Malvern Zetasizer Nano Series, User Manual

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Dilution Medium
The continuous phase of most samples can arbitrarily be put into one of two
categories:
.
Polar
dispersants are defined as those with a dielectric constant greater
than 20 e.g. ethanol and water.
.
Non-polar
or low polarity dispersants are defined as those with a dielectric
constant less than 20, e.g. hydrocarbons, higher alcohols.
Aqueous/Polar Systems
The aim of sample preparation is to preserve the existing state of the surface
during the process of dilution. There is only one way to ensure this is the case.
This is by filtering or centrifuging some clear liquid form the original sample and
using this to dilute the original concentrated sample. In this way the equilibrium
between surface and liquid is perfectly maintained.
If extraction of a supernatant is not possible, then just letting a sample naturally
sediment and using the fine particles left in the supernatant is a good method.
Zeta potential is not a size-dependent parameter using the approximation of the
Smoluchowski theory.
Another method is to imitate the original medium as closely as possible. This
should be done with regard to:
.
pH.
.
Total ionic concentration of the system.
.
Concentration of any surfactants or polymers present.
Non-Polar Systems
Measuring samples in insulating media such as hexane, isoparaffin, etc. is far from
easy. It requires the use of the universal dip cell. This is required because of its
chemical compatibility and the close spacing of the electrodes which allows the
generation of high field strengths without using excessively high voltages.
Sample preparation for such systems will follow the same general rules as for
polar systems. As there will be generally fewer ions in a non polar dispersant to
suppress the zeta potential, the actual values measured can seem very high, as
much as 200 or 250mV. In such non-polar systems, equilibration of the sample
after dilution is the time dependent step, equilibration can take in excess of 24
hours.
C H A P T E R 6
Z e t a s i z e r N a n o S e r i e s
P a g e 6 . 7