Structure of surfactants adsorbed on solid surfaces is a key knowledge in various technologies and applications. It is widely accepted in the literature that the surface− surfactant headgroup electrostatic interaction is a major driving force of adsorption of ionic surfactants on charged substrates. Our result shows that the adsorption of surfactants as monomers is driven by both electrostatic and nonelectrostatic interactions. Further adsorption of surfactants in aggregates is essentially driven by the tail−tail interaction. To a great extent, the substrate−tail interaction determines the structures of the adsorbed surfactant aggregates. Water and counterions influence the headgroup−substrate and tail−substrate interactions. We investigate two vastly different surfactants and substrates by molecular dynamics simulations: (1) SDS on alumina (SDS− Al 2 O 3 ), and (2) CTAB on silica (CTAB−SiO 2 ). We study the adsorption of a single surfactant at the solid surface by the density profiles and free energy of adsorption. In the SDS−Al 2 O 3 system, we analyze the free energy of adsorption on the substrate covered by aggregates of different sizes. We examine the configurations of surfactants and the distribution of water and ions at the liquid−solid interface as the number of adsorbed molecules on the substrate increases. In the SDS−Al 2 O 3 system, the headgroup adsorption is mediated by the Na + counterions; the adsorbed water molecules may be displaced by the surfactant headgroup but unlikely by the hydrocarbon tails. As a function of the surfactant adsorption, we observe single surfactants, aggregates of different morphologies, and bilayers. The CTAB−SiO 2 system combines both electrostatic attraction of the surfactant headgroup and affinity for the surfactant's hydrocarbon tail. At low surfactant adsorption, aggregates and single surfactant molecules lie on the substrate; hemimicelles form at intermediate adsorption; and micelles form at high surfactant adsorption. Our results agree with experimental observations and indicate two different surfactant adsorption mechanisms where the tail−tail and tail−substrate interactions play a fundamental role.
We use molecular dynamics simulations and molecular thermodynamics to investigate the formation of reverse micelles in a system of surfactants and nonpolar solvents. Since the early observation of reverse micelles, the question has been whether the existence of polar solvent molecules such as water is the driving force for the formation of reverse micelles in nonpolar solvents. In this work, we use a simple coarse-grained model of surfactants and solvents to show that a small number of polar solvent molecules triggers the formation of large permanent aggregates. In the absence of polar molecules, both the thermodynamic model and molecular simulations show that small aggregates are more populated in the solution and larger ones are less frequent as the system evolves over time. The size and shape of reverse micelles depend on the size of the polar core: the shape is spherical for a large core and ellipsoidal for a smaller one. Using the coarse-grained model, we also investigate the effect of temperature and surfactant tail length. Our results reveal that the number of surfactant molecules in the micelle decreases as the temperature increases, but the average diameter does not change because the size of the polar core remains invariant. A reverse micelle with small polar core attracts fewer surfactants when the tail is long. The uptake of solvent particles by a micelle of longer surfactant tail is less than shorter ones when the polar solvent particles are initially distributed randomly.
Temperature affects the aggregation of macromolecules such as surfactants, polymers, and proteins in aqueous solutions. The effect on the critical micelle concentration (CMC) is often nonmonotonic. In this work, the effect of temperature on the micellization of ionic and nonionic surfactants in aqueous solutions is studied using a molecular thermodynamic model. Previous studies based on this technique have predicted monotonic behavior for ionic surfactants. Our investigation shows that the choice of tail transfer energy to describe the hydrophobic effect between the surfactant tails and the polar solvent molecules plays a key role in the predicted CMC. We modify the tail transfer energy by taking into account the effect of the surfactant head on the neighboring methylene group. The modification improves the description of the CMC and the predicted micellar size for aqueous solutions of sodium n-alkyl sulfate, dodecyl trimethylammonium bromide (DTAB), and n-alkyl polyoxyethylene. The new tail transfer energy describes the nonmonotonic behavior of CMC versus temperature. In the DTAB-water system, we redefine the head size by including the methylene group, next to the nitrogen, in the head. The change in the head size along with our modified tail transfer energy improves the CMC and aggregation size prediction significantly. Tail transfer is a dominant energy contribution in micellar and microemulsion systems. It also promotes the adsorption of surfactants at fluid-fluid interfaces and affects the formation of adsorbed layer at fluid-solid interfaces. Our proposed modifications have direct applications in the thermodynamic modeling of the effect of temperature on molecular aggregation, both in the bulk and at the interfaces.
Efficient mixing, typically characterised by chaotic advection, is hard to achieve in low Reynolds number conditions because of the linear nature of the Stokes equation that governs the motion. Here we show that low Reynolds number swimmers moving in quasi-periodic orbits can result in considerable stretching and folding of fluid elements. We accurately follow packets of tracers within the fluid domain and show that their trajectories become chaotic as the swimmer’s trajectory densely fills its invariant torus. The mixing process is demonstrated in two dimensions using the Quadroar swimmer that autonomously propels and tumbles along quasi-periodic orbits with multi-loop turning trajectories. We demonstrate and discuss that the streamlines of the flow induced by the Quadroar closely resemble the oscillatory flow field of the green alga Chlamydomonas reinhardtii. Our findings can thus be utilized to understand the interactions of microorganisms with their environments, and to design autonomous robotic mixers that can sweep and mix an entire volume of complex geometry containers.
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