Study On The Interfacial Properties Of Surfactants And Their Interactions With DNA

Bearing a hydrophilic part and a hydrophobic part, surfactants can adsorb onto interfaces and lower the tension (y) of the interfaces, thereby enhancing the interfacial properties and leading to the applications of surfactants in cleaning, surface functionalization, foaming and emulsification. To understand how they work in these applications it is important to know the time-scales of the surfactant adsorption and desorption. This means that it is necessary to investigate the adsorption and desorption kinetics of surfactants, which have already been widely studied. However, traditional studies tend to make many assumptions, for example, extending the applicability of the equilibrium relations to the non-equilibrium cases. In this dissertation, using a bubble compression method, we first measured the equation of state y(Γ), followed the time-dependent surface concentration T(t) and clarified the adsorption/desorption process without using many assumptions.Cationic surfactants are receiving much interest for biological applications. The DNA/cationic surfactant system is of use in DNA extraction, DNA purification and gene delivery. 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 micellization processes of two novel cationic surfactants (gemini surfactants and ionic liquid surfactant) and their interactions with DNA.Chapter1presents the general introduction to surfactant micellization and its influences, interfacial properties of surfactants, the interactions between cationic surfactants and DNA together with the proposal for the thesis. The interplay of surfactant molecules between the interface and the bulk is presented. The adsorption and desorption kinetics of surfactants are included in the interfacial properties of surfactants, consisting the first important component of the thesis. The interaction the between cationic surfactant and DNA will be discussed in details, as it is the second essential part of this thesis.Chapter2mainly deals with the equation of state measurement and adsorption kinetics for two systems, including non-ionic surfactant C12E6and ionic surfactant CTAB with sufficient NaBr. By a single bubble compression measurement combined with a known equilibrium surface tension (yeq) value, the equation of state y(Γ) is determined, which is more accurate than the results from the traditional methods. Using the bubble compression method, the time-dependent surface concentrations Γ(t) for both systems are determined, showing that the adsorption is diffusion controlled at short times. The derived diffusion constants compare well with literature values.The desorption and adsorption processes are interrelated, accordingly, in Chapter3, we reported the desorption of surfactants from the air/water interface for different systems (C12E6, CTAB or TTAB with sufficient NaBr, anionic surfactant AOT with different counterions). For systems studied, the desorption processes are confirmed to be not purely diffusion-limited, showing the presence of an energy barrier in the desorption of surfactants from the interface to the sub-surface. The energy barrier can be affected by the alkyl chain length, but not the counterion type.In Chapter4, we mainly focused on the equilibrium and kinetic behaviors of the cationic gemini surfactant12-2-12-2Br at the air/water interface. It is found that in the absence of electrolyte, an electrostatic barrier exists during the surfactant adsorption at longer times. The effect of NaBr concentration on the dynamic surface tensions of the12-2-12-2Br solutions is also investigated. Addition of NaBr hardly affects the adsorption kinetics at times shorter than lag time, when the adsorption is diffusive. Comparing the systems at equilibrium,12-2-12-2Br is more sensitive to the presence of NaBr than CTAB. For the12-2-12-2Br system in the presence of100mM NaBr, the adsorption and desorption kinetics of surfactant molecules have also been studied.Chapter5presents the micellization of cationic gemini surfactant12-3-12’2Br and its interactions with DNA. Micellization of12-3-12·2Br is entropically driven and thermodynamically favored. Critical micelle concentration (CMC) increases slightly with temperature but decreases with ionic strength.12-3-12’2Br interacts strongly with DNA, resulting from the electrostatic attraction between12-3-12·2Br and DNA and hydrophobic interactions between the alkyl chains, correspondingly the DNA conformation is modulated. Salt screens the electrostatic attraction and promotes the aggregation of12-3-12·2Br. With increasing DNA concentration, critical aggregation concentration (CAC) remains constant, while saturation concentration (C2) increases. The effects of spacer length S on the micellization of12-S-12·2Br and its interaction with DNA are also investigated. Micellization of12-3-12·2Br has highest thermodynamic favorability. The12-S-12·2Br/DNA interaction weakens with increasing S.In Chapter6, we present a systematic study on the interactions between the cationic ionic liquid surfactant [C12mim]Br and DNA, using both experimental techniques and Molecular Dynamics (MD) simulation. The strong complexation occurs owing to the electrostatic attraction and the hydrophobic interactions. The aggregation of [C12mim]Br is thermodynamically favored driven by enthalpy and entropy change. Upon the addition of [C12mim]Br, the DNA chain undergoes compaction, conformational change, and also the change of net charges carried. MD simulation reconfirms the experimental results.