Table Of ContentInduced Crystallization of Polyelectrolyte-Surfactant Complexes at the Gas-Water
Interface
D. Vaknin1, S. Dahlke1, A. Travesset1, G. Nizri2, and S. Magdassi2
1Ames Laboratory and Department of Physics and Astronomy Iowa State University, Ames, Iowa 50011
2Institute of Applied Chemistry, Hebrew University of Jerusalem, Jerusalem, Israel 91904
(Dated: February 2, 2008)
4 Synchrotron-X-rayandsurfacetensionstudiesofastrongpolyelectrolyte(PE)inthesemi-dilute
0 regime(∼0.1M monomer-charges) withvaryingsurfactant concentrationsshowthat minutesurfac-
0 tant concentrations inducethe formation of a PE-surfactant complex at the gas/solution interface.
2 X-ray reflectivity and grazing angle X-ray diffraction (GIXD) provide detailed information of the
n topmostlayer,whereitisfoundthatthesurfactantformsatwo-dimensionalliquid-likemonolayer,
a with a noticeable disruption of the structure of water at the interface. With the addition of salt
J (NaCl) columnar-crystals with distorted-hexagonal symmetry are formed.
2
1 PACSnumbers: 82.70.Uv,82.35.Rs,81.07.Nb
]
t There has been a growing interest in the phase- ride (PDAC) (molecules are shown in Fig. 1). Sodium
f
o behavior, aggregation and precipitation of polymer-
s surfactantmixtures,inparticularofionicsurfactantsand
.
t oppositelychargedflexible polyelectrolytes(PEs)[1,2,3,
a
m 4, 5, 6], or semi-flexible PEs such as DNA[7] or actin[8].
In addition to the fundamental interest in the principles
-
d governing phase-behavior, aggregationand precipitation
n of polymer-surfactant mixtures, understanding the be-
o haviorofthese complexsystemsiscrucialfortechnologi-
c calapplicationsconcerningdetergents,paints,cosmetics,
[
andinDNAtransfection(i.e.,theincorporationofexoge-
1 nous DNA into a cell)[9, 10, 11] and others.
v
4 Some important aspects regarding the formation of
9 flexible polyelectrolyte-surfactant complexes present ex-
1 citing challenges, both experimentally and theoretically.
1 The role played by the interface in the growth and nu-
0
cleation of these complexes, for example, is to a large
4
extentunknown,althoughneutronandX-rayreflectivity
0
/ studies provided invaluable insight into the density pro- FIG.1: ChemicalstructureofSodiumdodecylsulfate(SDS)
at fileacrosstheinterfaceofthePE/surfactantsolutions[12, and Poly-diallyldimethylammonium chloride, (PDAC), used
m 13, 14, 15]. The role salt concentration has on aggre- inthisstudy. SurfacetensionasafunctionofSDSconcentra-
tion in water and, 0.1M NaCl and 2% PDAC both at 0.1M
gation and precipitation of PE- and PE-surfactant so-
- salt and without salt.
d lutions is to a large extent an open problem[6]. There
n are recent suggestion that at high salt concentrations,
o dodecylsulfate(SDS)(C12H27O4SNa;MW=290288.4)
macromoleculesmaybeover-screenedbycounterions,ef-
c and Poly-diallyldimethylammonium chloride, (PDAC)
fectivelyreversingtheircharge[16,17,18],andthatsame-
:
v chargemacromoleculesmayattracteachother inseveral (MW = 100,000-200,000; see Fig. 1) [C8H16NCl]n;
i n=685-1370; obtained from Sigma (Materials have cat-
X density-regimes[19].
