Structures of some surfactant polyelectrolyte complexes

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1 PRAMANA cfl Indin Acdemy of Sciences Vol. 61, No. 2 journl of August 2003 physics pp Structures of some surfctnt polyelectrolyte complexes REMA KRISHNASWAMY 1, V A RAGHUNATHAN 1 nd A K SOOD 2 1 Rmn Reserch Institute, C. V. Rmn Avenue, Bnglore , Indi 2 Deprtment of Physics, Indin Institute of Science, Bnglore , Indi Astrct. Structures of complexes formed in queous solutions y some nionic polyelectrolytes (doule nd single strnded (ds nd ss) DNA, poly(vinyl sulfonte) (PVS), nd poly(styrene sulfonte) (PSS)) with ctionic surfctnt system consisting of cetyltrimethylmmonium romide (CTAB) nd sodium 3-hydroxy-2-nphthote (SHN) hve een determined using smll ngle X- ry diffrction. All complexes re found to hve two-dimensionl (2-D) hexgonl structure t low SHN concentrtions. Anlysis of the diffrction dt shows tht the ds DNA CTAB complex hs n interclted structure, with ech DNA strnd surrounded y three cylindricl micelles. On incresing SHN concentrtion, DNA CTAB SHN complexes exhiit hexgonl-to-lmellr trnsition, wheres PVS complexes show hexgonl! centered rectngulr! lmellr trnsition. PSS complexes show yet nother sequence of structures. These results indicte the significnt influence of the chemicl nture of the polyelectrolyte on the structure of the complexes. Keywords. Surfctnt solutions; polyelectrolytes; DNA. PACS Nos E; Eq; By 1. Introduction Polyelectrolytes form complexes with oppositely chrged surfctnts in queous solutions, which re of considerle interest due to their potentil industril nd iomedicl pplictions [1]. A significnt frction of the counter-ions of highly chrged polyelectrolyte is condensed on to the polymer chin in solution [2]. These s well s the condensed surfctnt counter-ions re relesed on complextion. The resultnt increse in the trnsltionl entropy of the counter-ions is the min driving force for the formtion of these complexes [3]. Complexes of ds DNA with doule-tiled ctionic lipids hve een studied in detil using X-ry diffrction. These systems re found to exhiit either lmellr or 2-D hexgonl phse depending upon the lipids used [4,5]. The diffrction pttern of the lmellr phse is consistent with n interclted structure, with the DNA strnds sndwiched etween lipid ilyers. The hexgonl phse consists of close-pcked inverted cylindricl micelles, with the DNA strnds confined in their queous cores. A vriety of structures hs lso een oserved in complexes of polyelectrolytes with single chin surfctnts. Depending on the surfctnt concentrtion nd the flexiility of the polyelectrolyte, they re found to form either cuic or hexgonl structures [6 8]. Here we present results of smll ngle X-ry diffrction studies on mixed surfctnt polyelectrolyte complexes; some of the results on DNA polyelectrolyte complexes hve 447

2 Rem Krishnswmy, V A Rghunthn nd A K Sood Figure 1. Schemtic digrm of the interclted hexgonl phse, where ech DNA strnd (denoted y the open circles) is surrounded y three cylindricl micelles. The lttice prmeter, = p 3(R m +R DNA ),wherer m is the rdius of the cylindricl micelle (ο 2.0 nm) nd R DNA tht of hydrted DNA strnd (ο 1.25 nm). d d DNA Figure 2. Schemtic digrm of the lmellr phse of DNA surfctnt complexes, where the DNA strnds re sndwiched etween surfctnt ilyers. recently een pulished [9]. The surfctnt system used is mixture of the single-chined surfctnt CTAB, which forms cylindricl micelles in dilute solutions [10], nd SHN. The ddition of SHN to CTAB is known to led to cylinder to ilyer trnsformtion of the surfctnt ggregtes t reltive SHN concentrtion, α (=[SHN]=[CTAB]) ο 0.64 [11]. Complexes of ll the three polyelectrolytes hve hexgonl structure t lower vlues ofα. Anlysis of diffrction dt from ds DNA CTAB complex shows tht it hs n interclted hexgonl structure, where ech DNA strnd is surrounded y three cylindricl micelles (figure 1). At higher α lmellr complex is otined in the cse of DNA, whose structure is similr to tht found in the cse of doule-chined lipids (figure 2). In PVS complexes we find hexgonl! centered rectngulr! lmellr trnsition on incresing α. PSS complexes exhiit primitive rectngulr structure for intermedite vlues of α; thigher αnother structure is formed, which hs not yet een identified. These results indicte tht the chemicl nture of the polyion is n importnt prmeter in determining the structure of the complexes. 448 Prmn J. Phys., Vol. 61, No. 2, August 2003

