Figure 4. Correlation between experimental and theoretical dissolution activation energy values of (a) MOF-CH3@CH3 and (b) MOF-CH3@NH2 membranes. (c)E S of glucose solution, water and urea solution for MOF-CH3@CH3 and MOF-CH3@NH2 membranes. (d) Interaction energy between toluene and NH2-BDC, toluene, CH3-BDC. (e) Interaction energy between water and NH2-BDC, water, CH3-BDC. Grey for C, blue for N, red for O, white for H.
To further verify this hypothesis, molecule-molecule and molecule-pore interactions were respectively adjusted. Taking water as an example, glucose and urea solution (150 mmol L-1) were mixed with water to strengthen and weak the interactions between water molecules, respectively.[19,59] Figure 4c shows that E S of glucose solution (24.23 kJ mol-1) is higher than that of pure water (18.74 kJ mol-1), suggesting that the enhancement of water-water interactions elevates the energy for breaking hydrogen bonds from bulk state, thus lifting the Es of water. In contrast, urea solution displays weaker water-water interactions than pure water, thus the energy for molecule arrangement is reduced and brings lowerE S (15.99 kJ mol-1). For another, molecule-pore interactions were also adjusted. For instance, Figure 3c reveals that MOF-CH3@NH2membrane surface with –NH2 groups on pore entrances gives much lower E S (-4.16 kJ mol-1) for water than that of MOF-CH3@CH3 membrane surface with –CH3 groups (13.70 kJ mol-1). Moreover, compared with other nonpolar solvents, toluene and cyclohexane display much lower E S for MOF-CH3@NH2 membrane than that of MOF-CH3@CH3 membrane. This is due to the weak hydrogen bonds between –NH2 with active –H on cyclic hydrocarbon,[60,61] which compensates for the consumed energy considerably. This again delivers the fact that molecule-pore interactions exert paramount impact on molecular dissolution behaviors.