alog numbers L-4509 and 40,901-4 in the 2002 Sigma-
r Previous studies on surfactant-polyelectrolyte com- Aldrich catalog; shown in Fig. 1). PDAC at 2% (by
a
plexes have focused on bulk properties and the self- weight;concentrationofPEchargeswas0.137M)inpure
assembly as driven by surfactant concentration. The water (Milli-Q apparatus Millipore Corp., or Bedford,
present study focuses on interfacial behavior, in partic- MA;resistivity,18.2MΩcm)andin0.1MNaClwasused
ular on the role played by the gas-water interface in the in all experiments. After stirring for 10-20 minutes, so-
precipitation process, and how the self-assembly may be lutionswerepouredintoatemperature-controlledTeflon
controlled by the weakening of the electrostatic inter- Langmuir troughwhich was maintained at19oC anden-
actions (salt concentration). Herein, we report surface closed in a gas tight aluminum container,where surface-
sensitive synchrotron X-ray diffraction studies, both re- tension was measured with a microbalance using a Wil-
flectivity and diffraction at grazing angle of incidence helmy filter-paper-plate. The surface tension of the var-
(GIXD),onamodelsystemconsistingofsodiumdodecyl ious solutions was measured as a function of time after
sulfate (SDS) and Poly-diallyldimethylammonium chlo- pouring into the trough, or after compression or decom-
2
pressionofthesurfacewiththeLangmuirbarrier,andthe lution (0.1M NaCl) and in pure water for comparison
equilibrium value was averaged after a 30-60 min relax- are shown in Fig. 1, and similarly for 2% wt PDAC so-
ation (in all cases, the time dependent pressure showed lutions in pure water and in 0.1M NaCl solution. The
an exponential behavior, exp−t/τ;τ ≈2−5 min.). reduction in surface tension indicates the adsorption of
SurfacesensitiveX-raydiffractionstudiesofthe struc- surfactants at the surface, as inferred from the Gibbs
ture offree gas/solutioninterface were conducted onthe absorption equation[11]. The onset for the reduction in
Ames Laboratory Liquid Surface Diffractometer at the surface tension at 0.1M NaCl occurs at SDS concentra-
Advanced Photon Source (APS), beam-line 6ID-B (de- tions that are two orders of magnitude lower than those
scribed elsewhere[21]. The highly monochromatic beam of SDS in pure water[28]. Likewise, in the presence of
(8keVand16.2keV;λ=0.765334˚Aandλ=1.5498˚A), the polyelectrolyte, Figure 1 shows that the lowering
selected by a downstream Si double crystal monochro- of surface tension occurs at even lower SDS concentra-
mator, is deflected onto the liquid surface to a desired tions, suggestive of the formation of highly hydrophobic
angle of incidence with respect to the liquid surface by surfactant-polyelectrolyte complexes[10, 20]. Within ex-
a second monochromator [Ge(111)and Ge(220) crystals perimental error, our measurements in Fig. 1 show that
at 8.0 and 16.2 keV respectively] located on the diffrac- surface-tensionasfunctionofSDSconcentrationwith2%
tometer. Specular X-ray reflectivity experiments yield wt PDAC is not affected by the addition of simple salt
the electron density (ED) profile across the interface, (NaCl 0.1M) to water. The lowering of surface tension
and can be related to molecular arrangements in the by SDS to about 33 mN/m suggests that the main con-
film. The ED profile across the interface is extracted stituentsofthetop-mostlayeraresimilarwithandwith-
by refining a slab-model that best fit the measured re- out NaCl.
flectivity by non-linear least-squares method. The re- X-rayreflectivityandGIXDstudiesofPDAC-solutions
flectivity from the slab model at a momentum transfer surfaces were conducted at various SDS concentrations
Q , is calculated by R(Q ) = R (Q )e−(Qzσ)2 where, (with and without 0.1 M NaCl). Figure 2 shows a se-
z z 0 z
R0(Qz) is the reflectivity from step-like functions calcu- quence of normalized reflectivities, R/RF (where RF is
latedby the recursivedynamicalmethod, andσ is anef- the calculated reflectivity of an ideally flat water inter-
fective surface roughness,accounting for the smearingof face) for a typical SDS concentration, (10−4M) in pure
all interfaces due to thermal capillary waves and surface water and in PDAC solutions, and after adding 0.1M
inhomogeneities[21, 22]. The GIXD measurements are NaCl to the same solutions. At SDS concentrations
conducted with anevanescent incident beam that grazes greaterthan10−4M(inpurewater),thereflectivity[Fig.