3 2. Experimentl Structures of some surfctnt polyelectrolyte complexes Sodium slts of clf thymus ds DNA (30 50 kp) ws purchsed from Sigm. CTAB, 3- hydroxy-2-nphthoic cid (HNA), nd sodium slts of PVS nd PSS (MW=70000) were otined from Aldrich. M13 mp 18 ss DNA (7250 p) ws otined from Bnglore Genei. Sodium slt of HNA ws prepred y dding equivlent mounts of NOH to the cid solution. Other chemicls were used s received. The surfctnt solutions were prepred in deionized wter (Millipore), with the reltive SHN concentrtion, α, vrying from 0 to 0.7. The complexes precipitted out immeditely on dding the polyelectrolyte to the surfctnt solution. They were tken in glss cpillries long with some of the superntnt for X-ry diffrction studies. The reltive concentrtion of the polyelectrolyte, ρ = (weight of CTAB)=(weight of polyelectrolyte), ws vried over wide rnge out the isoelectric point, ρ iso, where the positive chrges of the CTA + ions re lnced y the HN ions nd the negtive chrges on the polyelectrolyte. Cu K α rdition from rotting node X-ry genertor (Rigku, UltrX 18) operting t 50 kv nd 80 ma ws used to produce the diffrction ptterns, which were collected on n imge plte (Mrreserch). 3. Results nd discussion 3.1 DNA CTAB SHN complexes These re found to e irefringent under polrizing microscope. Diffrction pttern of the ds DNA CTAB system shows three peks in the smll ngle region (figure 3). The mgnitudes of their scttering vectors, q, re in the rtio 1: p 3:2, corresponding to the (1,0), (1,1) nd (2,0) reflections from 2-D hexgonl lttice with lttice prmeter = 5:64 ± 0:09 nm. The pek positions nd their reltive intensities re independent of ρ nd CTAB concentrtion up to 300 mm. There re two possile wys of pcking DNA strnds nd CTAB micelles in 2-D hexgonl lttice, one of which is the interclted phse shown in figure 1, nd the other is the inverted hexgonl phse seen in some DNA lipid complexes [5]. Neither of them cn e ruled out on the sis of the oserved vlues of. Intensity(ritrry units) q (nm 1 ) Figure 3. Diffrction pttern of DNA CTAB complex t ρ = 7:2. Prmn J. Phys., Vol. 61, No. 2, August

