DYNAMIC LIGHT-SCATTERING STUDY OF SODIUM DODECYL-SULFATE ADSORPTION ON POLYSTYRENE LATEX-PARTICLES MODIFIED WITH AROMATIC AMINO-GROUPS

The adsorption of sodium dodecyl sulfate (SDS) to polystyrene latex pa rticles surface-modified with aromatic amino groups has been examined as a function of surfactant concentration by determination of the part icle hydrodynamic radius (RH) using dynamic light scattering. Adsorpti on isotherms were determined using a surface tension method. At pH 6, without added NaBr, SDS initially adsorbs with the sulfate head intera cting with the amino group by hydrogen bonding and with simultaneous c ontact between the alkyl tail and the polymer chains. The latter leads to aggregation of neighboring polymer chains through bridging by the alkyl tails. Combination of hydrophobic interaction and hydrogen bondi ng thus leads to an initially strong binding of SDS with the latex sur face, resulting in an L-type isotherm. Subsequently, SDS molecules ads orb on the polymer chains by hydrophobic interaction, with the sulfate heads extended toward the solution. Further adsorption occurs about t hese SDS molecules, and finally globular-like micelles are formed whic h are anchored on the polymer chains. Added NaBr, even at the 1 mM lev el, effectively screens the interactions between the sulfate head of S DS and the amino group. Hydrophobic interaction between the polymer ch ain and the alkyl tail, and also between the alkyl tails themselves, i s greatly enhanced. Adsorption isotherms are then of the S type in the presence of NaBr. At high ionic strength an adsorbed monolayer forms at the latex surface rather than as micelles sited on the polymer chai ns since there is a high density of polystyrene chains at the latex su rface. At pH 2, strong electrostatic attraction exists between the sul fate entity on the polymer chain and the amino group. This restricts f ree extension of the polymer chains and also hinders hydrogen bonding between SDS and the amino groups. SDS will adsorb on polystyrene chain s which are anchored in a looped. conformation on the surface, finally forming semiglobular aggregates at saturation. At pH 12, the polymer chain layer at the bare latex surface is strongly compressed. The amin o groups associated with OH- ions and the sulfate entities together gi ve a high charge density at the latex surface. Enhanced hydrophobic in teraction results on adding electrolyte (base) ions; i.e., the electro lyte promotes SDS adsorption with the alkyl tail in contact with the p olymer chain leading to cooperative adsorption of SDS molecules. Elect rostatic repulsion between the amino groups, the heads of the adsorbed SDS molecules, and the sulfate entities on the polymer chains extends the polymer chains on which SDS is adsorbed. The extended chains form a suitable steric environment for further SDS adsorption and allow a large adsorbed amount with formation of globular micelles strung along the polymer chains at the cmc. Above the cmc, further adsorption corr esponds to growth of the adsorbed micelles. At both pH 2 and pH 12, S- type adsorption isotherms were obtained.