Surface dilution phenomena

Achieving specific and covalent inhibition of hpolytic enzymes is a difficult task, because of mutuaUy non-exclusive processes such as interfacial denaturation, changes in interfacial quahty [45], and surface dilution phenomena [46]. Further-... 

Haldane su ested that life emerged in the homogenous phase (it was he who mentioned the hot, dilute soup ). In contrast, the Zn world concept considers primeval life as a surface-confined phenomenon. 

Manning (1969) suggested that there is a critical charge density above which counterions condense on the surface of the polyion. This phenomenon is most clearly illustrated by the simple case of iiffinite dilution. As 0->O in equation (4.11), the graph of P against Q falls into two parts about the critical point 2=1 ... 

It is to be hoped that future calculations will attempt to predict the diffusion coefficients of solutes in narrow pores. Measurements in such systems are extremely difficult to carry out and recent experiments in an admittedly broad pore (a 2 mm diameter capillary) are therefore of particular interest. Liukkonen and co-workers [61] found that the diffusion coefficient of NaCl in a dilute aqueous solution was 75% greater at the walls of this capillary than in the bulk solution, a result in line with the phenomenon of "surface conductivity [62]. Yet this finding clearly runs counter to the trend in the self-diffusion calculations in much narrower pores. It rather looks at this stage as if electrolytes near polar walls behave quite differently from non-electrolytes. 

The phenomenon of very diluted solutions is well known in radiochemistry. Carrier-free radioactive isotopes could be mentioned as an example. The term denotes a radioisotope of an element in pure form, that is, essentially undiluted, with a stable isotope. The chemical concentration of these radioisotopes is usually very low. For example, 1 kBq radioactivity (applied typically in a tracer experiment) is equivalent to cca. 2 10 12 mol in the case of 137Cs or 90Sr isotopes. In the case of such low concentrations, no chemical system can be considered homogeneous because all surfaces, the wall of the laboratory vessels, or any contaminants in the solution (such as air bubbles, small particles, great molecules, etc.) can initiate interfacial processes and the subsequent formation of heterogeneous phases (adsorption, colloid formation, precipitation, etc.). This is the result of the simple fact that the number of molecules on the surfaces is more than, or at least similar to, the number of particles in the solution. Even in a solution containing... 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

The last Issue to be dealt with Is the apparent irreversibility of the adsorption. One quite often encounters the opinion, especially In the older literature, that polymer adsorption would be an Irreversible phenomenon. These ideas are based on the hysteresis found In the adsorption isotherms desorption Isotherms (obtained by dilution with solvent) do not coincide with adsorption Isotherms (obtained by adding more polymer at given amount of solvent). Qualitatively, this was already discussed in sec. 5.3d. An experimental example Is given in fig. 5.31, for the adsorption of a polydisperse rubber from heptane on two types of carbon black (differing In specific surface area) ). The desorption Isotherms are found to He considerably above the adsorption Isotherms, the extent of desorption being very small. 

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