Impact of Some Organics on Structural and ... - ACS Publications

Langmuir 2002, 18, 581-596

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Impact of Some Organics on Structural and Adsorptive Characteristics of Fumed Silica in Different Media V. M. Gun’ko,*,† V. I. Zarko,† E. F. Voronin,† V. V. Turov,† I. F. Mironyuk,† I. I. Gerashchenko,† E. V. Goncharuk,† E. M. Pakhlov,† N. V. Guzenko,† R. Leboda,‡ J. Skubiszewska-Zie¸ ba,‡ W. Janusz,‡ S. Chibowski,‡ Yu. N. Levchuk,§ and A. V. Klyueva§ Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine, Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland, and A.V. Palladin Institute of Biochemistry, 9 Leontovich Street, 03030 Kiev, Ukraine Received March 13, 2001. In Final Form: June 19, 2001

Pristine fumed silica and powders with dried residuals of centrifuged aqueous suspensions of fumed silica, silica/PVP, or silica/protein were studied using nitrogen adsorption and IR spectroscopy methods. The aqueous suspensions of silica with added polymers, surfactant, or ethanol were investigated by means of photon correlation spectroscopy, 1H NMR, and adsorption methods. The impact of polymers on the suspension and dried powder characteristics depends on the adsorption mechanism and the conformation of polymer molecules (globular or unfolded) due to strong or weak intramolecular interactions. Globular proteins interacting with silica through the flocculation mechanism affect the textural characteristics of dried powders more weakly than do unfolded proteins but strongly impact the aqueous suspension, thus shifting the swarm size distribution toward larger sizes compared to the size distribution of poly(vinylpyrrolidone), PVP. PVP is adsorbed in the unfolded state and gives a nearly monomodal particle size distribution (PSD). The ionogenic surfactant 1,2-ethylene-bis(N-dimethyl carbodecyloxymethyl)ammonium dichloride at CAet < 0.01 wt % and ethanol at CEtOH ) 10-50 wt % impacts the silica’s PSD-dependent nonlinearly on the concentrations and pH.

Introduction Fumed silica composed of roughly spherical primary particles that form swarms (aggregates of primary particles, agglomerates of aggregates, and visible flocks) can undergo structural changes upon being suspended and dried and heated or after absorbing surfactants and polymers, etc. This initial material does not exist in the form of individual primary particles, and the characteristic size increases by 10 times at each subsequent level of the structural hierarchy.1-4 There is not a conventional model of primary particle bonding in aggregates of fumed silica. Some authors assume that hydrogen and electrostatic bonding occurs, but others believe that primary particle bonding occurs through Si-O-Si bridges.1-3 From SEM or TEM images,1,2 it is difficult to determine the nature of this bonding because some primary particles in ag* Corresponding author. E-mail: [email protected]. Fax: 38-044 444-35-67. † Institute of Surface Chemistry. ‡ Maria Curie-Sklodowska University. § A.V. Palladin Institute of Biochemistry. (1) (a) Basic Characteristics of Aerosil; Technical Bulletin Pigments, N11; Degussa AG: Hanau, 1997. (b) Ehrman, S. H.; Friedlander, S. K.; Zachariah, M. R. J. Aerosol Sci. 1998, 29, 687. (c) Briesen, H.; Fuhrmann, A.; Pratsinis, S. E. Chem. Eng. Sci. 1998, 53, 4105. (d) Vemury, S.; Pratsinis, S. E. J. Aerosol Sci. 1996, 27, 951. (2) (a) Barthel, H. Colloids Surf., A 1995, 101, 217. (b) Barthel, H.; Rosch, L.; Weis, J. In Organosilicon Chemistry II: From Molecules to Materials; Auner, N., Weis, J., Eds.; VCH Publishers: Weinheim, 1996; pp 761-778. (c) Barthel, H.; Heinemann, M.; Stintz, M.; Wessely, B. Proceedings of the International Conference on Silica Science and Technology (Silica 98), Mullhouse, France, September 1998; p 323. (3) (a) The Surface Properties of Silicas; Legrand, A. P., Ed.; Wiley & Sons: New York, 1998. (b) Iler, R. K. The Chemistry of Silica; Wiley & Sons: Chichester, 1979. (c) Kiselev, A. V.; Lygin, V. I. IR Spectra of Surface Compounds and Adsorbed Substances; Nauka: Moscow, 1972. (4) Gun’ko, V. M.; Zarko, V. I.; Leboda, R.; Chibowski, E. Adv. Colloid Interface Sci. 2001, 91, 1.

Table 1. Characteristics of Fumed Silicas and the Plateau Adsorption of BSA and Gelatin no.

Sar,a m2 g-1

COH, µmol m-2

CBSA, mg g-1

CBSA, mg m-2

Cgelatin, mg g-1

Cgelatin, mg m-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

102 148 197 192 270 293 300 275 308 308 368 384 390 410 411

4.8 3.8 3.6 3.5 3.2 2.9 2.9 3.0 3.0 3.0 2.6 2.8 2.6 2.4 2.4

200

1.96

210 230 310 280

1.06 1.19 1.14 0.95

320 350 380 450 410 430 380

1.03 1.14 1.03 1.17 1.05 1.05 0.92

170 243 295 200 275 320 346 315 307 380 250 420 470 450 200

1.67 1.62 1.49 1.04 1.01 1.09 1.15 1.14 1.0 1.23 0.68 1.09 1.21 1.10 0.48

gregates seem tightly sintered (as a result of collisions and sticking and fusing in the hot reaction zone of the flame, possibly with formation of Si-O-Si bonds between adjacent particles), but others have small-area contactsthat can be formed at lower temperatures at the flame periphery because of intermolecular (i.e., nonsiloxane)bonding. Obviously, the characteristics of a number of chemical and intermolecular bonds between adjacent primary particles in aggregates can be varied, depending on the temperature, on the collision frequency, and on other conditions. Therefore one can assume that both types of primary particle bonding in aggregates are possible. Each type of bonding has a different effect on the interactions of silica with polar compounds such as water, polymers, surfactants, and solvents. Clearly, any pre-

10.1021/la0103867 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/11/2002

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Table 2. Parameters of Aqueous Suspensions of Fumed Silica, Pure PVP, and PVP/Silica sample PVP A-300a A-300b A-300/PVPa A-300/PVPa A-300/PVPa A-300/PVPa A-300/PVPc A-300/PVPc

