THE JOURNAL OF b
PHYSICAL CHEMISTRY (Registered in U.8. Patent Office)
(Copyright, 1952, by the American Chemical Society)
Foz~iulcdby Wilder D. Bancroft VOLUME66
JUNE 18, 1959
NUMBER (i
ASSOCIATION BET\WEN POLYSILICIC ACID AND POLAR ORGANIC COMPOUNDS BY R. K. ILER Grasselli C‘iiemiculs .!)cpurlttwi~l, Experimentul Station, E. I . du Ponl de Neniours aiid Co., Inc. Received June 39, 1961
The relative degree of association between polysilicic acid and various types of organic compounds in a ueous solution has been related to the structure of the organic compounds. The relative degree of association was measuredly noting the solubilizing effect of the organic compounds on the precipitate obtained by mixing solutions of polysilicic acid and gelatin, other factors being held constant. In the absence of gelatin, the association of polar organic compounds with polysilicic acid is demonstrated by the fact that silica-organic complexes can be salted out of the aqueous mixture. Liquid complexes are obtained from freshly prepared solutions of polysilicic acid and polar organic compounds of low molecular weight. Solid precipitates are formed from aged solutions of the acid and organic compounds of higher molecular weight. Since the compounds which were found to associate with polysilicic acid all contained electron-donor atoms, it was inferred that hydrogen bonds between the electron donor atoms and the silanol group might be involved. Since the degree of association is also found to be related to the potential surface ener y of a number of the organic compounds, it is ostulated that the formation of the complex involves oriented adsorption of t i e polar molecules on the surface of the polysi&ic acid molecules.
No systematic study has heretofore bedn made of the complexes formed between soluble silicic acid and organic compounds. There have been previous indications of the existence of such complexes, such as the indefinite “adsorption compound” between silicic acid and methylene blue, * and the precipitate formed by adding egg albumin to silicic acid of high molecular weight as described by R. Willstiitter, Heinrich Kraut and Karl LoIn the course of tanning research in 1949, J. S. Kirk discovered that complexes were formed between silicic acid and ethers or amides, and that these could be salted out of solution as viscous oils or precipitate^.^ Also, Kirk discovered that these polar organic compounds were effective in inhibiting the reaction between silicic acid and gelatin or hide s ~ b s t a n c e . ~Since certain types of compounds, such as ethers and amides, which were found to be most effective in influencing the behavior of silicic acid, were also found by M. J. ( 1 ) L. Pelet-Jolivet and N. Anderaon, Bull. UOC. chim., [ 4 j S, 540 (1011). ( 2 ) R. Witlaflitter. Heinrich Kraut and Karl Lobiuger. Be,,., S8B, 2462 (1925); 61B, 2280 (1928). (3) J. 9. Kirk,‘U. S. Patent, 2,408,064-2,408.656 (1046). (4) J. 9. Kirk. U. e. Patent, 2,276,3144,276,315 (1912).
Copley and C. S. Marvels to enter into association with molecules such as chloroform through the formation of a hydrogen bond, it was logical to suspect that the association complexes with silicic acid were likewise the results of hydrogen bonding. The association of the polar compounds with the polysilicic acid probably involves an oriented adsorption, so that the surface activity of the compound is undoubtedly an important factor. However, since most of the compounds studied are nol “surface active agents” in the usual sense of the term, they will be referred to broadly as “hydrogcnbonding agents,” for the sake of convenience. General Behavior of the System GelatinPolysilicic Acid.-The observation that the precipitation of polysilicic acid by gelatin was inhibited by (‘hydrogen-bonding agents” offered a convenient point a t which to begin experimentation. In preliminary qiialitntive tests, using n silicic acid sol prepared from sodium silicate and sulfuric acid in a standardized manner and in an aqueous medium at a constant pH of about 2.5, observations were made of the tendency of gelatin to precipitate silica wit11 variation in age of the silica sol, p H , ( 5 ) M. J. Couley and C. S. h4arve1, J . Ani. Ckenr. SOC.,62, 3109 (1940).
G73
It. I
