Advanced Adsorption Studies with Dynamic Gravimetric Vacuum Sorption Instrument

Various sorption measurement techniques are available, including ambient flow and vacuum methods, gravimetric and volumetric methods, and static vacuum and dynamic vacuum methods. Most of the users of gravimetric instrument systems want to know whether a vacuum technique is better than an ambient flowing gas technique.

Advantages of DVS Vacuum Method

DVS Vacuum instruments are advantageous in a number of ways over ambient flow sorption instruments. Almost all types of materials can be dried using DVS Vacuum instruments.

Extensive outgassing/drying is required for some materials and is unfeasible with conventional drying techniques, such as thermal convection heating and dry gas flow. These materials include biomaterials, some strong hydrates, complex inorganic solids, nanopowders and fillers, chemical absorbents, and micro-porous solids.

Co-adsorption of gases, gas mixtures, and gas/vapor mixtures can be easily analyzed using the DVS Vacuum. Gas sorption measurement has become critical in areas such as catalysts, fuel cell, filtration and environmental protection.

Very low levels of vapor pressure (10^-10 Pa) and heat of evaporation of solid materials can be measured using the DVS vacuum. Determining the vapor pressure of solids helps in deducing their thermodynamic stability, and deriving their atmospheric inhalation exposure levels caused by volatilization.

DVS Vacuum is capable of working over a broad range of partial pressures, from atmospheric pressure down to 10^-6 Torr. This is a key capability for some sub-atmospheric applications such as capturing of low pressure gases.

Static and Dynamic Methods

Static Method

The volumetric sorption method is used to perform conventional vacuum sorption measurements at static mode. This method entails introducing the vapor into a sealed chamber and performing the analysis under static conditions (without gas flow).

It works well at relatively higher partial pressures, but provides erroneous measurements at low partial pressures in particular due to changes in the system pressure caused by various reasons. Vacuum leaks are the most common reason.

Dynamic Method

In the dynamic vacuum technique, both the upstream entry rate and the downstream exit rate of vapor are controlled by the instrument via an MFC and a butterfly valve, respectively. This, in turn, controls the residence time of the vapor within the chamber so that contact with the sample is established. Figure 1 illustrates the principle of the dynamic vacuum technique.

Figure 1. Principle of dynamic vacuum method

Volumetric and Gravimetric Methods

As a static process, the volumetric method may not be able to produce accurate results at low pressures because of system leaks. Moreover, this method measures the pressure change, from which the volumetric change is derived using the standard relationship:

PV = NkT

Where, P = Pressure of the gas; V = Volume of the gas; T= Temperature of the gas; N = Number of particles in the gas; and K = Boltzmann constant (1.38066 x 10^-23J/K).

Measuring vapor or gas pressure is not as precise and sensitive as measuring weight change. Therefore, the accuracy of the volumetric method is inferior to the gravimetric method. A large sample volume is required for the volumetric method to obtain the same sensitivity.

As a result, the time required to attain sorption equilibrium increases. Due to this, experimental costs will be more in the case of expensive samples such as drug samples for formulation research.

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SMS DVS Gravimetric Vacuum System

The SMS DVS Vacuum system is capable of working in both static and dynamic modes. As a result, this gravimetric system has the flexibility to study adsorption processes under vacuum. The following physical properties can be measured with the DVS Vacuum system:

• Adsorption and desorption isotherms
• Vapor pressure and heat of evaporation of solids

The use of Advance Analysis software allows for the measurement of additional properties, including activation energy, BET surface area, diffusion constants, heat of sorption, micro/meso-pore size distributions, and glass transition.

Application Examples

Vapor Pressure Measurement of Pesticide, Bifenthrin

Bifenthrin exhibits very low vapor pressure to be measured at 25°C. However, it is possible to extrapolate the vapor pressure of Bifenthrin from the DVS vacuum data collected at higher temperatures. Figure 2 shows the raw data measured at 65°C. The calculated vapor pressure at 25°C and heat of evaporation are 5.42 x 10^-7 Torr and 5.2 x 10⁴ J.Mol^-1, respectively.

Figure 2. Linear mass loss of bifenthrin at 65°C

CO₂ Sorption on MOFs at 25°C

Metal organic frameworks (MOFs) have highly crystalline clusters of metal ions linked by organic linkers. The large specific surface areas of up to 5,000m²/g and nano-sized pores of MOFs make them a potential candidate for high-capacity storage of natural gasses and other compounds.

The sorption behavior of CO₂ gas on MOF substrate is illustrated in Figure 3. It can be observed that the sorption has not attained equilibrium. MOFs can absorb much more above atmospheric pressures, making them suitable for carbon dioxide capture, storage, and transportation.

Figure 3. CO₂ gas sorption on MOFs

SO₂ Gas Sorption on Zeolites at 25°C

SO₂ sorption on three different zeolites shows irreversible sorption at 25°C and uptakes of between 30% and 35% of reference masses at low partial pressure. Therefore, zeolites are suitable for SO₂ capture.

Low Vapor Pressure Adsorption of 2-Hexanol on Activated Carbon

System leaks affect sorption in normal static vacuum techniques, especially at low pressures. As a result, the system pressure is not equivalent to the vapor partial pressure. This issue is addressed in dynamic vacuum by continuously introducing and removing the vapor in and out of the system using the down-stream butterfly valve.

Therefore, the pressure is controlled. Low pressure measurement of 2-hexanol on activated carbon is demonstrated in Figure 4. A volumetric system cannot provide such an accurate measurement at such low partial pressures.

Figure 4. Low partial pressure adsorption of 2-hexanol on activated carbon

Separation of Octane from Water by Activated Carbon at 25°C

Co-adsorption is gaining attention for use in filtration and separation processes. The co-adsorption of the mixture of octane and water at different ratios is depicted in Figure 5, showing the strong preference of the activated carbon for octane sorption. Therefore, activated carbon is a potentially useful agent for the separation of octane from water.

Figure 5. Co-adsorption of octane and water on active carbon

Conclusion

Dynamic and gravimetric vacuum sorption instruments provide a flexible research tool for the analysis of adsorption processes under static and dynamic analytical conditions. Like ambient flow sorption instruments, they can measure a range of properties, including activation energy, glass transition, diffusion constants, BET surface area, and micro/meso-pore size distributions. In addition to this, SMS DVS Vacuum instrument also provides measurements on vacuum drying, low pressure gas/vapor sorption, and vapor pressure and heat of evaporation of solids.

This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.

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