Dielectric Properties of Polyelectrolyte Aqueous Solutions:the Scaling ApproachPolyelectrolytes are macromolecules with manyionizable groups. In polar solvents, these groups can<strong>di</strong>ssociate, leaving charges on the polymer chain andreleasing counterions in solution. Polyelectrolytesfind widespread applications as functional poly-mersin many industrial processes as thickeners,<strong>di</strong>spersants, ion-exchange resins, enhanced oilrecovery agents etc. Moreover, DNA and a variety ofbiologically important macromolecules arepolyelecrolytes. Their properties depend on thefraction of <strong>di</strong>ssociated ionic groups, on the solventquality for the polymer backbone, the <strong>di</strong>electricconstant of the solvent and the salt concentra-tion.Fig. 1. Top - Different chain conformations areobserved in the <strong>di</strong>fferent concentration regimesand in <strong>di</strong>fferent solvents. Bottom: Scalingbehaviour of the <strong>di</strong>electric parameters as afunction of polymer concentration. a): theexpected exponent (α=1) for the “good solvent”model is retrieved using pure ethylene glycol (EG)as solvent, a good solvent for the polymeremployed; in contrast, the exponent α increasesabove 1 with the increase of the water content(poor solvent) in the mixture. b): the reversal isalso true: the expected exponent for a poorsolvent (1/3) is obtained for pure water solution.Long-range electrostatic interactions be-tweenpolyions and counterions and between <strong>di</strong>f-ferentcharged groups on the same polymer chainintroduces new scale lengths and the fine balancebetween long-range electrostatic and short-rangepolymer-solvent interactions gives rise to a com-plexphenomenology.We have recently investigated [1-3] the ra<strong>di</strong>o-wave<strong>di</strong>electric behaviour of aqueous polyelectro-lytesolutions in the light of the scaling approachproposed by Dobrynin and Rubinstein [4], as afunction of the solvent quality and in <strong>di</strong>fferentconcentration regimes, showing how the <strong>di</strong>fferentpolyion conformations reflect on the low-frequen-cypermittivity and electrical conductivity.As can be seen in Fig. 1 a) and b), for the par-tiallycharged PMVP-Cl polymer <strong>di</strong>ssolved in a mixedsolvent the pre<strong>di</strong>cted scaling behaviour is observe<strong>di</strong>n a wide concentration range. For this polymer,water and ethylene glycol [EG] behave as poor andgood solvent, respectively, and in both cases apower law is clearly evidenced. In particular, whenthe data are analyzed accor<strong>di</strong>ng to the “good solventmodel” (Fig. 1 a), the expected value of theexponent in the polyion concentration dependenceα=1 is attained for X=1 (pure ethylene glycol), whilea progressive deviation is observed as the quality ofthe solvent changes, approaching the “poor solvent”,i.e., as the mole fraction of water is progressivelyincreased. Analogously, when the same data areanalyzed accor<strong>di</strong>ng to the “poor solvent model” (Fig.1 b), the expected value of the exponent α=1/3 isretrieved at X=0 (pure water), whereas deviationsare apparent when the EG con-centration in thesolution increases (good-solvent con<strong>di</strong>tion). Theobserved scaling behaviour of the <strong>di</strong>electricproperties of polyelectrolytes in solvents of <strong>di</strong>fferentquality gives strong support to the existence of a“necklace” conformation of the chain, under poorsolvent con<strong>di</strong>tion.References[1] F.Bor<strong>di</strong>, C.Cametti, T.Gili, S.Sennato, S.Zuzzi,S.Dou, R.H.Colby, Phys. Rev. E 72, 031806 (2005).[2] F.Bor<strong>di</strong>, C.Cametti, T.Gili, S.Sennato, S.Zuzzi,S.Dou, R.H.Colby, Journal of Physical Chemistry122, 234906 (2005).[3] Bor<strong>di</strong>, C. Cametti, S. Sennato, S. Zuzzi, S. Dou,R. H. Colby, Phys Chem Chem Phys. 2006; 8:3653-3658[4] A. V. Dobrynin, M. Rubinstein, Prog. Polym. Sci.30, 1049 (2005).AuthorsF. Bor<strong>di</strong> § , C. Cametti § , S. Sennato § , S. Dou # , R. H.Colby #§Dip. <strong>di</strong> <strong>Fisica</strong>, Università <strong>di</strong> Roma ″La <strong>Sapienza</strong>″,Roma and INFM CRS-SOFT# Dept. of Material. Sci. and Engineering, ThePennsylvania State University, University Park, PA,USA.87SOFT Scientific <strong>Report</strong> 2004-06
Scientific <strong>Report</strong> – Self Assembly, Clustering, Structural arrestAging of <strong>di</strong>lute aqueous Laponite suspensions stu<strong>di</strong>ed bymeans of 23 Na Triple-Quantum NMR spectroscopyDuring the last years a drastic change has takenplace in application and stu<strong>di</strong>es 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 stu<strong>di</strong>es 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 concentrationabc<strong>di</strong>mpossible by standard NMR techniques based onsingle-quantum coherences.We set up a new NMR procedure based on standardrelaxation time measurements and advanced triplequantumso<strong>di</strong>um 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 so<strong>di</strong>um ions involved in electrostatic screening(bound counterions concentration) p b to describecounterions dynamical properties.Laponite is composed of rigid <strong>di</strong>sc-shaped crystalswith 1nm thickness and 30nm <strong>di</strong>ameter. 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 laponite<strong>di</strong>spersion 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 fin<strong>di</strong>ngs show that boundcounterions concentration, which determine theLaponite <strong>di</strong>sks electrostatic screening length, plays afundamental role in the <strong>di</strong>fferentiation of the route togelation. Two important observations have beendone. First of all, arrested phase in isotropicsolutions have been observed. Furthermore,<strong>di</strong>fference 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 <strong>di</strong>skes 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 <strong>di</strong> Roma “La<strong>Sapienza</strong>”, novembre 2005.Authors:S. Capuani (a), T. Gili (b), B. Maraviglia (c).(a) INFM-CRS-SOFT (CNR), Dip. <strong>di</strong> <strong>Fisica</strong>, Univ. “La<strong>Sapienza</strong>”, Roma, Italy. (b) Centro Enrico Fermi, Dip.<strong>di</strong> <strong>Fisica</strong> Univ. “La <strong>Sapienza</strong>”, Roma Italy.(c) Dip. <strong>di</strong><strong>Fisica</strong> Univ. “La <strong>Sapienza</strong>”, Roma, Italy.SOFT Scientific <strong>Report</strong> 2004-0688
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