pubs.acs.org/Langmuir © 2010 American Chemical Society
Highly Charged Inorganic-Organic Colloidal Core-Shell Particles Birgit Fischer,† Tina Autenrieth,‡ and Joachim Wagner*,§ †
HASYLAB, Deutsches Elektronensynchrotron, D-22603 Hamburg, Germany, ‡European Plastics Converters EUPC, Avenue de Cortenbergh 66, 1000 Brussels, Belgium, and §Institut f€ ur Chemie, Universit€ at Rostock, Albert-Einstein-Strasse 3a, D-18059 Rostock, Germany Received October 12, 2009. Revised Manuscript Received March 5, 2010
Highly defined, hybrid inorganic-organic colloidal core-shell particles consisting of a silica core and a shell of fluorinated acrylate are prepared in a two-step route. The core-shell structure of the particles is investigated by means of small-angle X-ray scattering (SAXS). Because of highly acidic sulfonic acid surface groups resulting from the radical initiator sodium peroxodisulfate at the organic shell, long-range electrostatic interactions lead to the formation of liquidlike mesostructures. Increasing the effective interaction by reducing the next-neighbor distances induces a freezing of the liquidlike structures, i.e., a transition to crystalline and glassy structures. Because of the high electron density in the core and the fluorinated polymer shell, these particles are strong X-ray scatterers. In combination with the large number of effective charges and the outstanding monodispersity, these core-shell particles are a promising model system for the investigation of the glass transition by photon correlation spectroscopy employing coherent X-rays.
Introduction In the recent decades colloidal particles attracted large scientific interest as well for applied as for fundamental research. Because of the ability to self-organization, colloidal particles show a phase behavior in analogy to molecular systems at enlarged scales of time and space. Liquidlike and crystalline mesoscale structures of colloidal particles interacting via pure hard-sphere exclusion as well as via long-range electrostatic interactions are thoroughly investigated.1-3 Also, glassy colloidal suspensions have theoretically and experimentally been investigated. Here, especially hardsphere systems acted as colloidal model systems for the ultraslow dynamics of glassy systems accessible via photon correlation spectroscopy. For hard-sphere colloidal particles a glass transition is theoretically predicted and experimentally verified4-8 at comparatively large volume fractions above a critical volume fraction of jc = 0.569 and is still under discussion.10 However, less is known about the glass transition of highly charged particles that has due to the long-range, screened Coulomb interaction been observed at significantly lower volume fractions in the range of j ≈ 0.20.11,12 In recent time, for slow dynamic processes *To whom correspondence should be addressed. E-mail: joachim.wagner@ uni-rostock.de. (1) Pusey, P. N. Colloidal Suspensions in Liquids, Freezing and Glass Transition; Elsevier: Amsterdam, 1991; pp 765-942. (2) H€artl, W.; Versmold, H.; Wittig, U.; Linse, P. J. Chem. Phys. 1992, 97, 7797– 7804. (3) Matsuoka, H.; Ise, N. In Polymer Analysis and Characterization; Advances in Polymer Science Vol. 114; Springer: Berlin, 1994; pp 187-231. (4) Pusey, P. N.; van Megen, W. Nature 1986, 320, 340–342. (5) Pusey, P. N.; van Megen, W. Phys. Rev. Lett. 1987, 59, 2083–2086. (6) Pusey, P. N.; Van Megen, W. Ber. Bunsenges. Phys. Chem. 1990, 94, 225–9. (7) van Megen, W.; Pusey, P. N. Phys. Rev. A 1991, 43, 5429–5441. (8) van Megen, W.; Underwood, S. M.; Pusey, P. N. Phys. Rev. Lett. 1991, 67, 1586–1589. (9) G€otze, W.; Sj€ogren, L. Rep. Prog. Phys. 1992, 55, 241. (10) Zaccarelli, E.; Valeriani, C.; Sanz, E.; Poon, W. C. K.; Cates, M. E.; Pusey, P. N. Phys. Rev. Lett. 2009, 103, 135704. (11) H€artl, W.; Versmold, H.; Zhang-Heider, X. J. Chem. Phys. 1995, 102, 6613– 6618. (12) H€artl, W.; Wagner, J.; Beck, C.; Gierschner, F.; Hempelmann, R. J. Phys.: Condens. Matter 2000, 12, A287–A293. (13) Gr€ubel, G.; Zontone, F. J. Alloys Compd. 2004, 362, 3 - 11; Proceedings of the Sixth International School and Symposium on Synchrotron Radiation in Natural Science (ISSRNS).
