# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 4 Future Outlook  
There are many applications of MOFs that depend on their adsorption properties. Hence, we have reviewed the utilization of IGC as a tool to investigate the physicochemical features of MOFs. IGC has helped to evaluate and predict the ability of MOF surfaces to contribute in both dispersive and specific interactions. Unlike an analytical GC technique, in IGC the roles of the mobile and stationary phases are reversed. This means that the material under study becomes the stationary phase and the injected well-known molecules within the mobile phase are probes, which are used to investigate the physicochemical properties of the stationary phase. An excellent agreement has been revealed between the IGC-measured enthalpies of adsorption, and those estimated using other surface energy analysis techniques, such as calorimetric, volumetric, or gravimetric measurements, and even molecular modeling measurements. Moreover, IGC showed shorter experimental times compared to other techniques without almost any specific preparation of samples for crystalline or amorphous materials. The probability of observing the differences associated with surface group orientations is one of the benefits of using IGC in researching acid- and base-specific properties. The total solid surface energy is represented by the summation of the dispersive and specific properties. From the thermodynamic properties of a material, its adsorption behavior can be interpreted and predicted. For instance, a plot of the Henry constants logarithm  
Figure 18: Linear dependence of RTIn(V\({}_{\text{N}}\)) vs. T for the adsorption of O\({}_{2}\)/N\({}_{2}\). Reprinted with permission from [78] Copyright (2020) Elsevier. versus the vapor pressure logarithm could be used to investigate whether the material's ability to discriminate between branched and linear alkanes is related to a size-exclusion property. The difference between enthalpy of adsorption and its vaporization analogue is how they demonstrate the strength of adsorption toward the tested probe. Imperfections due to the degradation of MOFs could also be detected by a large value for the entropy of adsorption. Moreover, the activity of the MOF material under examination could be measured and compared with classical microporous materials via the dispersive component of the surface energy as a function of London interactions and surface heterogeneity. Some considerations should be accounted for while using the IGC technique. A minimal amount of testing probes should be used in order to maintain a monolayer surface coverage. The peak symmetry is should also be carefully controlled, as the precise retention time is a basic source of the subsequent data. Optimum flowrate should be estimated via the Van Deemter curve, as a higher flowrate prevents the system's equilibrium, and a lower flowrate means broader peaks and longer experimental times. The surface area of the examined material must be measured accurately, as it becomes one of the basic properties utilized in further measurements. The molecular area of the polar probes most used in IGC is another issue to be considered as the literature shows a different value for those probes; however, a recent study [135] addressed that problem. Past research on a variety of materials has proved that the IGC technique can be fruitfully implemented to characterize the surface and bulk properties of solids based on their diverse morphologies and shapes. However, IGC as a tool for MOF characterization is still in its infancy and much of this area remains to be discovered. IGC is adaptable, accurate, and simple, attributes that can cement its position as an essential characterization technique in MOF research. While some MOFs have been investigated using IGC and revealed great potential, many others are still waiting. In addition, many more parameters of MOFs may be evaluated, such as shape, morphology, topology index [136], and surface area measurements [137]. Conceptualization, K.Y. ; methodology, K.Y. and O.S. ; software, K.Y. and O.S.