Diffusion basics

The octanol-water partition coefficient, or logP, measures the partition of a compound to between 1-n-octanol and water, which in turn is a model for the propensity of a compound to cross membrane barriers via passive diffusion. Basically. logP is the log ratio of the concentrations of the chemicals in the two solvents. logP = log [0]/[W]. where O is octanol and W is water. In addition. logP is a measure of the hydrophobicity and hydrophilicity of a chemical. Because the hydrophobic parameter is related to absorption, bioavailability, ligand-receptor... 

QM—semiempirical Good for predicting molecular geometry and energetics, predicting vibrational modes and transition structures (but does so less reliably than ab initio methods) Poor results for van der Waals and dispersion intermolecular forces (lack of diffuse basic functions)... 

The theory of Kamide (1990) allows determining the properties of the polymeric membrane covering a particle both in terms of number of pores, their conformation and apparent diffusivity. Basically, it is necessary to know the volumes ratio/ between solvent and polymer. These two phases are respectively identified as poor and rich phases. In our formulation and experimental activity we worked with ethylcellulose polymer and cyclohexane solvent. Consequently, the specific values defined in the following refer to such pairing. The membrane is formed by the separation of the polymer from the poor phase and the consequent coagulation of the polymer particles. It is possible to outline three main steps for the membrane formation ... 

The literature of surface diffusion is now quite extensive. A review of the basic ideas, with reference to many of the earlier papers, is given by Dacey [44], and a good selection of references including more recent work can be found in Aris [45]... 

A second approach modifies the CA resist chemistry. Eor example, researchers have introduced basic additives into the resist formulation to minimize the impact of surface contamination of the resist film (82,83). A resist that already contains added base (and consequendy requites a larger imaging dose) should be less affected by the absorption of small amounts of basic contaminants. Systems of this type have been claimed to have improved resolution as well. The rationalization here is that the acid that diffuses into the unexposed regions of the resist film is neutralized and does not contribute to image degradation (84,85). 

The time constant R /D, and hence the diffusivity, may thus be found dkecdy from the uptake curve. However, it is important to confirm by experiment that the basic assumptions of the model are fulfilled, since intmsions of thermal effects or extraparticle resistance to mass transfer may easily occur, leading to erroneously low apparent diffusivity values. 

The most basic oxygen transfer equation (6) states that the amount of diffusion is proportional to the surface of the interface (6,20) ... 

Oxidation of Silicon. Silicon dioxide [7631-86-9] Si02, is a basic component of IC fabrication. Si02 layers are commonly used as selective masks against the implantation or diffusion of dopants into silicon. Si02 is also used to isolate one device from another. It is a component of MOS devices, and provides electrical isolation of multilevel metalliza tion stmctures (12). A comparison of Si and Si02 properties is shown in Table 1. 

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

Another example is the siliciditing of tantalum, basically an oxidation— reduction reaction. The packing is sodium duoride and siUcon. After deposition, the coating diffuses continuously into the substrate, according to the following reactions ... 

The diffusion coefficient has been commonly reported ia cm /s ia many apphcations. Hereafter, irP/s will be used since it uses the basic SI units. 

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