Paper Technology International 2020 - Journal - Page 62

PAPERTECHNOLOGYINTERNATIONAL
Retention aids are often long and branched molecules with
little charge whereas fixatives are small with high charge. The same
is true for anionic trash – a synonym used for anionic interfering
substances – a rather small particle with high charge.
Small molecules and particles with high charge results in
velocity and reactivity.
Further charge carriers are dissolved salts, for example
HCO3 (hydro carbonate) which is also known as water hardness.
Salts also exist of anions and cations which decay in water. They
might originate from the tap water, process water and chemicals.
The amount and type of salts dissolved in water cause the
conductivity, noting that a high amount of dissolved salts will lead
to a high conductivity and as this material is non-volatile, we see
significant increases in conductivity in cases of high water closure.
This is a problem as conductivity does influence particle
surface charge and therefore should always be looked at.
Size maters
Let us return to the size of the particles for a moment.
Salts/ ions – or particles smaller than 1 nm - in solution
are considered as a real solution. The water is clear. Components
in a size range of 1 nm to 500 nm are considered as colloids. It is
important to note that a colloid simply refers to a state of size and
not to specific substances.
If we look at colloids in a water phase, the solution is
still clear but one property of colloid systems that distinguishes
them from true solutions is that colloidal particles scatter light.
If a beam of light passes through a colloid, the light is reflected
(scattered) by the colloidal particles and the path of the light
can therefore be observed. The scattering of light by colloids is
known as the Tyndall effect.
Finely dispersed solutions – 500 nm till 100 µm – are
already turbid. Coarse dispersed particles can be seen with
the naked eye. Particles with a size bigger than 1 nm – the
colloids – start showing the so-called electrokinetic effects or
electrokinetic phenomena. This is our friend when we want to
measure what is really going on in our furnish.
Figure 3: The electric double layer for an anionic particle
Electrokinetic effects
“Electrokinetic” in this context refers to a measurement
of movement or velocity and voltage or current. In fact, an
electrokinetic phenomenon results from the differential movement of
two phases where the interface is an electrical double layer.
The electrical double layer consists of ions, that are
electrostatically attracted by the surface charge of the particle.
These so-called counter ions establish a stationary layer
close to the particle surface, as well as a diffuse and mobile layer
further away from the particle. It is the mobility of the diffuse ion
layer, that allows us to measure charge:
•
•
Displacing the diffuse layer ions
causes a measurable potential
difference
Displacement achieved through
streaming liquid → streaming
potential in mV
Figure 4: Measurement of streaming potential.
Particle charge measurement with PCD
To measure the charge of a particle, we take advantage of
this electrokinetic phenomenon by inducing a shearing effect in the
liquid sample via a mechanical piston and measuring the voltage
potential. Addition of an automatic titration device to neutralize the
sample with an opposite charged ionic reagent allows us to build a
highly accurate and quantifiable measurement of particle charge.
Figure 5: (below) Particle charge measurement.
Figure 2: (above) Size and eletrokinetic impact.
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