Sorption Overview Series #5
Comparing Sorption Methods

In a standard experiment, the sample is loaded in the balance and isolated in a chamber. The chamber temperature is carefully monitored, alongside the pressure/concentration of the gas phase. Change in mass given by the balance has three contributions: the change in sample mass given by uptake in the sample, a change in net buoyant force given by the change in density of the gas phase and a drag force of a gas flow on the sample pan.

Buoyancy Force

The buoyancy force is small for experiments at ambient pressure and temperature but can play a role at high pressure (>a few bar) where corrections may be required.

Drag Force

The drag force is only relevant when a dynamic mode is used, as the gas flow impinges on the balance parts and may need to be corrected through a blank. A way of minimizing both drag and buoyancy corrections is to construct the balance symmetrically, having a sample and a reference pan in identical configurations in the flow path.

Experimental Design

• High temperature treatment under vacuum or inert gas is often required to dry and remove any residual molecules from the sample. In a gravimetric system, sample drying/activation can be followed directly and quantified.
• The probe molecule pressure can be in controlled in a pure or in a carrier configuration. Both static and dynamic modes can be used. The uptake is measured directly as a function of time, yielding the kinetics of mass transfer by default.
• Co-adsorption conditions (mixing of multiple probes) can be easily achieved in the fluid phase. Together with the ability to independently change total pressure and sample temperature, gravimetry is highly flexible when it comes to complex sorption programs: dosing or mixing of sequential components (co-adsorption/competitive adsorption), flowrate changes, temperature and pressure swings, in-situ activation under inert carrier or vacuum, stability testing, etc. SMS DVS systems are all gravimetric systems designed around the SMS UltraBalance design. Most DVS instruments are symmetric in design, except the DVS Intrinsic, DVS Endeavour and DVS Dual Vacuum.

Advantages of Gravimetric Methods

One of the key advantages of gravimetric sorption methods is their flexibility in sample size. For single-component measurements, as little as 1–5 mg of material is often sufficient to yield highly reliable results. At the same time, gravimetric systems can accommodate much larger sample sizes, handling several grams or tens of cubic centimeters, depending on the material and application.

The accuracy and range of measurements are directly tied to the performance and stability of the microbalance. For example, Surface Measurement Systems (SMS) offers advanced gravimetric analyzers with exceptionally high resolution:

• Low mass balance: up to 1 g total mass, with 0.01 µg resolution

• High mass balance: up to 5 g total mass, with 0.1 µg resolution

This combination of precision and versatility makes gravimetric methods ideal for both delicate, small-scale experiments and larger, industrially relevant studies.

Challenges and Considerations

While gravimetric sorption techniques are known for their high sensitivity and accuracy, there are a few practical considerations to keep in mind. Gravimetric systems typically require a stable laboratory environment, particularly with respect to temperature and vibration control, to ensure consistent results. Additionally, extended equilibration times may be needed for certain materials or conditions, especially when studying slow diffusion processes.

• In a standard volumetric experiment, the sample is weighed into a closed vial. If the material needs drying or activation, the vial is then taken to high temperature under inert gas or vacuum for a pre-defined time.
• The vial is re-weighed to determine the activated sample mass. It is then quickly transferred to the sorption device and connected to the dosing volume. This reference cell has a known volume and is separated from the sample cell by a valve.
• All air is removed from the two volumes using a vacuum pump. Then, the volume of the sample cell must be determined, a procedure which is performed by expanding a non-sorbing gas like helium – essentially performing a pycnometry experiment.
• Once the free cell volume is calculated, the reference volume is pressurized to a known pressure and the valve separating the cell is opened. The pressure decays as the gas is taken up by the sample and its decay is monitored to estimate whether the sample has reached equilibrium.
• Adsorbed amount can then be calculated for that pressure through an equation of state, which is followed by sequential doses of the probe molecule in increasing amounts.

Advantages of Volumetric Methods

• Volumetry is a straightforward method to measure pure single component sorption as it has few moving parts. Its simple construction also means that extreme conditions of low/high temperature and high pressure are easier to implement.
• It is often used for routine characterization experiments, and it can be parallelized by having the reference volume shared between two or more sample cells.
• The geometry and gas path of the sample cell can be easily changed, which means volumetry is easier to integrate with other measurements – XRD, FTIR, microscopy, etc.

Challenges and Considerations

Volumetric sorption techniques rely on indirect measurement via pressure, which introduces several important limitations:

• Higher sample requirements: At least 50–100 mg is typically needed to produce a measurable pressure drop. Maximum sample size is limited by the system volume (usually up to 1 g).
• Static dosing only: The amount of adsorbate must be known or optimized beforehand, as the system doesn’t allow for continuous dosing.
• Error propagation: Pressure errors from earlier doses can carry over and compound in subsequent measurements, increasing uncertainty over time.
• Temperature sensitivity: Because it’s a closed system, changing the temperature also changes pressure making isobaric measurements difficult or impractical.
• Mixture limitations: While mixtures can be dosed, partial pressures after sorption are unknown, so uptake calculations require assumptions.

Chromatographic and breakthrough methods are dynamic flow techniques used to study how probe molecules interact with packed columns. Most systems operate in a carrier mode for easier pressure control.

Probe introduction typically happens in two ways:

• Frontal (Breakthrough) Mode: A step change in concentration creates a front that moves through the column. Measuring breakthrough allows calculation of probe uptake by the material.
• Pulse (Inverse Chromatography) Mode: A brief concentration pulse is introduced, with the time to elute (retention time) revealing information about equilibrium uptake and molecular interactions.

Experimental Process

A sample-packed column is weighed before and after preparation, with thermal activation applied if needed. The system’s dead volume is measured using a non-interacting gas pulse. The probe pulse or front passes through the column, and sensors measure outlet concentration and flow to determine uptake via mass balance.

Challenges and Considerations

Controlling temperature, pressure, concentration, and flow precisely makes this method complex. It requires larger sample amounts (hundreds of mg to grams) and can be scaled for industrial simulation. Sample form affects performance, as fine powders may cause blockages.

Additional Capabilities

These methods uniquely provide multicomponent adsorption data and insight into adsorption kinetics critical for scaling processes. Surface Measurement Systems offers the iGC-SEA and BTA Frontier analyzers as leading tools for these analyses.