4 Rem Krishnswmy, V A Rghunthn nd A K Sood Intensity(ritrry units) (nm 1 ) q Figure 4. Diffrction ptterns of DNA CTAB SHN complexes t α = 0:7. Curves () nd () correspond to ρ = 14:4 nd 3.0, respectively. The rrow on curve () indictes in-plne DNA DNA correltion pek. ρ iso = 3:74 t α = 0:7. The hexgonl phse persists in the presence of SHN, with incresing grdully to 6.06 nm t α = 0:55. For α > 0:6 lmellr complex is otined (figure 4 nd ). In order to determine the structure of the hexgonl phse, we hve crried out detiled nlysis of the diffrction dt. Models for the electron density, ρ(x;y), in plne norml to the xis of the cylindricl micelles in the two possile structures were constructed nd their diffrction ptterns were clculted. In these models, the DNA strnd ws represented s circulr disc of uniform electron density ρ D. Note tht ρ D hs contriutions not only from the toms in the DNA molecule, ut lso from wter molecules nd counter-ions present in the solution. The rdius r D of this disc ws tken to e tht of DNA molecule with hydrtion shell round it (1.25 nm). Ech cylindricl micelle ws represented s circulr disc of electron density ρ c nd rdius r c corresponding to the chin region, surrounded y n nnulr ring of density ρ h nd width r h corresponding to the hedgroup region. The inverted micelle ws modeled s n nnulr ring of density ρ h nd width r h, corresponding to the hedgroup region, surrounding the circulr disc representing the DNA molecule. Vlues of ρ c, ρ h, ρ w (electron density of wter), r c,ndr h were tken from the literture [12]. Becuse of the different contriutions to ρ D, mentioned erlier, it could not e estimted. Therefore, the reltive intensities of the (1,1) nd (2,0) reflections with respect to tht of the (1,0) reflection were clculted from the two models for resonle rnge of vlues of ρ D (figure 5). As cn e seen from the figure, only in the cse of the interclted hexgonl phse the clculted nd oserved intensities mtch for prticulr vlue ofρ D, thus confirming this structure. As mentioned erlier, DNA CTAB SHN complexes exhiit hexgonl-to-lmellr trnsition ner the vlue of α t which the surfctnt ggregtes show cylinder-to-ilyer trnsformtion. This implies tht the shpe of the surfctnt ggregtes in the complexes is the sme s tht in the surfctnt solution. Therefore, this oservtion lso supports the interclted structure, where ech DNA strnd is surrounded y three cylindricl micelles (figure 1). Structure of the lmellr DNA complex is similr to tht found in DNA lipid systems, with periodicity of ο 5.5 nm (figure 2) [4]. At DNA concentrtions elow the isoelectric point (ρ iso = 2:81 t α = 0:6), we could not oserve pek corresponding to the DNA DNA seprtion in the ilyer plne, proly due to its proximity to the strong first-order 450 Prmn J. Phys., Vol. 61, No. 2, August 2003

5 Structures of some surfctnt polyelectrolyte complexes 0.15 i 2 reltive intensity 0.1 i i 2 i 3 i 3 i ρ DNA Figure 5. Vrition of the reltive intensities of the (1,1) (i 2 )nd(2,0)(i 3 ) reflections with respect to tht of the (1,0) reflection, with ρ D. The dshed lines correspond to the interclted hexgonl model nd the solid ones to the inverted hexgonl model. The dotted lines correspond to the oserved vlues. Note tht only in the cse of the inverted hexgonl model the clculted intensities mtch with the oserved ones for prticulr vlue of ρ D. lmellr pek (figure 4). However, t higher DNA concentrtions it is clerly seen in the diffrction pttern (figure 4). Our oservtions re consistent with rpid chnge in the DNA DNA spcing ner the isoelectric point, s found in DNA lipid complexes [13]. ss DNA complexes lso show hexgonl phse t lower vlues of α nd lmellr phse t higher α. The lttice prmeters in this cse re only out 0.5 nm less thn those in the cse of ds DNA complexes. 3.2 PVS CTAB SHN complexes These complexes re lso irefringent nd diffrction pttern of the SHN-free complex corresponds to 2-D hexgonl lttice, with = 4:64±0:08 nm (figure 6). The hexgonl phse is stle up to α ο 0:4. increses grdully with SHN concentrtion nd is equl to 5.16 ± 0.08 nm t α = 0:4. For vlues of α in the rnge , 2-D centered rectngulr lttice (cmm) is otined (figure 6), with lttice prmeters = 11:34 nm nd = 4:74 nm. Interestingly, for α > 0.7 lmellr phse is otined with spcing of 4.34 nm (figure 6c). 3.3 PSS CTAB SHN complexes These lso form hexgonl phse t low SHN concentrtions (figure 7). As in other cses, increses with α, from 4.64 nm t α = 0to5.05± 0.05 nm t α = 0:2. For α etween 0.4 nd 0.6 the peks in the diffrction ptterns (figure 7) cn e indexed on rectngulr lttice corresponding to the plne group pgg. The lttice prmeters nd vry from 9.34 nd 5.36 nm t α = 0: nd 5.56 nm t α = 0:6. At higher SHN concentrtions yet nother phse is formed, which we hve not een le to identify. The Prmn J. Phys., Vol. 61, No. 2, August