the surface an angle slightly below the critical-angle for 2(A)] is similar to that of a pure watersurface, although
total-reflectionfromthesurface,yieldingthein-planeor- with a surface roughness σ = 3.5 ˚A, significantly larger
dering within the penetration depth of the X-ray beam. thanthatmeasuredforawatersurfaceundersimilarcon-
ditions andwith the same instrumentalsetup (σ = 2.4
W
˚A). The enhanced surface roughness is evidence for the
presence of a dilute inhomogeneous SDS-film in a gas
phase at the air/water interface. The addition of NaCl
to the SDS solution modifies the reflectivity, and gives
risetoa minimumdue tothe formationofamorehomo-
geneous film, at Q ≈ 0.53 ˚A−1. The detailed analysis
z
in terms of a two-box model[22], yields the ED profile
shown in Fig. 2 (B) with a total film-thickness d =
total
12 ˚A comparedto the estimated stretchedSDS molecule
d = 19.3 ˚A. This implies an average molecular tilt-
st
angle,t=51.5degreeswithrespecttothesurfacenormal
[t=acos(d /d )]. Additional,reflectivitystudiesasa
total st
functionofSDSconcentrations(withandwithoutNaCl)
show systematic increase of total film-thickness, up to ≈
18˚A,withtheincreaseofSDSconcentration[24]. Assum-
inganaverageSDSmolecular-areaA,thenumberofelec-
FIG. 2: A) Reflectivity normalized to RF (RF is the reflec- trons per SDS molecule (including adsorbed molecules -
tivityfromideallyflatwaterinterface)for(a)10−4M SDSin H O or ions) N is given by,
water(b)10−4M SDS0.1MNaCl(c)2%wtPDACinpurewa- 2 ref
ter(d) 10−4MSDS in 2%wt PDACsolution (e) 10−4MSDS
N =N +N =A ρ(z)dz, (1)
2%wt PDAC after the addition of 0.1M NaCl. B) Electron ref SDS other Z
density profiles used to generate the fitted reflectivity (solid
linesinA),thestep-likefunctions(box-model)aregenerated where N = 148electrons andN is the number of
SDS other
assuming nosurface roughness (σ=0). electrons due to integrated water molecules or ions that
are inseparable from the top most layer. Using Eq. (1),
Surface tension versus SDS concentration in salt so- and the ED profiles, the lower limit for the SDS molecu-
3
FIG. 3: A) In-plane diffraction (GIXD) scans of pure wa-
ter, and 2%wt PDAC2x10−5M SDS and 0.1M NaCl as indi-
cated and the difference between the two scans. B) Similar
differences between solutions of 2%wt PDAC a) 0M SDS b)
2x10−5MSDSc)2x10−4MSDS.Thedisruptionofthestruc- FIG.4: A)AscanalongQx of(10)peakatQz =0.098˚A−1.
tureof water at theinterface is apparent. B) A rod scan along the same peak (logarithmic scale) also
revealingthe(11)peak. C)TheobservedpeaksintheQx,Qz
plane D) A schematic illustration of the suggested model
lararea(i.e.,N =0),tobe A ≈35.6˚A2,whereas structurewiththeunitobliquecell. Thelongaxesofcylindri-
other min cal micelles are parallel to the liquid surface with each layer
assuming two bound water molecules per SDS molecule
separated by thepolymer.
yields A ≈40.4˚A2 , in agreement with values extracted
from surface tension isotherms[23]. The two-box model
showninFig.2(B)reflectsthemolecularED,withaelec-
tronrichsulfatehead-groupcomparedtothelowdensity (2 ×10−4M). The difference between the two patterns
hydrocarbonchains. The ED of the slab associatedwith is shown in Fig. 3 at two SDS concentrations. The
the hydrocarbon chains (0.27 e/˚A3) is much lower than broad peak in the diffraction at Qr = 1.34 ˚A−1 of an
that of closely-packed hydrocarbon chains ≈ 0.34 ˚A3 in average 4.69 ˚A d-spacing is due to scattering from 2D-
alkanes, for instance, indicating that the SDS molecules liquidhydrocarbonchains(comparedtotypicald-spacing
are not closely packed, most likely in a 2D liquid-like for hexagonally ordered hydrocarbon chains is 4.2 ˚A).
state. The reflectivity from a 2% PDAC in water (with The linewidth of the peak, ∆Q= 0.31 ˚A−1 with average
noSDS)isshowninFig.2todemonstratethattheeffect correlation-length ξ ≈ 20 ˚A, is further evidence for the
of the PE on the surface even at this high concentration 2D disordered chains[26].
is negligible showing a small step of lower density at the At small angles, the GIXD reveals several discrete
interface. TheadditionofSDStothe2%PDACsolution Bragg reflections, evidence of a diffraction pattern from
bringsthe minimuminR/RF toQz ≈0.38˚A−1,demon- crystalsthatarehighlyorientedwithrespecttothewater
strating the film is thicker (dtotal = 22.46 ˚A) and more surface. In fact, these reflections are found to be related
organized than that of SDS in water or in salt solution. to those observed in the reflectivity. As shown in Fig.
