What is a Langmuir monolayer?

So a while back, I wrote a quick piece describing surface tension. I had hoped to develop that further in future blog posts, and I was partly busy with other things – but mostly I forgot. So today I’d like to carry on from that article and explain what a Langmuir monolayer is. Also I’ll hopefully shed a little light on a great forgotten experimentalist.

Avogadro’s constant

As many of you may have heard of, there is a constant called Avogadro’s constant – which is the number of individual molecules in 12g of carbon…. Yeah, that definition helped no-one. Okay, for various reasons it’s important to chemists (and physicists) that it’s possible to have a unified unit system for the elements. That unified system is to say that a mole of a material is the amount of material that contains 6.02e+23 molecules. So carbon 1 mole = 12g but for silicon 1 mole = 28g because silicon is a much bigger molecule.

This concept was first described in 1811 by  Amedeo Avogadro. Which, while helping to revolutionise our understanding of reactions and chemistry, was also a little bit before the invention of a microscope capable of counting individual molecules (squinting really hard wasn’t developed till the early 1900s). So while the –concept?– had been proven on paper no-one had yet devised an experiment to prove it.  Thankfully, about 50 years later Benjamin Franklin dropped some oil into a pond and provided science with a solution.

Franklin & Rayleigh

Inspired by some notes of Pliney the Elder, Benjamin Franklin was fascinated by the effect of oil on the surface of water (I think he was also interested in a few other things, but they are unimportant). Mr Franklin described in a paper, how a drop of oil appeared to spread out evenly over a pond until it had formed a covering the size of which depended on the amount of oil added. He was so amazed by this effect he allegedly carried small samples of oil with him whenever he went on walks so that he could test this effect in every pond/water system he could find.

What Franklin was observing was that the molecules of the oil were spreading out over the water to form a monolayer. Oil molecules are what is know as amphillic – which is to say that they are hydrophobic (hates water) at one end and hydrophilic (has a water fetish) at the other. When the oil is spread on the water the molecules orientate themselves so that the hydrophobic end is as far from the water as possible.

However, Franklin didn’t know this at the time and it wasn’t until a man called Lord Rayleigh came along in the late 1800s and realised that the oil was spreading out to form a single layer of molecules (a monolayer).

Lord Rayleigh reasoned that by knowing the size of the molecule he was using (which he estimated based on other work) and the amount added, then he could calculate the exact number of molecules of oil in a single drop, if he measured the area that the drop spread to cover. By applying this method he found the calculated amount closely matched the theoretical calculations of Amedeo Avogadro, which experimentally proved Avogadro’s constant.

However, the biggest revolution in our understanding of monolayers came not from men with awesome moustaches (photo of Lord Rayleigh) but from a highly intelligent woman doing the washing up. I should be clear I’m not being sexist-ly flippant, the washing up really was a critical component (allegedly).

Pockels’ monolayers

Agnes Luise Wilhelmine Pockels 1862 – 1932 (thanks wikipedia) is now known as one of the great experimentalists of the late 1800s and early 1900s. As a woman in the late 1800s, she had little to no access to education beyond a basic level and despite a great childhood love of physics, had no prospects of attending University or creating a life of science. What she did have however was a sibling (Friedrich) who was equally passionate about science and somewhat crucially, didn’t have any University disqualifying ovaries. Through Friedrich, Agnes Pockels continued to read and learn all she could about physics, while also acting as carer to her parents.

While acting as a carer, Pockels began to do what I would term ‘kitchen science’ and experimented with materials she had to hand around the home. There is a fanciful idea that Pockels developed her ideas while staring at her washing up. She never said this, but it does make a nicer story than ‘she spent years reading the scientific literature’.

One area that was of particular interest to her were the experiments of Lord Rayleigh which, given her ready access to both water and a range of oils in her kitchen, was an ideal area .

It was during these experiments that she devised two very important methods. First using a button, some string & a pivot balance she developed a method of measuring the surface tension (see previous post) of a liquid. This homemade experimental setup then led on to her discovering that the films that Lord Rayleigh saw, weren’t just floating on the surface of the water but were interacting with it and altering the surface tension.

She also discovered that these films of oil could be manipulated by ‘skimming’ the surface of the water with barriers. This ‘skimming’ allowed for the compression and expansion of the oil monolayer materials and for the first time, allowed Agnes Pockels to demonstrate that a monolayer of molecules could be controlled for study in a basic homemade water trough (a small tin).

With the help of Lord Rayleigh, whom Anges Pockels had contacted about her work, Anges Pockels’ discoveries were published in Nature in around 1891 with the title “Surface Tension”.

Pockels’ Langmuir monololayers

By the end of the 1800s, the exploratory work by Franklin, Rayleigh and Pockels had set in place all the tools and ground work required to understand floating monolayers. It’s at this point that (with impeccable timing) Irving Langmuir became interested in the field.

With the funding and resources of the mighty General Electric Corporations (GE) Irving Langmuir set about building a larger and more advanced trough based on Pockels’ published designs. This trough allowed for finer control and manipulation of the films, and he used it to demonstrate firstly, that the films were true monololayers (previously it had been difficult to get accurate measurements) and that they had the hydrophobic-hydrophillic orientation. These experiments earned him a Nobel Prize in 1932.

Irving Langmuir applied this refined setup to a huge range of problems, and produced a huge wealth of new research – vastly expanding our knowledge of chemical reactions and electrical properties. Through these experiments, Irving Langmuir typified a world of chemistry previously unobservable, and allowed scientists to manipulate nanometer thick films of material – and for the first time in the laboratory, study thin biological structure such as cell membranes.