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correlation spectroscopy using coherent X-rays (XPCS) with practically no limitation to the accessible Q-range has been available.13,14 By this method distances much smaller than the ones accessible via laser light can be probed. Such small interdistances are of special interest in concentrated suspensions with small next-neighbor distances close to a freezing transition. Because of the comparatively low flux of coherent X-rays, strongly scattering particles are required for this method. In addition to high electron density, a large number of effective surface charges and a size distribution as narrow as possible are mandatory properties. In this contribution we describe the preparation and structural characterization of a hybrid, inorganic-organic core-shell system consisting of silica cores giving a high contrast for X-ray scattering and an organic shell consisting of fluorinated polymers with a comparatively high electron density and a large number of effective charges. These particles combining a high electron density and a strong electrostatic interaction exhibit an outstanding narrow size distribution for particles with an overall diameter around 100 nm. For particles of this size, the structural correlations of interest are easily accessible via small-angle X-ray scattering (SAXS). The formation of structures close to a freezing transition by self-organization is evident from the experimentally observed structure factors.
Small-Angle Scattering of Interacting Core-Shell Particles Particle Form Factor. Small-angle scattering arises from inhomogeneities in the scattering length density at mesoscale length scales, i.e., in the range from few nanometers to several hundred nanometers. The contrast for X-ray scattering is related to the different electron density of the suspended particles on the one hand and the electron density of the suspending medium on the other. For core-shell particles two scattering length differences have to be taken into account: let F1 = Fc - Fm be the difference between the scattering length densities of the core Fc
(14) Robert, A.; Wagner, J.; Autenrieth, T.; H€artl, W.; Gr€ubel, G. J. Magn. Magn. Mater. 2005, 289, 47–49.
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and the suspending medium Fm and F2 = Fs - Fm the corresponding difference between the shell Fs and the medium Fm. Let I0 be the intensity of the incident beam, V the illuminated sample volume, and N the number density of the colloidal particles. Then the scattered intensity originating from a system of equally sized spherical core-shell particles with the core radius Rc and the outer shell radius R can be factorized as IðQÞ ¼ I0 VNSðQÞP0 ðQ, Rc , R, F1 , F2 Þ
leading in many cases to an analytical expression for a polydisperse form factor is given by the Schulz-Flory distribution.15 Assuming the same relative width of the distribution functions for the core size and the shell size, i.e., a constant core to shell ratio β = Rc/R, an analytical expression for the polydisperse form factor has been derived.16 For a constant shell thickness R - Rc, taking only the polydispersity of the core into account, also an analytical expression has been derived by Bartlett and Ottewill.17 In the most general case, however, two independent distribution functions for the size of the core and the size of the shell have to be considered. Since the polydispersity of the outer shell radii R is for our inorganic/organic hybrid particles much smaller than the one of the core particles, we use here a distribution function for the total particle radius, i.e., the