lecture 6 zeta potential

8/13/2019 Lecture 6 Zeta Potential

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The Electric Double Layer Development of a net charge at the

particle surface affects the distribution of

ions in the surrounding interfacial region,resulting in an increased concentration ofcounter ions (ions of opposite charge tothat of the particle) close to the surface

An electrical double layer exists aroundeach particle.

The liquid layer surrounding the particleexists as two parts; an inner region (Sternlayer) where the ions are strongly boundand an outer (diffuse) region where theyare less firmly associated

Within this diffuse layer is a notionalboundary known as the slipping plane,

within which the particle acts as a singleentity The potential at this boundary is known

as the Zeta Potential

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Zeta potential is an abbreviation for electrokinetic potential incolloidal systems. In the colloidal chemistry literature, it is usuallydenoted using the Greek letter zeta, hence -potential. From atheoretical viewpoint, zeta potential is electric potential in theinterfacial double layer(DL) at the location of the slipping planeversusa point in the bulk fluid away from the interface.

In other words, zeta potential is the potential difference between thedispersion medium and the stationary layer of fluid attached to thedispersed particle.

Zeta potential should not be confused with electrode potential or

electrochemical potential (because electrochemical reactions are generally

not involved in the development of zeta potential).
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Zeta Potential

If all the particles have a large negative or positive zeta potentialthey will repel each other and there is dispersion stability

If the particles have low zeta potential values then there is no force toprevent the particles coming together and there is dispersion instability

A value of 25 mV (positive or negative) can be taken as the arbitraryvalue that separates low-charged surfaces from highly-charged

surfaces.

Zeta Potential [mV] Stability behavior of the colloid: from 0 to 5, Rapid coagulation or flocculation

from 10 to 30 Incipient instability from 30 to 40 Moderate stability from 40 to 60 Good stability more than 61 Excellent stability

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The Influence of Zeta Potential

Zeta Potential and pH

The most important factor that affects zeta potential ispH

A zeta potential value quoted without a definition of it's environment (pH,

ionic strength, concentration of any additives) is a meaningless number

Imagine a particle in suspension with a negative zeta potential If more alkali

is added to this suspension then the particles tend to acquire more negative

charge

If acid is added to this suspension then a point will be reached where the

charge will be neutralized

Further addition of acid will cause a build up of positive charge

In general, a zeta potential versus pH curve will be positive at low pH and

lower or negative at high pH There may be a point where the curve passes through zero zeta potential

This point is called the isoelectric point and is very important from a

practical consideration

It is normally the point where the colloidal system is least stable


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In the above example it can be seen that if the dispersion pH is below 4 or above 8 there

is sufficient charge to confer stability. However if the pH of the system is between 4 and

8 the dispersion may be unstable. This is most likely to be the case at around pH 6 (the

isoelectric point)

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Methods for experimental determination of zetapotential

Zeta potential is not measurable directly but it can becalculated using theoretical models and anexperimentally-determined electrophoretic mobilityor dynamic electrophoretic mobility.

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Electrophoretic velocity is proportional toelectrophoretic mobility, which is the measurableparameter. There are several theories that linkelectrophoretic mobility with zeta potential. They arebriefly described in the article on electrophoresisandin details in many books on Colloid and Interface

Science.

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Electrophoresis: The movement of a charged particle

relative to the liquid it suspended in under theinfluence of an applied electric field

This technique finds application in the measurementsof zeta potentials of model systems (like polystyrene

latex dispersion) to test colloidal stability theory To asses the stability of coarse dispersion

In identification of charge groups

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The particles move with a characteristic velocity which

is dependent on the strength of the electric field(measured by the instrument), the dielectric constantand the viscosity of the medium (known fromliterature) and the zeta potential

The velocity of a particle in a unit electric field isreferred to as its electrophoretic mobility

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Zeta Potential (Smoluchowskis Formula)

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Protective colloid

The addition of large amount of the hydrophile, to a

hydrophobic colloid stabilize the system, the hydrophilepossibly being adsorbed as a monomolecular layer on thehydrophobic particles. This phenomenon is known as

protection.

The added hydrophile is known asprotective colloid

Gold number: Is the minimum wt in mg of the protectivecolloid required to prevent a colour change from red to violet in10 ml of gold sol on the addition of 1ml of a 10% NaCl

solution

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lecture 6 zeta potential

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