CPVP, wt %

CA300, wt %

ζ, mV

pH

Def, µm

CA30, wt %

CPVP, wt %

Cegg alb., mg g-1

Cgelatin, mg g-1

5 5 5 5 5 5 5 5

1.5 -15.1 -12.9 -8.0 -15.6 -12.2 -14.1 -7.1 -6.2

6.27 5.21 5.43 5.93 6.16 6.16 6.07 5.72 5.75

>50 0.78 0.76 0.50 0.47 0.37 0.34 0.31 0.36

3 3 3 4 4 4

0 0.15 0.3 0 0.2 0.4

170 80 30 180 102 40

380 210 116 224 190 100

5

0.13 0.25 0.38 0.5 2.5 5.0

Table 3. Impact of PVP Immobilized on A-300 on Protein Adsorption from the Aqueous Solution

aMCA suspension was ball-milled for 5 h and stored for 1 month. b

Fresh suspension was prepared in the ultrasonic bath for 9 h. PVP addition to the MCA suspension of silica 5 min before the measurements.

c

Table 4. Parameters of the Interfacial Water Layer for Pure Silica and Silica/PVP Suspensions (CSiO2 ≈ 6 wt %) system A-300 A-300a + 0.3 wt % PVP A-200 1 wt % PVP A-200 + 1 wt % PVP A-200 + 5 wt % PVP

-∆Gs,b kJ mol-1

-∆Gw,b kJ mol-1

Csuw,c mg g-1

Cwuw,c mg g-1

γS,d mJ m-2

3.2 2.5

1.3 1.0

700 500

700 900

253 186

3.0 1.6 3.0

1.4

680

279

0.9

730 300 520

1400

220

3.0

1.0

1100

1700

403

a S 2 -1 for A-300 and A-200, respectively, N2 ≈ 300 and 190 m g in PBS. b ∆Gs and ∆Gw are the changes in the Gibbs free energy of strongly and weakly bound waters, respectively. c Csuw and Cwuw are the concentrations of the unfrozen waters strongly and weakly bound to the surfaces. d γS is the change in the surface free energy.

Figure 1. Nitrogen adsorption-desorption isotherms for pristine A-300 and dried silica and (a) silica/proteins and (b) silica/PVP prepared by drying of the corresponding aqueous suspensions.

treatment of fumed silica possessing structurally nonrigid swarms, whose stability typically decreases with size, can alter many silica properties that are important in different applications.1-7 Concentrated aqueous suspensions of (5) Adsorption On New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Elsevier: Amsterdam, 1996.

fumed silica (CSiO2 ≈ 5 wt %) , which frequently possess multimodal particle size distributions (PSDs)4 because of preservation of the swarms, are relatively stable, and silica does not lose its adsorptive ability during long storage periods.4,7 Besides, at CSiO2 ≈ 9-10 wt %, “jellification” of the silica dispersion is observed after its exposure to air for several hours because of the formation of a continuous three-dimensional network of interparticle bonds. At the same time, the diluted suspensions (CSiO2 < 1 wt %) are less stable than the concentrated suspensions and are characterized by broader PSDs.4 Chemical modification of oxide surfaces, immobilization of polymers on them, and addition of different surfactants or solvents (e.g., alcohols) to the aqueous suspensions may be responsible for alterations in the hydrogen bond network, the free energy of the interfacial layers, and the surface charge distribution, etc., which can affect the dispersion stability and other characteristics of both suspensions and powders prepared from these components.4,7 Such powders with adsorbed molecules (e.g., drugs, PVP, proteins, cellulose, etc.) on their surfaces are of interest in medicine and biotechnology. The aim of this work was to study the impact of polymers (i.e., bovine serum albumin (BSA), egg albumin, gelatin,and PVP), an ionogenic surfactant (the drug Aethonium (6) Proceedings of the International Conference on Silica Science and Technology (Silica 98), Mullhouse, France, September 1998 (7) (a) Gun’ko, V. M.; Turov, V. V.; Zarko, V. I.; Dudnik, V. V.; Tischenko, V. A.; Voronin, E. F.; Kazakova, O. A.; Silchenko, S. S.; Chuiko, A. A. J. Colloid Interface Sci. 1997, 192, 166. (b) Mironyuk, I. F.; Gun’ko, V. M.; Turov, V. V.; Zarko, V. I.; Leboda, R.; SkubiszewskaZie¸ ba, J. Colloids Surf., A 2001, 180, 87. (c) Kazakova, O. A.; Gun’ko, V. M.; Lipkovskaya, N. A.; Voronin, E. F.; Pogorelyi, V. K. Kolloidn. Zh. Submitted for publication. (d) Gun’ko, V. M.; Mironyuk, I. F.; Zarko, V. I.; Voronin, E. F.; Turov, V. V.; Pakhlov, E. M.; Goncharuk, E. V.; Leboda, R.; Skubiszewska-Zie¸ ba, J.; Janusz, W.; Chibowski, S. J. Colloid Interface Sci. In press.

Impact of Organics on Fumed Silica

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Figure 2. (a, c, d) Changes in the free energy and (b) water adsorption potential distributions for pure PVP solutions and suspensions of silica, silica/PVP, and silica/PVP/PBS computed on the basis of 1H NMR data. ∆G is a function of (c) the filled pore volume and (d) the pore radius for silica (6 wt % A-200) and PVP/silica (1 wt % PVP, 6 wt % A-200) suspensions.

or 1,2-ethylene-bis(N-dimethyl carbodecyloxymethyl)ammonium dichloride), and polar solvent mixtures with water-ethanol on the characteristics of fumed silica in aqueous suspensions and dried powders. Materials and Techniques Materials. Fumed silica (pilot plant of the Institute of Surface Chemistry, Kalush, Ukraine; 99.9% purity, specific surface area Sar ) 102-411 m2 g-1, Table 1) was heated at 773 K for several hours to remove residual HCl and other adsorbed compounds. Samples A-200 (SN2 ≈ 190 m2 g-1) and A-300 (SN2 ≈ 300-340 m2 g-1) at the concentration CSiO2 ) 1-6 wt % were treated in a ball mill for several hours (mechanochemically activated (MCA) suspensions), strongly sonicated for 5 min by means of an ultrasonic disperser, or gently sonicated in an ultrasonic bath (CSiO2 ) 0.1-6 wt. %). These samples were used to explore the properties of the polymer/silica suspensions. Egg albumin (molecular weight ≈ 4.4 × 104 g/mol), bovine serum albumin, gelatin (molecular weight ≈ 3.5 × 105 g/mol), ethanol, and Aethonium (pharmaceutical purity) were used as received. Adsorption of BSA and gelatin (Table 1) was studied without strong pretreatment of oxides and with no addition of

