We’re going with the ‘talking to yourself’ gimmick again?
Yup, it had a pretty luke-warm reception last time but I’m optimistic that despite not changing anything, people will love it. Besides, I couldn’t persuade anyone to interview me.
*sigh* Okay, so what are we/I here to talk about today?
I had a paper published!
How nice, do you want a cookie?
Hey, I’m still a relatively young researcher – this is a big deal. I’ve only got 5 papers and I’m allowed to be naively excited when a new one comes out!
Okay, fine, I’ll play along. What’s your paper about?
Okay, so during my PhD, I worked with covering fibre optics with sensor materials. This paper is based on some of that work and focuses on a technique I trialled to improve my ability to coat/cover my sensor with material.
Why, what was wrong with the other way of doing it? Surely it’s either covered in stuff or it’s not.
Well no, there’s a big difference between a pizza with a handful of cheese in one corner and a pizza with a nice even covering of mozzarella and brie (it’s delicious try it!).
The problem with the old technique is one of packing. The method of coating is called Langmuir-Blodgette (which I’ve covered in more detail before) and is, simplistically, based on squeezing molecules together in a floating layer (a monolayer) and then transferring them to a surface, by moving that surface through the layer. However, the problem with this technique is that the packing/density of the squeezed monolayer greatly affects how well it sticks to the surface you’re coating.
Err… okay. So as I was saying – the density is vital. If your monolayer is too loosely packed then it won’t coat the surface at all; and if it’s too compressed, then it becomes unstable and instead of coating, it simply buckles and becomes a messy complicated multilayer (translation: a random pile) which is no good for thin coatings.
Surely you can just compress the monolayer to the right density and use that?
Well yes, and no. Firstly, with some materials that density is a very small range, secondly that density might be great for coating but might be terrible for using those molecules for sensing. Being forced to only use one narrow range is very limiting on the way we can shape and design sensors for certain applications.
So what was your ‘amazing’ solution, oh Great One?
I’ll ignore that. You know how I said that these were floating monolayers? Well what they are floating on is pretty important. One of the factors that affects the lowest density at which the monolayer will coat a surface is it’s ‘attachment’ to the liquid. The attachment I’m talking about is the hydrogen bonds it forms with the water molecules. For coating, the more packed the material, the less favourable the hydrogen bonds are and thus the easier the material can separate from the surface.
Really, that seems conveniently simple?
Well… err… maybe I glossed over a few things. The maths and energy kinetics governing all this are more than a bit complicated and this is one of many forces involved – but at the moment it’s the only one we’re interested in.
Weren’t you telling us your great idea? This all sounds like more background!
I am getting to that! I wanted to set the scene – you know, add atmosphere and suspense…
So if the density is, at least in part, about overcoming the strength of the hydrogen bonding of the water, then I reasoned that by lowering the hydrogen bonding potential of the water in the first place, would lower the density requirement.
Water molecules form between 3 and 4 hydrogen bonds, either to surrounding water molecules or at the surface to anything floating on it – like a monolayer. My idea was to replace the water with something that formed less hydrogen bonds. The reasoning was the less hydrogen bonds there are then less energy is required to break free of their grip.
Great, so you replaced the water with something with no hydrogen bonds and called it a day?
Well no, that wouldn’t help either as the monolayer only forms because of the hydrogen bonding – so we still need it. Just less of it. My solution to this was alcohol.
You got drunk?
No, I got the water drunk.
Alcohol molecules come in many shapes and sizes but all have one thing in common : an -OH group connected to a carbon chain. In the case of simple alcohols (ethanol, methanol etc) the length of the carbon chain directly affects their ability to hydrogen bond.
So what I realised is that if you add progressively longer chain alcohols to the water, this should lower the overall potential of the water to hydrogen bond making it easier for the floating layer to lift off.
I’m assuming that this worked, otherwise the last 800 words have been a bit of a waste of time?
Yes! I showed that by adding these alcohols the surface pressure (a measure of density) required to lift the floating layer off dropped in line with the reduction of hydrogen bonding.
While it’s not included in the paper I did also show that if you add hydrogen peroxide (which has 4-5 hydrogen bonds) it went the other way and the density required increased!
And the coating?
Same thing happened. In the coating I only compared water to water+ethanol because coating experiments take ages as you need to run them with several different conditions.
No, just pragmatic. To back up my data for the coating experiment I ran it in several different conditions and on a sensor surface to make sure I could clearly show differences in coating. As this was the critical proof of my theory I wanted to make sure I had more than one data source.
And all the coating experiments showed that the material was being released from the monolayer at a lower surface pressure (measure of density) just as predicted by my hand-wavy theory.
‘Your’ theory? Aren’t there like, 7 authors on that paper?
Fair point… Okay so in the order they appear on the paper, here are the author contributions.
- Me – All the lab work presented in the paper; most of the data analysis; and writing of the draft paper
- Rebecca Wong – A fellow PhD student who made the fibre optic sensors I used to test the theory
- Mike Collins – My industrial sponsor but more importantly, listened to me rant in the pub one evening about hydrogen bonding and helped to develop the theory with beer and napkins.
- Stephen James – Functional PhD supervisor and person I went to anytime my data started giving me a headache. Also fibre optics guru who helped design the fibre testing aspect of the paper.
- Frank Davis – Synthetic chemist who gave me the floating materials
- Ralph Tatam – Department head, owns the labs I use (and possibly my soul…)
- Seamus Higson – Actual PhD supervisor
That’s refreshingly upfront. Also, oddly passive aggressive at times…
Yup, I think so. Hopefully now I’ve explained it, the paper will now make much more sense to anyone who might have an interest in this area. And thanks to some twitter awesomeness you can all download a version of this paper for free at the link below.
Modifying Monolayer Behaviour by Incorporating Subphase Additives and Improving Langmuir Blodgett Thin Film Deposition on Optical Fibres
Buckets full of oil measured by fibre made with a magic laser · 1 October 2014 at 13:00
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