The role of long tails in steric stabilization and hydrodynamic layer thickness

In steric stabilization, the ''thickness'' of the adsorbed layer on the particles is of key importance in determining the stability of the colloid. There is evidence to suggest that both the adlayer thickness, as measured by hydrodynamic means, and the colloid stability are determined solely by the most outward-extending tails in the adlayer, even when the tail density is quite low. The present study seeks to investigate experimentally and to quantify the relationship between tail density, hydrodynamic adlayer thickness and colloid stability for a model system consisting of a bimodal adlayer of triblock copolymers anchored to the surface of a latex colloid. The low molar mass component of the adlayer has a thickness too small to impart significant steric stability by itself, thereby isolating the stability attributable to the long tails provided by the high molar mass component. By systematic variation of the adlayer composition, the density of tails required to impart marginal stability can be determined and compared to the effect of tail density on the hydrodynamic thickness. For the system investigated, the adlayer proportion of long tails required for marginal stability was less than 0.5%, in good agreement with the proportion required to significantly increase the hydrodynamic adlayer thickness. The practical inference is that the most efficient steric stabilizers (on a per unit mass basis) are highly polydisperse mixtures with only a very small proportion of the highest molar mass constituent.