outer shell radius R, and a second distribution function for the core to shell ratio β. For the distribution of the outer shell radius, we use again the exponential Schulz-Flory distribution 1 ZR þ 1 ZR þ1 ZR ZR þ 1 R exp R cR ðR, R, ZR Þ ¼ ΓðZR þ 1Þ R R ð4Þ R¥ that is normalized, i.e., 0 cR(r,R,ZR) dr 1 with R = ÆRæ denoting the mean outer shell radius and ZR describing the polydispersity of the particles. For a Schulz-Flory distribution, the polydispersity can be written as p = (ÆR2æ - ÆRæ2)/ÆRæ2 = 1/(ZR þ 1). For the distribution of the core to shell ratio, we need again a normalized distribution function cβ(β,β0,Zβ) with finite probability for 0 < β < 1, asymptotically R approaching cβ(β,β0,Zβ) 0 in the limits β f 0 and β f 1 and 10cβ(β,β0,Zβ) dβ 1. In analogy, the parameters β0 and Zβ are related to the mean core to shell ratio Æβæ and the width of the distribution function. Substituting y = tan(βπ/2) and dy = π/2(1 þ tan2(βπ/2)π/2)) dβ, the Schulz distribution can be mapped from the interval 0 < y < ¥ to the interval of interest for the core to shell ratio, i.e., 0 < β < 1. Hereby we obtain the mapped distribution function
ð1Þ
with structure factor S(Q) accounting for interparticle correlations. The scattering vector Q = 4π/λ sin(θ/2) is determined by the angle θ enclosed by the transmitted and diffracted beam and the wavelength λ. The form factor P0 (Q,Rc,R,F1,F2) is related to intraparticle correlations, i.e., the correlation function of scattering length densities within a single particle. The latter quantity describing the scattering function for a single particle is for spherical particles given by the Fourier-Bessel transform of the scattering length density correlation function. Z Rc Z R P0 ðQ, Rc , R, F1 , F2 Þ ¼ ½ 4πF1 r2 j0 ðQrÞ dr þ 4πF2 r2 j0 ðQrÞ dr2 0
Rc
ð2Þ Here, j0(Qr) = sin(Qr)/(Qr) denotes the zero-order spherical Bessel function. If we normalize this function to the forward scattering (Q f 0) of a particle, we obtain the form factor P(Q,Rc,R,F1,F2) 2R 32 RR Rc 4πF1 r2 j0 ðQrÞ dr þ Rc 4πF2 r2 j0 ðQrÞ dr 0 5 PðQ, Rc , R, F1 , F2 Þ ¼ 4 RR R Rc 2 2 0 4πF1 r dr þ Rc 4πF2 r dr ð3Þ Introducing the core to shell ratio β = Rc/R, the form factor can be rewritten in the form P(QR,β,F1,F2) which only depends on the product QR with R denoting the outer shell radius. Even highly defined colloidal suspensions, however, consist of particles with different sizes. A suitable size distribution function 1Zβ þ1
0
B π C B Zβ þ 1 C C B 1 þ tan2 πβ0 A 2ΓðZβ þ 1Þ@ tan 2 R1 that is still normalized, i.e., 0cβ(β,β0,Zβ) dβ 1. Here, however, the parameter β0 is different from the mean value Æβæ. Since no simple relation between β0 and Æβæ exists, its mean value Æβæ has to be calculated by numerical integration. The joined probability p(R,β,R,β0,ZR,Zβ) to find a particle with the overall radius R and the core to shell ratio cβ ðβ, β0 , Zβ Þ ¼
1 0 ! !Zβ B Zβ þ 1 πβ πβ πβ C C B tan C tan expB - @ πβ0 2 2 2 A tan 2
β = Rc/R is given by the product of both normalized distribution functions pðR, β, R, β0 , ZR , Zβ Þ dR dβ ¼ cR ðR, R, ZR Þ dR cβ ðβ, β0 , Zβ Þ dβ ð6Þ
Hence, the form factor of particles with the probability densities cR(R,R,ZR) and cβ(β,β0,Zβ) can be expressed as
PðQ, R, β0 , ZR , Zβ , F1 , F2 Þ ¼ Z 0
¥Z 0
1
cR ðR, R, ZR Þcβ ðβ, β0 , Zβ ÞR ðR, β, F1 , F2 Þ
ð5Þ
3
2
!2
ðQRÞ3 ðβ3 F1 þ ð1 -β3 ÞF2 Þ
fF1 ½sinðβQRÞ - βQR cosðβQRÞ
- F2 ½sinðQRÞ -QR cosðQRÞ - sinðβQRÞ þ βQR cosðQRÞg2 dβ dR
ð7Þ
with (15) Schulz, G. Z. Phys. Chem., Abt. B 1949, 46, 155–193. (16) Wagner, J. J. Appl. Crystallogr. 2004, 37, 750–756. (17) Bartlett, P.; Ottewill, R. H. J. Chem. Phys. 1992, 96, 3306–3318.
