The Growth of OFM SAMs by Ellipsometry

Ben Fry*, Hugh Spikes, Janet Wong

Tribology Group, Department of Mechanical Engineering, Imperial College, London SW7 2AZ, UK

INTRODUCTION: Organic Friction Modifiers (OFMs) are additives which change the friction of a lubricated contact in the mixed and boundary lubrication regimes. They alter the friction by adsorbing their polar head group onto the surface with the hydrocarbon tail group protruding out towards the solution forming Self Assembled Monolayers (SAMs), as shown in Figure 1. SAMs can alter the physical properties of the surface such as wettability and friction.

Figure 1 – A schematic showing the SAM formed by OFM additives on a surface. Blue circles represent polar head groups of OFMs.

Knowledge of the formation of these SAMs can lead to better understanding of the mechanism of friction reduction of these OFM additives. The adsorption properties for OFMs onto metals have been studied in situ using various techniques such as a quartz crystal microbalance (QCM) ¹ and sum frequency generation (SFG) ². QCM was used to measure the change in mass on the surface of the quartz crystal when OFM solution is added. SFG has been used to show the conformational difference in the film formed by molecules with differing levels of unsaturation.

Ellipsometry is a non-invasive laser based technique for measuring film thickness and other optical properties of a surface. This technique is regularly used for measuring SAMs on silicon or gold surfaces from aqueous solutions^3,4 and has been used to measure the thickness of inorganic films (such as MoS₂ ⁵) and ZDDP tribofilms⁶. Ellipsometry has still not been used for monolayer growth from an oil based solution because it is thought that the refractive index difference between the oil (1.43) and the monolayer (1.45-1.55) ⁷ would be too small for the growth to be observed. 

The ability for ellipsometry to measure multiple layer thicknesses simultaneously will increase our understanding of additive interactions on tribofilms. Measuring OFM film formation is a step towards being able to study this.

In this work, in situ ellipsometry will be used to investigate the growth of an OFM SAM from an oil based system. This will show whether ellipsometry can be used accurately to measure OFM SAMs in oil based solutions. The results will be correlated with other in situ techniques such as QCM and AFM.

METHODS:  Spectroscopic ellipsometry⁸ will be used to observe the growth of OFM SAMs from hexadecane on different surfaces including silicon and iron oxide. For this an Accurion nanofilm EP3 imaging ellipsometer⁹ with a solid-liquid cell attachment will be used to measure Ψ and Δ by nulling ellipsometry. These 2 values are then fit into a model to determine the film thickness. The results will be confirmed by measuring the adsorption by QCM-D and other surface sensitive techniques.

Figure 2 – The change in Delta Δ after the addition of 1 mM octadecylamine in hexadecane at t = 0 to a solution of hexadecane on silicon.

RESULTS & DISCUSSION: A baseline is taken in pure hexadecane and once the octadecyalmine solution is added (t = 0), Δ drops, as shown in Figure 2. The decrease in delta with minimal change in Ψ shows a growth of a thin film on the surface⁸. The surface can then be modeled presuming a homogenous layer is growing on the surface to give a thickness to the layer grown. The final height of the SAM is estimated to be 1.0 - 1.25 nm, suggesting a tilt angle of the molecules to be 60 - 67° from normal.

This results from 1 mM is in agreement with previous AFM results of octadecylamine on mica ¹⁰ which shows the formation of islands with a height of 0.9 to 1.4 nm.

This shows that Ellipsometry can be used for measuring the growth of the monolayer from hexadecane solutions despite the refractive index values being so close.

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