#1 Introduction to Sorption Techniques

We begin our series with an explanation of associated terminology around the important sorption measurable variables and modes of controlling the introduction of probe molecules. Then we will provide a global view on the aforementioned measurement principles and discusses their advantages and disadvantages.

Throughout this series, we will use the concepts of gas and vapor interchangeably, as referring to molecule in the gaseous phase used to probe the material, or the sorbate. In a strict definition, a gas is a fluid which cannot turn into liquid by increasing its pressure – i.e. a fluid above its critical temperature.

There are several commonly used experimental techniques that can be used to measure gas and vapor sorption, including volumetric (sometimes called manometric), gravimetric (DVS) and chromatographic (sometimes called breakthrough) techniques. These techniques have wide practical applicability, first in determining fundamental material properties such as surface area, porosity, surface energy, hygroscopicity, contact angle and second in assessing a material’s performance towards various applications that rely on sorption behavior – heterogenous catalysis, API stability, water harvesting, gas separation, etc.

• The choice of experimental technique will determine the type of information obtained and have different limitations and inherent advantages. Some examples that inform the choice of technique include:
• Measurements on large or small volumes of sample, or samples with very high or very low surface area and porosity.
• Sample sensitivity to temperature, vacuum, and pressure changes.
• Importance of mimicking process and real-world conditions – multiple gases/vapors, presence of air and humidity, constant or varying temperature and pressure, etc.
• Complexity, cost, and ease of use of the instrument.

• Importance of determination of sorption kinetics, equilibrium times, diffusion, and general time-resolved data.
• Number of samples and number of gases/vapors to be analyzed at the same time.
• Interest in probing the material’s fundamental properties – such as density, surface area, pore size distribution, surface energy, specific gas-material interaction properties, cohesion & adhesion, etc.
• Options to combine multiple measurements at the same time (hyphenations) e.g. XRD, Raman, IR, calorimetry, etc.