Free fatty acids in oils and many emulsifier such as Tween can contribute negative charges in emulsions.

In fact,

In manufacturing of Tween and other surfactants, they used free fatty acids as a substrate

I would recommend you to buy/or use new Tween 80 bottle. You will suffer more with old Tween 80

The presence of Tween 80 as an emulsifier leads to shift of the share plane position in the electrical double layer and hence a reduction in the magnitude of zeta potential. Despite the decrease observed in the magnitude of zeta potential, as compared to the absence of the emulsifier, the emulsion may be stable since non-ionic interactions between droplets are operating.

Best regards,

Amane JADA

Dear Poonam,

do not worry, the 'rule' of absolute value of Zeta potential to obtain stable dispersions only applies (if at all) for purely electrostatically stabilized dispersions. Even in this case you will find exemptions, e.g. silica near isoelectric point.

It cannot apply for sterically or electrosterically stabilized dispersions. Tweens give sterical stabilization. Slightly negative Zeta potential is known for ethoxylated surfactants and is ascribed to adsorption of ions.

Stabilization of emulsion by nonionic surfactants is due to steric repulsion.Your Zeta potential measurements indicate that the interface is negatively charged. This charge will enhance the stability of the emulsion by causing double layer repulsion between droplets. There is no relationship between stability and certain value of Zeta potential. The origin of interface charge depends on the composition of oil, and pH and electrolytes present in the water phase.

I agree with the many good answers already given: I guess that most likely your case is as Atikorn says --- (1) Ionic contaminants exist in many nonionic surfactants as contaminants and sometimes they are surface actives -- so these contaminants bring some negative charge to emulsion surfaces. One thing you might try --- if you measure the conductivty of your Tween 80, a nonionic surfactant, do you actually find conducitivity? If so, you have salts present, and potentially surface active contaminants.

Less Likely effects to describe your result(s) -- (2) Adsorption of neutral polymers and species shift the position of the plane of shear away from the particle surface (Hunter discusses this briefly in the old book "Zeta Potential in Colloid Science" (1981) in Chapter 8.4. This only describes shifts in zeta potential if you already have charge on your particle surface and the nonionic is in a sense partially burying the charge under a steric brush. Not quite your situation since you thought you were starting with no charge on the particle, but a real effect. (3) Hunter also points out in the above book that sometimes neutral polymer adsorption influences chemispecific adsorption of ions on the surface of the particle -- thereby also altering zeta potential of particles. This could perhaps be an effect in your case, depending on the ions in your continuum phase, but it still seems like option (1) above, as mentioned by Atikorn, is your most likely candidate.

I concur with Amane. Your stability profile and thus definition of stable state depends on several criteria: (1) on the effect of the surfactant on the diffusive layer thickness - you can determine this by measuring zeta potential in the absence and presence of surfactant and calculate the enhancement of the layer due to adsorption; (2) the time you are required to have prior to destabilization - you can calculate a flocculation time using Gregory's approximation and compare that to your constraint; (3) miscelleneous - amount of surfactants adsorbed, excess may cause destabilization/ electrolytes may significantly alter your stability profile/ dielectric constant of your oil

Good Luck,

@ Atikorn: I have considered similar causes. I will try with fresh Tween and verify the results. Free fatty acid from oil can also be the reason for negative charge.

@ Amane, @Titus, @Abdul : very helpful explanations.

@ Mike: I have not yet measured the conductivity of Tween 80. I am also considering the other effects that you have mentioned.

@Christian: In absence of surfactant I will not get an emulsion. Even if a course emulsion is formed, it will destabilize soon. Will it not affect the zeta potential measurement?? (Just a thought)

@Stoyan: Thanks for the reference

In the case, POONAM, you have a decent surfactant, you don't need to worry about ZETA potential. OSMOTIC REPULSION FORCE DUE TO ADSORBED SURFACTANT is much stronger than any DLVO forces. Paper #1 on my RG site discusses this subject.

AM also experiencing the same problem as POONAM, I've also prepared emulsion sunscreen with potassium cetyl phosphate (Anionic) as my surfactant and zeta-potential ranges from -6.5-7.5mV. what could be the effect?

Catalina, a solution has no zeta potential only particles and droplets. It shows that as often you may measure something and use in-built calculation which delivers garbage.

It would be very difficult to define the zeta potential of micelles, but maybe there are theories for this which you can search for. For sure you will need a tailor made model to calculate a reasonable zeta potential instead of using the inbuilt software designed for trivial cases.

One side remark to the down-vote function. I noticed that it is used often unreasonable as dislike. If you do not like the answer it does not mean it is unreasonable. I communicated this already twice with the Researchgate team with no reaction. So if you use this function think twice and provide strong arguments. 

following the question of poonam singha:

I have prepared an emulsion using a plant extract as an emulsifier. the extract is rich in saponins (around 4%), and sugars (more than 50%). It also contains 1% of proteins. I used this crude extract (2% w/w) to preparre o/w emulsions (10% oil). The emulsions were pretty stable at all ph values except of 1 and 2 were there was a huge increase in the drople size (from 110 nm to 270 nm and 627 nm respectively). However, they were relatively stable at ph 3 (157 nm). When I measure te zea potential it gives me values of -31 mv, -18 mv and  0.21 mv for ph 3, 2 and 1 respectively. I believe the active compound is saponins butt this molecules have a pka of 3.6 which makes me wonder why the emulsions were stable at ph 3 (below the pka).

I have used tween 20 in my formulation and I get negative zeta potential is that possible? How?

Harshita Patel Please start a new question. You will get a better response. When you post the question include more detail such as the composition of the formulation.

I have prepared an microemulsion have using tween 20, transcript p and oleic acid . After measuring the zeta potential value I found that it ranges between - 3.66 mV

So plz suggest why it give negative zeta potential.

Why is the value important in your case?

You need to look at the quality and variability of your results. -3.66mV may simply be zero. Read the answers given to the original question - there's a lot of useful information for your situation.

It is also important to know that interpreting zeta potentials for liquid-liquid systems is not the same as for liquid-solid systems. You don't specify your oil phase.

Dear Prof. Ernandes,

AT which pH and Salinity? are you performing zeta after dilution or as such?

Generally, negative zeta potential is attributed to oil.

Best regards.

Hamid

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