A monolayer is a thin layer of atoms, molecules or particles that is only one atom, molecule or particle thick. A Langmuir trough and a force-based tensiometer can be used to measure the properties of the monolayer. Monolayers are of fundamental importance in many industrial sectors.
What are monolayers?
A monolayer is an ultra-thin layer of atoms, molecules or particles deposited on another surface. This layer is only as thick as a single atom, molecule or particle. A characteristic feature of monolayers is their ability to self-organise. The molecules on the surface interact with each other and with the surface itself. These interactions lead to the formation of regular lattice structures. The type of interactions and the geometry of the molecules determine the type of lattice structure.
The properties of monolayers often differ greatly from those of the corresponding bulk material, i.e. the three-dimensional solid. Quantum mechanical effects are more pronounced in a monolayer, and electrical and optical properties change significantly.
How can monolayers be examined?
A Langmuir trough and a force-based tensiometer, such as those in the DCAT series from DataPhysics Instruments, enable detailed investigation of monolayers. The Langmuir trough method is used to determine the surface pressure of a molecular layer on a water surface. For the measurement, a trough is filled with water. The monolayer is applied to the water surface. The trough is bounded on the right and left by movable barriers that allow the monolayer to be compressed. A Wilhelmy plate is immersed vertically between these barriers.
A Wilhelmy plate is generally used to measure the surface tension precisely. This is a small, rectangular plate made of a platinum-iridium alloy that is immersed vertically in the water surface. The Wilhelmy plate is connected to a high-precision balance to determine the force resulting from the liquid film adhering to the plate. Changes in this force caused by the molecular arrangement are recorded by the tensiometer and converted into surface pressure. This surface pressure describes how much the molecules on the surface reduce the surface tension of the water.
At the same time, the Langmuir trough has movable barriers that can be used to compress the monolayer on the surface in a targeted manner. During this compression, the surface pressure is measured continuously. In this way, a characteristic diagram can be created depending on the available area per molecule: the pressure-area isotherm. This diagram shows how the surface pressure changes as the molecules transition from a widely spaced arrangement to a more densely packed, liquid-ordered phase to a solid-ordered, compressed structure.
The measurement method thus allows quantitative analysis of the properties of monolayers. The shape and transitions of the isotherm allow conclusions to be drawn about the molecular size, packing density, interactions and stability. It is crucial that the method does not make the molecules directly visible, but rather reveals their arrangement and behaviour indirectly by precisely measuring the surface pressure.