Dynamic vapor sorption
Encyclopedia
Dynamic vapor sorption is a gravimetric
technique that measures how quickly and how much of a solvent is absorbed by a sample: such as a dry powder absorbing water. It does this by varying the vapor concentration surrounding the sample and measuring the change in mass which this produces. Water vapor
is most commonly used, but it is also possible to use a wide range of organic solvents. DVS was originally developed to replace the time and labor intensive desiccator
s and saturated salt solutions to measure water vapor sorption isotherm
s.
isotherms. In general, a vapor sorption isotherm
shows the equilibrium amount of vapor sorbed as a function of steady state relative vapor pressure
at a constant temperature. For water sorption isotherms
, water relative vapor pressure is more commonly expressed as relative humidity
. In a DVS experiment this is accomplished by exposing a sample to a series of step changes in relative humidity
and monitoring the mass change as a function of time. The sample mass must be allowed to reach gravimetric equilibrium at each step change in humidity before progressing to the next humidity level. Then, the equilibrium mass values at each relative humidity
step are used to generate the isotherm. Isotherms are typically divided into two components: sorption
for increasing humidity steps and desorption
for decreasing humidity steps. Sorption can be further divided into adsorption
(sorbate located on the surface) and absorption (sorbate penetrates the bulk).
result from a DVS experiment for a microcrystalline cellulose sample. The kinetic data (Figure 1a) shows the change in mass and humidity as a function of time. From the kinetic results, the rate of water uptake and water diffusion coefficients can be determined. The equilibrium mass values at the end of each humidity step were used to calculate the sorption
and desorption isotherms (Figure 1b). The difference in water vapor uptake between the sorption and desorption isotherms is called the hysteresis. The shape and location of the isotherm hysteresis
can elucidate information about the sorption
mechanism and sample porosity. Although an isotherm experiment is the most common use of a DVS instrument, humidity (or other vapor) ramping experiments can be performed to investigate vapor-induced phase changes. These changes include: glassy to rubbery transitions, amorphous to crystalline conversions, and sample deliquescence.
vapor sorption isotherms and vapor sorption kinetic results can yield vital information for materials ranging from pharmaceuticals to fuel cells. Although water sorption
experiments are most common, the use of organic vapor in DVS experiments can reveal additional sample properties. The below sections highlight how DVS experiments are utilized in several industries.
and solvate formation. Gravimetric vapor sorption experiments are one of the most sensitive methods for determining amorphous contents, which may have a detrimental impact on the stability, manufacturability and dissolution characteristics of the formulated drug product.
properties of food products are recognized as critical factors in determining their storage, stability, processing and application performance. DVS is also used to measure moisture and flavor diffusion properties for packaging and barrier applications. Further, moisture sorption plays critical roles in the storage and performance of agricultural products like pesticides, herbicides, fertilizers, and seeds.
applications related to the personal care industry include the dehydration of contact lenses and superabsorbent polymer
s.
is vital to its performance: if the hydration level is too low, the polymers exhibit greatly reduced ionic conductivity
; if hydration level is too high, excess water can flood the pores in the gas diffusion layer and impede mass transport within the electrode structure. For these reasons, DVS has been used to study the water sorption and transport properties of proton exchange membranes.
Gravimetric analysis
Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid...
technique that measures how quickly and how much of a solvent is absorbed by a sample: such as a dry powder absorbing water. It does this by varying the vapor concentration surrounding the sample and measuring the change in mass which this produces. Water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
is most commonly used, but it is also possible to use a wide range of organic solvents. DVS was originally developed to replace the time and labor intensive desiccator
Desiccator
thumb|right|A desiccator and a vacuum desiccator - note the stopcock which allows a vacuum to be applied. The blue [[silica gel]] in the space below the platform is used as the [[desiccant]]....
s and saturated salt solutions to measure water vapor sorption isotherm
Moisture sorption isotherm
At equilibrium, the relationship between water content and equilibrium humidity of a material can be displayed graphically by a curve, the so called moisture sorption isotherm....
s.
Water sorption isotherms
The main application of DVS is to measure water sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
isotherms. In general, a vapor sorption isotherm
Sorption isotherm
A sorption isotherm describes the equilibrium of the sorption of a material at a surface at constant temperature. It represents the amount of material bound at the surface as a function of the material present in the gas phase and/or in the solution...
shows the equilibrium amount of vapor sorbed as a function of steady state relative vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
at a constant temperature. For water sorption isotherms
Moisture sorption isotherm
At equilibrium, the relationship between water content and equilibrium humidity of a material can be displayed graphically by a curve, the so called moisture sorption isotherm....
