The Colloid Chemistry of Silica - American Chemical Society

3 The Formation and Interfacial Structure of Silica Sols John D. F. Ramsay , Stephen W. Swanton, Akihiko Matsumoto , and Dhanesh G. C. Goberdhan 1

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Downloaded by UNIV OF TEXAS EL PASO on August 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch003

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Atomic Energy Authority Technology, Harwell Laboratory, Oxfordshire, OX11, United Kingdom

Several techniques, including small-angle neutron scattering (SANS), ultracentrifugation, photon correlation spectroscopy, and Si NMR spectroscopy, were used to investigate the nature of the oxide-water interface of silica sols and its significance in the formation and growth of colloidal particles in aqueous solution. These studies were performed with a range of commercial silica sols of different diameters in the range~7-30nm. When the diameter is small the sols contain a significant proportion of oligomeric silicate species that may be associated at the surface of the particles. For sols of the largest diameter, the relative proportion of oligomers is much smaller. In all the sols the core of the particles has a highly condensed Si-O-Si structure. 29

THE CLASSIC DESCRIPTION of the

structure and mechanisms of formation of silica sols by the hydrolysis and condensation of silicates i n aqueous media was given by Her in 1979 (I). According to Her, polymerization may occur i n essentially three stages: (1) the polymerization of monomers to oligomers and then to primary particles, (2) growth of particles, and (3) particle aggregation to form networks that eventually give rise to a gel Corresponding author. Current address: Centre National de l a Recherche Scientifique, Institut de Recherches sur l a Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne, France. O n assignment from Department of Chemistry, Faculty of Science, C h i b a University, Yayoi, C h i b a 260, Japan. Present address: Esso Chemical Research Centre, Abingdon, Oxon, United Kingdom. 2

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0065-2393/94/0234-0067S08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

68

T H E C O L L O I D CHEMISTRY O F

SILICA

structure extending throughout the liquid medium. Stages 2 and 3 depend on the p H and salt concentration, as discussed by Her. These general processes have subsequently been confirmed by many workers using a range of microscopic techniques (2) such as S i N M R and IR spectroscopy, light scattering, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), although less attention has been given to stage 2. Considerable advances in understanding stage 1 have been obtained from S i N M R investigations (3-5) and stage 3 from scattering measurements (light, X-rays, and neutrons) (6, 7). Stage 1 has been shown to give rise to a range of complex poly silicic acid structures. In general extensive condensation takes place in which monomers form ring structures that associate and condense further, eventually to form the core of silica particles containing few silanol groups. In stage 3 the present understanding of the double-layer interaction between particles and the mechanisms of aggregation to give gel formation is extensive and has been advanced by descriptions based on fractal theory (8). In this chapter we will describe some recent investigations of the formation and interfacial structure of a series of commercially produced silica sols (Nalco, Dupont, and Ludox) with different diameters in the range « 7-30 nm. This type of sol, which has wide industrial applications, has been used as a model system in numerous studies of colloidal silica, although in general, the nature of the silica-water interface has received little attention. The interfacial structure may well explain some of the unusual properties of silica sols, such as the high surface charge and exceptional colloidal stability together with the enhanced capacity for sorption and complexation of ionic species in solution. Indeed in the past these features have been ascribed to a surface "gel layer" resulting from extensive hydroxylation within a few angstroms at the surface (9), although definitive evidence of this gel layer is still lacking. Here we have applied several techniques (photon correlation spectroscopy (PCS), SANS, analytical ultracentrifugation, and S i N M R spectroscopy) to explore the properties of these silica sols. Particular interest attaches to the sol of smallest diameter because here we anticipate that any effects due to incomplete condensation of oligomeric components will be relatively more marked, especially in the diffusion behavior and effective diameter of the spherical particles. 29


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Experimental Details Silica Sols. Concentrated (ca. 15 to ca. 4 0 % w/w) silica sols of different particles size (SI to S4) were obtained commercially from designated sample batches, viz., SI is Nalcoag 1115 from Nalco Chemical; and S2, S3, and S4 are Ludox S M , H S , and T M respectively, from DuPont. These sols are prepared by


3.

RAMSAY ET A L .

