Bearing a hydrophilic part and a hydrophobic part, surfactants can adsorb onto interfaces and lower the interfacial tension (γ), thereby enhancing the interfacial properties and leading to the applications in cleaning, surface functionalization, foaming and emulsification. Charged surfactants are also used in biological applications, in particular to extract and purify DNA, or for gene delivery. In this thesis we have studied the adsorption properties of surfactants, both to air/water interfaces and onto DNA to form complexes. The first part of the thesis concentrates on interfacial studies of surfactants. To understand how they work in these applications it is important to know the time-scales of the surfactant adsorption and desorption. Thus it is necessary to investigate the adsorption and desorption kinetics, which are already widely studied. However, traditional studies tend to make many assumptions, for example, extending the applicability of equilibrium relations to non-equilibrium cases. In this dissertation, the adsorption of two different surfactant systems has been investigated, non-ionic surfactant C12E6 and ionic surfactant CTAB with sufficient salt. A single bubble compression measurement combined with a known equilibrium surface tension (γeq) value allows the determination of γ(Γ), which is more accurate than results from traditional methods. The time-dependent surface concentrations are measured, showing that the adsorption is diffusion controlled at short times.Having shown that adsorption is diffusion controlled, we report desorption of surfactants from the air/water interface for different systems. The desorption processes are confirmed not to be purely diffusion-limited, showing the presence of an energy barrier. The energy barrier is influenced by the alkyl chain length, but not the counterion type.In the second part of the thesis we concentrate on DNA/surfactant systems. Although the interaction between cationic surfactant and anionic polyelectrolyte has been extensively studied, there still remains need to further understand the complex system, especially to rationalize the choice of surfactants to reach controllable DNA binding ability and low toxicity to the organism. In this dissertation, we introduced the systematic investigation on the interactions of two cationic surfactants with DNA.The first surfactant used is a cationic gemini surfactant 12-2-12∙2Br. Before using it with DNA a thorough characterization has been carried out. The equilibration of 12-2-12∙2Br onto an air/water interfaces in the absence of electrolyte is very slow. Addition of NaBr hardly affects the adsorption kinetics at short times, during which the adsorption is diffusive. However, the adsorption equilibrates much faster. The micellization of cationic gemini surfactant 12-3-12·2Br has been investigated. The critical micelle concentration (CMC) increases slightly with temperature and decreases with ionic strength. 12-3-12·2Br interacts strongly with DNA, due to the electrostatic attraction between the two and the hydrophobic interactions between alkyl chains. Salt screens the electrostatic attraction, while increasing spacer length of gemini surfactant weakens its interaction with DNA.Another surfactant has also been studied for its DNA binding ability and we present a systematic study on interactions between cationic ionic liquid surfactant [C12mim]Br and DNA by experimental techniques and Molecular Dynamics (MD) simulation. By adding [C12mim]Br, DNA chains undergo compaction, conformational changes, with the change of net charges carried by the DNA/surfactant complex. MD simulation confirms the experimental results.
