114 cP is quite a high viscosity for a size measurement if laser diffraction is utilized.  Are you measuring via laser diffraction?  The fact you input the viscosity appears to indicate that this is a DLS experiment and that you have entered the viscosity at one temperature but are conducting the experiment at another. For a water in oil emulsion the dilution must be in the oil + stabilizer/surfactant phase.  More details are required as to your specific system. How did you measure your viscosity?  Water-in-oil emulsions are one of the more difficult systems to measure for particle size.

I measure the viscosity using the electrode viscometer and yes I work on temp ,25C .. but I do not know how I can measure it and what is the dilution factor I have to use..yes I see it is very demanding task to measure of for microemulsion system.... thanks in advance 

What technique are you using to try to measure particle size?  What instrument?

Zetasizer 

Ya dilution is the very crucial thing in the w/o microemulsion for particle size measurement because after dilution may your microemulsion convert. So for that you have to optimize the dilution factor to achieve your desired particle size. Coming to viscosity it is also very high.

Article which is attached below may help you.

http://www.sciencedirect.com/science/article/pii/S0378517312000154 

In vitro and in vivo evaluation of a simple microemulsion formulation for propofol.      DOI: 10.1016/j.ijpharm.2012.01.011

Thanks

thank u vey much

Which solvent (oil) you have used for preparation of w/o microemulsion?

Oleic acid 

So in my point of view, first try to measure droplet size of microemulsions using viscosity of Oleic acid (viscosity = 27.64 cP at 298 K) by DLS. I think, then you don't need to dilute the sample and you will get an idea about droplet size.

One other route to measure the viscosity is to add a known size standard (e.g. very small quantity of 100 or 150 nm material) and adjust the viscosity value (it's inversely proportional with size in the Stokes-Einstein equation) until the standard reads correctly.  This approach was used in the measurement of the size of sucrose:

http://link.springer.com/article/10.1007/s11051-007-9317-4

Oleic acid melts at about 14 C so this material is only just a liquid at room temperature.

I agree with Kaushik Kundu.It should be fine to measure droplet size of microemulsions using viscosity of Oleic acid (viscosity = 27.64 cP at 298 K) by DLS.

Dilution should change of microemulsion composition and it should possibly not be good idea. 

Is the viscosity of the medium or the total system necessary? My understanding is solvent viscosity. Can anyne please clarify, if otherwise is true.

For similar sytemes, we could measure droplet size using Malvern Zetasizer (NanoZS)

The viscosity required is that of the dispersant liquid - the phase within which the particles move.  Obviously it's required for the Stokes-Einstein equation.  Problems comes with more concentrated or crowded systems where we have particle-particle interactions and the effective viscosity is not the same as where particles would be in infinite dilution. Dilution would be with the 'mother liquor'  - the same effective composition of the background liquid in the original formulation.  Now this could be recovered by centrifugation retaining a little of the original for the measurement.  In this instance the particles enter the same phase that they had in the original system.  The system cannot simply be oleic acid and water - a surfactant is needed to bridge the aqueous and organic phases and keep the water droplets stable without separation or settling.

I note that we have 2 conflicting viscosities for the oleic acid: 114 cp at 25 C (Ruba) and 27.64 cP at 298 K (Md. Abu).  Obviously the same temperature.  The latter seems correct: http://www.sjlipids.com/c181.htm One also notes that the viscosity is considerably reduced at 90 C [4.85 (90)] and this will obviously increase the diffusion constant of water droplets.  Perhaps an elevated temperature measurement with the Peltier on the Zetasizer is a possibility?

Dear Alan F Rawle ,

Thanks for the explanation.

For w/o microemulsion, the partcle size to be measured is for the droplet, which comprises mainly the reverse micelles formed  by association of the surfactant species which are dispersed in the oil phase. Water is confined in the core of the reverse micelle. Therefore should the oil phase not be considered the dispersant liquid?

Dilution should surely be a problem here. If you add water progressively, the system will turn to a micelle dominated system via a bicontinuous microemulsion and the droplet size will change. Therefore after dilution(if it is inevitable from high viscosity consideration) the droplet size must not correspond to that of the original w/o microemulsion.

114 cP is the viscosity of microemulsion solution and 27.64 cP is for Oleic acid only.

I feel that elevated temperature measurement with the Peltier on the Zetasizer will not be good choice. The rise in temperature will affect the droplet size and the phase diagram , It might happen that microemulsions are phase separated at high temperature. Even if the measurement is possible at hiht temperature, the drplet size will not correspond to that of the original w/o microemulsion.  Dynamic percolation behavior may be considered and transition from w/o to o/w via bicontinuous microemulsions with change in temperature should play role.

Md. Abu  I agree with your comments with respect to the viscosity - it's the oil phase viscosity required for a DLS experiment of this nature.  I find it a little surprising that the viscosity rises to such a high level in the microemulsion (114 cP) in relation to the oleic acid (27.6 cP) but would believe Kaushik's measured values.  If this indeed the case, then this must point to an extremely crowded system with a high concentration of water droplets - indeed is this a oil-in-water emulsion rather than a water-in-oil emulsion?.  Is the H2O concentration in the microemulsion known - perhaps by Karl-Fisher? 

Yes, if we pass through the CMC (critical micelle concentration) then changes in size are inevitable as the emulsion inverts. We need more information from the original poster (Ruba Malkawi ).

Higher viscosity of microemulsion compared to oil medium may be due to the highly viscous liquid surfactant like Tween-80 or Tween-85 or Span.

Because it is well-known that when we formulate Ionic liquid-in-oil microemulsion, the viscosity of microemulsion become much larger than its bulk oil phase.