Guest diffusion

In both the above cases, we have 2D processes. Following nucleation, the reaction may be either phase boimdary controlled (i.e. the rate is limited by the rate at which the interlayer space expands to accommodate the guest) or diffusion controlled (i.e. the reaction rate is controlled by the rate at which the guests diffuse between the layers - the interlayer spacing expands instantly as the guests move). 

In addition to the above, there are further possibihties. When the rate of guest diffusion between individual layers is very large compared with the rate of nucleation at the edge of the crystal, there exists a situation in which the individual layers appear to fill instantly. In this case, when Avrami kinetics are applied to the system, the diffusion process being observed is not the diffusion of guest species between the layers, but the diffusion of filled layers parallel to the c-axis. Such ID processes will consist of nucleation followed by diffusion control in the vast majority of cases, although phase boundary control is also possible if the rate of advancement of the phase boundary is also very rapid with respect to nucleation. In this case, instantaneous nucleation is not a possibility [18]. 

The kinetic rate constant for the association process (7cjN) has an upper limit set by diffusion. In other words, the rate of the fastest association processes cannot exceed the rate by which the host and the guest diffuse to encounter in solution. The maximum value of kD can then be estimated using the well-known Smoluchowski equation8 ... 

Calculation results at 177 K led to the prediction that methane diffuses from one a-cage to another at a rate of 1.9 x 1010 per sorbate per second in NaY zeolite and 24.3 x 1010 per sorbate per second in NaCaA zeolite. Thus, this is another example of a guest diffusing faster through a host with smaller pore windows (the so-called ring effect ), a phenomenon that had been observed previously only for noble gas atoms. 

Processes like nucleation and guest diffusion in binary clathrate mixtures are difficult to study because of limitations on accessible simulation timescales and lengthscales. Peters and Trout have recently developed a method that calculates reaction coordinates of complex... 

Figure 3.4.15 Schematic drawing of surface motion transfer, which can facilitate guest diffusion within narrow pores surrounded by flexible organic moieties of the host skeleton.

Figure 3.4.25 Generation and diffusion of crystal phase boundary in mass-induced phase transition in the single crystal. The cases of smooth guest diffusion (a) and not smooth (b) are shown. This can explain the pressure selective on-off adsorption shown in Fig. 3.4.24.

Though maximizing affinity is a profitable way to boost thermodynamic selectivity, there is an upper limit to it due to practical limitations. Taking the well-studied avidin-biotin system as a prototypical example of a high-affinity supramolecular complex (ATasj,"10 M ) and assuming the fastest possible association of host and guest (diffusion controlled rate sec ), one arrives at... 

Venditto V, De Girolamo Del Mauro A, Mensitieri G, Milano G, Musto P, Rizzo P, Guerra G (2006) Anisotropic Guest Diffusion in the b Crystalline Host Phase of S5mdiotactic Polystyrene Transport Kinetics in Films with Three Different Uniplanar Orientations of the Host Phase. Chem. Mater. 18 2205-2210... 

Figure S.5 Picture of melt-blended films of LLDPE and 0.18% w/w Cl-RG (top) and Cl-YB (bottom) stretched at room temperature to a draw ratio of 500%. Pictures were taken under excitation with UV light of a wavelength of 365 nm. The melt-processing technique produces films of much higher optical brightness and contrast than guest-diffusion techniques. Reproduced with permission from Ref [34].

The availability of SPS films with three different kinds of uniplanar orientation not only allows establishment of fine structural features of SPS crystalline and co-crystalline phases (e.g., experimental evaluation of the orientation of transition-moment-vectors of host and guest vibrational modes, with respect to the host chain axes) [106,107] but it can also be relevant for practical purposes. For instance, it allows guest orientation control [108,109] for cocrystalline phases and guest diffusivity (and hence permeability) control [97,98,110,111] for the nanoporous phases. (See following subsection 10.3.3.3). 

Guest Diffusivity and Crystaiiine Phase Orientation Guest sorption studies from dilute aqueous solutions and from gas phases as well as desorption studies have been conducted for SPS hlms presenting the three different kinds of uniplanar orientation of the nanoporous 8 phase (a,/ Cu, an Cl, and fli C//). These investigations have been affected mainly by FTIR measurements combined with gravimetric measurements. 

Venditto, V, De Girolamo Del Mauro, A., Mensitieri, G., Milano, G., Musto, R, Rizzo, R, Guerra, G. Anisotropic guest diffusion in the d crystalline host phase of syndiotactic polystyrene Transport kinetics in films with three different uniplanar orientations of the host phase. Chem. Mater., 18,2205-2210 (2006). 

It is reasonable to assume that guest diffusion in a hydrate will not be too facile without the presence of defects.The Bjerrum D- and L-defects (Figure 31) are common in ice and hydrates. To test the effect of defects on the migration of guest molecules in S-I hydrate, nanosecond MD calculations employing empirical potentials were performed on model hydrogen sulfide and... 

Durao, J., Gales, L., 2013. Guest diffusion in dipeptide crystals. CrystEngComm 15,1532-1535. 

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