Wetting Properties Of Surfactant Systems

The research of surfactant systems is one of the important areas in soft condensed matter physics. Large amount of amphiphathic molecules exist in chemical and biological systems, most of them can be regard as surface active agent. Their amphiphilic behaviors result various kind of interfacial/surface phenomena, and also effect greatly on the physical characters of complex fluid systems. However, lots of questions related to surfactant have yet to be answered or studied. In this thesis, we treat these problems in a physical way, and study the interfacial/surface wetting and spreading phenomena of polydimethylsiloxane oil/ surfactant solution systems. Background and current theory are outlined in first and second chapters. From the third chapter, we present our experiments, data, results and discussions.When a drop of polydimethylsiloxane oil touches the surface of sodium dodecylsulfate solution, it spreads immediately and forms an oil layer with uniform thickness. Then because of the existence of surfactant, in a certain concentration range, after a while, the spread oil layer will retract and reform an oil lens again, the rest of the surface is covered by a thin oil film. This wetting status is between the complete wetting and partial wetting, and it is named as pseudopartial wetting. In sodium dodecylsulfate solution systems, this state only occurs at concentrations near cmc(critical micelle concentration), at higher or lower concentrations, the wetting states are always complete wetting. A series of experiments are conducted, including:1), by utilizing ellipsometer technique, the thickness of oil layer can be measured at the surfaces of each wetting status. The relation between film thickness and wetting states is analyzed, and by combining the measurement of surface/interfacial energy, we calculated the two Hamaker constants of van der Waals energy term.2), by using teflon particles as tracers, the spreading dynamic process can be recorded. We summarized all the spreading behaviors at each concentration and explained the relation between spreading and surface energy balance. In the intermediate concentrations, the existing theory does predict the right form of spreading rate because of its simplified energy term. By adding an extra energy term, we successfully explained our data of spreading rate at the whole concentration range.3), after the retracted oil lens formed, its radius varied through time. For each concentration, we recorded the surface status for over 100 hours. The results show that the variations of oil lens radius are related to concentration, surface energy status, spreading coefficient and etc.4), the surface/interface status and cmc value of surfactant solution change along with temperature. Different surface states at different temperatures are observed. Wetting status changes dramatically at high temperatures.Similar experiments of systems of another surfactant-Triton X-100, are also conducted. The major difference between Triton X-100 and sodium dodecylsulfate is that the latter is ionic and the former is nonionic. The molecular status of Triton X-100 inside water is a full molecule. Hence, the energy balances at surface maybe also different in the two kind of surfactant systems. From this point of view, we studied the wetting behavior of PDMS/Triton X-100 systems:1), the dynamic spreading process is utterly different from the conventional theory of liquid/liquid spreading. In this case, at low concentration with very high spreading coefficient, the system shows extremely slow spreading rate. The spreading behavior is similar to the liquid/solid case, hence we named this particular spreading phenomena as "solid-like spreading". Moreover, in these systems, the spreading rate increases as the spreading coefficient decreases.2), the short time scale equilibrium states are also different from the former systems. At higher concentrations, the wetting states transit from complete wetting to pseudopartial wetting, and never return, i.e., at sufficient high concentrations, the retracting oil lens will always be formed. At very high concentrations, the spreading-retracting behaviors are even faster than on lower concentrations.3), however, after a very long time(＞3hours), the reformed oil lens will disappear again at very high concentration range, and the wetting state goes back to complete wetting in the end. This second time disappearing of oil drop is a special character of this system. The final disappearing time decreases along with increasing concentration.All these special spreading/wetting phenomena of PDMS/Triton X-100 system have not been officially reported before. A lot of them contradict to current theories, or can not be fully explained by theory. This indicates that unknown energy effect does exist in this system and can not be probed by conventional techniques such as surface tension measurement. We speculate these kind of effect may come from the micellar formation in bulk solution. The electrostatic/volume effect may related to surface, and finally change the wetting status.