, water relative vapor pressure is more commonly expressed as relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
. In a DVS experiment this is accomplished by exposing a sample to a series of step changes in relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
and monitoring the mass change as a function of time. The sample mass must be allowed to reach gravimetric equilibrium at each step change in humidity before progressing to the next humidity level. Then, the equilibrium mass values at each relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
step are used to generate the isotherm. Isotherms are typically divided into two components: sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
for increasing humidity steps and desorption
Desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption . This occurs in a system being in the state of sorption equilibrium between bulk phase and an adsorbing surface...
for decreasing humidity steps. Sorption can be further divided into adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
(sorbate located on the surface) and absorption (sorbate penetrates the bulk).
Typical Results
Figure 1 shows a typical water sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
result from a DVS experiment for a microcrystalline cellulose sample. The kinetic data (Figure 1a) shows the change in mass and humidity as a function of time. From the kinetic results, the rate of water uptake and water diffusion coefficients can be determined. The equilibrium mass values at the end of each humidity step were used to calculate the sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
and desorption isotherms (Figure 1b). The difference in water vapor uptake between the sorption and desorption isotherms is called the hysteresis. The shape and location of the isotherm hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
can elucidate information about the sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
mechanism and sample porosity. Although an isotherm experiment is the most common use of a DVS instrument, humidity (or other vapor) ramping experiments can be performed to investigate vapor-induced phase changes. These changes include: glassy to rubbery transitions, amorphous to crystalline conversions, and sample deliquescence.
Applications
DVS measurement has applications over a wide range of industries. Both equilibriumEquilibrium
Equilibrium is the condition of a system in which competing influences are balanced. The word may refer to:-Biology:* Equilibrioception, the sense of a balance present in human beings and other animals...
vapor sorption isotherms and vapor sorption kinetic results can yield vital information for materials ranging from pharmaceuticals to fuel cells. Although water sorption
Sorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
experiments are most common, the use of organic vapor in DVS experiments can reveal additional sample properties. The below sections highlight how DVS experiments are utilized in several industries.
Pharmaceuticals
The moisture sorption properties of pharmaceutical materials such as excipients, drug formulations and packaging films are recognized as critical factors in determining their storage, stability, processing and application performance. Further, vapor sorption experiments can be used to study hydrateHydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
and solvate formation. Gravimetric vapor sorption experiments are one of the most sensitive methods for determining amorphous contents, which may have a detrimental impact on the stability, manufacturability and dissolution characteristics of the formulated drug product.
Food science
The moisture sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
properties of food products are recognized as critical factors in determining their storage, stability, processing and application performance. DVS is also used to measure moisture and flavor diffusion properties for packaging and barrier applications. Further, moisture sorption plays critical roles in the storage and performance of agricultural products like pesticides, herbicides, fertilizers, and seeds.
Personal care products
DVS experiments have been widely used in the study of personal care materials. For instance, the moisturization of hair samples with different chemical (i.e. conditioning, coloring, and bleaching) and mechanical (i.e. perming, combing, and blow-drying) treatments. The hydration behavior of skin samples has also been studied by DVS. Other moisture sorptionSorption
Sorption refers to the action of absorption* Absorption is the incorporation of a substance in one state into another of a different state ....
applications related to the personal care industry include the dehydration of contact lenses and superabsorbent polymer
Superabsorbent polymer
Superabsorbent polymers are polymers that can absorb and retain extremely large amounts of a liquid relative to their own mass....
s.
Building materials
In particular to building materials, moisture sorption has significant implications for cements, woods, insulation materials, and fibers. Moisture damage is a significant factor limiting a building’s lifespan. As well, moisture infusion through a building’s outer structure can have a significant effect on indoor air quality and air-conditioning load.Proton exchange membranes
A critical parameter affecting the performance of proton exchange membranes is the water content. Water is typically supplied to the fuel cell by humidifying the gas feed stream. The level of hydration within the proton exchange membraneProton exchange membrane
A proton exchange membrane or polymer electrolyte membrane is a semipermeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen...
is vital to its performance: if the hydration level is too low, the polymers exhibit greatly reduced ionic conductivity
Ionic conductivity
Ionic conduction is the movement of an ion from one site to another through defects in the crystal lattice of a solid. Ionic conduction is one aspect of current....
; if hydration level is too high, excess water can flood the pores in the gas diffusion layer and impede mass transport within the electrode structure. For these reasons, DVS has been used to study the water sorption and transport properties of proton exchange membranes.