Sedimentation stability and aging of aqueous dispersions of Laponite in the presence of cetyltrimethylammonium bromide

摘要

This work discusses the sedimentation stability and aging of aqueous suspensions of Laponite in the presencenof cetyltrimethylammonium bromide (CTAB). The concentration of Laponite was fixed at a constant level Cl = 2 %wt, which corresponds to the threshold between equilibrium gel IG1 and glass IG2 states. The concentrationnof CTAB Cs was within 0–0.3 %wt. In the presence of CTAB, the Laponite aqueous suspensions were unstablenagainst sedimentation and separated into the upper and bottom layers (U and B layers, respectively). Thendynamic light-scattering technique has revealed that addition of CTAB even at a rather small concentration,nCs = 0.0164 %wt (0.03 cation exchange capacity), induced noticeable changes in the aging dynamics of thenU layer. It was explained by equilibration of CTAB molecules that were initially nonuniformly distributednbetween different Laponite particles. Accelerated stability analysis by means of analytical centrifugation withnrotor speed ω = 500–4000 rpm revealed three sedimentation regimes: continuous (I, Cs < 0.14 %wt), zoneliken(II, 0.14 < Cs < 0.2 %wt), and gel-like (III, Cs > 0.2 %wt). It was demonstrated that the B layer was “soft”nin the zonelike regime. The increase of ω resulted in its supplementary compressing and collapse of “soft”nsediment above certain critical centrifugal acceleration. The physical nature of the observed behavior, accountingnfor enhancement of hydrophobic interactions between Laponite particles, is discussed.