If the droplets in your emulsion are large (diameter above about 2 micrometers) then that would explain your poor results with DLS. In that case, your sample is outside the instrument's range of operation.

Instead, you could use static light scattering, e.g. Malvern Mastersizer. However you'll need the instrument's cell  for small sample volumes as you should use the same diluant as you used for DLS.

No, once I can do the analysis with 100-200 nm and another ones faced to error (large PDI).

Dear Yousef,

The upper size limit, depends on the instrument. For example, the maximum measurement size by Malvern Zetasizer ZS  is 10 micrometers according to the manual. However, you can still measure the sizes up to 20 or 30 micrometer but with more uncertainty. Thus, the size measurement limitation may not be the case here. 

Since there is surfactant in your sample, the viscosity is different from that you might expect from your background solution. Have you measured the real viscosity of your sample and enter it in the settings of the software? Viscosity is an important parameter in size measurement procedure as it is used in the relationship between the speed of particle motion and their size—Stokes-Einstein Eq. 

DLS, does not care how much the concentration of your sample is as long as you can gain an appropriate count rate. This is not mentioned anywhere but according to my experience a count rate of 500-5000 is alright.  

There are other methods of size measurement, of course, depending on your purpose whether you want to obtain the hydrodynamic diameter (Z-average) which is the diameter with which the particles motion can be described or you want to determine physical sizes of particles with visualization (TEM, SEM, optical microscopes, etc). The more sophisticated techniques include nanoparticle tracking analysis (NTA) and field flow fractionation (FFF) which can be used to monitor the aggregation of particles as well. 

Hope this helps,

Peyman

Dear Peyman,

Thank you for your helps, certainly they're useful. However, today, I diluted my inverse emulsion (polyacrylamide aqueous solution in cyclohexane in presence of the TW85) in mixture of cyclohexane-TW85 for DLS cell. In this way, I have some questions:

1- I put viscosity of cyclohexane in the software setting. What's your view about this? or is it better, first, I measure the viscosity of diluting cyclohexane-Tw85 mixture, and then put the real ones?

2- I obtained the results in range of 130-200 nm by above diluting method. The count rates were 200-300 kcps, but you said 500-5000, is it possible to be wrong the data?

Thanks in advance

You are welcome Yousef. 

As for the viscosity, if the concentration of the surfactant is few percent it might not affect the viscosity of your solution, but if it is say 10%, you should check the deviation in viscosity and enter the real one in the software settings. 

We cannot actually say that the low count rate in range of 200-300 kcps cause wrong result because what I mentioned was a rule of thump and thus the obtained range in your sample is still close to it. However, it indicates that the concentration of colloids in your sample is low which might increase the uncertainties of the measurement. 

By the way, what is the size that you expect form DLS? I should mention that the hydrodynamic size determined by DLS is always larger than physical size measured by TEM/SEM. For example, if the reported size in the literature is around 50-100 nm measured by TEM/SEM, then your measurement size of 130-200 nm is completely reasonable because hydrodynamic diameter is the size with which the particle movement can be considered, that is the size of particle, stern layer and part of diffuse layer of ions around it. 

In addition, have you ever looked into the zeta potential to see whether the condition of your sample is aggregating or not? 

Dear Peyman, Thank you for useful guides. Yes, you're right. I am going to do both size measurement by TEM and DLS.

About the zeta potential, you do mean I measure the zeta p, if there is contrast charges, I should report the aggregation? Could you help me how to do this analysis?

Do not think zeta potential can be calculated reasonably for the oil continuous emulsion.

If you look at your emulsion under the microscope, do you see no droplets as your DLS method suggests?

Dear Titus, Could you tell me more why DLS cannot measure the particle size in oil continuous phase?

I wrote zeta potential. Due to low conductivity I doubt that values are meaningful.

Of course DLS is an option to measure in nonaqueous media. Still suspect most of the droplets are larger than 1 µm, which can be checked under the microscope.

Yeah, you're right. I found the preparation procedure for DLS. I should dilute the inverse emulsion once with cyclohexane and second by mixture of cyclo-surf for DLS cell.

For measurement of zeta potential you should use option PALS and manually to give on electrodes high voltage - up to 200 V.

For viscosity it is necessary to use values of viscosity of the environment in which the particle makes Brownian motion. As the size of drops of water is much more than a size of surfactant micelles, it is necessary to use viscosity of surfactant solution (as Peyman Babakhani say), but not pure solvent. For comparison   results of DLS with TEM it is necessary to use n - average values of hydrodynamic diameter.