Acid diffusion effect

Synthesis ofp-Ethyltoluene. j )i7n7-Ethyltoluene, the feedstock for j )-methylstyrene, is difficult to separate from the products of toluene alkylation with ethane using conventional acidic catalysts. The unique configurational diffusion effect of ZSM-5 permits -dialkylbenzenes to be produced in one step. In the alkylation of toluene with ethene over a chemically modified ZSM-5, -ethyltoluene is obtained at 97% purity (58). 

Lin, TH Phillies, GDI, Prohe Diffusion in Polyacrylic Acid Water—Effect of Polymer Molecular Weight, Journal of Colloid and Interface Science 100, 82, 1984. 

Acid diffusion. Acid catalyzed resist systems are particarly noteworthy for their high sensitivity toward radiation. However it has been suggested that the amplification effect observed with catalytic resist systems is achieved only at the expense of lost resolution. Some diffusion of catalyst is necessary to achieve sufficient loss of BOC groups in order to impart sufficient difference in polarity for discriminatory film dissolution. Yet unlimited acid diffusion would result in loss of resolution. 

The clearest evidence for microscopic diffusion control in nitration comes from the kinetic studies of Coombes et al. (1968), with low concentrations of nitric acid in 68.3% sulphuric acid as solvent. In this medium, the concentration of nitronium ions is proportional to the concentration of molecular nitric acid as required by (24) and, since the concentration of nitronium ions is very small, the concentration of molecular nitric acid is effectively equal to the stoicheiometric concentration of nitric acid. At a given acidity, the reactions have the kinetic form (25). Nitric acid is written out in full in this equation to show that the rate coefficient is calculated with reference to the stoicheiometric concentration of the acid. This convention assists the comparison of reaction rates over a wide range of acidity. 

Capillary phenomena may arise from other causes. Thus a solution of ammonium acetate turns both red and blue litmus papers violet. The drops are more bluish in the center and more red at the edge. It appears that the paper hinders the diffusion of ammonium hydroxide to a greater extent than it does the diffusion of acetic acid. The effect is even more evident when the reaction is produced with lead acetate. Around the center of the drop is found a blue region (adsorption of lead hydroxide) which is surrounded by a red region due to the diffusion of the acetic acid. These observations account for the discrepancy found in medical books regarding the reaction of lead acetate. It is impossible to determine accurately the reaction of this salt with litmus paper. It can be done, however, with methyl red solutions. The reaction resulting from the hydrolysis of salts such as sodium acetate and ammonium chloride is demonstrated readily by means of indicator papers. 

As expected, the confinement of phosphoric acid in the PBI matrix does not give rise to any relevant electroosmotic drag. Of course, the main reason is the fact that proton conductivity is dominated by structure diffusion, that is the transport of protonic charge carriers and phosphoric acid are effectively decoupled. The other reason is that protonic charge carriers are produced by self-dissociation of the proton solvent (phoshoric acid), that is the number of positively and negatively... 

Diffusion. Acid diffusion in pH modified foods was investigated with fluorescent dyes or color indicators (18). Colorimetric pH profiles compared well with electromagnetic measurements. Effective mass diffusivities for acetic, citric, gluconic and phosphoric acids in potatoes were on the order of lO10 nr/sec. Diffusion occurs mainly in an unsteady state in heterogeneous, multilayer, cellular systems (19). Diffusion can be seen mathematically predicted by one-dimensional solutions of the second Fick equation ... 

Albright, Hanson, et al (1,2,3 ) have reviewed the work of previous investigators and concluded that the criteria sometimes used to determine the rate-limiting step were not adequate. The reaction kinetics measured by some workers (4,5 ) exhibit considerable differences even though they claimed to be measuring intrinsic kinetics. Such differences may result from diffusion effects. Recently, Cox and Strachan (6 ) reported the nitration of chlorobenzene to be kinetically controlled at a nitric acid concentration of 0.032 mole/liter in 70 weight percent sulfuric acid for the nitration of toluene in the same acid, the rates of chemical reaction and diffusion were of comparable magnitude. 

Yoshimura, H. Shiraishi, J. Yamamoto, and S. Okazaki, Correlation of nano edge roughness in resist patterns with base pol3nners, Jpn. J. Appl. Phys. 32, 6065 (1993) T. Itani, H. Yoshino, S. Hashimoto, M. Yamama, N. Samoto, and K. Kasama, Polymer structure effects on dissolution characteristics and acid diffusion in chemically amphfied deep ultratviolet resists, J. Vac. Sci. Technol. B. 15(6), 2541 (1997). 

---