Adsorption and Crystallization of Particles at the Air–Water Interface Induced by Minute Amounts of Surfactant

Abstract

Controlling the organization of particles at liquid–gas interfaces usually relies on multiphasic preparations and external applied forces. Here, we show that micromolar amounts of a conventional cationic surfactant induce, in a single step, both adsorption and crystallization of various types of nanometer- to micrometer-sized anionic particles at the air–water interface, without any additional phase involved or external forces other than gravity. Contrary to conventional surfactant-induced particle adsorption through neutralization and hydrophobization at a surfactant concentration close to the critical micellar concentration (CMC), we show that in our explored concentration regime (CMC/1000-CMC/100), particles adsorb with a low contact angle and maintain most of their charge, leading to the formation of two-dimensional assemblies with different structures, depending on surfactant (C s) and particle (C p) concentrations. At low C s and C p, particles are repulsive and form disordered assemblies. Increasing C p in this regime increases the number of adsorbed particles, leading to the formation of mm-sized, highly ordered polycrystalline assemblies because of the long-range attraction mediated by the collective deformation of the interface. Increasing C s decreases the particle repulsion and therefore the interparticle distance within the monocrystalline domains. A further increase in C s (≈CMC/10) leads to a progressive neutralization of particles accompanied by the formation of disordered structures, ranging from densely packed amorphous ones to loosely packed gels. These results emphasize a new role of the surfactant to mediate both adsorption and crystallization of particles at liquid–gas interfaces and provide a practical manner to prepare two-dimensional ordered colloidal assemblies in a remarkably robust and convenient manner.

Collective Transport of Magnetic Microparticles at a Fluid Interface through Dynamic Self‐Assembled Lattices

“…After 60 min, the small vessel is brought to its initial position, where the aqueous suspension is the bottom phase and the monolayer of adsorbed particles can be easily visualized with a straight microscope (the method is described in detail in refs. [76,77]). The main advantage of this second method is that it does not require of the application of magnetic field gradients, which may promote the formation of permanent aggregates.…”

Photoswitchable Dissipative Two‐Dimensional Colloidal Crystals

“…We recently showed that micromolar amounts of conventional cationic surfactants (for example, dodecyltrimethylammonium bromide, DTAB) induced the adsorption of anionic particles by decreasing the adsorption barrier at the air/water interface . At such low surfactant concentrations (≈CMC/1000–CMC/100, where CMC is the critical micelle concentration), the particles adsorbed with a low contact angle (≈30°) and remained highly charged, forming disordered assemblies or polycrystalline patches in a particle‐ and surfactant‐concentration‐dependent manner.…”

“…Note that 10 μ m of AzoTAB is about three orders of magnitude lower than its CMC (12.6 m m and 14.6 m m for the trans and cis isomers, respectively), resulting in a surface tension comparable to that of pure water, regardless of irradiation conditions (Supporting Information, Figure S1). The particles were brought to the air/water interface of a suspension in a cylindrical cell by flipping it up and down using our previously described protocol (Figure C, top panel). We found that, similarly to the case of non‐photosensitive surfactants, almost all particles adsorbed at the air/water interface and accumulated at the center of the cylindrical well containing the sample due to the slightly concave shape of the meniscus.…”

Photoswitchable Dissipative Two‐Dimensional Colloidal Crystals

“…We recently showed that micromolar amounts of conventional cationic surfactants (for example,dodecyltrimethylammonium bromide,DTAB) induced the adsorption of anionic particles by decreasing the adsorption barrier at the air/water interface. [30] At such low surfactant concentrations (% CMC/ 1000-CMC/100, where CMC is the critical micelle concentration), the particles adsorbed with al ow contact angle ( % 308 8)a nd remained highly charged, forming disordered assemblies or polycrystalline patches in ap article-and surfactant-concentration-dependent manner.H ere,t o explore the possibility to dynamically switch particle assemblies at the air/water interface at constant composition, we used anionic polystyrene particles (diameter 5.1 mm) at af ixed concentration (0.01 mg mL À1 )a nd 10 mm of the AzoTAB photosensitive surfactant [31][32][33] (Figure 1A). AzoTAB is ac ationic surfactant with an azobenzene moiety in its hydrophobic tail, which can isomerize from trans to cis upon suitable light irradiation ( Figure 1B).…”

“…Note that 10 mm of AzoTAB is about three orders of magnitude lower than its CMC (12.6 mm and 14.6 mm for the trans and cis isomers, respectively [34] ), resulting in as urface tension comparable to that of pure water, regardless of irradiation conditions (Supporting Information, Figure S1). Thep articles were brought to the air/water interface of as uspension in ac ylindrical cell by flipping it up and down using our previously described protocol [30] ( Figure 1C,t op panel). We found that, similarly to the case of non-photosensitive surfactants,a lmost all particles adsorbed at the air/water interface and accumulated at the center of the cylindrical well containing the sample due to the slightly concave shape of the meniscus.I nt he absence of light irradiation, the particles formed adisordered structure containing voids ( Figure 1C-i).…”