Guest diffusivity

Figure 16.5 Design principle of nanoparticle gated mesoporous silica nanospheres. The nanoparticles cap the pores by covalent bonds trapping previously diffused guest molecules. The bonds are then broken under specific stimuli allowing the guest molecules to diffuse away.

K. Borowska, B. Laskowska, A. Magon, M. Mysliwiec Pyda, S. Wolowiec, PAMAM dendrimers as solubilizers and hosts for 8-methoxypsoralene enabling transdermal diffusion guest, InL J. Pharm. 398 (2010) 185-189. 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

The primary question is the rate at which the mobile guest species can be added to, or deleted from, the host microstructure. In many situations the critical problem is the transport within a particular phase under the influence of gradients in chemical composition, rather than kinetic phenomena at the electrolyte/electrode interface. In this case, the governing parameter is the chemical diffusion coefficient of the mobile species, which relates to transport in a chemical concentration gradient. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

Fluorescence correlation spectroscopy can be used to measure the binding dynamics of host-guest complexes when the fluorescence quantum yields for the free and bound hosts are different. Analysis of fluorescence correlation spectra depends on the profile for the excitation pulse, which impacts the shape of the emission profile and mechanistic assumptions are made with respect to the diffusion of the various species in solution.58 For each chemical system different assumptions are made. 

This technique was employed to study the binding dynamics of Pyronine Y (31) and B (32) with /)-CD/ s The theoretical background for this particular system has been discussed with the description of the technique above. Separate analysis of the individual correlation curves obtained was difficult since the diffusion time for the complex could not be determined directly because, even at the highest concentration of CD employed, about 20% of the guest molecules were still free in solution. The curves were therefore analyzed using global analysis to obtain the dissociation rate constant for the 1 1 complex (Table 12). The association rate constant was then calculated from the definition of the equilibrium constant. 

Guerra, G. Manfredi, C. Musto, P. Tavone, S., Guest conformation and diffusion into amorphous and emptied clathrate phases of syndiotactic polystyrene, Macromolecules 1998, 31, 1329 1334... 

The diffusion of entrapped guest molecules out of dendritic boxes was un-measurably slow over a period of several months. However, Meijer and coworkers have demonstrated that the shape-selective liberation of encapsulated guests could be achieved by removing the closed shell in two steps (Scheme 3) [15]. First, hydrolysis of the tBOC groups of the dense shell with formic acid gave a partly open dendritic box . At this point, small guests such as p-nitroben-zoic acid and nitrophenol could diffuse out of the box by dialysis. Susequent and complete removal of the outer shell by refluxing with 12 AT HC1, lead to the... 

The first type of cointercalation is a possible means of improving a given host as an electrode adding a second larger guest might prop the lattice apart and improve the diffusion of the first. In electrochemical cells. 

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]. 

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