The action of polysilicic acid on mixed monolayers of cholesterol and lecithins

Silicic acid produces marked expansion in dipalmitoyl lecithin films and in mixed cholesterol-dipalmitoyl lecithin films with more than 50 % phospholipid. In the presence of silicic acid, these films no longer exhibit a transition region between the liquid-expanded and liquid-condensed states. Mixed cholesterol-dilauroyl lecithin.films are also expanded by silicic acid. In the absence of silicic acid, the addition of cholesterol produces greater condensation in dilauroyl lecithin films than in films of dipalmitoyl lecithin, but the reverse is the case if the substrate contains silicic acid. These results have been interpreted in terms of a possible electrostatic interaction between negatively charged dissociated silicate ions and the positively charged trimethyl ammonium group of the choline fragment.