Beckman Coulter LS 13 320, User Manual

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PN B05577AC B-6
Sample Handling
Liquid Sample Dispersion
Chemical Methods of Liquid Dispersion
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Wetting agents: used to lower surface tension (e.g., hydrophobicity) between diluent (typically
water) and particle (usually nonionic agent).
•
Surfactants: “Surface active agents” increase the surface charge of the particles in order to
cause them to repel one another so that they remain properly dispersed while in the
suspension. Types of surfactants include:
Anionic: imparts negative charge
Cationic: imparts positive charge
Amphoteric: imparts both negative and positive charges
Nonionic: wets particle without imparting charge
Stabilization by surface charge
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Adjustment of pH for surface ionization. Adjusting the pH so that the surface is charged.
—
Amine, hydroxyl, and carboxyl groups all adsorb a hydrogen ion below their isoelectric pH
value (obtained via zeta potential analysis) and result in a positive (+) charge on the group.
—
Amine, hydroxyl, and carboxyl groups all lose a hydrogen ion above their isoelectric pH
value and result in a negative (-) charge on the group.
—
Typically, 2 pH units above or below the isoelectric pH value will result in stabilization.
Stabilization occurs when the zeta potential is at least +30 mV.
•
Common Ions
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A solution containing a dissolved ion which is the same as one found in the sample's
molecular lattice may help disperse that sample by adsorbing and charging the surface of
the particles. However, be careful for solvation effects.
•
Multiple-charged Ions
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For ionic particles or particles with polar bonds in water, multiply-charged ions that are not
part of the crystal lattice, may adsorb to give a surface charged with soluble salts.
(Examples: polyphosphates; hexametaphosphate, pyrophosphate, polysilicate ions)
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For nonpolar organic particles in polar organic media, the surface can be charged by
adsorbing a neutral ion-pair. Dissociation then occurs with one part of the ion-pair,
desorbing and leaving a charged particle. (Example: trimethyldodecylamine
hydroxybenzoate dissociates into a quaternary amine(+) and a polar organic acid(-).)
•
Surfactant Ions which charge the surface
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Organic powders: these can adsorb the organic ion of a surfactant with the inorganic
counter-ion dissolved in solution, allowing the particle to be wetted and charged, thus
repelling each other.
—
Organic amines adsorb a hydrogen ion to become positively charged when pH is below the
pKb.
—
Organic acids lose a hydrogen ion to become negatively charged when pH is above the pKa.
—
Again, zeta potential of + 30 mV is achieved when pH is 2 units above pKa or 2 units below
pKb.