Thethicknessofthe slabassociatedwiththehead-group 4, these peaks are sharp characteristic of 3D ordering,
region(dhead ≈11˚A)isevidencethatthefilmconsistsof with no rod-like scattering typical of quasi-2D system.
PDAC-SDS complex at the interface. The thickness and The positions of the peaks observed and their layout as
the ED of the two slabs at the air interface, associated showninFig.4(C)areconsistentwithaslightlydistorted
withthehydrocarbontailsthatarenotlooselypackedas hexagonal structure, as depicted in Fig. 4(D) with a =
in a liquid state. The most dramatic effect in the reflec- 40.3± 0.5 ˚A, b = 44.6± 0.6 ˚A, and γ = 121 ± 1 deg.
tivity is observedwiththe additionofsaltto the PDAC- The peaks observed are similar although not the same
SDS solution, where Bragg reflections are superimposed as those observed by Chu and co-workers[1, 2] in small
onthereflectivityatQI ≈0.165˚A−1 andatQII ≈0.345 angleX-rayscatteringexperimentsfromrelatedsystems.
z z
˚A−1. These two peaks, as will be shown below are the Based on the anisotropy observed, we propose a simple
first and second order Bragg reflections from hexagonal structure of stacked cylindrical micelles, with their long
structurethatisnormaltothescatteringplane(inreflec- axisparalleltothewater-surface,whereeachlayerissep-
tivity configuration). Similar neutron reflectivity studies aratedby a layerof the PEs,as depicted in Fig.4, form-
of the dilute PDAC solutions with SDS and NaCl are ing a 2D polycrystalline system with preferred orienta-
consistent with the present findings[15]. tionwithrespecttothesurface. Thedistortedhexagonal
The picture of a liquid-like film with disordered SDS diffraction pattern shows the growth is anisotropic, and
at the gas-water interface is corroborated by our GIXD impliestheinterfaceplaysanimportantroleininitiating
studies. Figure 3(A) shows the diffraction patterns complexation (aggregation) processes. Another impor-
of pure water surface[25], and that of the PDAC-SDS tantresultistheeffecttheadditionofsalt(NaCl)hason
4
factant concentration lowers surface tension (see Fig. 1)
and initiates the micellization. We argue that micelle-
formation is initiated at the interface as the ideal linear
bulk-separation among surfactants is ≈ 250 ˚A (surfac-
tant concentrations (≈10−4M). The addition of NaCl to
the PE/surfactant solution screens electrostatic interac-
tions, leading to cylindrical-micelles[28]. Absorption of
PE to micelles is then expected to be enhanced by the
mechanism of counterion release[29], and we speculate
that micelles will then self-attract by a similar correla-
tion mechanism as has been recently observed in multi-
valent ions[30], eventually condensing into an hexagonal
(columnar) crystal, and thus growing crystals from the
interface.
FIG. 5: A) Schematic pathways of complexation and sub-
In summary, we have shown the fundamental role
sequentcrystallizationA)Negligiblesurfactantconcentration
playedby the interface innucleating PEsurfactantcom-
the PE is repelled from the interface. B) Below the CAC
plexes, and how crystallization may be induced by salt
a PE surfactant is formed at the interface. C) Beyond the
concentration at tiny surfactant concentrations. Our
CAC micellization occurs D) The addition of salt transforms
micelles to cylindrical shape and crystallizes them. studyalsoshowsthecapabilitiesavailablebyX-rayscat-
tering techniques.
WewishtothankM.MuthukumarandD.S.Robinson
promoting PE/surfactant crystallization. Our heuristic for helpful discussions. The MUCAT sector at the APS
interpretation of the aggregation and subsequent crys- issupportedbytheU.S.DOEBasicEnergySciences,Of-
tallization is depicted in Fig 5. The PE, with no sur- ficeofScience,throughAmesLaboratoryundercontract
factants or salt added, is highly soluble in water and is no. W-7405-Eng-82. Measurements were supported by
repelled from the air/water interface due to the discon- the U.S. DOE, Basic Energy Sciences, Office of Science,
tinuity in dielectric constant[27]. Minute increasein sur- under contract no. W-31-109-Eng-38.
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