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RðR, β, F1 , F2 Þ ¼
4π ðF β3 þ F2 ð1 - β3 ÞÞR3 3 1
ð8Þ
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Here the integrand is obtained by performing the integration in eq 3 and weighting the result by the forward scattering R2(R,β, F1,F2) of a given particle and the relative probability to find such a particle. The double integration in eq 7 is numerically performed using an adaptive Gauss-Kronrod algorithm. The Mesoscale Structure of the Suspension. While the form factor solely accounts for intraparticle correlations, the interparticle correlations between the particles centers of mass are described via the structure factor S(Q): this quantity is the Fourier transform of the pair correlation function g(2)(rij) with rij denoting the distance between the particles center of mass. For noninteracting particles without mutual exclusion, we have a completely random structure with g(2)(rij) 1. Interactions between different particles, however, lead to the formation of a short-range order for colloidal liquids or even a long-range order for colloidal crystals. In our case, the colloidal particles are negatively charged macroions due to the dissociation of strongly acidic sulfonic acid surface groups introduced to the polymer chains by the radical initiator K2S2O8. Because of the protons as counterions always present in the vicinity of colloidal particles, these macroions with a distance rij repel according to the DLVO theory18 via a screened Coulomb or so-called Yukawa interaction Vðrij Þ ¼
1 Zeff 2 e0 2 expð -Krij Þ 4πε rij
ð9Þ
whereby ε is the dielectric constant of the suspending medium, Zeff the number of effective charges of a macroion, and e0 the electron charge. The inverse Debye-H€ uckel screening length κ strongly depends on the ion strength in a suspension. In the presence of a mixed bed ion exchanger resin stray ions quantitatively can be removed. In this way, the inverse Debye-H€uckel screening length κ can be minimized when only negatively charged macroions and protons as their counterions are present. In this case even particles at distances much larger than the particle size repel each other. On the other hand, by addition of stray ions, the electrostatic repulsion can be screened leading to completely disordered structures with S(Q) 1. According to eq 1, the structure factor can be obtained by division of the scattered intensity of a concentrated suspension;scaled by the inverse ratio of particle number densities; by the scattered intensity of a completely disordered suspension. With known functional form of the particle interaction, the structure factor can be calculated by means of statistical mechanics. Since the particles interact not only via direct pair interactions but also via interactions mediated by other particles, the structure depends according to the Ornstein-Zernike equation on both contributions.19 To overcome the infinite recursion implicated by the Ornstein-Zernike equation, a closure relation has to be applied as an approximation to treat mediated interactions. For charged colloidal particles, the rescaled mean sphere approximation (RMSA)20-23 has widely been applied to describe the structure of dilute liquidlike ordered systems.
Experiments Preparation of Inorganic-Organic Core-Shell Particles. The preparation of the hybrid, inorganic-organic particles (18) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (19) McQuarrie, D. A. Statistical Mechanics; University Science Books: Sausalito, CA, 2000. (20) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 108. (21) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (22) H€artl, W.; Versmold, H. J. Chem. Phys. 1984, 80, 1387. (23) Wagner, J.; H€artl, W.; Lellig, C.; Hempelmann, R.; Walderhaug, H. J. Mol. Liq. 2002, 98, 183–190.