the electrolyte buffer solution. Silica (40 mg oxide/5 mL protein solution or 1 g oxide/125 mL solution) was added to the protein solution (0.6 wt %) and agitated for 0.5 h; adsorption was measured after exposure for 1 h (plateau adsorption), and the suspension was centrifuged at 6000 rpm for 15 min. The Biuret reactant (4 mL) was added to the supernatant (1 mL). After the solution was agitated, it was exposed for 0.5 h, and then its optical density was measured at λ ) 540 nm to calculate the amount of absorbed protein compared to that in the initial solution. This method was described in detail elsewhere.8 Notice that the pH dependence of albumin adsorption onto fumed oxides was studied previously.9 The protein/silica residual was dried at 313 K and degassed at 333 K for 2 h before nitrogen adsorption measurements were made. Poly(vinylpyrrolidone) (-CH2CH(NC4H6O)-)n, n ≈ 100, molecular weight ) 12600 ( 2700, pharmaceutical purity) was used as received. The PVP solution was added to the MCA suspension of silica, or fumed silica powder was added to the PVP solution (8) (a) Laboratory Investigation Methods in the Clinic; Menshikov, V. V., Ed.; Medicine: Moscow, 1987. (b) Weichselbaum, T. Am. J. Clin. Pathol. 1946, 16, 40. (c) Kochetov, G. A. Practical Enzymology Manual; Vysshaya Shkola: Moscow, 1980. (9) Gun’ko, V. M.; Vlasova, N. N.; Golovkova, L. P.; Stukalina, N. G.; Gerashchenko, I. I.; Zarko, V. I.; Tischenko, V. A.; Goncharuk, E. V.; Chuiko, A. A. Colloids Surf., A 2000, 167, 229.

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Figure 3. Particle size distributions with respect to the light scattering (solid lines), particle volume (dashed lines), and particle number (dot-dashed lines) for the aqueous suspensions of fumed silica A-300 at CSiO2 values of (a) 0.25 wt %, (b, c, f) 1 wt %, (d) 0.1 wt %, and (e) 0.5 wt % as a function of pH. Suspensions a-c were strongly sonicated by the disperser for 5 min and suspensions d-f were gently sonicated (pH ≈ 5.3-5.6) in the ultrasonic bath for 30 min. For d-e, the exposure time interval between each measurement was ˜90 s and the regularization parameter was R ) 0.01. and then agitated (1000 rpm) for several hours and sonicated for 5-6 min. The ratio γ ) CPVP/CSiO2 ) 0-1 and γ e 0.1 corresponds to practically irreversible adsorption of PVP because it was not washed from the silica. The estimated statistical monolayer (Θ

) 1) corresponds to γ ≈ 0.2. Several experiments with PVP/silica were performed using the physiological buffer solution (PBS) with NaCl (5.5 g), KCl (0.42 g), CaCl2 (0.5 g), MgCl2 (0.005 g), and NaHCO3 (0.23 g) per liter of distilled water. PVP adsorption


Figure 4. Particle size distributions in the aqueous suspensions of fumed silica A-300 (CSiO2 ) 1.0 wt %, pH ) 5.5) at different light-scattering angles (Θ): 90°(curves 1 and 6), 60° (2), 45° (3), 30° (4), and 15° (5). The time interval between the measurements was ˜90 s from 90 to 15°. was determined spectrophotometricaly at λ ) 420 nm. This wavelength was selected on the basis of the calibrated graph using the corresponding mixtures of the liquid residuals of the centrifuged PVP/silica suspensions and the aqueous solutions of citric acid (38.4 g/L distilled water) and iodine (0.81 g of I2 + 1.44 g of KI/L of distilled water), which formed characteristic complexes with PVP. Infrared Spectroscopy. IR spectra (Specord M80; Karl Zeiss, Jena) of A-300 and PVP/A-300 were recorded. The samples were prepared with the solid residual of the centrifuged dispersion dried at room temperature, heated at 350 K for 3 h, and pressed (28 × 2 mm, ∼10-16 mg) or (PVP ) 0.5-0.6 mg) stirred with KBr (∼60 mg). BSA/A-300 samples (22 mm × 5 mm, ∼12 mg) were prepared using dried (room temperature, 24 h), stirred, and pressed solid residual of the centrifuged (6000 rpm, 30 min) BSA/silica dispersions (CSiO2 ) 5 wt % and various CBSA) that had been exposed for 1 h. Photon Correlation Spectroscopy (PCS). PSD investigations were performed using a Zetasizer 3000 (Malvern Instruments) apparatus (λ ) 633 nm, Θ ) 90°). Deionized distilled water was used to make the suspension (CSiO2 ) 0.1-1.0 wt %) that was sonicated for 5 min using an ultrasonic disperser (Sonicator Misonix Inc.; 22 kHz, 500 W). The pH values measured by a precision digital pH meter were adjusted by addition of 0.1 M HCl or 0.1 M NaOH solutions, and the salinity was 0 or 10-3 M NaCl. To compute the PSD and Def (average hydrodynamic diameter, i.e., the particle diameter plus the double shear-layer thickness) values, the Malvern Instruments software (version 1.3) was used, assuming that the particles were roughly spherical. The ζ-potential measurements were made in conjunction with the PSD study of the concentrated aqueous suspensions (pure or with PVP) at CSiO2 ) 5 wt % (Table 2) using a ZetaPlus (Brookhaven Instruments) ζ-potential apparatus under conditions shown in Table 2. Fumed silica or PVP/silica interactions with living flagellar Proteus mirabilis 187 (PM 187) at a concentration of ∼106 mL-1 and a solution salinity of 0.9 wt % of NaCl was explored using a PCS apparatus (λ ) 632.8 nm, Θ ) 15-90°, correlation time interval ) 3.0 × 10-5-2.0 × 10-3 s) described elsewhere.10 The PCS autocorrelation functions were analyzed with respect to the PSD and the PM mobility using the corresponding kernels in the integral equations. These equations were solved with the modified regularization-SVD (singular value decomposition) procedure (10) Levchuk, Yu. N.; Osokin, V. M. Tekhnika Sredstv Svyazi 1989, 3, 72.

