IGC 5-Fold Way
Any single measured IGC property may tell you little about what is going on at a surface. Via the 5-Fold Way, the effects of topography (surface shape) and chemistry (what's on the surface) can be disentangled.
This is a beta attempt to tackle a big issue facing IGC - the lack of a direct link from interactions to measured values. Criticisms and suggestions are most welcome.
IGC 5-Fold Way
Background to the 5-Fold Way
Before explaining the sliders and outputs, it is necessary to explain the background theory.
A typical set of IGC experiments with linear hydrocarbon probes might result in a "surface energy" which is likely to be meaningless. An additional set of experiments with, say, i-octane and cyclooctane might reveal something about the surface structure via IM, the index of morphology and the Relative IM, RIM. With some polar probes (a word used loosely within IGC because it includes molecules like benzene) some insights might become apparent about polar interactions via the Specific Interaction index, ISP. And with some acid/base probes some measure of the acid/base properties of the surface might be possible.
The reason for saying that the measured surface energy is likely to be meaningless is that any step, void or hole in the surface is likely to allow probes to encounter 2x, 3x or 4x the interaction energy compared to the 1x interaction on a planar surface. Even if the real surface energy of such topologies is unchanged from the plain surface, the standard measurement technique will give a value (much) higher.
With branched and cyclic alkanes, the chances of interacting strongly with such topologies is reduced, so we get a lower measured surface energy than we might have expected. However, if the surface has some sort of surface treatment (coating) then more interesting probes might interact preferentially with the treatment, giving a large IM or RIM, especially if a more polar probe is used.
So IGC results are a mixture of topology and chemistry. This app provides a simplistic set of calculations of the sorts of interactions that might be possible with various probes, and lists the 5 key values that a set of measurements might produce:
- γd: the so-called dispersive surface energy which is a mix of the surface energy and topological factors.
- IM: The morphology index that tries to measure whether γd reduces when a branched or cyclic hydrocarbon is used.
- RIM: The Relative IM which sees whether the probe is attracted more than it should be, revealing some extra chemistry of attraction
- ISP: The Specific Interactions of polar probes with more polar and H-bonding parts of the surface
- Acid/Base: The extent of acid-base or base-acid interactions between surface and probe.
On old-fashioned IGC machines, getting a full 5-fold dataset with a range of hydrocarbon and polar probes is somewhere between difficult and impossible. But on a modern, automated, precision machine, it is routine. And with the 5-fold results we start to see the bigger picture:
- Pure topology: The chemical interactions with the surface are of no great significance, but there are a number of high energy sites which might have 2x, 3x or 4x interactions depending on whether they are steps, voids or holes
- Pure chemistry: There are no high energy sites, but the chemical treatments lead to various interactions with different probes that can be chemically disentangled.
- Mixed mode: Both topology and chemistry are involved.
Once the 5-fold way becomes the norm in IGC, we will much better be able to understand, within a given material, the effects of:
- Different processes: How the basic production processes affect the topology and chemistry.
- Different treatments: How temperature, chemistry, dispersants etc. affect the topology and chemistry.
- Different handling: How packing, grinding, sieving etc. affect the topology and chemistry.
- Storage effects: Our materials change with time, temperature, RH, but how? Again, it might be topology, chemistry or both
- Effects in use: Our materials change as they are used. Again, it might be topology, chemistry or both
We can choose a probe with Dispersive, Polar and H-Bonding characteristics defined by their Hansen Solubility Parameters, plus their Molar Volume to reflect their overall ability to interact, plus a code saying (Yes/No) if they are linear and/or cyclic.
Then we choose a surface which has its own range of Dispersive, Polar and H-Bonding characteristics, along the lines of HSP. Such values are routinely obtained by seeing how "happy" or "unhappy" the material is within a range of solvents, the classic method for determining HSP. Then the extra features on the surface are defined in terms of X/4 - whether they are plain (X=1) up to porous (X=4) along with the % of high energy sites. The number of of such features is controlled by the % High Energy slider. Finally we have a notional thickness of some chemical treatment such as a dispersant.
The text outputs are as outlined above.
As a bonus we get the Absorption Energy Distribution Function. For any given surface/probe there's a main peak of energies, then the additional components can create higher peaks that have a disproportionate influence on the retention times and the calculated values. The Heterogeneity is the ratio of the extra peak to the overall normalised peak.
Although the 5-fold way is a powerful methodology for understanding the surface, a measurement of the AEDF (via IGC-FC, as described in the Isotherms app) is important in its own right. If you have a good IGC machine then the best of all worlds is the measurements for the 5-fold way plus the AEDFs for a few well-chosen probes.
The IGC apps are based on the inputs kindly provided by Dr Eric Brendlé of Adscientis who are specialists in IGC measurements.