Fatty acids have been investigated as boundary structures to construct artificial cells due to their dynamic properties and phase transitions. Here we have explored the possibility that fatty acid systems also demonstrate movement. An oil phase was loaded with a fatty acid anhydride precursor and introduced to an aqueous fatty acid micelle solution. The oil droplets showed autonomous, sustained movement through the aqueous media. Internal convection created a positive feedback loop, and the movement of the oil droplet was sustained as convection drove fresh precursor to the surface to become hydrolyzed. As the system progressed, more surfactant was produced and some of the oil droplets transformed into supramolecular aggregates resembling multilamellar vesicles. The oil droplets also moved directionally within chemical gradients and exhibited a type of chemotaxis.

https://pubs.acs.org/doi/pdf/10.1021/ja0706955

Fatty acid chemistry at the oil-water interface: self-propelled oil droplets.

Marangoni instability and spontaneous non-linear oscillations produced at liquid interfaces by surfactant transfer.

Periodic Marangoni convective instability has been observed in a biphasic system during the mass transfer of cetyltrimethylammonium bromide (CTAB) from an aqueous to a dichloromethane organic phase. Visualization of the convective fluxes was possible thanks to the CTAB crystals that are formed in the aqueous phase at a temperature below the Krafft point. Surface tension and electrical potential oscillations have been shown to be correlated with the fluid motion. Surface tension measurements, representative of the adsorption state, showed fast adsorption during the convective stage, followed by a slower desorption process in the quiet stage. To account for the electrical potential data, two components need to be taken into account. In the quiet stage, the signal was comparable to surface tension, and the main contribution would result from the electrical double layer formed at the interface by charged surfactants. In the convective stage, the electrical potential was furthermore related to the velocity of the fluid in the aqueous layer. Perturbations of the charge distribution in the Gouy-Chapman layer due to tangential flows could be at the origin of the phenomenon.

Periodic marangoni instability in surfactant (CTAB) liquid/liquid mass transfer.

A combined study devoted to chemo-Marangoni convection and the underlying kinetics is presented for a biphasic system in which surfactants are produced in situ by an interfacial reaction. The pattern formation studied in a Hele-Shaw cell in both microgravity and terrestrial environments initially shows an ensemble of chemo-Marangoni cells along a nearly planar interface. Soon, a crossover occurs to periodic large-scale interfacial deformations which coexist with the Marangoni cells. This crossover can be correlated with the autocatalytic nature of the interfacial reaction identified in the kinetic studies. The drastic increase in the product concentration is associated with an enhanced aggregate-assisted transfer after the critical micellar concentration is approached. In this context, it was possible to conclusively explain the changes in the periodicity of the interfacial deformations depending on the reactant concentration ratio.

Chemo-Marangoni convection driven by an interfacial reaction: pattern formation and kinetics.

The results of theoretical and experimental studies of spontaneous nonlinear oscillations produced at the liquid/liquid interface by surfactant transfer from a point source situated in one of the bulk phases are presented. The theoretical analysis is based on the direct numerical simulation of the system evolution. The experiments are performed for the heptane/water interface using middle-chain aliphatic alcohols as surfactants. The results for the oil/water interface are compared with the corresponding data obtained for the air/water interface. The presented results allow the conclusion that auto-oscillations at the air/liquid and liquid/liquid interfaces are governed by very similar mechanisms but their characteristics are strongly dependent on the properties of the two contacting media, in particular, on the surfactant partition coefficient.

Nonlinear spontaneous oscillations at the liquid/liquid interface produced by surfactant dissolution in the bulk phase.

Oscillations of the electric potential and of the interfacial tension were simultaneously recorded in a water−oil−water system (cetyltrimethylammonium bromide (CTAB) in water/picric acid in dichloromethane/water). Both types of oscillations were perfectly correlated in time and shape. In each oscillatory cycle, the fast jumps in the electric potential are related to a rapid adsorption of the surfactant at the interface while the slow motion relaxations correspond to the liberation of the interface from the surfactant by the formation of hydrophobic cetyltrimethylammonium picrate ion pairs that diffuse into the CH2Cl2 phase. The fast inflow of CTAB to the interface is probably caused by periodically occurring convective Marangoni instabilities. The transitions to the convective regime are assumed to be controlled by the fractional surfactant coverage of the interface.

Jean-Claude Micheau†

Papers

Correlation between Electric Potential and Interfacial Tension Oscillations in a Water−Oil−Water System