of pressure, composition and temperature).
PCT
Acronym for pressure-composition-temperature (Isotherm) and is a commonly used for absorption isotherms determined from metal hydrides. These data are also plotted using a different convention with pressure on the y-axis and composition (the amount absorbed) plotted on the x-axis and relates to the conventions of phase diagrams: The boundaries of Hydride phase(s) are often marked.
Physical Adsorption
See Physisorption.
Physisorption
Weak Adsorption due predominantly to dispersive (Van der Waals) intramolecular forces. For example, the adsorption of nitrogen at LN2 temperature used in BET surface area analysis. Adsorbate molecules with inherently low Van der Waals interaction such as water vapor are very sensitive to surface chemistry.
Pore Size Distribution
Used to characterize (typically) microporous materials, such as activated carbons, that have an amorphous skeletal structure.
Pore Volume
The volume of the pores, or voids, within a porous material. The ratio of this volume to the volume occupied by the solid (the skeletal structure) of a porous solid can be used to express its porosity. [return to top] S
Sievert’s Method (Apparatus)
See Volumetric.
Solubility
The magnitude of Absorption which might be expressed as a concentration in defined conditions or as a solubility constant. For example, an ideal (dilute) isothermal solid solution is described by Sievert’s solubility constant, ks, which is the ratio of adsorbed concentration to the square root of gas pressure.
Sorbate
A molecule capable of being adsorbed (see Adsorption) or absorbed (see Absorption).
Sorbent
A solid or liquid material capable of interacting with the Sorbate.
Sorption
Generic term for Adsorption and Adsorption interactions proposed by J. W. McBain in 1909.
Specific Surface Area (SSA)
The surface area of a unit mass of material (e.g. cm2 g-1). For porous materials in particular, this is highly dependent on the method used.[return to top] T Thermal Decomposition
The decomposition of a compound by heating. As determined using thermogravimetric analysis (TGA).
Thermal Stability
The thermodynamic stability of a compound. A stable compound will decompose at high temperatures, an unstable compound will decompose at lower temperatures. For reversible metal hydrides, high stability results in a low plateau pressure in the pressure-composition isotherm (PCI), low stability results in a high plateau pressure, for a given temperature. [return to top] V
Van ‘t Hoff Plot
For metal hydrides, this is a plot of ln (P) against 1/T, where P is the plateau pressure of the hydride at a temperature, T. This typically results in a straight line with a gradient equal to the enthalpy of formation or decomposition and an intercept equal to the entropy.
Vapour Sorption
The sorption of a fluid in its vapour phase. For example, the sorption of water at different relative humidities.
Volumetric Method
The isothermal measurement of sorption from the pressure change by dosing between calibrated volumes, also known as Sievert’s method. The origins of this method date back to the 18th century and it is still in widespread usage in conventional BET analyzers. It is most widely applied to the study of near ideal gases. [return to top] |
