The concept of zeta-potential is defined only for phase interfaces for a liquid or solid surface in contact with an immiscible liquid. At pH where the folic acid dissolves, there is no zeta potential to measure, since a true solution is a single phase. If you could disperse folic acid as solid particles, then you have a possibility of measuring the electrophoretic mobility and zeta potential. The attached paper shows how we studied an organic monomethine oxonol pigment at different pH (see Figs 8 and 9). This pigment dissolved rapidly as pH was raised above 7.

Try to titrate starting pH = 10 down, adding folic acid only. If you can't obtain pH =2, folic acid is too weak, so, lower values of pH are not obtainable.

The concept of zeta-potential is defined only for phase interfaces for a liquid or solid surface in contact with an immiscible liquid. At pH where the folic acid dissolves, there is no zeta potential to measure, since a true solution is a single phase. If you could disperse folic acid as solid particles, then you have a possibility of measuring the electrophoretic mobility and zeta potential. The attached paper shows how we studied an organic monomethine oxonol pigment at different pH (see Figs 8 and 9). This pigment dissolved rapidly as pH was raised above 7.

Yes I agree with John. you can measure the zeta potential of nanoparticle, or dispersion.

John Texter is a MASTER, you can trust him. I do.

Thank you all for your valuable comments.

But how do people report the Zeta potential values of Bovine Serum Albumin then? is it because it is a protein? The sample preparation for recording zeta values is preferentially done in water.Because bovine serum albumin does dissolve in water thus becoming a single phase but still gives zeta values which are published.

Dear Dr. Hoojan:

a1. This is an example, the surfaces of protein molecules in Bovine Serum are charged.

a2. Yes, is due charge on the protein molecule surface.

Please, consider read the instructional material in attachment, BovineSerumColoids.

a3. See, DNA, long-chain polymeric molecules and another as some carbohydrates can be classified as a colloid. Original systems are based on the set up experimental to work in aqueous ambient. Fortunately, great part of modern system can be used in non-aqueous fluid as alcohol etc..etc... In fact, when pH is involved this parameter is defined to water only. In this sense, any measurement of pH in a non-aqueous fluid is erroneous. This event occurs because the number indicated is non-adequate. But there are some papers that describe the necessary correction and its support.

In a brief on-line course there is a Fig. 01 showing particles positioned at shear plane. Well, these "particles" can be charged protein-molecules, consider see:

http://www.colloidmeasurements.com/zeta.html

or long-chain polymer molecules see second paper in attachment, ZetaPotentialResins.

Also, there is the toplic pharmaceuticals in:

http://www.zeta-meter.com/5min.pdf

The definition of shear plane is presented in the well enginnered material in attachement ShearPlaneDefinition. This material belong to small group that make mention to a concept extremally important that is: "There are at least two striking errors in this methodology: the history of the system is ignored, and the effect on the zeta potential is not only due to the pH of the system, but is also due to the increase in ionic strength."

Best regards

Marcos Nobre

Maneka, You raise some good questions about apparent exceptions to the phase boundary definition of zeta potential. I point out to my students that I prefer dealing with electrophoretic mobilities, because such mobilities are measureable quantities, whereas zeta potentials are theoretical constructs that must be calculated from such mobilities. We use the Smoluchowski [kappa x a >> 1] or Hueckel [kappa x a

Dear John,

thank you much for your detailed explanation. Nevertheless, I have some doubts regarding your last sentence. Partly, it maybe a definition of strong adsorption (difficult to desorb or amount of saturation adsorption). Though a matter of discussion whether, e.g. anionic surfactants adsorb onto negatively charged surfaces like silica, I think they do so on 'active' centres in relatively low amount. Most paper I know state that surfactant adsorption on polar surfaces of opposite charge is negligible or zero. I see no reason why interaction through hydrophobic chains should be more favourable at the polar oppositely charged surface  than in the continuous phase.

Titus, I think it is fair to say that we know from micellization of charged surfactants, that dispersion forces typically dominate. Additionally, industrial colloid scientists know that most aqueous colloids tend to be intrinsically negatively charged, and are further stabiiized by adsorbing additional negatively charge surfactant. Charge does not repel alkyl chains, only same-sign charges. See the attached papers for a handful of examples where intrinsically negatively charged colloids adsorb negatively charged surfactant; these examples are only the tip of the "iceburg." Did you mean to say "that surfactant adsorption on polar surfaces of identical charge is negligible or zero"? I categorically disagree with such an assertion, and the adsorption data in the attached papers prove my point. See the paper attached to my first answer above (OMT adsorption onto a monomethineoxonol with intrinsic benzoic acid ionizable surface groups), and the Colloids Surfaces paper attached below with intrinsic ring-opening carboxy groups and three or four different negatively charged surfactants. If in fact you meant charged surfactant adsorption onto surfaces of opposite charge, you have the same hydrophobic driving force for adsorption and some additional electrostatic driving force. Best wishes, John

I meant classical adsorption of e.g. cationic - via anionic surfactants on silica, cellulose

opposite positively charged alumina.

When cellulose based fibres are concerned, result in papers of literature are anionic surfactants are adsorbed through e.g. divalent cations or onto fibres containing waxes etc. (rather absorbed).

 How do I run a zeta potential on a ph range from 2-10 for folic acid?

Pl add very few drops of Tweens 20 and measure ZP.

The following link will be useful

http://www.sciencedirect.com/science/article/pii/0021979784903060

maybe you mean another link than the one leading to

The use of zeta-potential measurements in organic solvents to determine the donor—acceptor properties of solid surfaces ???

good answer, Man Singh!

Use of the nonpolar surfactant at CMC (for example, peo-ppo block copolymer) would solve both problems: increase the stability of molecular folic acid in acidic solution and measure the zeta potential of the micelle doped by folic ion at any pH.

PS It should be clarified that, indeed, the concept of zeta potential is only applicable to the heterophase system. In the case of ideal solutions we can only talk about the mobility of ions. But the use of micelles allows you to simulate the colloidal system consisting of condensed matter of the studied ions (in the case, if the ion is able to be integrated into the structure of the micelles).

Man Singh / A.B. Scherbakov. I respectfully disagree. Creating micelles will bring nothing to this study. You do not know how much folic acid is bound to the micelles or whether it is on the outside or the inside.

Folic acid does not have a zeta potential. And if it did, diffusion broadening due to Brownian motion would make it impossible to measure accurately.

The best approach, in my opinion, is simply to do a classic acid/base titration that allows you to determine the molecular charge. See the link to a nice old paper on the titration curve of gelatin.

Once you know the charge, you can apply the theory of your choice to derive the parameters that you need. But be careful :-).

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1269871/pdf/biochemj01080-0029.pdf

Alan Parker, mainly you are right. Of course, the molecular solution has no zeta potential, because it is the property of the interphase boundary. But if I understand correctly, asker wants to measure the zeta potential of folic acid, assuming that it would be a solid matter - how to do it when the molecule is fully soluble? Let's imagine that the folic acid is covalently attached to the surface of the solid polymer nanosphere, the study would be correct? Or soft nanosphere? Or physically adsorbed on the surface of soft nanoparticle? What is the difference with the micelle (it’s known that the charged group of organic ion in the nonpolar micelle immersed in polar solvent is always oriented into the solution)?

By the way, zeta-potential is measure of electrokinetic phenomena, in the case of electrolytes (ideal solutions) it is some similar to ions mobility. Yes, the acid-base titration enables to study the pH of equivalence/isoelectric point or point of zero charge (for sols), but it is not the best method to determine the molecular charge density and the mobility of ions.

how to decide pH of flotation on the basis of zeta potential studies ??