Researches On Single-Stimulus/Multi-Stimuli Response Of Mixed Systems Based On Several Fatty Acid Salts

In this thesis,a series of external stimulus-responsive mixed micellar systems have been constructed using cationic surfactant or additive and fatty acid salt surfactant based on an ionic self-assembly strategy.Stimulus-responsive micellar systems can change their aggregate microstructure and rheological properties by responding to external physical or chemical stimuli（such as temperature,p H,CO₂,light,etc.）due to their specific functional groups.The rheological properties,microstructure,stimulus-responsive mechanism of mixed systems have been explored by some technical means,such as rheology,dynamic light scattering（DLS）,infrared spectroscopy（FT-IR）and ¹H nuclear magnetic resonance spectroscopy（¹H-NMR）,and molecular dynamics simulation（MD）method.（1）The stimulus-responsivenesses to temperature,p H and CO₂ for the anionic/cationic surfactant mixed systems constructed by sodium stearate（SS）and cetyltrimethylammonium bromide（CTAB）have been explored.Experimental results indicate that this kind of mixed systems have good temperature sensitivity,their viscosities decrease by 3-4orders of magnitude when the increase of temperature is not larger than5 K.The 100 mmol·kg^-1 CTAB/20 mmol·kg^-1 SS mixed system has been selected to explore its triple stimulus-responsivenesses to temperature,p H and CO₂,and it shows good cyclic reversibility for these stimuli.The reasons why this system shows stimulus-responsivenesses to temperature,p H and CO₂ have been explored based on the analysis and comparison of MD simulation results such as aggregate structures,distance distribution probabilities,radial distribution functions and interaction energies.（2）The temperature stimulus-responsiveness of the mixed systems constructed by anionic surfactant sodium myristate（SM）and cationic surfactant CTAB or N,N’-bis（2-（decanoyloxy）ethyl）-N,N,N’,N’-tetramethyl-1,3 propane diammonium dibromide(C₁₀DEG₃)has been studied.Experimental results illustrate that the CTAB/SM mixed system shows good temperature sensitivity,and 2-3.5 K increase in temperature leads to reduction in viscosity by 3-5 orders of magnitude.The C₁₀DEG₃/SM mixed system also shows temperature sensitivity.In comparison with CTAB,gemini type ester-containing quaternary ammonium salt C₁₀DEG₃ shows better biodegradability,and it can self-assemble with SM to form temperature-sensitive hydrogels at lower concentrations such as 0.5 or 1 mmol·kg^-1.（3）The stimulus-responsivenesses of the mixed systems constructed by anionic surfactant sodium laurate（SL）with sodium chloride as additive have been studied.The experimental results illustrate that the viscosities of the mixed systems decrease by 5-6 orders of magnitude when the temperature increases about 5 K,showing good temperature sensitivity.The 50 mmol·kg^-1 SL/150 mmol·kg^-1 Na Cl system has also been selected to investigate its p H stimulus-response,and the experimental results indicate that its p H stimulus-response shows cyclic reversibility.The MD simulation results illustrate that for this kind of mixed system,when the temperature increases,the diffusion coefficients of the surfactant ions and their counterions increase,the rates of molecular thermal motion increase,the hydrophobic interaction between the surfactant tail chains and the electrostatic attraction between the ionic headgroup of L^-and counterion Na⁺decrease significantly.For this kind of mixed system,as the p H decreases,part of SL is protonated to form lauric acid.Both the distance between the L^-ionic headgroups and the distance between the ionic headgroup and Na⁺increase,and the solvent accessible area of L^-increases.These factors can weaken the self-assembly ability of the surfactant,leading to the microstructural transition of the aggregates,which in turn affect the phase behavior and rheological property of the mixed system.