InFo - Decemeber 2011
Unravelling the mysteries of formulated products using atomic force microscopy
Chris Hodges and Simon Lawson*
Over the last 20 years the Atomic Force Microscope (AFM) has emerged from being a specialist laboratory tool into a general surface characterisation apparatus. Industries such as the silicon chip manufacturing sector use AFM as a routine quality control technique. This has led to an improvement in reliability since small faults are more easily discovered and adjustments in the manufacturing process made earlier, saving costs and improving customer satisfaction.
AFM concept
The AFM concept is a very simple idea: if a small, sharp probe is pressed gently against a surface, and some way of detecting the movement of the probe is used, then an accurate map of the surface may be made without significantly affecting the surface. It is now straight forward to be able to produce very small sharp probes for this purpose. The picture to the left shows a common AFM probe.
Formulation Features
Typically the radius of curvature of the AFM probe tip is less than 10nm, allowing formulation features such as adsorbed surfactant micelles (see figure to the right) to be examined. The AFM may be used in both air and liquid environments, over wide pH and ionic strength range.
The micelles shown in this figure are only 7-8nm across and are neatly distributed across the surface. The lower part of the image illustrates where the AFM tip was deliberately pressed harder to reveal the silica substrate underneath. This technique may be used on commercial adsorbates from formulations to investigate the ease of removal from a particular substrate.
Instead of scanning a sample laterally, AFM can be used to scan vertically above a particular location. This allows the variation of the bending of the AFM cantilever with nearness to the surface to be investigated. Typical “force-distance” curves can be very informative. The graph below shows an example force curve for the micellar system shown above
Micelle layer stiffness
The black line shows what happens to the AFM cantilever as the probe approaches the micelle covered surface and the red line shows how they behave as the two surfaces separate from each other. Just before the top of the black curve there is a kink showing where the AFM probe pushes through the adsorbed micelle layer. This can provide information regarding the stiffness of the layer. Using different probes, the AFM can measure the stiffness of surfaces upto values comparable for some of the tougher polymer surfaces.
Adhesion studies
The red curve shows a large jump from a minimum in the force. This tells us about the adhesion between the probe and the sample. Adhesion studies are commonly carried out by AFM. Sometimes particles are stuck onto the AFM probe to be able to vary the material for adhesion (so called “colloid probes”), and also to provide a more reliable contact area.
Surface scanning
With a larger scanner an AFM may image a significant area (upto 100 x 100mm2) and this can include areas of significant roughness. The next image shows a sample consisting of many nanoparticles that had been deposited onto a substrate at a concentration of 200ppm.
The sharp ridges formed by the piles of nanoparticles have self-ordered and are parallel to each other. The height on the image represents the maximum range of heights from the lowest point to the highest, but any particular feature may be examined if required.
A Case Study - Linking Bulk and Interfacial Properties.
A company was interested in both how Laponite clay nanoparticles structured on a typical substrate surface and at what concentrations could the nanoparticles be added to the formulation to create a surface structuring effect. This reflects a common industrial problem: as more products are designed from the ‘bottom up’, how do the bulk and interfacial properties link.
Different concentrations of Laponite solutions were deposited onto a silica based substrate. The solutions were left to evaporate and the nanoparticle deposit formed. The images shown below are spread over concentration range of 6 orders of magnitude (a being the highest) and the AFM clearly picks out structure even at the lowest concentration (last figure)
The uniformity of the nanoparticle coating changes as the concentration changes, and that the ability to control the nanoparticle deposition on drying is also concentration dependent. In addition, an investigation of the difficulty of removing these nanoparticles from the substrates was investigated and the AFM data demonstrated that in general these nanoparticles are not removed.
The Future
At present, AFM remains a specialist technique requiring expert operators. However, AFM has come a long way over the last 20 years and it is being established as an invaluable R&D and QA tool for formulators who want to understand and relate particle, surface and coating characteristics of their formulations with their bulk behaviour and properties.
* Corresponding author
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Adhesion Science for Practical Formulators: One day training course - 12th July - http://t.co/GifpjrQW
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RT @nanoktn FP7 Funding call - Multiscale modelling for nanomaterial design http://t.co/dfcpOcCd
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Microbes Good and Bad in Personal Care, 19th June 2012 - http://t.co/5oWqPJuu
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Good to see @GrahamClayton (formerly of @IF_Tweets ) at refurbed Kings X station 2day. Pleased to see he still has his taste for #Latte
