Colloids in Flatland: a perspective on 2D phase-separated systems, characterisation methods, and lineactant design

Abstract

In 1861 Thomas Graham gave birth to a new field of science, today known as colloid science. Nowadays, the notion "colloid" is often used referring to systems consisting of two immiscible phases, one of which is finely dispersed into the other. Research on colloids deals mostly with sols (solids dispersed in a liquid), emulsions (liquids dispersed in liquid), and foams (gas dispersed in a liquid). Because the dispersed particles are small, there is a lot of interface per unit mass. Not surprisingly, therefore, the properties of the interface have often a decisive effect on the behaviour of colloids. Water-air interfaces have a special relevance in this field: many water-insoluble molecules can be spread on water and, given the right spreading conditions and enough available surface area, their spreading proceeds until a monolayer (a one-molecule thick layer) eventually remains. Several 2D phases have been identified for such monolayers, like "gas", "liquid expanded", "liquid condensed", and "solid". The central question of this review is whether these 2D phases can also exist as colloidal systems, and what stabilizes the dispersed state in such systems. We shall present several systems capable of yielding 2D phase separation, from those based on either natural or fluorinated amphiphiles, to polymer-based ones. We shall seek for analogies in 3D and we shall try to clarify if the lines between these 2D objects play a similar role as the interfaces between 3D colloidal systems. In particular, we shall consider the special role of molecules that tend to accumulate at the phase boundaries, that is, at the contact lines, which will therefore be denoted "line-actants" (molecules that adsorb at a 1D interface, separating two 2D colloidal entities), by analogy to the term "surfactant" (which indicates a molecule that adsorbs at a 2D interface separating two 3D colloidal entities).

Reactions Coupled Self‐ and Co‐Assembly: A Highly Dynamic Process and the Resultant Spatially Inhomogeneous Structure

“…Because the internal interactions such as aurophilic interactions, and the inter‐ligand interactions, were broken during the rupture, they were not as stable as the fully assembled ones, so the ruptured assemblies can reassemble with the remaining strings to regain the intermolecular aurophilic interactions and inter‐ligand interactions, leading to the spatially inhomogeneous structure to disperse the internal stress. And the spatially inhomogeneous, micro‐phase separated co‐assemblies are very common in life system as part of its complexity, however, they are very difficult to achieve in a one‐step process with monomers as starting materials in artificial systems . Although we can still not demonstrate the functions of such structure in the current system so far, it's true that phase structure is highly important for the performances of high performance polymer and functional polymer …”

“…And the spatially inhomogeneous, microphase separated co-assemblies are very common in life system as part of its complexity, [50] however, they are very difficult to achieve in ao ne-step process with monomersa ss tarting materials in artificial systems. [51] Although we can still not demonstrate the functions of such structure in the current system so far,i t's true that phases tructure is highly important for the performances of high performance polymer and functional polymer. [52,53] What's more, the highlyd ynamic assembly system can also lead to different final structures by changing the pH values duringt he assembly.W hen the pH values of Au I -3MPA-co-Cys were changed from 4.25 to 6.17 after it had assembled for 1min at pH 5.10, the absorption peak positions of the assemblies change from 386 nm to 368 nm ( Figure S10), indicating the dynamic change of its structures.…”

Line tension and its influence on droplets and particles at surfaces

“…Lineactants are the 1D analog of surfactants; surfactants preferentially adsorb at the surface between two bulk phases and decrease the associated surface tension. Lineactants have primarily been studied at the perimeter of 2D monolayer structures at surfaces [157][158][159] (Fig. 1e) which possess a "2D line tension".…”

Dramatic Shape Modulation of Surfactant/Diacetylene Microstructures at the Air–Water Interface

“…However, assembly and controlled modulation of condensed domains within Langmuir monolayers, which could be further transferred from the water surface onto solid supports, are still considered a challenging task. Multi‐component monolayers, in particular, have been shown to promote formation of organized condensed domains through phase segregation between the two (or more) amphiphilic molecules comprising the monolayers 19–21…”

“…Multi-component monolayers, in particular, have been shown to promote formation of organized condensed domains through phase segregation between the two (or more) amphiphilic molecules comprising the monolayers. [19][20][21] We demonstrate the assembly of remarkably diverse condensed microstructures in binary Langmuir monolayers comprising a surfactant molecule, octadecylmelamine (OM), and a diacetylene monomer, N-(2-aminoethyl)tricosa-10,12-diynamide (TEA). OM has been previously shown to display interesting self-assembled structures at the air-water interface.…”