Biology Reference
In-Depth Information
Oncea chain isstarted itpropagates according to
M
j
+
→
M
M
j
+
1
As the length of the chain increases the molecule becomes
decreasingly water soluble, until it eventually comes out of the
solution and forms a primary particle. Thermal motion in the
solution causes collision between primary particles, leading to
coagulation and fusion into larger particles. These are spherical
because interfacial tension acts to minimize the interfacial area.
Sincetheinitiatorfreeradicalistypicallyawater-solubleionicgroup
suchasSO
3
−
or OSO
3
−
, it imparts a charge to the primary particle.
As primary particles coagulate, the surface charge density of the
growing sphere increases. This leads to electrostatic repulsion,
slowing and eventually stopping further coagulation. It is often easy
to produce latexes of a very narrow size distribution, described as
“monodisperse.” Synthetic methods appropriate to the construction
of micron and sub-micron sized latex electrochemical labels include
the synthesis of polystyrene (PS) latex colloids [54], which are then
present during the synthesis of polystyrenesulphonate (PSS) [55],
leading to a negative PSS shell around the PS core; the copoly-
merization of styrene and acrylic acid to produce a polystyrene-
co
-
acrylic acid (PSA) coploymer [56], which has a negative charge due
to acrylic acid deprotonation. Other than sulphonate and sulphates,
functional groups which can be introduced to the latex by the
initiatorinclude alcohols,carboxylic acids, and
=
NH
2
+
[57].
8.5.3
Latex Solution Properties
In solution the latex spheres will experience van der Waals forces
of attraction, which at a separation
r
will be proportional to
r
−
6
.
For coagulation to not occur, these forces must be balanced by the
repulsive electrostatic force arising from either the ionic functional
groups on the latex, or adsorbed ionic surfactant. Hence, a latex
particleinanelectrolytewillsupportatightlyboundlayerofoneion
balancedbyadiffuselayerofanoppositelychargedion.Thisdiffuse
layer is equivalent to the diffuse layer at an electrode-solution
interface and so can be described by Gouy-Chapman theory [46].
Therefore, the width of the diffuse layer will be equal to the Debye
