Biology Reference
In-Depth Information
Oppositely charged PEs can also form complexes in solution
[known as “interpolyelectrolyte complexes” (IPEC)] [69, 70]. Solid-
state nuclear magnetic resonance spectroscopy has shown that
these are structurally similar to l-b-l films, and hence l-b-l films
can be thought of as stacked layers of IPECs. Interestingly, IPEC
formation is almost entirely driven by the entropy increase [71],
which suggests that forces other than electrostatic attraction may
be used to form l-b-l films. This has been demonstrated for
hydrogen bonding[72-73], hydrophobicinteractions[74], andDNA
hybridization[75, 76].
Where electrostatic attraction
is
used for l-b-l film formation,
increasing the ionic strength of the solution will generally increase
thefilmthickness[77,78].Thisisthoughttobebecauseahighersalt
concentrationincreasestheshieldingaroundtheionicgroupsofthe
polyelectrolyte, causing itto adopt amorecoiled, compactform [71,
79, 80].
8.5.5
Layer-by-Layer Modification of Latex
8.5.5.1 Latex surface charge excess
To modify latex spheres by the l-b-l method, there must be a
stable charge excess on the colloid surface (also required to prevent
coagulation). In the case of PS latex commercial samples are
available, from suppliers such as Sigma, bearing sulphate groups.
Otherwise, those groups can be imparted by synthesizing PSS in the
presence of PS [55]. PSS is a strong electrolyte and therefore can be
expected to be fully dissociated. In the case of PSA latex copolymers
[56] the negative charge will be dependent on the polyacrylic acid
(PAH) deprotonation and therefore on the contacting pH. This
deprotonation was studied in detail recently for sub-micron PSA
spheres by Li
et al
. [81]. The dissociation proceeded as
(latex)
−
COOH
+
OH
−
→
(latex)
−
COO
−
+
H
2
O
Based on UV absorbances measured after latex dissolution, it was
found that the PAH:PS ratio in the solid was 0.34, resulting in
latexesthatwerehighlyhydrophobic.Thismeantthatdeprotonation
only extended approx. 1.5 nm into the sphere, which for the
diameter of 0.265
μ
mmeansthatofthetotalPAHintheparticle
