Effect of Span-80 in n-Hexadecane on Surface Tensions at High Temperatures

Abstract

Drop formation and release, spreading and wetting of substrate, etc., are interfacial phenomena that are critical to the performance of many high-speed processes in digital printing and fabrication. In these instances, dynamic surface tension (surface tension as a function of time) in the presence of a surfactant is more important in evaluating the performance of the process than its equilibrium surface tension. It is known that temperature change can cause a change in the adsorption kinetics of ionic and nonionic surfactants at interfaces resulting in a change in equilibrium surface tension measured with temperatures [1-4]. We have recently reported [5] high-temperature measurements (up to 200 °C) of surface tensions using optical imaging of the liquid/gas interface inside a tapered micropipette created within a comparable sized micro-capillary that was heated by coating an electrically conductive, transparent, tin-doped indium oxide (ITO) thin film. This novel method is further used in the current study for: (i) determining high-temperature equilibrium surface tensions of a non-ionic surfactant, Span-80 in n-Hexadecane (up to 160 °C) and comparing the results with those of the pure liquids6 and; (ii) investigating the effects of this surfactant on the dependence of surface tensions on temperature and time when the equilibrium is perturbed by a rapid temperature-change. The interesting aspect of the surfactant selection is that it is not normally surface active at the air/ n-hexadecane interface at room temperatures but is extremely surface active at high temperatures. Results show that, as expected, surface tension decreases as temperature increases and vice-versa. The values determined by optical measurements closely match the values obtained using theory of corresponding states. Also, the time to attain equilibrium is very short (a fraction of a second) and there is hysteresis in dynamic surface tensions between the temperature increase and decrease cycles with the decrease cycle lagging behind the increase cycle. Surfactant concentration is important in affecting dynamic surface tension but does not seem to play a significant role in the reduction of equilibrium surface tensions.