Surface energy γ, is the principle characteristic measured by the IGC. For a solid to exist, it must be energetically favorable (of lower energy) for its molecules to form bonds with each other. It follows that the molecules at the solid’s surface are in a less favorable state (of higher energy), as they are not surrounded by other bulk molecules. It is the same intermolecular forces which are responsible for the attraction between powder particles and other solids, liquid and vapor molecules which can occur via long range van der Waals forces (dispersion forces) and short range chemical forces (polar forces). Thus, surface energy values (dispersive and polar) correlate to several key solid properties including wetting, dispersability, powder flowability, agglomeration, process-induced disorder, adhesion/cohesion, static charge, adsorption capacity and surface chemistry.
Experimental Technique for Measuring Surface Energy
There are a range of techniques available for measuring the surface energy of solid particulate materials. Though contact angle measurement is by far the most common method, it is rarely used for particle and other non-planar materials due to experimental limitations leading to inaccurate and unreliable results. Inverse gas chromatography is now the proven and preferred method for surface energy measurements, and surface energy heterogeneity in particular.
Inverse Gas Chromatography
Inverse Gas Chromatography (iGC) is a gas-solid technique for characterizing surface and bulk properties of powders, particulates, fibers, films and semi solids. A series of vapor pulses are injected through a column packed with the sample of interest. Unlike traditional analytical gas chromatography, iGC is a physical chemistry technique using vapor probes with known properties to characterize the unknown surface/ bulk properties of the solid sample.
What is iGC-SEA (Inverse Gas Chromatography-Surface Energy Analyzer)?
iGC- SEA or Inverse Gas Chromatography-Surface Energy Analyzer is an instrument that uses the iGC principle. The heart of its innovation is the patented injection manifold system which generates accurate solvent pulse sizes across a large concentration range, resulting in isotherms at unprecedented high and low sample surface coverages. This allows for the accurate determination of surface energy heterogeneity distributions. The fully automated iGC-SEA can be operated at different solvent vapor, flow rate, temperature, humidity and column conditions.
iGC-SEA has a unique data analysis software called Cirrus Plus, specifically designed to measure surface energy heterogeneity, isotherm properties and related physical characterization parameters. Further, bulk solid property experiments resulting from probe-bulk interaction and using solubility theory are now possible. It automatically and directly provides a wide range of surface and bulk properties of the solid samples and gives more accurate and reliable data than manual calculations. It also has a humidity control option. The impact of humidity and temperature can be determined for the physicochemical properties of solids such as moisture induced Tg, BET specific surface area, surface energy, wettability, adhesion and cohesion. Read product information or request application notes.
Founding Principle of iGC-SEA
iGC is a gas-phase technique for characterizing surface and bulk properties of powders, particulates, fibers, films and semi-solids. An experiment consists of a series of vapor pulses or frontal injections eluting through a column packed with the sample under examination.
The vapor’s retention time is measured by a Flame Ionization Detector (FID). Varying the vapor probe molecule, flow rate, temperature, or column conditions elucidates a wide range of surface and bulk properties of the sample.
iGC-SEA compared to other common techniques
iGC-SEA compared to Contact Angle (CA) and Atomic Force Microscopy (AFM)
|Surface Energy Analyzer (SEA)||Contact Angle (CA)||Atomic Force Microscopy|
|Sample Form||Powder, fibres, films, nano particles, granules and semi-solids||Flat samples- easy Particulate – difficult||Flat smooth samples-easy Particulate- difficult|
|Nature of Measurement||Vapour adsorption isotherm method||Liquid-solid contact-Dissolution-Swelling||Contact/adhesive force between probe and sample|
|Reproducibility and Repeatability||Very good reproducibility and repeatability (RSD%=1)||Reproducibility varies-need to consider contact angle hysteresis Cannot do repeats||Significant data scatter in both-surface topography can affect data. Difficult to get statistically significant data sets|
|Experimental Conditions||Controllable T and %RH||Typically ambient T and %RH condition||Strict controlled T- poor humidity control|
|Theoretical Validity||Well developed theoretical models for determining surface energy||OK for flat surfaces, very problematic for particulates||Need JKR or DMT theory to analyse data based on the known radii of the AFM probe|
|Time for Measurement||Single set of measurement 1 hour||Set of measurements less than 1 hour||Set of measurements less than 1 hour, but many hours to set up|
|Mapping of Properties||Surface energy heterogeneity well established||Possible in theory, but not practical||Possible in theory, but not practical because of data variability|
iGC-SEA compared to Traditional iGC
|Home Built iGC||SEA|
|Injected phase||Liquid with a special tool (headspace) it can be vapour||Vapor|
|Column design||Circular or U shape glass tube||Straight silanized glass tube|
|No. of sample tested at the same time||1||Twin sample holders|
|Detector||FID or TCD||FID only due to the humidity option|
|Operating range (Temp)||40˚ to 400˚C||20˚ -180˚C|
|Injection unit||Volume||Moles or coverage|
|Analysis Software||Peak analysis and manual data analysis||Peak analysis and provides physicochemical data for the solid sample|
|Carrier gas flow rate||Soap bubble flow meter||Mass Flow Controller|
|Dead Volume Measurement||Manual||Automatic|
|Pressure Drop Measurement||Manual with special device||Automatic|
|Sample packing||Non-standard apparatus||Column packer accessories|
|Humidity Control||Rare||Standard Option|
|Sample Size||10mg to ≈1g||1mg to 10g|