F. Blockhaus et al., ADSORPTION-DESORPTION BEHAVIOR OF ACRYLIC-MALEIC ACID COPOLYMER AT CLAY-MINERALS, Journal of colloid and interface science, 186(2), 1997, pp. 234-247
The interactions between acrylic-maleic acid (PAA-PMLA) copolymer with
clay minerals are investigated in terms of adsorption/desorption beha
vior. The adsorption isotherms were obtained as a function of clay min
eral structures, pH and ionic strength by sensitive polyelectrolyte ti
tration and radiotracer methods. The nature and the location of bindin
g sites for PAA-PMLA copolymer on aluminol sites at the edge surface c
an be stated. In the case of the adsorption on kaolinite, the pi-I-dep
endent interaction in relatively low ionic strength is discussed not o
nly in terms of electrostatic contributions, but evidences for a ligan
d exchange mechanism are also presented. Increasing the ionic strength
also enhances the adsorption by a screening of the charges in the ads
orbed layer especially when the charges of the copolymer and the surfa
ce are of the same sign. The adsorbed PAA-PMLA copolymer can be displa
ced by phosphate compounds such as monophosphate and sodium tripolypho
sphate (STP). The observed desorption process results from a competiti
on between PAA-PMLA copolymer and phosphate compounds for the same bin
ding sites on the kaolinite surface. Adsorption equilibrium constants
can thus be derived from single and mixture adsorption isotherms. Effe
cts due to the heterogeneity of the kaolinite surface and to the confo
rmation of the adsorbed PAA-PMLA copolymer are also discussed. Strongl
y bound PAA-PMLA copolymer traces support the Ligand exchange mechanis
m at the aluminol sites of the edge surface. Clay minerals may thus ac
t as natural barriers in the soil transport of the polycarboxylates us
ed in phosphate-free detergents. (C) 1997 Academic Press.