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is a two-step procedure consisting of a St€ ober polycondensation for the fabrication of silica cores and covering them by a polymer shell in a subsequent step. The silica particles are prepared from a solution of 10 g of tetraethoxysilane (TEOS) in 500 mL of absolute ethanol. Under stirring at room temperature, the polycondensation is started by addition of 10 mL aqueous ammonia (25 wt %). The formation of silica particles is indicated by increasing turbidity of the suspension. After stirring for 24 h, the resulting suspension is filtered and dialyzed against distilled water for 1 week. Hereby, the suspending medium is exchanged from ethanol to water and impurities with small molecular weight are removed. The polymer shell is produced by the radical induced polymerization of 1H,1H,5H-octafluoropentyl acrylate. The monomer is prepared by the reaction of 1H,1H,5H-octafluoropentanol (purchased from Fluorochem, Old Glossop, UK) with a stochiometric surplus of acryloyl chloride (Sigma) in presence of copper powder as a catalyst. The resulting ester is purified by vacuum destillation. The preparation of core-shell particles starts from a dilute suspension of the dialyzed silica spheres in distilled water. Approximately one-quarter of the silica spheres obtained from the first step is stirred in 1500 mL of distilled water and heated to 65 C under a nitrogen atmosphere to remove oxygen. To the suspension a Fe2þ/Fe3þ/HSO3- redox catalyst prepared from 5 mg of (NH4)2FeSO4 and 300 mg of NaHSO3 is added. By means of a two-channel peristaltic pump, during 24 h 20.0 g of 1H,1H,5H-octafluoropentyl acrylate and 60 mg of K2S2O8 dissolved in 20 mL of distilled water are added via Teflon capillaries immersed in the strongly stirred suspension. Hereby the concentration of radicals and monomer is kept low to suppress the formation of new nuclei forming pure polymer colloids and to promote the growth of long polymer chains entangling the silica spheres. At the end of the reaction, the suspension is filtered and again dialyzed for 1 week against distilled water. Stray ions are removed by means of a mixed bed ion exchanger leading to opalescent colloidal liquids indicating short-range order of the colloidal particles at interdistances comparable to visible light. SAXS Experiments. The small-angle X-ray scattering experiments were carried out at the high brilliance beamline (ID02) and the TROIKA beamline (ID10C) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The dilute, disordered samples of silica core particles and silica/polymer core-shell particles contain 1 10-3 mol L-1 potassium chloride to screen the electrostatic interactions. The experiments are carried out at two detector distances of d = 10 m and d = 2 m at ID02 using 12 keV X-rays with a wavelength of λ = 0.995 A˚. The data collected employing a Frelon CCD with 2048 2048 pixels are corrected for the parasitic scattering of a water-filled capillary and for detector efficiency and radially averaged. The structure factors of the higher concentrated suspensions are obtained at the ID10C beamline at ESRF employing 8 keV (λ = 1.5347 A˚) photons and a direct illumination CCD. Stray ions in the suspensions are removed by a mixed bed ion exchanger at the bottom of sealed quartz capillaries. A concentration gradient is obtained by careful centrifugation. Afterward, the number density of the particles depends on the vertical position of the capillary with respect to the primary beam. The structure factors are obtained by division of the scattered intensity of a concentrated suspension by the one of a dilute suspension whose electrostatic interactions are screened by 1 10-3 mol L-1 potassium chloride. Finally, the data are rescaled by the ratio of number densities of the dilute to the concentrated sample. Dynamic light scattering experiments (DLS) are carried out using a goniometer system purchased from ALV (Langen, Germany) employing a HeNe laser (λ = 628 nm) with a power of 35 mW. All light scattering experiments are carried out at ϑ = 90 and 25 C . The time correlation function is obtained by pseudo-cross-correlation of the signals from two photomultipliers DOI: 10.1021/la903872a
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Figure 1. SAXS from a highly dilute suspension of the silica cores used for covering with a fluorinated polymer shell in a subsequent step. The solid line represents a fit assuming Schulz-Flory distributed spheres with a diameter of σcore = 46 ( 1 nm and a polydispersity of pcore = 0.1383.
Figure 2. SAXS from a highly dilute suspension of core-shell particles. The large number of at least 12 pronounced minima indicates the high monodispersity (pshell = 0.0321) of the particles with respect to the total diameter σshell = 96.8 ( 0.5 nm. to suppress noise. The size distribution of the particles is determined using the CONTIN algorithm.24,25
Results Figure 1 displays the small-angle scattering of a dilute sample of the silica core particles containing 10-3 mol L-1 potassium chloride. Here only two smooth minima in the particle form factor can be observed. The experimental data can be described assuming Schulz-Flory distributed spherical particles with a diameter of σcore = 46 ( 1 nm and exponential parameter Zcore = 51 ( 2 corresponding to a polydispersity of p = 0.1383. Such a polydispersity is usual for comparatively small silica particles. The size of the particles is in accordance to a slightly larger hydrodynamic diameter of 48 nm as determined by DLS. After the covering process with polymer in the small-angle scattering of the core-shell particles an unusual large number of at least 12 minima can be distinguished indicating an extremely small polydispersity of the particles (Figure 2). Opposite to homogeneous spherical particles, the first two minima at small wave vectors are comparatively diffuse, whereas the higher orders at large wave vectors are still considerably sharp. Such a behavior can be reproduced by core-shell particles with well-defined outer (24) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229. (25) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 523.
