Water-based latex dispersions 4. Exchange dynamics and residence times of nonylphenol ethoxylate in concentrated latex dispersions

The equilibrium residence times of the nonionic surfactant nonylphenol ethoxylate (NP100) in a latex dispersion were determined using NMR diffusometry. At 16% w/w particle concentration and 0.12, 0.43 and 0.81% w/w NP100, the residence times of the surfactant were 0.16, 1.02 and 4.73 s in solution (τ_A) and 0.3, 0.37 and 0.61 s on the surface of the particles (τ_B), respectively. At even higher particle concentration (>45% w/w), τ_A and τ_B were 1.47 and 2.2 s. Calculating the number of collisions that ought to result in adsorbed species, at 16% w/w, only 2, 5 and 2‰ (corresponding to 0.12, 0.43 and 0.81% w/w NP100) resulted in adsorption, whereas at >45% w/w, only 12‰ resulted in adsorption, which suggested that the surfactant was irreversibly adsorbed on the particles. The small increase in collision frequency with increased particle concentration could be a result of a diffusion controlled adsorption, while an energy barrier for desorption controlled the overall exchange dynamics in the dispersion. The slow dynamics in the dispersion was controlled, mainly by the nonylphenol group, which gave NP100 a strong preference to surfaces. In addition, the chain length of the poly(ethylene glycol) (PEG) group changed the solution behavior from being that of a typical surfactant to that of a polymer.