The information gained in the study will be invaluable in aiding our understanding of the structure and transport properties of the cell membrane as well as helping in the development of vehicles to improve the delivery of nucleic acids (i.e. gene). In this experiment we would like to investigate the intermolecular interactions that occur between nucleic acids (i.e. siRNA/DNA) with mixed phospholipid/cholesterol monolayers at the oil-water interface in the presence of the divalent cation, Ca++, using neutron reflectometry in combination with contrast variation. The conformational order of the film as function of DNA/siRNA concentration will be studied and compared with comparable films at the (simpler) air-water interface.