Langmuir, Vol. 18, No. 3, 2002 585 CONTIN11 at unfixed or fixed (0.01) regularization parameters and under the nonnegativity condition. Nitrogen Adsorption. The specific surface area (Sar,Table 1) was evaluated using a Jemini 2360 (SVLAB) apparatus with argon adsorption. Nitrogen adsorption-desorption isotherms (Figure 1) were also recorded at 77.4 K using a Micromeritics ASAP 2010 adsorption analyzer. The specific surface area (Sar, SN2) was computed using the BET method.12,13 The pore volume (Vp) was estimated from adsorption at a relative pressure p/p0 ≈ 0.98-0.99 by converting the volume of adsorbed nitrogen to the volume of liquid nitrogen. The regularization procedure11,14 was used to calculate the pore size distributions f(Rp) for pristine silica and suspended centrifuged-dried polymer/silica using the overall adsorption isotherm equation described elsewhere.15 The desorption data were used to compute f(Rp) under the nonnegativity condition with a fixed regularization parameter R ) 0.01. The Fowler-Guggenheim (FG) equation describing localized monolayer adsorption with lateral interaction16,17 was used as the kernel in the integral equation

Θ(T,p) )

∫

xmax

xmin

Θl(T,p,x) f(x) dx

(1)

to compute the nitrogen adsorption energy distributions f(E). f(x) is an unknown distribution function of a given parameter x.11 1H NMR Spectroscopy. The 1H NMR spectra of the interfacial water were recorded using a high-resolution WP-100 SY (Bruker) NMR spectrometer with a bandwidth of 50 kHz. Relative mean errors were (10% for signal intensity and (1 K for temperature. The amount of fumed silica in the suspensions was CSiO2 ≈ 6 wt % and CPVP ) 0.3, 1, and 5 wt %. The amount of unfrozen water (Cuw) in the frozen aqueous suspensions was calculated by comparing the signal intensity (I) for the interfacial liquid water to that of the water adsorbed on silica powder from the gas phase using the calibrating graph I ) f(Cuw). The calculation technique that was used to estimate the change in the free energy (∆G) of the interfacial water and the amount of unfrozen water strongly (Csuw) and weakly (Cwuw) bound to the silica surfaces or to PVP molecules was described in detail elsewhere.7 The adsorption potential distribution f(A) with respect to the interfacial water disturbed by silica or silica/ PVP was computed from f(A) ) -dCuw/dA where A ) -∆G is the differential molar work. Additionally, ∆G as a function of the pore radius (Rp) or the volume (Vp) filled by unfrozen water was calculated using the pore size distribution f(Rp) ) dVp/dRp, assuming that water can be frozen in narrow pores at lower temperature than it can be frozen in larger pores. Because stable aggregates of primary particles exist in the aqueous suspension of fumed silica,4,7 one can assume that the channel structure (i.e., gaps between primary particles) in these aggregates does not change very much in the aqueous suspensions; however, agglomerates can be rearranged significantly up to total decomposition. Because unfrozen water is located mainly in aggregates or on their outer surfaces, ∆G(Cuw) can be transformed to ∆G(Vp) or ∆G(Rp), assuming that the unfrozen water density is ∼1 g/cm3 and that this water fills the pores in the same way that liquid nitrogen, which was used to estimate f(Rp) and Vp, does. (11) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. Provencher, S. W. Comput. Phys. Commun. 1982, 27, 229. (12) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: London, 1982. (13) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley & Sons: New York, 1997. (14) Szombathely, M.v.; Brauer, P.; Jaroniec, M. J. Comput. Chem. 1992, 13, 17. (15) (a) Nguyen, C.; Do, D. D. Langmuir 1999, 15, 3608. (b) Nguyen, C.; Do, D. D. Langmuir 2000, 16, 7218. (c) Gun’ko, V. M.; Do, D. D. Colloids Surf., A. In press. (16) Hill, T. L. Statistical Thermodynamics; McGraw-Hill: New York, 1956. (17) (a) Choma, J.; Jaroniec, M. Langmuir 1997, 13, 1026. (b) Jaroniec, C. P.; Jaroniec, M.; Kruk, M. J. Chromatogr., A 1998, 797, 93.

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Figure 5. Particle size distributions with respect to light scattering (solid lines), particle volume (dashed lines), and particle number (dotted/dashed lines) for aqueous suspensions of silica/PVP at CSiO2 ) 0.25 wt % (a-c) and 1 wt % (d, e) at CPVP/CSiO2 ) 0.04.

Results and Discussion Polymer Adsorption. The increasing stability of particle swarms with decreasing primary particle size1-4

plays an important role in polymer adsorption. Globular protein molecules can interact with silica, mainly at the outer surfaces of undestroyed aggregates when theaggregate channel radii (Rp < 5 nm) are less than the polymer


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Figure 6. (a) PSDs for pure silica suspensions (MCAsuspension), living microorganisms Proteus mirabilis 187, a mixture of silica and PVP/SiO2 with P. mirabilis 187 and (b) velocities of P. mirabilis 187 and their mixtures with silica or PVP/silica computed at unfixed regularization parameters.

molecule size. An increase in Sar (Table 1) enhances theplateau adsorption of BSA or gelatin in the concentration range of oxide studied at CSiO2 ) 0.8 wt % and Cprotein ) 0.6 wt %. Note that these silica samples (that were synthesized under various conditions (described in detail elsewhere7b,d) are characterized by different hydrophilicity values, even for similar Sar values, which may explain the scatter in protein adsorption versus Sar data. The adsorption (mg/m2) of the surface area decreases with with decreasing Sar because of the diminution of the concentration of accessible silanols (COH), which are the main adsorption sites on silica. Besides, the adsorption rises with COH independent of the nature of the proteins (Table 1). The COH values were determined from the integral intensity of the IR band at 3750 cm-1 according to a method described elsewhere.18 PVP molecules that are simpler and smaller than protein molecules do not have strong intramolecular bonds as proteins do, despite the availability of electron-donor N-CdO groups. Proton-donor groups are absent in PVP. Adsorbed PVP enhances the pH of the isoelectric point (IEP) nearly linearly with CPVP, and the ζ potential is reduced with increasing CPVP at a constant CSiO2 (Table 2). A large number (∼100/PVP molecule) of polar electrondonor N-CdO bonds that are between the pyrrolidone rings may be responsible for practically irreversible (18) Sobolev, V. A.; Tertykh, V. A.; Chuiko, A. A. Zh. Prikl. Spektrosk. 1970, 13, 646.

Figure 7. Particle size distributions with respect to the light scattering (solid lines), particle volume (dashed lines), and particle number (dotted/dashed lines) for aqueous suspensions of silica/egg albumin at CSiO2 ) 0.25 wt % and Calb/CSiO2 values of (a) 0.172, (b) 0.23, and (c) 0.276.