6 Rem Krishnswmy, V A Rghunthn nd A K Sood Intensity(ritrry units) c q (nm 1 ) Figure 6. Diffrction ptterns of PVS CTAB SHN complexes. α nd ρ for the different curves re: 0, 1.1 (); 0.4, 1.1 (); 0.7, 1.1 (c). Intensity(ritrry units) c q (nm 1 ) Figure 7. Diffrction ptterns of PSS CTAB SHN complexes. α nd ρ for the different curves re: 0, 1.8 (); 0.6, 4.5 (); 0.7, 12.0 (c). diffrction pttern of this phse shows three peks in the smll ngle region without ny specific reltion etween the corresponding q (figure 7c). The hexgonl phse of PVS nd PSS complexes with CTAB SHN cn e expected to consist of cylindricl micelles ridged y polyelectrolyte chins, s in the cse of poly(crylic cid) (PAA) CTAB complexes [7]. The lttice prmeters of the rectngulr phse of PVS nd PSS complexes do not depend on the polyelectrolyte concentrtion. Therefore, these structures most proly consist of rion-like ggregtes rrnged on 2-D rectngulr lttice (figures 8 nd 9), s found in some surfctnt systems in etween the hexgonl nd lmellr phses [14,15]. 4. Conclusions The complexes formed y some nionic polyelectrolytes in queous solutions with ctionic surfctnt system hve een studied using smll ngle X-ry diffrction. These complexes re found to show sequence of structures on chnging the surfctnt composition. 452 Prmn J. Phys., Vol. 61, No. 2, August 2003

7 Structures of some surfctnt polyelectrolyte complexes Figure 8. Schemtic digrm of the centered rectngulr cmm phse seen in PVS complexes t intermedite SHN concentrtions, consisting of rion-like surfctnt ggregtes ridged y polymer chins. Figure 9. Schemtic digrm of the rectngulr pgg phse oserved in PSS complexes t intermedite SHN concentrtions, consisting of rion-like surfctnt ggregtes ridged y polymer chins. Different structures re lso otined with different polyelectrolytes. The chrge densities of ll the four polyelectrolytes used re comprle. The persistence length of ss DNA nd PVS re ο1 nm, wheres those of ds DNA nd PSS re out 50 nd 10 nm, respectively. Therefore, differences in the structures otined with different polyelectrolytes cnnot e ttriuted to differences in the vlues of these prmeters. Hence we my conclude tht the results presented here show the importnce of specific interctions in the formtion of these complexes. Acknowledgements We thnk Dominique Lngevin, M Muthukumr nd Srirm Rmswmy for helpful discussions. AKS thnks K R K Eswrn for his involvement in the erly stges of this work. References [1] See, for exmple, Interctions of surfctnts with polymers nd proteins edited y E D Goddrd nd K P Annthpdmnhn (CRC Press, Boc Rton, 1993) [2] F Oosw, J. Polym. Sci. 23, 421 (1957) G S Mnning,J. Chem. Phys. 51, 924 (1969) Prmn J. Phys., Vol. 61, No. 2, August

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