Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Self Assembly, Clustering, Structural arrestAging of dilute aqueous Laponite suspensions studied bymeans of 23 Na Triple-Quantum NMR spectroscopyDuring the last years a drastic change has takenplace in application and studies based on NuclearMagnetic Resonance (NMR), as a result of theavailability of high-field spectrometers and powerfulcomputers. As a consequence, a whole series of newmethods, such as multiple-quantum NMRspectroscopy, has risen. These methods haveproduced a qualitative change in NMR research byallowing dynamical studies in a way previouslyp b[%]τ c[ns]QCC [MHz]100959085807570656055503230282624222018160,800,750,700,650,600,550,501.2%1.4%0 50 100 150 200 250Correlation Time1.2 %1.4 %1.7 %2.0 %2.4 %2.8 %0 50 100 150 200 250t w[days]Slow DynamicsQuadrupolar Coupling Constant1.2 %1.4 %1.7 %2.0 %Slow Dynamics2.4 %2.8 %0 50 100 150 200 250t1.7%2.0%LCDaysBound 23 Na Counterions1.2% w/w1.4% w/w1.7% w/w2.0% w/wt w[days]HCFig. 1: (a) bound counterions, (b) correlationtime, (c) quadrupolar coupling constant, asfunction of aging time. (d) Schematic laponitesuspensions arrested phase at lowconcentration (LC) and at high concentrationabcdimpossible by standard NMR techniques based onsingle-quantum coherences.We set up a new NMR procedure based on standardrelaxation time measurements and advanced triplequantumsodium signal detection to monitoring agingin low density colloidal systems. This is obtained bymeans of measurements of NMR parameters fromwhich we extracted rotational correlation times τ c,quadrupolar coupling constant QCC and total amountof sodium ions involved in electrostatic screening(bound counterions concentration) p b to describecounterions dynamical properties.Laponite is composed of rigid disc-shaped crystalswith 1nm thickness and 30nm diameter. Each crystalis composed of roughly 1500 unit cells with anempirical chemical formula:Na + 0.7[(Si 8Mg 5.5Li 0.3)O 20(OH) 4] -0. 7.NMRmeasurements were performed on 23 Na nuclei usinga Bruker Avance 400 system with a 9.4 Tsuperconducting ultrashield magnet. Six laponitedispersion in water solution (D 2O+H 2O) have beenprepared at concentrations below the transitionisotropic liquid – isotropic gel (C w=1.2%, 1.4%,1.7%, 2.0%, 2.4%, 2.8% and I=10 -4 ).We monitored τ c, QCC and p b as function of agingtime (Figure 1). Our findings show that boundcounterions concentration, which determine theLaponite disks electrostatic screening length, plays afundamental role in the differentiation of the route togelation. Two important observations have beendone. First of all, arrested phase in isotropicsolutions have been observed. Furthermore,difference we observed in the way to reach thearrested phase as function of laponite concentration.At low concentrations (C w=1.2%, 1.4%, 1.7%) thearrested phase is due to the formation of clusters,while at high concentrations (C w=2.0%, 2.4%,2.8%) the single laponite platelets seems to be theelementary constituent of the entire system (Figure1). Thus, if for low concentrations we speak of acluster driven arrested phase achievement, as far ashigh concentrations is concerned a diskes networkdriven, arrested phase transition should be invoked.References[1] T. Gili, NMR Multiple Quantum Spectroscopy:application to the study of materials and biologicalsystems, PhD thesis, Università di Roma “LaSapienza”, novembre 2005.Authors:S. Capuani (a), T. Gili (b), B. Maraviglia (c).(a) INFM-CRS-SOFT (CNR), Dip. di Fisica, Univ. “LaSapienza”, Roma, Italy. (b) Centro Enrico Fermi, Dip.di Fisica Univ. “La Sapienza”, Roma Italy.(c) Dip. diFisica Univ. “La Sapienza”, Roma, Italy.SOFT Scientific Report 2004-0688