Diffusion

DIFFUSION, the gradual penetration of one substance into another or through another. This name designates a whole group of phenomena, the common feature of which is that the diffusing substance, through random molecular movements, spreads from places of higher to places of lower concentration, approaching uniform distribution. D. occurs especially rapidly in gases. If two gases come into contact with each other, then both of them fill the entire volume available for their spread. The same result is also obtained in the case of D. of gases through a porous partition. The latter hinders the simple mixing of both gases and thereby allows for more precise observation of the phenomenon being studied. Measurements have shown that D. of gases through porous partitions (effusion) occurs at a speed proportional to the pressure of the gas and inversely proportional to the square root of its density (or its molecular weight). By passing a mixture of gases of different molecular weight through a porous tube, one can cause their partial separation. D. through various solid bodies (for example, rubber) or through liquid films occurs in a similar manner. In the latter case, in addition to the density or molecular weight of the gas, its solubility in the wall substance also affects the speed of D. D. in liquids. In liquids and particularly in solutions, phenomena of D. have been subjected to especially numerous and detailed studies, which have led to their further subdivision. The name D. is retained here only for the free spread of dissolved substances in the mass of the liquid (often also for their spread through porous walls, the pores of which are very large compared to the diffusing molecules). The passage of liquid and substances dissolved in it through membranes is called osmosis (depending on direction - endosmosis or exosmosis); the term 'dialysis' is specifically used for the passage of dissolved substances through membranes. When studying D., it is necessary to most carefully eliminate the possibility of mechanical mixing of liquids. For this, D. is conducted against the action of gravity: the denser solution is placed at the bottom of the vessel, and on top of it is carefully layered a lighter liquid (usually - pure solvent). To avoid convection currents, a strictly constant temperature is maintained in the diffusion vessel. Nevertheless, it is difficult to completely eliminate all secondary influences that disrupt the proper course of D. This is achieved much better when measuring D. through a porous wall. D. of various dissolved substances was studied in detail by Graham (Graham). However, Pick is credited with establishing the mathematical law that quantitatively expresses its speed. We shall call the concentration gradient the difference in concentration between two points located 1 cm apart in the direction of D. (or: the ratio of the difference in concentration at two points to the distance between them). According to Fick, the speed of D., i.e., the amount of dissolved substance passing per unit of time through a given cross-section, is proportional to the product of its area and the concentration gradient of the solution. The coefficient of proportionality, called the diffusion coefficient, represents for each substance (at a given t°) a characteristic constant. It shows how much of a given substance passes per unit of time through one square centimeter at a concentration gradient equal to one. The unit of time is usually taken as a day, since calculation for one second would yield too small values. In the following table, diffusion coefficients (D) of some dissolved substances are given. Substance D (cm/day) NaCl........... 1,17 0,81 0,31 0,13 0,042 Sucrose........ Gum arabic ...... As the molecular weight increases, the diffusion coefficient gradually decreases. Therefore, as Graham already established, colloids, having large and heavy particles, differ from crystalloids by extremely slow D. A quantitative relationship can also be established between the diffusion coefficient (D) and the molecular weight (M): D = k/M. It is sufficiently well satisfied at least for substances having similar chemical structure. The given formula is even used in some cases for approximate determination of the molecular weight of the substance being studied from its D: D1/D2 = M2/M1. However, in those cases where (as is the case with colloids) the dissolved particle is many times larger than the solvent molecules, it is necessary to use another formula proposed by Einstein (Einstein) to calculate its size and weight. D. can proceed in dense colloidal gels at almost the same speed as in solutions. - Phenomena of D. play a role of primary importance in the life processes of the organism and the cell. The entry of oxygen through the lungs into the blood and its release from the blood to the tissues, the absorption of digestive products from the intestine can serve as examples of processes determined by D. in the broad sense of the word, i.e., equalization of concentrations. Of course, in this case, mainly not processes of free diffusion, but of osmosis, penetration of diffusing substances through one or another membranes and shells take place.