Dynamic Light Scattering Study and DLVO Analysis of Physicochemical Interactions in Colloidal Suspensions of Charged Disks

Abstract

The interparticle interactions in colloidal suspensions of charged disks of Laponite clay in water were investigated using dynamic light scattering (DLS) and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We studied the effects of clay concentration (C(L)), the concentration of externally added salt (C(S)), and temperature (T) on the microscopic dynamics of the clay suspensions. The fast (τ1) and mean slow relaxation times (⟨τ(ww)⟩) of Laponite suspensions were extracted from intensity autocorrelation functions measured at different waiting times (t(w)) after sample preparation. Comprehensive Laponite concentration-salt concentration-temperature-time superpositions of both the microscopic diffusive time scales and the stretching exponent corresponding to the slow relaxation process highlight the self-similar nature of the energy landscapes of the Laponite suspensions. The evolution of the sodium ion concentration in the aging suspension with tw, measured for several values of CL, CS, and T, was used in a DLVO analysis of the free energy of the suspension for two charged disks parallely approaching one another. This analysis confirms that, in addition to repulsive interparticle interactions, attractive interactions also play a pivotal role in the microscopic dynamics of spontaneously evolving Laponite suspensions.

Kinetics of the glass transition of fragile soft colloidal suspensions

“…In our experiment, it is to be noted that while the proportion of attractive and repulsive interaction changes with C S , the softness of interparticle interactions and the compressibility of the Laponite particles do not change. This is estab- lished from the observed self-similarity of the potential energy landscape with C S reported earlier [26]. Hence, the apparent insensitivity of D to changes in C S confirms several earlier simulations and experimental results on colloidal and molecular glass formers [8,9,31].…”

“…Since the number density of Laponite particles in suspension is directly proportional to C L , multi-body interactions are enhanced with increasing C L , thereby shifting the onset of the glass transition to earlier times ( figure 2(a)). This feature is reminiscent of a previous observation in dense colloidal suspensions [29,30] and has been also discussed earlier in the context of the aging of Laponite suspensions [17,26].…”

“…< τ ww > is plotted for different values of T with C L =3.0% w/v and C S =0.05 mM in figure 2(e). It is seen that increase in T accelerates the time-evolution of < τ ww > [26,32]. In figure 2(f), D (•) and t ∞ α ( ) are plotted for the data shown in figure 2(e).…”

“…• C. It is seen from the plots that the evolution of < τ ww > becomes faster with increase in C L [26]. The fragility parameters D (•) and the Vogel times Kob-Anderson (KA) model glass formers has a very weak dependence on density [27].…”

“…• C [26]. D (•) and t This is due to the enhancement of the screening of the interparticle repulsive interactions and the increasingly important role that interparticle attractions play in spontaneously evolving Laponite suspensions.…”