Aleksandrs Novikovs

What system are you studying?

You probably know that it's possible to obtain an aggregate index from DLS by comparing the forward and back scattering results (where the larger aggregates are favored in forward scattering). See attached application notes.

As DLS is looking at diffusion coefficients which may need many seconds (old units were cm2s-1) to determine accurately. Hence your aggregation rates should be fairly slow (over a few minutes minimum) to capture the growth. I have done this successfully with clays (unpublished work) where I could watch the (slow) first order growth, sonicate the sample outside the cuvette, and repeat the process.

I study transition metal chalcogenide dispersions in non-polar solvents, where particle size is less than 100 nm. I want to be able to determine stability ratio from DLS data. I have hydrodynamic radius dependance from time, I can calculate slope from it. I grasp that this slope is somehow related to aggregation rate constant.

Aleksandrs Novikovs

I'm a little confused. The SI units for reaction rate are concentration of product formed per unit time - thus moles/second (for example). If you know the molar mass of your reactants then the growth in time (referred to moles) is the reaction rate. If you're looking to the order of the reaction then that's a different matter.

In the case of your system a linear increase in volume (r3) per unit time is the sign of Ostwald ripening. I use the example of louching (and a calculation) in this webinar (registration required). Slides 50 onward are relevant:

'The spontaneous emulsion formation (louching) effect'

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150218-emulsion-formation-effect-alcohol.html

This provides some references (e.g. Sitnikova: N L Sitnikova, R Sprik, G Wegdam, E Spontaneously Formed trans-Anethol/Water/Alcohol Emulsions: Mechanism of Formation and Stability ' Langmuir, 21, 7083-7089 (2005)).

Other useful references:

https://en.wikipedia.org/wiki/Particle_aggregation

https://en.wikipedia.org/wiki/Reaction_rate_constant

Hello Aleksandrs Novikovs

this might be a good reference for you Article Absolute Aggregation Rate Constants in Aggregation of Kaolin...

Thank you. I am a chemist. For me it is weary hard to comprehend this article. Maybe you can recommend introductory literature about this topic?

Dear Aleksandrs Novikovs , you could give a try to an in-situ real-time analysis with Cordouan technologies Vasco Kin apparatus (https://www.cordouan-tech.com/products/vasco-kin/). It could run during your sample aggregation inside your reaction chamber/container. This could be really interesting to follow z-average evolution.