ADSORPTION OF CATIONIC SURFACTANTS ON MEDICAL POLYMERS - EFFECTS OF SURFACTANT AND SUBSTRATE STRUCTURES

Cationic surfactants with two and three alkyl chains were adsorbed on the surfaces of expanded Teflon and knitted Dacron surgical grafts. Didodecyldimethylammonium bromide (DDAB) showed saturation adsorption, forming monolayers. Tridodecylmethylammonium chloride (TDMAC) formed vertically oriented multilayers, without a saturation limit, on both these polymers. Electron spin resonance spectra of coadsorbed 12-DOXYL-stearic acid methyl ester showed that the adsorbates were flat-lying and immobile at low surface densities but vertically close-packed and mobile at high densities. At intermediate binding levels they existed in separate immobile and mobile phases, without any intermediate states. TDMAC formed multilayers even before completion of a saturated monolayer, evidently due to the preferential filling of narrow channels in these polymers. Equilibration of TEMPO between an aqueous phase and surfactant-coated polymers showed that only vertically oriented mobile surface layers could accommodate a secondary lipophilic adsorbate. This finding is important for the binding of antibiotics and other drugs to surface-modified surgical grafts.