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Figure 8. Particle size distributions with respect to the light scattering (PSDI, solid lines), particle volume (PSDV, dashed lines), and particle number (PSDN, dotted/dashed lines) for the H2O/CH3CH2OH suspensions of silica at CSiO2 values of (a-e) 0.1 wt %, (f-j) 0.5 wt %, and (k-o) 1 wt % at CEtOH values of 10 (a, f, k), 20 (b, g, l), 30 (c, h, m) , 40 (d, i, n), and 50% (e, j, o). pH ) 4.8-5.9.

adsorption of the polymer molecules on silica. PVP molecules are not washed from the silica surfaces at CPVP/CSiO2 e 0.1 because of bonding in multicentered adsorption complexes, and approximately two-thirds of CdO groups at Θ < 1 form hydrogen bonds with silanols, as simultaneous breaking of all these bonds is unlikely. However, changes in ∆G upon adsorption of polar or ionogenic polymers on solid surfaces from the aqueous solution are relatively small (-∆∆G ≈ 4-5 kBT).19,20 Interaction between PVP and tSiOH groups can cause a buildup of the surface charge density on the silica, depending on CPVP and the electrolyte concentration.4 However, strong adsorption of PVP enhances the pH of the IEP and reduces -ζ (Table 2) because of shielding of the oxide surfaces. Nevertheless, Def of PVP/silica swarms (Table 2) decreases because of decomposition of agglomerates by PVP adsorbed in the unfolded state; adsorbed PVP can reduce the adsorption of albumin or gelatin with CPVP (Table 3) because of significant shielding of surface silanols that are responsible for strong intermolecular interaction with proteins.

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The adsorption potential computed with respect to the disturbed interfacial water using 1H NMR spectra with the completefreezing-out of the bulk water and the layer freezing-out of the interfacial water at 210 < T/K < 273 shows that the concentration of weakly bound water (Figure 2b, A < 1 kJ/mol, Table 4, Cwuw) is lower for the pure silica suspension or the pure PVP solution. The decline in the linear portion of the ∆G(Cuw) graph at small ∆G values (Figure 2a) that correspond to weakly bound water is larger for the pure silica suspension than that for the PVP/silica suspension (i.e., the boundary between the disturbed and undisturbed water is more robust without PVP). However, in the case of the pure PVP solution, Cwuw cannot be estimated, while nearly linear ∆G(Cuw) allows one to calculate only Csuw, as the capability of noninogenic PVP to disturb the water over a large distance is small. This effect can be also caused by formation of PVP oligomers in the pure aqueous solution because Def > 50 nm (Table 2) and the “outer surface area” of the oligomers is small. The f(A) intensity (Figure 2b) increases at CPVP ) 5 wt % (as well as for Csuw and Cwuw (Table 4)), but at CPVP ) 1 wt %, the opposite effect is observed at A > 1 kJ/mol for Csuw and the free surface energy γS compared to the same parameters as those for the individual silica suspension (or Csuw for PVP/PBS/silica). On the other hand, changes in ∆G versus Vp (Figure 2c) or Rp (Figure 2d) show that the impact of silica on water in the pores at Rp ) 1-10 is significantly larger than that for the PVP/silica suspension (CPVP ) 1 wt %, γ ) CPVP/CSiO2 ≈ 0.17) because the free energy is lower. This effect is due to reduction in the long-range components of surface forces for silica shielded by PVP. Clearly, in the case of large interparticle distances in agglomerates, unfrozen water can be located close to the silica particles (i.e.,close to the outer surfaces of aggregates); therefore, formation of a denser PVP layer at the silica surfaces at Θ < 1 leads to a decrease in Csuw. At large values of CPVP (CPVP ) 5 wt %, γ ≈ 0.83), a major portion of PVP is in the nonimmobilized state or else weakly interacts with silica, but the amount of water weakly and strongly bound to PVP increase significantly (Table 4, Figure 2a). Thus, at γ < 0.2 (i.e., Θ < 1), polymer molecules adsorb strongly, and the number of free pyrrolidone groups can be less than the number of bonded groups (i.e., the adsorbed PVP layer is relatively dense, and oxide particles or PVP molecules can disturb only a relatively thin interfacial water layer). At large values of CPVP, a significant portion of PVP molecules has free tails or loops that do not interact with silica but effectively interact with water molecules, thus disturbing their hydrogen bond network. However, a decline in the ∆G dependence on Cuw, Vp, or Rp (especially at ∆G > -0.7kJ/ (19) (a) Principles in Protein Adsorption. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985; Vol. 2, (b) Landau, M. A. Molecular Mechanism of the Action of Physiologically Active Compounds; Nauka: Moscow, 1981. (c) Proteins at Interfaces: Physicochemical and Biochemical Studies; Brash, J. L., Horbett, T. A., Eds.; ASC Symposium Series 342; American Chemical Society: Washington, DC, 1987. (d) Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983. (e) Sato, T.; Ruch, R. Stabilization of Colloidal Dispersions by Polymer Adsorption; Marcel Dekker: New York, 1980. (20) (a) Gun’ko, V. M.; Zarko, V. I.; Goncharuk, E. V.; Turov, V. V.; Voronin, E. F.; Pakhovchishin, S. V.; Pakhlov, E. M.; Guzenko, N. V.; Leboda, R.; Chibowski, E.; Chuiko, A. A. Colloids Surf., A. Submitted for publication. (b) Goncharuk, E. V.; Pakhovchishin, S. V.; Zarko, V. I.; Gun’ko, V. M. Kolloidn. Zh. 2001, 63, 1. (c) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloids Surf., A 1997, 123-124, 283. (d) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Goncharuk, E. V.; Voronin, E. F.; Kazakova, O. A. Teor. Eksp. Khim. 2001, 37, 73.


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Figure 9. Particle size distributions with respect to the light scattering (PSDI, solid lines), particle volume (PSDV, dashed lines) and particle number (PSDN, dotted/dashed lines) for pure silica (lines) or water/Aethonium/silica (shaken for 0.5 min after addition of Aethonium) (lines + symbols) suspensions at CSiO2 ) 1 wt %, 0.001 M NaCl. (d) Suspensions sonicated for an additional 1 min.

mol) is small for a relatively thick layer of weakly bound water (Figure 2). Notice that in the case of the adsorption of protein hydrolysate, fermentatively hydrolyzed bull

blood proteins with fractions from 4000 to 25 000 Da on fumed silica (CSiO2 ) 3 wt %, Cprotein ) 1 wt %, Cuw ) 3001200 mg/g7a) have smaller values of Cuw (i.e., small proteins

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Figure 10. Particle size distributions with respect to the light scattering (PSDI, solid lines), particle volume (PSDV, dashed lines) and particle number (PSDN, dotted/dashed lines) for pure silica (lines) and water/Aethonium/silica (lines + symbols) suspensions (shaken for 0.5 min after addition of Aethonium) at CSiO2 ) 0.1 wt %, 0.001 M NaCl.