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Figure 3. Small-angle scattering of a colloidal glass and a dilute, disordered suspension of colloidal particles (scaled by the ratio of number densities). The electrostatic interactions in the dilute sample are screened by addition of 1 10-3 mol L-1 KCl. The ratio of both quantities is the structure factor S(Q) whose maximum Smax(Q) > 2.85 indicates the existence of a glassy structure.
radii and a significantly broader distribution of the core to shell ratio as discussed in the preceding section. A least-squares fit with additional resolution correction reasonably reproduces this experimental result. Deviations at small scattering vectors Q that correspond to lengths much larger than the particle size have to be attributed to the destabilization of the charged particles by the addition of salt, i.e., the beginning of agglomeration. The best parameters are σshell = 96.8 ( 0.5 nm for the mean total particle diameter and β = 0.49 ( 0.08. This corresponds with Zβ = 8.22 ( 2.3 resulting in Æβæ = 0.47 ( 0.08 and a mean core diameter of Æσcoreæ = 45 ( 4 nm, which is in very good agreement with the result obtained for the core particles prepared in the first step. For the parameter ZR = 995 ( 40 corresponding to a polydispersity pshell = (Æσshell2æ/Æσshellæ2 - 1)1/2 = 0.0321 for the whole particle is obtained which is outstanding small for particles with a diameter less than 100 nm. The parameter Zβ = 8.2 ( 2.3 corresponds to pβ = (Æβ2æ/Æβæ2 - 1)1/2 = 0.21 ( 1.10. Within the experimental accuracy, this is in accordance with the relative polydispersity of the core particles. The best parameter for the half-width of the Gaussian resolution function is with (6 ( 2) 10-3 nm-1 roughly half the minimum accessible scattering vector Qmin. DLS experiments confirm the presence of only one population of particles with a mean hydrodynamic diameter of 98 nm and narrow size distribution. Again, the slightly larger hydrodynamic radius is in very good accordance with the size determined from the SAXS data. The variation of the salt content does not influence the hydrodynamic radii of the particles within experimental accuracy as long as no agglomeration is induced by stray ions. The self-organization in highly concentrated suspensions where stray ions are removed by a mixed bed ion exchanger is visible as opalescence in backscattering geometry using visible light. At small wavevectors, pronounced correlation maxima are observed in the scattering patterns of ordered suspensions (Figure 3). The quotient of the intensity scattered by an ordered suspension and the one of a disordered suspension is the structure factor S(Q). At the smallest number density near the top of the capillary a typical liquidlike structure factor indicating a short-range order is visible (Figure 4). A least-squares fit of the structure factor employing the rescaled mean sphere approximation (RMSA) reasonably reproduces the experimental structure factor and reveals a number density of N = 1.3711 1020 m-3. For spherical particles with a diameter of around σtotal = 97 nm this corresponds Langmuir 2010, 26(9), 6201–6205
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Discussion
Figure 4. Structure factors of a liquidlike ordered (top left), crystalline (top right and bottom left), and glassy suspension of highly charged colloidal particles. The solid line in the top left figure represents a fit according to RMSA. From this liquidlike structure factor, an effective number of Zeff = 371 ( 4 electron charges is derived.
