ADSORPTION OF ALKYLTRIMETHYLAMMONIUM BROMIDES ON NEGATIVELY CHARGED POLYSTYRENE LATEX-PARTICLES USING DYNAMIC LIGHT-SCATTERING AND ADSORPTION-ISOTHERM MEASUREMENTS

The adsorption of the cationic surfactant dodecyltrimethylammonium bro mide (DTAB) on polystyrene latex particles of low negative charge has been examined as a function of surfactant concentration by estimation of the hydrodynamic radius by dynamic light scattering measurements. A dsorption isotherms were also determined by a surface tension techniqu e. The ''hairy'' layer at the latex surface is considered to be an imp ortant factor determining surfactant adsorption. The DTA(+) ions are i nitially firmly bound at the extremities of the polystyrene chains thr ough both charge attraction between the quaternary ammonium head group s of DTAB and the terminal sulfate entities of the polymer chain and b y hydrophobic interaction between the alkyl tails and the polymer back bone. This leads to L-type isotherms in the salt-free system and at 1 mM added NaBr. Further adsorption of surfactant subsequently occurs ar ound the initially adsorbed DTA(+) ions through hydrophobic interactio n between the alkyl tails of the surfactant, finally leading to coilin g of the polymer segments and formation of semiglobular aggregates at the latex surface. The clear two-step character of the isotherms and t he corresponding contraction of the adsorbed layer reflect this adsorp tion mechanism. At high ionic strength, enhanced adsorption leads to c lose packing of the DTA(+) ions because of effective screening of the interactions between the quaternary ammonium head groups. Thus semiglo bular aggregates of greater area can form at the latex surface. Furthe r adsorption above the cmc will eventually cause coalescence of adjace nt aggregates, forming an adsorbed monolayer comprised of a continuous hydrophobic phase intercalated by the heads of surfactants and the su rface. Effective screening of interactions between the head groups of the adsorbed DTA(+) ions as well as enhanced hydrophobic interaction b etween the alkyl tails of the surfactants results in an S-type isother m at high ionic strength. The adsorbed amount decreases with increasin g alkyl chain length of the cationic surfactant and this effect is con sidered to derive from the effect of the ''hairy'' layer at the latex surface. Enhanced hydrophobic interaction promotes bridging of adjacen t polymer chains on adsorption of the cationic surfactant leading to a ggregation of chains and decreased adsorption.