can shield the silica surface more strongly than larger globular proteins or PVP can because Cuw is lower). Particle Size Distributions. The PSDs in the silica or polymer/silica suspensions (Figures 3-10) can be multimodal or monomodal, depending on the component concentrations and the pH.4 However, small aggregates (dPCS < 100 nm) are the main contributors to the distributions related to both light scattering (PSDI) and particle volume (PSDV) or number (PSDN) for all pure silica suspensions (Figures 3a-c, 4), for some samples with PVP (Figures 5 and 6), for silica suspended in ethanol/ water (Figure 8), or for aqueous suspensions of silica/ Aethonium (Figures 9 and 10). This effect is caused by decomposition of agglomerates and large aggregates of primary silica particles because of the presence of solvent or polymer molecules and by electrostatic repulsion between negatively charged silica particles. Smaller aggregates are the main contributors not only to PSDN

but also to PSDV (Figures 3 and 5), but in comparison with PSDN or PSDV, PSDI shifts toward larger dPCS values at λPCS ) 633 nm because the light-scattering intensity is greater for particles with sizes close to λPCS. The fumed silica dispersion stability decreases with decreasing CSiO24, and even short time exposure leads to marked growth of silica particle swarms, especially at low CSiO2 ) 0.1 wt % (Figure 3d-f). However, at CSiO2 ) 1 wt %, changes in the PSDs during several minutes are relatively small (in Figure 3f, the positions of the maxima of the first and last curves are very similar). Typically, recording of the autocorrelation function in PCS is carried out at Θ ) 90°, which corresponds to maximum light scattering.4 The study of the suspension CSiO2 ) 1 wt % that was sonicated for 30 min in the bath, aged for two months, and agitated before measurements were made gives similar PSDs at other scattering angles (Figure 4), but they are slightly shifted because of agglomeration in the suspension that occurred upon exposure of the sample for ˜90 s between each measure-


ment (Θ ) 90-15°). Additionally, after two month’s storage of the suspension, there is growth of the average particle size compared to the average particle size of the initial suspension (Figure 4, curve 6). Similar results related to the PSD displacement toward larger dPCS because of longtime aging of the fumed silica suspensions were observed at different CSiO2 values; however, the increase in CSiO2 typically reduces agglomeration.4 Features of PVP/silica swarms are reflected in interactions with living mobile flagellar microorganisms such as P. mirabilis 187 (CPM ≈ 106/mL), depending on CPVP. PVP/ silica impacts the PSDs more strongly than does pure silica, as PSD markedly shifts toward larger dPCS values only for PM/PVP/silica (Figure 6a). The proper mobility of flagellar PM decreases significantly (Figure 6b) because of its interaction with PVP/silica, in contrast to the interaction of PM/pure silica with PVP/silica in the diluted suspensions (CSiO2 ) 0.1 wt %). Consequently, even at a low concentration, PVP/silica reduces the vital functions of PM more significantly than pure silica does because of the strong adhesion properties of PVP. Note that these features with respect to cell surfaces are used in some drug compositions. For egg albumin/silica mixtures that are suspended, centrifuged, dried, and suspended, the PSDs (Figure 7) that are shifted toward larger dPCS values in comparison with those for pure silica (Figure 3) or PVP/silica (Figures 5 and 6) depend on the albumin concentration (Calb) as the distributions become more broadened and complex with increasing Calb. At the maximum Calb value, a large peak of PSDV appears at dPCS > 1 µm (Figure 7c), corresponding to swarms in which the main contribution is from protein molecules or their oligomers because PSDI has low intensity in this region. Clearly, the capability of dissolved proteins to change the light-scattering intensity is lower than that of silica swarms. Small particles at dPCS > 30 nm correspond to both washed albumin molecules and their oligomers or to albumin swarms with silica particles because PSDI has low intensity at dPCS values between 30 and 100 nm. For all the protein/silica samples, the main PSDV peaks lie to the left of PSDI peaks that correspond to the availability of large agglomerates containing protein molecules or to oligomers containing smaller amounts of silica particles. Therefore, one can assume that the interaction of egg albumin with the silica surface is weaker than intramolecular albumin interactions (i.e., the globular structure of proteins is not decomposed; therefore, the protein can be washed from the silica upon secondary suspension7a), in contrast to PVP molecules that interact with silica more strongly than they do with other PVP molecules. Intramolecular interactions in PVP are weak. These features of polymer-silica interactions have different effects on the pore-formation process that occurs when the solid residual of the centrifuged polymer/silica suspensions is dried. One can assume that changes in the medium (e.g., water/ethanol frequently used to study the adsorption of compounds that are not water soluble) of silica suspensions result in alterations in their PSDs and in other characteristics. Changes in the concentration of ethanol (CEtOH) in the fumed silica suspensions (CSiO2 ) 0.1-1.0 wt %, pH ) 5.9-4.5) influence the Def value only slightly but nonlinearly (Figure 11a); these changes also influence the swarm size distributions (Figure 8). Note that the pH value of similar suspensions decreases linearly with the addition of ethanol: pH ≈ pH0 - 0.028CEtOH.21 In contrast to (21) Zaporozhets, O. A.; Lipkovska, N. A.; Ivan’ko, L. S.; Sukhan, V. V.; Pogorely, V. K. Functional Materials 2000, 7, 2.

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Figure 11. Dependence of the effective diameter Def on (a) CEtOH and (b) CAet containing 0.001 M NaCl at different CSiO2 values.

changes induced by ethanol, Aethonium significantly changes the PSDs (Figures 9, 10, and 11b), especially at pH values close to the pH of the IEPSiO2 when the negative surface charge density is low enough to prevent strong interaction with the positively charged surfactant ions. With the presence of NaCl (0.001 M) at CSiO2 ) 1 wt % and low CAet, the multimodal distributions can be observed and characterized by the availability of small aggregates (15-30 nm) (Figure 9) close to the size of the primary silica particles. Similar particles are frequently observed in the concentrated MCA suspensions of fumed silicas.4 In the case of smaller CSiO2 values (CSiO2 ) 0.1 wt %), similar small particles in the presence of Aethonium (Figure 10) and in the aqueous ethanol medium (CEtOH ) 10% and CSiO2 ) 1 wt % (Figure 8k)) are also observed./ However, for other aqueous ethanol suspensions at larger CEtOH values, dPCS > 50 nm, and both monomodal and multimodal PSDs are observed (Figure 8). The addition of small

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Figure 12. RS plots for (a, b) pristine and dried silica and silica/proteins and (c, d) pristine and dried silica and silica/PVP.

amounts of Aethonium results in different changes in the size of the smallest particles (Figure 9); however, the Def value always increases with the addition of Aethonium (Figure 11b). Thus, addition of different amounts of ethanol gives smaller changes in the PSDs than do significantly smaller amounts of Aethonium (Figures 8-11) or polymers (Figures 6 and 7). Note that a high concentration of Aethonium cations causes the formation of flocks and that the silica particles are negatively charged over the pH range used in this work. These changes are nonlinear with respect to the additive concentration because of nonrigid swarm structures that are easily rearranged by surfactant ions and by polar (PVP) or charged (protein) polymer molecules. However, these changes in the PSDs are not dramatic, and solid sediment is not formed; consequently, the aqueous suspensions of fumed silica are relatively stable with regard to changes in the polarmedium composition. Such composition changes are important in fumed silica applications in other media. It is of interest to study the impact of this composition on the corresponding dried-complex powders (these were suspended, centrifuged, and then dried) that can be used for different purposes.

