Soft Report - Dipartimento di Fisica - Sapienza

Aging in Charged Colloidal SystemsColloidal dispersions are a very interesting class ofmatter. Due to their rich phenomenology andproperties they can be used to test a large variety oftheoretical models, or vice versa theories can besubjected to stringent experimental tests. In somecases in fact the inter-particle interactions can betuned almost ad-hoc, e. g. by changing the ionicstrength in the system. In this context the study ofLaponite system is particularly intriguing. LaponiteRD is a disk like clay particle with a well definedthickness of 2H=1 nm and a diameter of about2R=30 nm (fig. 1A). When laponite is dispersed inwater a strong negative charge appears on faceswhile, depending on the pH of the solution, a weakpositive or negative charge appears on the rim.Therefore in this colloidal system long rangeelectrostatic repulsion and short range attractiveinteractions are both present and the competitionbetween the two interactions makes Laponitesuspensions even more interesting.In particular a new phase diagram has shown as wecan be in presence of phase separation, liquid,attractive gel, attractive or repulsive glass, justchanging the clay amount and/or the presence ofsalt in the solutions [2]. For this reasons we canconsider Laponite suspensions as a model system todescribe disk like charged particles in presence ofboth short range attraction and long range repulsion.We performed a dynamic light scattering study of theaging phenomenology in laponite dispersions inwater, varying clay and salt concentrations. Allsamples studied, surprisingly also those predicted tobe in a liquid stable phase, experience aging (fig.1B).After a certain waiting time, starting when thesample is filtered, a non ergodic state is reached asshown from an incomplete decay of theautocorrelation function (fig.1B). More the sample isconcentrated less is the waiting time necessary toobtain an arrested phase. Moreover, at a fixedlaponite concentration, this time decreases withadding salt, reflecting the screening of the repulsivepart of the interaction between particles. All theautocorrelation data show two dynamical regimes.The fast “diffusive” behavior is described by a singleexponential decay with a fast relaxation time τ 1. Theslow one is well described by a stretched exponentialdecay defined by the relaxation time τ 2 and thestretching exponent β. From the analysis of thewaiting time behavior of the slow dynamics it ispossible to identify two dynamical routes to reach afinal arrested state, one at "low" (open circles in fig.1C) and one at "high" (full circles in fig. 1C) clayconcentrations. This dynamical difference should bea signature of two different structuralrearrangements of the system realized because ofthe competition between the repulsive and theattractive parts of the potential. These new resultsdetermines a redrawn of a part of the phase diagram(fig. 1C). In particular the predicted liquid region (ILin fig. 1C) is found not to be a stable phase, aspreviously determined, but an arrested phase.Moreover the presence of two different arrestedstates, one at "low" and one at "high" clayconcentrations, and a transition line between themare identified [3],[4] .References[1] A. Mourchid, A. Delville, J. Lambard, E. Lecolier,and P. Levitz, Langmuir 11, 1942 (1995).[2] H. Tanaka, J. Meunier, and D. Bonn, Phys. Rev. E69, 031404 (2004).[3] B. Ruzicka, L.Zulian and G. Ruocco, Phys. Rev.Lett. 93, 258301 (2004).[4] B. Ruzicka, L.Zulian and G. Ruocco, Langmuir,22, 1106 (2006).Fig.1: (A) Laponite crystal. (B) Agingphenomenon in sample with C w=1.0%, C s=2x10 -3 M. (C) Phase diagram of Laponite dispersion, thered line indicates the hypothesized new transitionline between two different arrested states.AuthorsB. Ruzicka (a), L. Zulian (b,a), G. Ruocco (c,a)(a) CRS SOFT-CNR-INFM, c/o Universita’ di Roma« La Sapienza » P.le A. Moro 2, 00185 Roma.(b) Dipartimento di Fisica Universita’ degli studi diPerugia, via A. Pascoli 06123, Perugia, Italy.(c) Dipartimento di Fisica, Universita’ Roma « LaSapienza », P.le A. Moro 2, 00185, Roma, Italy.79SOFT Scientific Report 2004-06