COLLOIDS 69 



reactions of a certain type may be encouraged over other reactions, even although, the 

 necessary reacting substances may be present in the solution (specific adsorption). The 

 adsorbing substance itself is not, however, usually susceptible of chemical change 

 even when it exists as very minute particles, as in the case of colloidal solutions. 

 Nevertheless, adsorption may accelerate chemical reactions by bringing together in 

 concentrated form substances of high chemical reactivity. In such cases the adsorbing 

 substance itself does not enter into the chemical reaction, and can be recovered at the 

 end in an unchanged condition. It acts as a catalyst (page 72). As we shall see 

 later, enzymes act in this way i. e., their rate of reaction is controlled by adsorp- 

 tion.* 



The distinguishing feature of such adsorption phenomena is that a curve of . the 

 reaction (drawn by plotting amount of chemical change against concentration of react- 

 ing substances) is a parabola, indicating that the laws of mass action (page 23) are 

 no longer followed. In order to be able to determine whether some particular process 

 as, for example, a fermentation process, or the absorption of oxygen by blood is 

 caused by adsorption, we must compare its curves, constructed according to the same 

 principles, with the typical adsorption curve. A formula may be used in constructing 

 the curves. In arriving at this formula, two facts have to be remembered: (1) As ad- 

 sorption proceeds and less and less of the free energy on the adsorbing surface re- 

 mains to be neutralized, the reaction slows off, until equilibrium is reached. The more 

 dilute the solution, the greater is the proportion of its contents to be adsorbed, which 

 means that if a is the amount of substance adsorbed from a certain solution, then, 

 from a solution of twice that strength, somewhat less than 2 a will be adsorbed i. e., 

 a multiplied by some root of 2. Although the formula is one belonging to the class 

 known as parabolic, it must not be assumed that every reaction which happens to give 

 such a parabolic curve (such as the combination of O with hemoglobin under certain 

 conditions) (see page 396) must be one dependent on adsorption. 



It must be understood that although the substance that is removed from a solution 

 by adsorption is no longer capable of contributing to the conductivity or the osmotic 

 pressure of the solution, it is nevertheless not so firmly fixed that it can not be set 

 free again by purely mechanical means, as by constant dilution of the fluid. If char- 

 coal which has adsorbed sugar is placed in a dialyzer made of membrane, the pores of 

 which allow sugar but not charcoal to pass through, the sugar will gradually be re- 

 moved if the dialyzer is immersed in running water. A certain equilibrium exists be- 

 tween the substance adsorbed and the same substance still remaining in solution. If 

 the latter is constantly diminishing by dialysis, the adsorption compound must break 

 down to maintain the equilibrium. It is clear, however, that the process of removal 

 will be extremely slow. The ability of adsorbed substances to withstand removal by 

 washing is taken advantage of by nature in holding back foodstuffs in the soil. 



*Another instance of the influence of surface energy on the course of chemical reactions is seen 

 in the accelerative influence of charcoal on such reactions as the oxidation of formic acid, glycerol, 

 etc. Surface tension may also cause retardation of chemical reactions, as is seen in the turbidity 



(due to the separation of chloroform) which gradually develops when a ^ NagCOs solution is 

 mixed with a -^-chloral hydrate solution. The surface remains clear, because surface energy has 

 prevented the reaction. 



An important effect of surface tension on chemical reactions is also seen in the relationship 

 between it and the absorption coefficient of gases (volume of gas dissolved by unit volume of 

 liquid). The lower the surface tension, the greater the solubility of the gas. Oxygen and nitrogen 

 are, for example, much more soluble in alcohol, hydrocarbons or oil than in water. This shows 

 the futility of attempting to prevent the loss of gases from fluids such as blood by covering them 

 with oils or hydrocarbons. 