Nitrogen Adsorption. Changes in the initial fumed silica properties or pretreatment conditions on drying of the solid residual of the centrifuged aqueous suspensions result in marked differences in the nitrogen adsorptiondesorption isotherms (Figure 1) and RS plots (Figure 12), which reflect alterations in the textural characteristics of solids (Table 5). The incline in the hysteresis loops of dried powders is larger than that for the initial silica (i.e., the mesopore size distributions of the powders differ because of changes in the large channels in aggregates and in interaggregate space in agglomerates on suspending, centrifuging, and drying (Figure 13). A similar shape of the adsorption isotherms, which can be assigned to type II with a narrow hysteresis loop,12 is observed for silica/ PVP and silica/protein powders but is characterized by decreased adsorption because of the reduced porosity compared to that of dried pure silica (Vp, Table 5) and reduced specific surface area (SN2). These changes are clearly observed in the RS plots (Figure 12, Si-1000 was used as a reference material22) that show a marked diminution of the adsorption in narrow pores and the (22) Jaroniec, M.; Kruk, M.; Olivier J. P. Langmuir 1999, 15, 5410.


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Table 5. Structural Parameters of Pristine Powders and Dried Suspensions of Fumed Silica, Silica/PVP, Silica/ Egg Albumin, and Silica/Gelatin powder

CX, wt % (X ) PVP, alb)

aA-300 bA-300 bA-300/PVP bA-300/PVP

5 10

aA-300* bA-300* bA-300*/alb bA-300*/alb bA-300*/alb bA-300*/gelatin bA-300*/gelatin

17.2 23.0 27.6 21.1 31.3

SN2, m2 g-1

Vp, cm3 g-1

342 182 170 157 322 275 175 157 138 143 123

0.566 0.612 0.923 0.383 0.613 1.147 0.903 0.868 0.724 0.745 0.701

aPristine powder. bDried powders from solid residual of the suspensions.

Figure 13. SEM image of aggregates of suspended and dried fumed silica A-300.

opposite effect in large mesopores of the dried powders. Note that only the interior volume in aggregates is filled by nitrogen at p/p0 f 1, and external surfaces of aggregates may be covered by several monolayers of nitrogen because interaggregate volume (>15-20 cm3/g) in agglomerates is significantly larger than Vp and is not filled by nitrogen (or Ar or H2O) at p/p0 f 1. The availability of PVP gives a stronger structural effect in comparison with proteins. For example, the normalized RS plots are nearly the same for all the samples with silica/albumin or silica/gelatin and are independent of concentration and nature of the components (Figure 12b), in contract to behavior of silica/ PVP (Figure 12d). Thus, structural changes caused by suspension and drying of silica and silica/polymers can be observed for both narrow and large pores of dried solid residuals. Structural features of silica particles that are suspended and dried are clearly observed in SEM images of relatively large agglomerates >5 µm (Figure 13). Secondary particles become denser in comparison with those of pristine fumed silica;1,2 however, secondary particles are less dense than silica gel particles because transformation of fumed silica to silica gel requires more rigid reaction conditions. For example, HTT of A-380 in an autoclave at 150 °C for 6 h reduces SN2 to 192 m2/g but increases Vp to 1.29 cm3/g (initial Vp ) 0.67 cm3/g). Both changes are approximately 2-fold but are opposite from each other. At the same time, these changes are close to those observed on suspending and drying A-300 (Table 5)

Figure 14. Nitrogen adsorption potential distributions for pristine and dried silica and (a) silica/proteins and (b) silica/ PVP prepared by drying of the corresponding aqueous suspensions.

and are linked to dissolution and condensation of the silica fragments.1,3 Because the direction of these processes is connected to the dissolution of smaller particles and to the growth of larger particles with simultaneous compacting contacts between adjacent particles,3c the specific surface area decreases. The Vp value was determined using the nitrogen adsorption increases due to compacting of agglomerates (Figure 13), which enhances the capillary condensation of nitrogen and causes its adsorbed volume to increase when p/p0 f 1 (i.e., the estimated Vp rises). Changes in the shapes of the isotherms (Figure 1) and in RS plots (Figure 12) are in agreement with the adsorption potential distributions f(A) ) -da/dA, where a is the amount of absorbed nitrogen and A ) -∆G ) RgT ln(p0/ p), (Figure 14). These curves also show alterations related to the adsorption in narrow pores (at A > 1 kJ/mol) and

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Figure 15. Pore size distributions with respect to (a, b) dVp/dRp and (c, d) dS/dRp for pristine fumed silica A-300 and powders prepared by drying aqueous suspensions of silica, egg albumin/silica, gelatin/silica, silica, and PVP/silica.

large mesopores (A < 0.1 kJ/mol) in comparison with that for the pristine silica. Changes in pristine silica are caused by changes in the silica particle morphology on drying and filling of the gaps between the particles by polymer molecules. More detailed information about structural changes in dried silica and silica/polymer powders can be obtained from analyses of the pore size distributions f(Rp). Suspending, centrifuging, and then drying polymer/ silica mixtures changes the pore structure according to the polymer that is used and its concentration (Figure 15). The availability of egg albumin or gelatin at different concentrations causes a weak effect that is mainly related to the filling of pores by protein molecules or to the formation of large protein oligomers, in contrast to PVP, and results in markedly rearranging secondary silica particles and pores in the aggregates. Albumin and gelatin can adsorb on silica surfaces in the globular form because of strong intramolecular forces and hydrophobic interactions between nonpolar side groups of amino acids.4,7,19,23 (23) (a) Norde, W.; Lyklema, J. Colloids Surf. 1989, 38, 1. (b) Norde, W.; Favier, P. Colloids Surf. 1992, 64, 87. (c) Norde, W.; Anusiem, A. C. I. Colloids Surf. 1992, 66, 73. (d) Urano, H.; Fukuzaki, S. J. Ferment. Bioeng. 1997, 83, 261.

