DNA-surfactant complexes at solid surfaces

In this work, we report on the adsorption of complexes between DNA of diffe rent molecular weight and a cationic surfactant, cetyltrimethylammonium bro mide (CTAB), on hydrophobized and hydrophilic negatively charged silica sur faces as measured by ellipsometry. We will demonstrate how the adsorption i s affected by the state of the DNA-surfactant complexes formed in bulk solu tion. High molecular weight DNA molecules, which condense (transform from c oil to globule state) on addition of small amounts of cationic surfactants, do not adsorb on hydrophilic silica prior to phase separation. However, DN A-surfactant complexes formed from low molecular weight DNA were found to a dsorb. For these complexes surfactants interact with DNA, without condensat ion of the DNA. Adsorbed DNA-surfactant complexes can easily be removed fro m the hydrophilic silica surface when replacing the bulk DNA-surfactant sol ution with pure salt solution. At the hydrophobic surface the DNA adsorbs w ithout addition of cationic surfactant. However, with addition of a very lo w amount of surfactant, a rapid increase in adsorbed amount and a simultane ous decrease in adsorbed layer thickness are observed. This compaction of t he adsorbed layer is to some extent reversible when replacing the bulk DNA- surfactant solution with pure salt solution.