to a volume fraction of j = πNσ3/6 = 0.065. At a residual ionic strength of (6 ( 3) 10-6 mol L-1 the effective charge is determined to Zeff = 371 ( 4 electron charges. Essential for the self-organization of charged particles is the effective pair potential between the centers of two particles. This potential is related to an effective number of charges Zeff that is much smaller than the total number of charges accessible by chemical titration. A main advantage of this method is avoiding any manipulation of the sample with the risk of introducing traces of stray ions that heavily can influence the structure. For the determination of the effective number of charges, here the same sealed quartz capillary under the same conditions is used. Because of the particles outstanding small polydispersity of p = 0.0321, the volume fraction can be determined neglecting structural effects resulting from polydispersity. Significant influences to the structure factor arise from polydispersities p g 0.1.26 Increasing the volume fraction further to j = 0.120, i.e., reducing the interparticle distances and enhancing electrostatic repulsions, leads to the formation of long-range ordered colloidal crystals indicated by Bragg reflections. Here the diffraction peaks (111), (200), (220), (311), and (222) can be observed, whereby the last two ones overlap in a single maximum. Nevertheless, the width of these powder reflections is much smaller than the width of the maxima in the liquidlike structure factor. The diffraction pattern indicates the existence of fcc crystals whose Bragg reflections are indicated by the vertical lines in Figure 4. At a volume fraction of j = 0.136, the diffraction peaks become broader, the (200) reflection is reduced to a shoulder in the most intense (111) diffraction peak, and the (220) reflection is still visible as a resolved maximum in the scattered intensity. Close to the bottom of the capillary, again a liquidlike structure factor is observed. The maximum of the structure factor is with S(Qmax) = 3.0 clearly above the Hansen-Verlet freezing criterion predicting a freezing when S(Qmax) J 2.8.27 Hence, from a structural point of view, here a glassy system is present. (26) D’Aguanno, B.; Klein, R. J. Chem. Soc., Faraday Trans. 1991, 87, 379–390. (27) Hansen, J.-P.; Verlet, L. Phys. Rev. 1969, 184, 151–161.
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By covering comparatively small silica particles with a 1H,1H,1H-octafluoropentyl acrylate shell, extremely monodisperse and highly charged colloidal particles with a total diameter around 100 nm and 370 effective electron charges are accessible. Typical interparticle distances of structures formed by selforganization of these electrostatically repelling particles can easily accessed by SAXS experiments. Because of the combination of the high electron density of both the silica core and the perfluorinated polymer shell, these particles are strong X-ray scatterers and well suited for experiments with comparatively low primary flux like XPCS. Because of the extremely narrow size distribution the volume fraction of the structures can be determined with high accuracy. This is hardly possible for particles underlaying a rather broad size distribution. Hence, these inorganic-organic hybrid particles are a well-suited model system for the investigation of the glass transition of highly charged colloids. As a consequence, the existence of a glassy structure can here not be attributed to the suppression of crystallization by the particles polydispersity: at intermediate concentrations colloidal fcc crystals formed by the same particles are observed. The existence of an undercooled liquidlike structure or a colloidal glass has here to be attributed to the strong electrostatic interaction of the highly charged particles at sufficiently short interparticle distances. Because of these strong interactions as visible by the static structure factor, the mobility of the particles is rather low. Hence, crystallization to the energetically lowest fcc configuration is suppressed and a metastable glassy structure is observed. The observation of colloidal glasses at intermediate volume fraction indicates that not the particle interdistances are important for the formation of a glassy structure but the effective interaction at the mean particle distance. Hence, in presence of a long-range repulsive potential, the freezing to a colloidal glass is achieved at significantly larger particle distances than for shortrange hard-sphere-like potentials. The existence of colloidal glasses formed by highly charged particles is reported in several studies11,12,28,29 at comparable volume fractions in the range of j ≈ 0.2, which is nearly by a factor of 3 smaller than the critical volume fraction jc = 0.56 for the glass transition of hard-sphere particles. However, the charged particles used in these studies are significantly larger in the diameter than the present system and are, as a consequence, less suitable for SAXS experiments since extremely small angles have to be accessed to probe the characteristical distances. Obviously, in analogy to the crystallization, which can for highly charged particles opposite to hard-sphere particles be observed at volume fractions smaller than 1%, also the glass transition of highly charged colloidal particles occurs at smaller volume fractions. We will investigate the slow dynamics of these glassy structures by means of 2d-XPCS in dependence on Q using this system as a highly defined and strongly scattering model system. Acknowledgment. The authors thank Theyencheri Narayanan and Aymeric Robert for support during the experiments at ESRF and for fruitful discussions. (28) Beck, C.; H€artl, W.; Hempelmann, R. J. Chem. Phys. 1999, 111, 8209–8213. (29) Sirota, E. B.; Ou-Yang, H. D.; Sinha, S. K.; Chaikin, P. M.; Axe, J. D.; Fujii, Y. Phys. Rev. Lett. 1989, 62, 1524–1527.
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