Formation and Interfacial Structure of Silica Sols

69

hydrothermal treatment of sodium silicate solutions (J) and contain N a as the counterion. These sols have already been studied extensively (6, 10, J J). The mean diameters of the sol particles, as previously determined from transmission electron microscopy (JO) together with the effective particle radius as derived from the specific surface areas, SBET, of the outgassed gels (JO) are given with other properties in Table I. Measurements were made with sols of different concentration by diluting the stock samples with demineralized water. The stock sol, S I , was also dialyzed repeatedly against water (denoted S1 ) and used in further studies. +

D


Table I. Properties of Silica Sols

Stock Particle Concentration Diameter (nm) (w/v)

Sol

of Gel, SBET

(m /g)

a

SI S2 S3 S4

16.1 32.8 40.3 47.2

Particle Radius (nm)

Surface Area

b

2

8 12 16 30

3.3 5.2 6.5 10.5

410 260 210 130

Na+

Content (mg/mL) 5.6 6.0 4.9 3.2

pH 10.5 10.1 9.8 9.0

"From electron microscopy. From SBET, assuming silica density of 2.2 g/mL. fo

Small-Angle Neutron Scattering. Measurements were made as described previously (JO, J J) using a multidetector instrument installed in the P L U T O reactor at Harwell and also with the D l l instrument at the Institut Laue-Langevin, Grenoble, France. Photon Correlation Spectroscopy. Measurements were made with a commercial 96-channel photon correlator (Malvern K7023) using a helium-cadmium laser and with a 256-channel correlator (Malvern K7032) using a helium-neon laser with a standard spectrometer system (Malvern 4700), adopting procedures as described previously (J2). Analytical Ultracentrifugation. Sedimentation coefficients were determined with an ultracentrifuge (Beckman L 8 - 7 0 M ) fitted with a schlieren analytical attachment. Photographic images of schlieren (refractive index gradient) profiles were analyzed with a profile projector (Nikon, model V I 0 ) using standard procedures (J3). S i N M R Spectroscopy. S i N M R spectra of the stock sols contained i n 1mL plastic vials were recorded at a field strength of 39 M H z using a commercial instrument (Bruker-Physics CXP200) at a 10-s pulse repetition rate. 29

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Results and Discussion S m a l l - A n g l e N e u t r o n S c a t t e r i n g . The intensity of small-angle scattering, I(Q), for a concentrated colloidal dispersion of identical particles is given by (14) I(Q) =

K{ - ) Vln P(Q)S(Q) Pp Ps

2

p


(1)

70

T H E C O L L O I D CHEMISTRY O F SILICA

where Q is the scattering vector, defined as |Q| = 4ττ sin θ/λ

for a scattering angle 2Θ and wavelength λ; pp and p are, respectively, mean scattering length densities of the particles and solvent; Vp is volume of each particle; n is the particle number density; Κ is experimental constant; and F(Q) is the particle form factor, which spheres of radius R is given by s


P

P(Q)

=

Γ 3[sin(QR)-QHcos(QR)] Q R 3

the the an for

(2)

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The structure factor, S(Q), is determined by the nature of the particle interaction potential (7); for noninteracting systems S(Q) = 1 (viz., for systems in the limit of very low concentration where electrical doublelayer interactions are negligible). Extensive SANS investigations have been performed previously with silica sols of the present type (6,10,11). The scattering behavior of the sols can be closely described on the basis of the foregoing theoretical treatment for monodispersed spherical particles. Furthermore, detailed analysis of the form of the scattering curves (relative intensity i(Q) vs. Q) for sols of different concentration has provided information on the size of particles and the nature of the interaction potential. Typical values of R, derived from experimental fits of extensive scattering data (I J) to equation 1, are —5 nm for S I , ~ 7 nm for S2, ~ 9 nm for S3, and —15 nm for S4. These values confirm that R, although slightly larger, is in satisfactory accord with that determined from transmission electron microscopy (TEM). There is more discrepancy with the sol of smallest particle size. The possible explanation for this discrepancy will be discussed subsequently. The structure and composition of the particles themselves are deter mined during the formation process. Information on the nature and composition of the particles can be obtained from contrast variation studies, as described previously (7, 14). Thus for the simple case of a uniform and homogeneous particle, the scattering length density, pp, can be derived from variations of the solvent scattering length, p . This derivation is readily achieved with water using H 2 O - D 2 O mixtures (see Table II). Op is derived from the solvent composition of zero contrast (i.e., PP = p ). This fact is illustrated by the scattering of sol S2 in different H 2 O - D 2 O mixtures (Figure 1), in which scattering is negligible for a composition of —65% (/v) D 2 O . A precise determination of p is obtained from the linear relationship of I(Q) vs. %v/v D 2 O (cf. equation 1) as illustrated in Figure 2. The experimentally determined value of pp (3.6 X s

