Auto-Oscillation of Surface Tension: Effect of pH on Fatty Acid Systems

Abstract

Spontaneous nonlinear oscillations of surface tension produced by transfer of either octanoic or nonanoic acids from a droplet situated in the bulk water to the air/water interface are studied experimentally. It is shown that the oscillation amplitude decreases significantly with the increase of pH of aqueous phase. At pH > 6.5, detectable oscillations for the two fatty acids studied do not exist. The results are discussed in terms of the mechanism proposed recently for spontaneous oscillations produced by transfer of nonionic surfactants.

Ionic Strength and pH as Control Parameters for Spontaneous Surface Oscillations

“…The Frumkin isotherm, which is more appropriate for describing the adsorption of fatty acids, was not used here because consideration of any exponential term is very timeconsuming for numerical simulations. The surfactant mixture is considered to be ideal, 18 and it is supposed that the value of the saturation adsorption Γ m is the same for both surfactants, which is a reasonable assumption according to the results obtained in ref 16. In this case, the dimensionless form of the adsorption isotherms is…”

“…The goal of this study is to find such a formulation that enables one to obtain oscillations with detectable amplitude at high enough pH, where the oscillations have small amplitude or are nondetectable if the only additive present in the solvent is the buffer. 16 The two parameters considerably affecting the oscillation amplitude are surfactant solubility and surface activity. The addition of salt changes both of these parameters.…”

“…The main reason for the decrease of the oscillation amplitude is the transition of fatty acid from the more surface active protonated form to the much less surface active dissociated form. 16 For the confirmation of this explanation, an inorganic salt is added to increase the surface activity of the dissociated form of fatty acid, which should result in restoring the oscillatory dynamics with a high enough amplitude at high pH. Correspondingly, the model proposed in refs 13−15 was modified to take into account (i) the difference in surface activity of the dissociated and protonated forms of the used fatty acid (octanoic acid), as well as the dependence of the surface activity of the dissociated form on the ionic strength in solution, and (ii) the dependence of the local dissociation degree of the acid on its local concentration.…”

Maze solving using temperature-induced Marangoni flow

“…23 This is attributed to the fact that pH can Please do not adjust margins Please do not adjust margins change the surface tension of the solution of fatty acid in a more pronounced way than the temperature can. 31 Another effect that needs to be taken into account arises from the fact that a horizontal temperature gradient between the entrance and the exit of the maze induces the Rayleigh-Bénard convection 32 (near the cooled down metallic sphere the fluid moves downwards and continues its motion at the bottom of the channel towards the region of higher temperature, and due to inertia the fluid moves in opposite direction near the liquid-air interface, i.e., towards the lower temperature region). In effect, the flow responsible for maze solving is driven, in general, by both the Marangoni flow and the Rayleigh-Bénard convective cell established in the maze.…”

Effects of Surfactants and Electrolytes on Chemical Oscillation at a Water/Nitrobenzene Interface Investigated by Quasi-elastic Laser Scattering Method

“…This process is referred to as chemical oscillation or chemical excitation. Kovalchuk et al [21][22][23][24][25][26][27][28][29][30][31][32] showed that the dynamics at an air/water or a water/ organic interface affect the heterogeneity of the interfacial tension caused by the addition of an alcoholic surfactant through a capillary. Using time-resolved quasi-elastic laser scattering (QELS) measurements, [33][34][35][36][37][38][39][40][41][42][43] Ikezoe et al [33][34][35] found that surfactant molecules are rapidly adsorbed and gradually desorbed when an aqueous surfactant solution is introduced to the interface between aqueous and organic phases with a capillary.…”