Gemini surfactant/DNA complex monolayers at the air-water interface: Effect of surfactant structure on the assembly, stability, and topography of monolayers

The formation of complex monolayers between gemini surfactants (C_sH_2s-α,ω-(C_mH_2m+₁ N⁺ (CH₃)₂Br^-)₂, abbreviated as C_12-C_s-C₁₂, s = 3, 4, 6, 8, 10, 12) and DNA at the air-water interface was systematically investigated. The polyion-complex monolayers formed in situ through the electrostatic attraction between the ammonium groups of gemini surfactants and the phosphate groups of DNA. The effect of surfactant spacer length on the surface properties was investigated. A turning point of the surface properties (extrapolated molecular area and collapse pressure) of the gemini surfactant/DNA complex monolayers appears when the surfactant spacer is above a certain length (s = 6). The gemini surfactant spacer taking a reverse U-shape conformation at the air-water interface is proposed to interpret the turning point. A quantitative kinetic analysis of the decay curves further confirms that the turning point appears at the surfactant spacer above its critical length, s = 6. Moreover, the surface topographies of the gemini surfactant/ DNA complex monolayers were controlled by the spacer length of the gemini surfactants, which may be important in surface patterning and nanofabrication.