Studies on Adsorption and Wetting Phenomena Associated with Solid Surfaces in Aqueous Synthetic and Natural Surfactant Solutions

Adsorption of surfactants at air-liquid and solid-liquid interface and wetting of solid surfaces are closely interdependent. The performance of many physicochemical process and fundamental understanding depends on these two important phenomena. Because of the importance of these phenomena this study mainly focuses on adsorption of different surfactants at air-water and solid-water interfaces and wetting of those surfactant solutions at flat solid surfaces. The main emphasis of this study is plant-based natural surfactants; however some synthetic surfactants are also studied as a reference for comparison. The effects of electrolytes, alcohols, and naturalsynthetic surfactants mixtures are also studied. Electrolytes are most powerful inexpensive additive enhances the adsorption capacity of ionic surfactants at interfaces which in turn also enhances the interfacial behaviour. Adsorption kinetics and isotherm of anionic (dodecylbenzene sulfonate, SDBS), cationic (cetylpyridinium bromide, CPB), and non-ionic (TX-100) surfactants in the presence and absence of electrolytes on PTFE-water interface are studied. The kinetics of adsorption fits well pseudo-second-order kinetic model for the three surfactants studied here. Adsorption isotherms of TX-100 follow Langmuir type, whereas SDBS and CPB follow Freundlich type. However, in the presence of electrolytes both the ionic surfactants show better fitting with Langmuir type isotherm. The effect of electrolytes on the surfactant concentration far below the CMC shows there is a linear increase in amount adsorbed with the increase in ionic strength of the electrolyte mainly due to reduction in headgroup repulsion and finally reaches a plateau level when the equilibrium concentration reaches CMC at that electrolyte concentration. The structure of tailgroup of non-ionic surfactants also plays an important role in both adsorption and wetting behaviour. To get some insight about the fact, the adsorption and wetting behavior of two nonionic surfactants (TX-100 and Igepal CO-630) having the same headgroup but structurally different tailgroups has been compared. The change in contact angle with the concentration of surfactant follows a trend similar to that for adsorption onto a PTFE surface. At low surfactant concentration, Igepal CO-630 shows a slightly higher adsorption density and better wetting properties than TX-100. Both surfactants show lower adsorption densities at the PTFE–water interface than at the air–water interface