Cclxxxvi . Physical Chemistry of Lipoids . Iii . Relations between the Iodine Value of Monophosphatides and Their Reactions to Neutral Salts

IT has been reported in a previous communication [Spiegel-Adolf, 1932, 1] that, contrary to the Hofmeister series, colloidal solutions of egg-lecithin show a greater decrease in viscosityon addition of bromides than on addition of chlorides, iodides and thiocyanates. These results combined with more recent interferometric studies [Spiegel-Adolf, 1932, 2] were explained as due to reaction of molecular bromine with the unsaturated groups of the lecithin molecules. The experiments to be reported here were started with the aim of verifying this theory. As the number of unsaturated groups can be measured by their power to add iodine, determinations of the iodine value for the material under physicochemical investigation were started. These experiments of Spiegel-Adolf [1932, 1, 2] had been made on egglecithin (Merck). Therefore we first determined the iodine value in a colloidal solution of egg-lecithin, as we could not find references to the iodine value of colloidal lecithin. In order to exclude the factor of ageing [Spiegel-Adolf, 1932, 1], a two months-old lecithin sol was used, made in the way previously [Spiegel-Adolf, 1932, 2] described. The determinations of the iodine value were made according to Hiibl [1924], after the lecithin had been precipitated by a mixture of CaCl2, MgCl2 and HC1. The result was an iodine value of 41. Levene and Rolf [1926] obtained 47 as the iodine value for a lecithin made from egg-yolk by precipitation with a cadmium salt. In order to test further the supposed relation between iodine value and reactions to bromides different ways were tried of modifying the iodine value. 1. Irradiation by ultraviolet light is known to decrease the iodine value of lecithin. By exposing samples of 1 % lecithin sol in quartz tubes for 4 hours at a distance of 40 cm. to the irradiation of a mercury quartz lamp (Hanovia, 220 V, D.C.), it was possible to reduce the iodine value in another sample from 35 to 30. The viscosity showed a slight increase, thus confirming findings of Nagata [1930]. A shorter period of irradiation has a lowering influence on the viscosity of the lecithin sol. But whilst with the non-irradiated sample, as has been previously shown [Spiegel-Adolf, 1932, 1], bromide caused a sharper decline of the viscosity than did chloride, the difference disappeared completely with the irradiated sample. Table I shows the values for a typical experiment.