Brownian motion substances

General Linearized Brownian Motion. Our discussion has not so far passed beyond the Langevin approximation, which must be modified for realistic description of the motion of molecules not vastly more massive than the molecules of the solvent medium. This is particularly necessary for the discussion of pure substances. 

Efflux by transmembrane diffusion can also contribute to the poor diffusion of substances within brain parenchyma. Diffusion within the interstitial space of the brain is dependent on Brownian motion and the production of metabolic free water as driving forces and so is very slow (Cserr, 1984 Cserr and Berman, 1978). However, efQux by non-saturable (and saturable) mechanisms can further reduce the distance a substance will ultimately diffuse. For example, the less lipid soluble drug atenolol can diffuse about 3 times further into brain tissue than can the more lipid soluble drug acetaminophen (de Lange et al., 1993). 

Brownian motion The random movement of a particle as a result of collisions with fluid molecules or other parHcles Catalyst A substance that increases the rate of a chemical reacHon without being consumed. Catalysts usually work by providing a lower acHvaHon energy pathway Cation An ion with a positive chaise Cation exchange capacity In the past, this property has been expressed in milliequivalents per 100 g of material (meq/lOOg). In SI units, it is expressed in moles of posiHve charge (p ) per unit mass. Often in centimole per kilogram, cmol (p )/kg. The old and new units are numerically equivalent... 

PCS measures the diffusion coefficient of particles in the size range between 3 nm and a few micrometres. Particle size measurements for particles and/or aggregates smaller than 1 pm were performed on a Malvern Photon Correlation Spectrometer (PCS) Autosizer 4700 (633 nm, 5 mW, He-Ne laser). It is essentuial to use a red laser due to the fluorescence spectra of the humic substances (Goldberg and Weiner (1989)). A round quartz cell was used and temperature adjusted to 25 C. The method measures the diffusion coefficient (Brownian motion) of particles and is limited to about 3 nm... 

Bronsted acid a substance that donates protons Bronsted base a substance that accepts protons Brownian motion random movement in suspensions caused by collisions with molecules... 

The essential requirement for a substance to be rubbery is that it consist of long flexible chainlike molecules. The molecules themselves must therefore have a backbone of many noncollinear single valence bonds, about which rapid rotation is possible as a result of thermal agitation. Some representative molecular subunits of rubbery polymers are shown in Figure 1.1 thousands of these units linked together into a chain constitute a typical molecule of the elastomers listed in Figure 1.1. Such molecules change their shape readily and continuously at normal temperatures by Brownian motion. They take up random conformations in a stress-free state but assume somewhat oriented... 

Brownian motion itself, as its name reveals, was discovered in 1827 by the English botanist Robert Brown (1773-1858). Looking through a microscope at little particles of pollen suspended in water, he was fascinated by their random dances . The particles were moving by themselves, apparently with no external encomagement. So a lot of people decided that there must have been some hving power causing the motion (because the flowers were animate ) They reckoned this had proved that there was some mysterious substance which made the animate different from the inanimate. 

Eventually, the answer was found by Albert Einstein and the Polish physicist Marian Smoluchowski (1872-1917), then a professor at the University of Lviv. The title of one of Einstein s papers on the theory of Brownian motion is rather telling On the motion of particles suspended in resting water which is required by the molecular-kinetic theory of heat . Einstein and Smoluchowski considered chaotic thermal motion of molecules and showed that it explains it all a Brownian particle is fidgeting because it is pushed by a crowd of molecules in random directions. In other words, you can say that Brownian particles are themselves engaged in chaotic thermal motion. Nowadays, science does not make much distinction between the phrases Brownian motion and thermal motion — the only difference lies back in history. The Einstein-Smoluchowski theory was confirmed by beautiful and subtle experiments by Jean Perrin (1870-1942). This was a long awaited, clear and straightforward proof that all substances are made of atoms and molecules. ... 

Many liquid and semi-liquid pharmaceutical preparations are disperse systems. Disperse systems are defined as systems in which a substance is distributed as particles (discontinuous) into a dispersion medium (continuous). Three types of disperse systems will be discussed which are pharmaceutically relevant colloidal systems, suspensions and emulsions. In both colloidal systems and suspensions, solid particles are dispersed in a liquid. The difference is that in colloidal systems the particles do not settle, while they do in suspensions. This difference is caused by the size of the particles. In colloidal systems, the particles are so small (1 mn - 1 pm) that the Brownian motion (diffusion caused by thermal energy) is stronger than the force of gravity so that they remain suspended in the liquid and do not settle. In suspensions, the particles are larger (>1 pm) and as a consequence the force of gravity is stronger than the Brownian motion which makes them settle (if the density of the particles is larger than that of the dispersion medium). Emulsions consist of non-miscible liquids. Two types of emulsions will be discussed oil drops dispersed in water (oil-in-water emulsion or o/w emulsion) and water drops dispersed in oil (water-in-oil emulsion or w/o emulsion). There are also more complex structures such as w/o/w emulsions and bi-continuous systems. However, these systems will not be discussed. 

Brownian Motion When we place a blot of ink in still water, the colored region expands with time but the color fades, eventually filling the entire water in the container. The final state is a uniform concentration of the ink. Spreading of a substance throughout accessible volume is called diffusion. The phenomena is made possible by microscopic movement of water molecules. 

All real substances are viscoelastic. How they respond in particular situations depends on the rate of testing compared with the rate of spontaneous structural reorganization at the molecular level [1]. As depicted in Fig. 3.3, the neighbors of molecules in an ordinary liquid weU above its glass-transition temperature Tg change rapidly through the action of Brownian motion. Local structural memory - the... 

Colloid chemistry studies the nature and effects of surfaces and interfaces on the macroscopic properties of substances. These studies involve the investigation of Brownian motion/" surface tension, interfacial tension (the tension that exists in the plane of contact between a liquid and a solid, or between two liquids), wetting and spreading of Uquids on solids, adsorption of gases or of ions in solution on solid surfaces, etc. 

Diffusion The mixing of substances by molecular motion to equalize a concentration gradient. Applicable to gases, fine aerosols and vapors. (See Brownian diffusion.)... 

Colloidal solutions may also be made by dispersing in the solvent j a solid or liquid substance which is normally insoluble, such as gold, ferric oxide, arsenious sulfide, etc. A colloidal solution of this sort consists of very small particles of the dispersed substance, so small that their temperature motion (Brownian movement) prevents them from settling out in the gravitational field of the earth. 

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