Principle of Physisorption Analysis
Physisorption involves reversible adsorption of gas molecules driven by weak van der Waals forces. Unlike chemisorption, no chemical bonds are formed, allowing for non-destructive assessment of the material. The technique produces high-sensitivity adsorption/desorption isotherms that provide quantitative insight into:
- Specific surface area
- Pore size distribution (micro-, meso-, and macropores)
- Total pore volume
- Adsorption energetics
Nitrogen gas at 77 K (-196°C) is the most commonly used adsorptive due to its well‑documented properties, accessible temperature, and stable molecular behavior.
Analytical Capabilities of Gas Adsorption Analyzers
Modern physisorption instruments automate the precise dosing of adsorbate gas and measurement of equilibrium pressures to produce high-resolution isotherms. Key analytical functions include:
Surface Area Analysis
- BET Method (Brunauer–Emmett–Teller): The standard model for multilayer adsorption.
- Supports single-point and multipoint BET measurements.
- Sensitivity down to 0.01 m²/g with nitrogen.
- With krypton, detection extends to ~0.0005 m²/g, ideal for low surface area materials.
- Langmuir Method: Available for monolayer-dominant systems.
Pore Size and Pore Volume Analysis
- BJH (Barrett–Joyner–Halenda): For mesopores (2–50 nm) and macropores.
- Micropore Analysis:
- Horvath–Kawazoe
- t-plot
- Density Functional Theory (DFT) with high resolution to ~0.35 nm pores.
- Pore volume reporting: Total and region-specific volumes.
Additional Analysis Options
- Heat of adsorption
- Repetitive cycling of isotherms
- Advanced modeling (GAB, Sips, etc.)
- High-stability pressure transducers for accurate low-pressure measurements (down to ~1.3 × 10⁻⁹ P/P₀)
The Role of Liquid Nitrogen
Liquid nitrogen (LN₂) is essential for establishing the cryogenic conditions required for nitrogen adsorption at 77 K/-196°C. In physisorption, LN₂ cools the sample, while the nitrogen used as the adsorptive remains in the gas phase.
Function of LN₂ in Physisorption
- Creates the required low temperature: Cooling the sample to 77 K (–196 °C) reduces molecular motion and allows gas molecules to adsorb more easily onto the surface.
- Provides stable measurement conditions: A constant cryogenic temperature ensures that adsorption and desorption data are collected under consistent, controlled conditions.
- Enables accurate surface area measurements: The low temperature allows gases like nitrogen to form well‑defined monolayers on the material, which is essential for calculations such as BET surface area.
Dewar and Temperature Control Considerations
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Sufficient cryogen volume: A properly sized dewar ensures that the sample remains submerged in liquid nitrogen for the full duration of the analysis.
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Refill capability: Many setups allow topping up LN₂ during a run without disrupting temperature stability.
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Uniform cooling: Isothermal jackets or insulating components help keep both the sample tube and reference tubes at a steady temperature.
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Consistent immersion depth: Systems that control or monitor free space help ensure the liquid nitrogen level remains constant around the sample.
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Accurate pressure reference: A dedicated sensor to monitor the saturation pressure of nitrogen (P₀) improves the accuracy of relative pressure (P/P₀) calculations throughout the measurement.
Conclusion
Physisorption analysis is a foundational technique for understanding the surface area, pore structure, and adsorption behavior of porous materials. When paired with controlled cryogenic cooling using liquid nitrogen, gas adsorption analyzers deliver high‑resolution data essential for materials development, quality control, and scientific research.
This combination of precise temperature regulation, advanced pressure measurement, and robust analytical models provides researchers with a reliable, non‑destructive method for characterizing a broad range of materials.
Get in touch with Noblegen today to discuss how on-site generators can support you with a stable, reliable source of liquid nitrogen.