In my research I use DLS to characterize how does polyelectrolytes behave in different media. In this case I added 25mM of NaCl into polyanion solution in deionized water. My observation was that the hydrodynamic diameter increased about ten times in comparison with diameter measured in phosphate buffer solution. I've read about the charge shielding effect caused by NaCl addition, thus aggregation occures and this might be an explanation of Dh increasement. I would be grateful if someone would confirm if my line of thought is correct.
DLS is only giving you the diffusion coefficient of the polyanion. You should double check your result with SLS or SAXS to ensure this statement. As far as I know. strong charge may lead to two modes (two characteristic diffusion times): one faster and one slower than that of neutral particles with the same size.
What was the polyanion, and what was its concentration? DLS gives you a relaxation spectrum,. What analysis method did you apply to interpret the spectrum?
The limited description is also possibly consistent with micelle formation. We seem to be giving you more choices than you might have wanted, which I cannot help easily.
First of all I would like to thank for all answers. My polyanions are four and six-arm star-shaped polymethacrylates witch are statistical copolymers of methyl methacrylate with methacrylic acid. Concentration was matched to the molecular weight of polymer (like it was recommended in Malvern Zetasizer Nano user manual). I'm not sure if I understand correctly your question about the analysis method.
Quasielastic light scattering actually measures the intensity-intensity correlation function of the scattered light. The correlation function in question decreases monotonically with increasing delay time. Qelss spectra contain a rather limited amount of information 5-8 parameters is usually optimistic. You then need some process to extract parameters from the spectrum.
"Concentration was matched to the molecular weight of polymer " OK, what were the molecular weight of the polymer, the hydrodynamic radius based on the chemical structure if you know it, and the concentration in g/L or the like. Is there anything in this manual discussing the applicability of their formula to neutral or polyelectrolyte polymers?
As a suggestion, repeat the experiment with the polymer being diluted repeatedly. Also, repeat the measurements on the same sample several times; if the polymer is slowly aggregating, D will be a function of time.
I occasionally consider writing a book on the method.
If you take any handbook on light/x-ray/neutron scattering, you will find a formula that describes the intensity of radiation scattered at scattering vector q in the form similar to:
I(q) = K c M P(q) S(q),
where
K - contrast factor characteristic for a given solvent and solute, wavelength etc.
c - weight concentration
M - molecular weight
P(q) - form factor - describes the q-dependence of I(q) for infinitely dilute solution of your paricles (provided that they don't change upon dilution)
S(q) - structure factor - describes the effect of interactions between solute particles. At low q it strogly decreases with increasing interactions (up to ~1000 times).
At constant concentration c, the increase of the salt contents will reduce the repulsive interactions of colloids. This may result either in the aggregation effects or in the flattening of the S(q) function. If the particles are small (below 50-100 nm), it is very difficult to distinguish between the two effects using just light scattering because you are in the low-q limit and both phenomena will result in similar observable effects: apparent increase of scattered light intensity(at low q) and apparent increase of the hydrodynamic radius. What you need is a physical model of your system (RMSA, Rogers-Young etc) or a set of data from another technique covering a wider range of q (SANS, SAXS, XPCS).
If you think that the concentration is low enough to forget about the interactions, a simple check if aggregation really occurs is to find if the ratio of scattered intensities is equal to the cube of the ratio of hydrodynamic radii. In such a case it would be only the molecular weight that changes in the formula above (M~r^3).
The study of polyelectrolytes by DLS is difficult and tricky. If they are strong polyelectryoltes, i.e. they have a high linear charge density, they exhibit strong intermolecular interactions-repulsions that affect dynamic behaviour of polyions. By DLS one measures the diffusion coefficient on the basis of fluctions of scattered light intensity. Single chains diffuse fast and result in a high diffusion coefficient. The highlighted repulsion appears as and additiona (small) diffusion coefficient in a DLS measurement. This "anomalous diffusion" which is often termed as a slow mode. In order to "get rid " of this slow mode, the salt concentration has to be high enough in order to efficiently screen the electrostatic repulsions. For many polyelectrolytes around 0.1 M salt (e.g. alkali halides) are sufficient, but for highly charged ones (DNA being an exmaple) higehr salt concentration is needed. However, this depends also on the nature of added salt (electrolyte).
Of course one has to know the system. If there are any possibilities of aggregation between chains, this has to be taken into account. However, in your case with 25 mM salt (which is rather low) and if the poyion is highly charged I do not expect any aggregation/assiciation. The result you report is logical. Phosphate buffer has a much higher ionic strength (concentration) than 25 nM NaCl and is evidently able to completely screen electrostatic repulsion. Therefore, you do not observe any more the slow mode, which has a low diff. coeff. and this results in a high value of "an apparent" hydrodynamic radius.
I hope this will help you in performing reliable DLS measurements with polyelectrolytes.
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