Ion HYDRATION effects shoud be accounted for detaled picture of Electrokinetics of Zwitterionic Surfactant. I don't believe that such a work exist, but one never knows.

A zwiterionic surfactant I used contains two functional groups : amminium and sulfonate

The zeta potential values at a specific range of concentration of surfactant is about -5 to -10 mV for pH around 6-7

I would like to mention that the isoelectrical point (IEP), i.e. The pH at which the zeta potential is zero, differs from the point of zero charge (PZC) i.e. the pH at which the surface charge density is zero. Concerning the zwitterionic (amphoteric) surfactants, they have both cationic and anionic centers attached to the same molecule. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. The anionic part can be more variable and include sulfonates, carboxylates, etc. Thus the surfactants that you have used have likely carboxylate as anionic groups which have pKa values around 3-4, and this may explain you observed IEP=3.

Best regards

Amane Jada

Regardless small ZETA potential, SURFACTANT Stabilization id due to the OSMOTIC REPULSION FORCE at Overlapping Adsorbed Monolayers condition. Detailed Mechanism is described within Paper #1 at my RG page.

Dear Atthaphon

In my opinion, the effect which you seen (the negative charge of an amphoteric surfactant in solution at pH 7) is not necessarily caused by error.

Theoretically, equilibrium constants the two processes of dissociation amphoteric surfactant molecule (acid and alkaline) shall be equal, since they deal with the balance of processes involving the same ion (proton). Therefore, in theory at pH 7 system should be electrically neutral.

However, it often happens - especially for amphoteric surfactants with very asymmetrical structure (closeness of various structural elements in the vicinity of acidic and basic groups) - that a part of the structure of the molecule interacts very strongly to the acid group, or to the base group, causing disturbance acid-base imbalance at pH = 7, this in turn leading to effect, that in pH=7 we observed either negative or positive charge.

So, for example, amino acids at pH = 7 always produce excess of negative charge.

Your surfactant contains sulfonate instead of carboxylate, it is known that the sulfonate groups are insensitive to the aqueous phase pH.i.e these groups are negatively charged in the pH range 2-13. On the other hand the amine groups of your surfactant are positively charged in the pH range 2-pKa of the corresponding ammonium. groups.Therefore at pH=3, a compensation of the positive charges with the negatively should occur, whereas at pH=7 the negative charges of the sulfonate groups exceed those of the ammonium groups, and at pH below 3, the positive charge of ammonium groups exceed those of sulfonate.

Best regards

A. Jada

The comments and assumptions you guys provided are very useful and informative and I appreciate it. However, I will try hard to read other related phenomena and physical chemistry to futher gain understanding about the zeta potential of zwitterion ionic surfactants.

Thank you again :)

Besides the other comments, you may wish to have a look in recent publications by Faruk Nome and coworkers. They report what they call chameleon like behavior of zwitterionic surfactants ascribed to preferential adsorption of ions (follows Hoffmeister series and is more predominant for anions).

I agree with Amane's explanation. Without knowing the exact molecular structure of the surfactant it is hard to say more. Also I assume that you are working at concentrations exceeding the critical micellar concentration. Best regards,

Shankar

A ZETA potential exibited by adsorbed surfactant has only small effect on overall colloidal stability. Non-Ionik Surfactant of BRIJ seria will provide stability as strong as any ionic or Zwetterronic Surfactant. Non ionic surfactamt is actually better, as its CMC is largely incensitive to background electrolite concentration. That is OSMOTIC REPULSION FORCE DUE TO ADSORBED SURFACTANTS is responsible for large forces, order or more higher in magnitude, than anything DLVO can predict.

Misha, Luboi SURFACTANT in ionic solution has some ZETA potential. But take background ioni concentration 1 M/L or higher No ZETA potential exist, but, nevertheless surfactanr stibilizes colloid, BRIJ could not care less about background electrolite concentration. I worked 40 years, trying to explain this problem, succeeded only recently,

results are in paper #1 on my RG page. Every one who is interested can take a look, full size of the paper in COLLOIDS and SUDFACES B is posted.

I think you are observing what some researchers have called chamaleon-like nature of zwitterionic surfactants, that is, they are able to attract both cations and anions that are in solution. Anions (and the most polarizable ones - or the big ones) are the most effective. I suggest you have a look at recent publications from Faruk Nome on this. They also quote quite negative zeta potential values