z34 NATIONAL BUREAU OF STANDARDS [.I.F.I.
from zero, that is, compounds which had no effect at all on the system, to 3, compounds which caused all of the powder to be suspended in the "oily" butanol or at the interface, with the aqueous phase completely clear. The ratings were arrived at by comparing the test tubes with controls containing no surface-active agent. Sets of each of the five test powders were used to test 68 different surface-active agents. The results showed that 11 of the agents could be given a top rating, 10 a zero rating, and that the remainder fell in between. Although these ratings appeared to be strongly dependent on the pH of a particular group, the increasing order of the affinity of the groups for the hydrated particle surfaces of the calcified materials could be divided into four types: hydroxyl (less affinity than water), amino, carboxyl, and groups theoretically capable of forming a 5-atom amphoteric chelate ring with calcium.
This information is being used to design "surface-active comonomers" which can displace water from the tooth surface and also copolymerize with the dental resin. One such surface-active comonomer (the adduct of N-phenyl glycine and glycidyl methacrylate) has given average adhesion values as high as 780 psi. after the bond was soaked one or more days in water. The controls without the surface-active comonomer averaged 10 psi. or less. The experiments are continuing so that data may be derived for other surface-active comonomers which might further improve the adhesion between dental resins and tooth surfaces.