Typically, 300-600 mg of albumin or gelatin per gram of oxide are adsorbed on fumed silica through the flocculation mechanism, and maximum adsorption is observed at pH values close to the IEP of the proteins. These protein have the lowest cross sections on their surfaces at pH ≈ ΙEP because of their dense structures that are compacted by strong attractive intramolecular bonds in the absence of repulsive electrostatic forces between charged groups.4,9,19,23 However, strong adsorption of PVP corresponds to lower amounts of adsorbed albumin or gelatin, approximately 100-150 mg per gram of silica.4,19 PVP concentrations above 10 wt % (γ ) 0.1) disturb a major portion of accessible ≡SiOH groups, as the band at 3750 cm-1 practically disappears at CPVP ) 17.5 wt % (Figure 16a). In the case of BSA/silica samples (Figure 17a), this band is observed at BSA concentrations that are significantly larger (e.g., CBSA ) 30 wt %) than the corresponding PVP concentrations (CPVP). Besides, a baseline level in the IR spectra of BSA/silica samples increases with CBSA (Figure 17) in contrast to the PVP/silica signal (Figure 16) caused by scattering from larger protein/silica particles (also observed by the PCS method). These results confirm that PVP molecules adsorb in the unfolded form, but proteins


Langmuir, Vol. 18, No. 3, 2002 595

Figure 17. IR spectra of silica and BSA/silica at CBSA ) 0 (curve 1), 5 (2), 10 (3), 20 (4), 30 (5), 40 (6), 50 (7), and 100 wt % (8) for (a) the SiO-H band at 3750 cm-1 and (b) ν between 1250 and 2100 cm-1. Figure 16. IR spectra of silica and PVP/silica at CPVP ) 0 (curve 1), 5 (2), 10 (3), and 17.5 (4) wt % for (a) SiO-H band at 3750 cm-1 and (b) ν between 1250 and 2100 cm-1. Pure PVP is represented by curve 5.

keep their globular structures in the liquid media or in the adsorption state. Additionally, strong interaction between unfolded PVP molecules and primary particles of fumed silica leads to decomposition of a significant portion of particle swarms, as nearly linear PVP molecules can penetrate into the channels of aggregates and decompose them. Notice that in the atmosphere of the saturated water or ethanol vapors, PVP molecules can be well distributed on the surfaces of agitated fumed silica similarly to that observed in the aqueous suspension (i.e., 10-30 wt % water or ethanol can promote unfolding and transportation of PVP molecules and the decomposition of a large portion of secondary silica particles). Therefore, decreased silica-silica interaction between particles covered by PVP (Θ g 1) results in textures that are characterized by markedly lower mesoporosity in comparison with that for dried pure silica or protein/silica powders (Figure 15). For instance, at CPVP ) 10 wt % (Θ < 1), f(Rp) has only a small peak at Rp ≈ 15 nm that is shifted in comparison with that for pure dried silica, silica/

PVP at CPVP ) 5 wt %, or silica/proteins. At CPVP ) 5 wt %, the pore volume increases (pore volume also increases for other dried powders with the exception of silica/PVP at CPVP ) 10 wt %) (Table 5); however, the specific surface area decreases for all the dried samples. The distributions fS(Rp) computed with respect to dSN2/dRp show that narrow pores are the main contributors to the specific surface area (Figure 15c and d), which decreases strongly for dried powders. In the aqueous suspensions, polymer molecules can penetrate into pores and block ≡SiOH groups, whose interaction is necessary to form the ≡Si-O-Si≡ bridges on drying. All these effects lead to various morphologies of the dried powders with silica or silica/polymer (Table 5, Figure 15). Changes in the nature of the particle surface and its topography influence the adsorption potential (Figures 15 and 2) and nitrogen adsorption energy (Figure 18) distributions. In the case of silica/protein powders, the intensity of the second peak (adsorption occurs mainly on the mesopore walls) is lower than that for pristine silica. However, a f(E) peak at 17-18 kJ/mol appears because of changes in the nature of the surface as nitrogen is adsorbed on protein globules or between them and silica

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at CPVP ) 10 wt %, as the nonuniformity of the surfaces is greater (as well as narrow pore contribution) at lower CPVP because the silica surfaces are only partially covered by PVP. Notice that f(Rp) for the PVP/silica powder at CPVP ) 5 wt % differs strongly from that at larger CPVP values (Figure 15) because of the availability of free silica patches at the lower CPVP value. Conclusions According to the IR spectra, 20 wt % of PVP and 40 wt % of BSA adsorbed on the fumed silica surfaces disturb all the SiOH groups appearing as the band at 3750 cm-1 on pure silica. This effect can be explained by decomposition of agglomerates and large aggregates as the total surface area becomes accessible for both unfolded PVP and globular BSA molecules. Consequently, one could assume that aggregates formed because of the siloxane bridges, which are not decomposed on polymer adsorption, should be relatively small, but larger aggregates are formed because of nonsiloxane linkages (i.e., hydrogen and other types of intermolecular bonding). The ionogenic surfactant Aethonium (CAet < 0.01 wt %) and ethanol (high concentrations of 10-50 wt %) have a smaller effect on the particle size distributions than do PVP or albumin. Globular proteins (albumin and gelatin) have a weaker influence on the silica structure upon suspending and drying than does PVP because of the difference in their adsorptions. Proteins are characterized by strong intramolecular bonds, but unfolded PVP molecules rearrange silica particle swarms. The suspension and drying of silica and silica/polymers leads to a reduction of the specific surface area, but the effective pore volume that is accessible to adsorbed nitrogen increases in comparison to the amount of pristine fumed silica. Polymers covering the surfaces of primary particles and their aggregates provide additional diminution in S, but the relative reduction in the pore volume compared to that of the dried pure silica suspension is smaller, and addition of 5 wt % PVP even enhances Vp. Covering of the silica surfaces by polymers that block surface silanols changes the adsorption potential distributions for adsorbed water as well as the nitrogen adsorption energy distributions. The adsorption potential is reduced at low coverage, as the narrow pore contribution decreases, but is enhanced at greater coverage, corresponding to the secondary filling of large mesopores.

Figure 18. Nitrogen adsorption energy distributions calculated for (a) silica and silica/proteins and (b) silica and silica/ PVP.

Acknowledgment. This research was supported by NATO (Grant EST.CLG.976890), the Polish State Committee for Scientific Research, and the Ministry of Higher Education and Science of the Ukraine (Grant 2M/303-99).

surfaces on the most active sites in the narrow pores. The PVP impact on f(E) at CPVP ) 5 wt % is larger than that

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