Turbidity-wavelength technique is used by many to determine the 'n' value and hence the stability of the microgels. The formula used for the same is n=-(dlog tur/dlog lambda). Here turbidity can be found out by absorbance, but how to specify lambda? For example I have seen some papers where for PNIPAM microgels lambda was taken as 600nm. How would I decide this?
You could also try to do rheological measurements.
Do not know your materials, but generally samples need a pre-shear and then you switch to a constant stress mode and measure the compliance J.
Look at Figure 4.19 in:
https://books.google.nl/books?id=lnPyCQAAQBAJ&pg=PA334&lpg=PA334&dq=viscosity+compliance+stability+measurements+%22constant+stress%22&source=bl&ots=9OxBo8U8Tk&sig=DXgEeFwqB3mRt0-4AGSg25Gdags&hl=en&sa=X&ved=0CCIQ6AEwAGoVChMI_NW-n8mQyQIVw7wUCh0tOATO#v=onepage&q=viscosity%20compliance%20stability%20measurements%20%22constant%20stress%22&f=false
Just take one applied stress level, e.g. 1 Pa after the pre-shearing and measure the compliance in time. in Fig 4.19 the compliance increases, but for microgels that built up a viscosity in time till a certain level J becomes constant, but as they showing instability or even syneresis J will go down after some time. For low viscosity microgels (at low shear ...) it will be not easy to measure, but else it should work.
The advantage of such rheological measurements is that you mostly can measure on the original samples without diluting, which may affect he equilibrium of the adsorbed material which is related to the stability you want to measure.
Otherwise discuss my proposal with a rheologist, he will understand what I mean. Edit: sorry I just saw that you have rheology in your profile.... so my suggestions may overlap your knowledge?
Particle size measurements as f(time) could be another route. Particle size itself and the poly dispersity will increase in time when instability occurs. Try to measure as concentrated as possible to avoid the influence of the dilution on the stability. This are very fast measurements and done with a fixed laser angle (90 degrees) machine may work already for this case.
Dear Mithra,
you may make it more easy. Extinction ratio measured at two different wavelength depends on size of particles. Measuring the so called turbidity ratio will indicate you flocculation of particles. Change in transmission at a defined wavelength, e.g. 470 nm will indicate when due to changes in microgel environment you get a particle dispersion, i.e. transmission will change from near 100 % to markedly lower values. In microgel state refractive index is nearly that of the solvent.
Titus, your suggestion looks interesting, certainly when you have no laser particle size equipment available.
Mithra:
Look for some articles on it, do a search. Like this one or the ones that are referring to it (as it is from 1981)
http://deepblue.lib.umich.edu/bitstream/handle/2027.42/24510/0000787.pdf
Or use Google book search such as:
https://books.google.nl/books?id=0AueaypD1BwC&pg=PA291&lpg=PA291&dq=turbidity+ratio+stability+dispersions&source=bl&ots=BwSquEHEJ5&sig=cOviBon2hko4bIJUXKeiKnnp2Jw&hl=en&sa=X&ved=0CCwQ6AEwAWoVChMIzcKB2OeUyQIVDEkaCh3Ncg-u#v=onepage&q=turbidity%20ratio%20stability%20dispersions&f=false
Tamás Haraszti I understood the fact that I can measure around a central wavelength and then take the data around it. Thanks for the suggestion.
Andre Roelofs I have done rheology on macrogels, and for this particular microgel sample I have not done rheological measurements. I would certainly do what you suggested. I have done the particle size measurements with DLS.
Titus Sobisch Sir, Suppose I measure the transmittance for a range of wavelength 200-1200nm. I will next see where the transmission is changing to a lower value. But if the change is gradual I will get a range of wavelengths, how will I define change in transmission at a defined wavelength ( Ex: 470nm). ?
Dear Mithra
Measuring transmission in a larger range of wavelength gives you also information about particle size distribution. There are a few researchers dealing with this approach, e.g. Maria Teresa Celis. This involves more complicated maths. For turbidity ratio approach it is sufficient to measure at 2 wavelength, e.g. 650 and 470 nm. If only concentration changes ratio of corrected extinctions is constant, if there is also a change in size ratio changes.
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