Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 194

0.50

0 0 0 0 0 0

0.45

1

0.40

0.1

0.35

0.01

0.30

0.001

0.25

apparent separation (nm)

0.20

0.15

0.10

0.05

0.00

0 20 40 60 80 100

apparent separation (nm)

Figure7.14 Normalisedforce-distancecurvefortheinteractionofcellulosesphereandkraft
ligninfilmatapHof8.5inabackgroundelectrolyteofNaClwithconcentrationof0.1mM.
The data were fit to DLVO theory in the limits of constant charge (upper fit) and constant
potential(lowerfit).Thefittingparameterswere ψ cell =− 3 mV, ψ lig =− 75 mV, κ −1 = 30 nm.

Insetshowsthesamedataonalog-linearscaletodemonstratetheexponentialdecay.Adapted
with permission from Notley and Norgren (2006). Copyright (2006), American Chemical
Society.

film, a short range steric force was observed consistent with the increase in solubility
of the polymer.
The determination of the surface charge and the surface potential of the lignin films
as a function of aqueous solution conditions allow an understanding of the surface
interactions apparent in a number of industrial applications with a prominent example
being paper-making where wood fibres with lignin rich surfaces interact in aqueous
media. In the paper-furnish, there are many other components with which lignin may
interact, in particular, polymeric additives that may be used to improve strength properties
of the finished paper or to retain inorganic filler particles. Thus, knowledge of the
interaction of lignin surfaces with a variety of polymers, polyelectrolytes and soft-matter
complexes is of great importance. Lignin model films have been used to study the
adsorption of polyelectrolytes in terms of both the kinetics and surface excess under
variable solution conditions such as pH, ionic strength and polymer concentration.
Polyelectrolytes of opposite charge to the kraft lignin films, that is, cationic poly-
electrolytes have been used in two studies. In the first, poly(allylamine hydrochloride)
(PAH), a polymer with weakly ionisable charged groups was used (Norgren et al . 2007).
There was an observed increase in adsorbed amount measured using the quartz crystal
microbalance as the pH of the solution was increased which is as expected for the
adsorption of a polyelectrolyte to an oppositely charged surface through purely ionic
interactions. Interestingly, though, in this study, a significant amount of an anionic
polyelectrolyte, poly(acrylic acid) (PAA) also adsorbed to the kraft lignin surface indi-
cating that nonionic interactions are also possible. When polyelectrolyte complexes of
PAH and PAA were prepared in different ratios which influences both size and charge,
both cationic and anionic charged complexes adsorbed to the surface as presented in