of Gibbss Theory of Surface- Concentration, 485 



The results of two emulsion experiments are given 

 below : — 



Caffeine solution (approx. 0*125 per cent.). 



Experiment 

 no. 



Drop-number 



before 

 emulsification. 



Drop-number 



after 

 emulsification. 



Area of 

 oil-surface. 



1 



160 160, 160£ 

 160, 160} IfiOi. 1604- 



87,130 cm. 2 

 127,310 cm. 2 



2 









The results show that there was no readable difference in 

 the concentration of the solution before and after emulsification — 

 the difference being a small fraction of a drop. Special experi- 

 ments were undertaken to determine the accuracy of the 

 readings made with the pipette, and it was found that for a 

 small drop-number such as 160 the readings were reproduc- 

 able to within about Jth of a drop. 



No very reliable quantitative relations can therefore be 

 obtained from these determinations, as we are evidently 

 dealing with a very small change in concentration, i. e., a 

 very small adsorption. The important feature is this — that 

 the calculated value of Y (viz. 10~ 8 grm./cm. 3 ) icould also 

 correspond to a small fraction of a drop as indicating 

 concentration change ; while a value for Y of the order 

 obtained in the case of sod. glycocholate and the dyes 

 (viz. 10 -6 grm./cm. 2 ) would have been indicated by a 

 change of 4-5 drops, which would of course have been 

 perfectly readable. These experiments therefore show that 

 caffeine is very much less adsorbed than sodium glycocholate, 

 Congo red, or methyl orange. 



It is perhaps going beyond the limits of accuracy warranted 

 by the drop-pipette method to make an approximate calcu- 

 lation of the r from the data given above. Taking the 

 second experiment, the change in concentration corresponds 

 to about Jth of a drop. This is only approximate, and in all 

 probability would give a maximum value for Y. From a 

 curve * obtained by plotting the drop-numbers against con- 

 centrations given in Table IV. it is found that ^th of a drop 

 corresponds to a change in concentration of 0*00094 per cent., 

 and hence the total amount of caffeine removed from the 

 500 c.c. emulsified is 0*0047 gram. The total adsorbing 

 surface is 127310 cm. 2 , hence 



T = 3-7 xlO" 8 grm./cm. 2 , 



r calculated = 2*4 x 10 -8 grm./cm. 2 



* The curve is a straight line between drop-numbers 155-200. A 

 fraction of a drop can therefore be read fairly accurately. 