s

v

P

1/2


3.

Formation and Interfacial Structure of Silica Sols

RAMSAY ET AL.

71

ΙΟ c m ) is in close accord with that calculated for amorphous silica (3.47 Χ 1 0 c m ) assuming a framework density of 2.2 g/mL (see Table II). This agreement indicates that the sols have a dense silica core and that any surface-hydroxylated species are readily exchangeable with D 2 O . The form of the scattering curves in Figure 1, which show a maximum at Q « 3 Χ 1 0 and a decrease in intensity at lower Q, is due to the maximum in the structure factor S(Q) (cf. equation 1). This feature becomes progressively pronounced as the sol concentration is increased and indicates that appreciable interaction occurs between the sol particles at the concentra tion (0.15 g/cm ) here. -10

-2

10

-2


- 2

-3

c

Φ

Φ >

Φ

ce

Figure 1. SANS results for silica sol S2 (0.15 g/cm ) in different D2O-H2O mixtures. Key (% v/v D2O): O , 0; •, 30, Δ , 50; V , 55; Θ, 65; X , 80; and ·, 100. (Reproduced with permission from reference 18. Copyright 1991 Academ ic Press.) 3

Ultracentrifugation Studies. O n ultracentrifugation the silica dis persions show a single solute boundary separating from the meniscus, which is observed as a peak in the schlieren (refractive index gradient) profile of the cell. Schlieren photographs recorded for a dilute dispersion of S4 are shown in Figure 3 and are typical of those observed for all the silicas. W i t h increasing sedimentation time the boundary broadens as a



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T H E COLLOID

25

75

50

CHEMISTRY

O F SILICA

100

% D 0 % 2

Figure 2. SANS contrast variation results for silica sol S2 in D2O-H2O mixtures. Intensity corresponds to Q (À ) of 2.5 X JO" . 2

-1

Table II. Molecular Scattering Lengths, Σώι, and Corre sponding Coherent Scattering-Length Densities, p, for Neutrons for Silica and Water of Mass Densities, δ

Compound H2O D2O S1O2 Si02exp

lib,- (10- /cm) 12

δ (g/cm ) ρ (10 /cm- ) 2

10

-0.168 1.914

1.00 1.10

-0.56 6.36

1.575

2.20

3.47 3.6

fl

2

"Experimentally determined value.

result of diffusion and the centrifugal separation of particles of slightly different size. The observed boundary shapes indicate that the dispersions are reasonably monodisperse but with some increase i n polydispersity for the smaller sized sols SI and S2. The effect of sol concentration on the sedimentation coefficients, s , of all four sols is illustrated in Figure 4. The values of s decrease as the 25

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3.

Formation and Interfacial Structure of Silica Soh

RAMSAY ET AL.

73


A Figure 3, Schlieren photographs of silica sol S4 taken during ultracentrifuga tion at 1100 rpm after (a) 3.5 min, (b) 5.5 min, and (c) 7.5 min. The vertical lines toward the left side of the photographs indicate the air-solution meniscus; sedimentation is from left to right.

1000 500 brO—0-.-o-o._._

2

1001

-u

1

50

10

•

_·__

1-0

20

—·

30

b

α

50

60 3

Silica

70 1

concentration / 10" g.ml"

Figure 4. Variation of the sedimentation coefficient, s , with concentration for diluted silica sols as determined by analytical ultracentrifugation for (a) SI, (b) S2, (c) S3, and (d) S4. 25

size of the sol particle becomes smaller, as would be expected, and furthermore s is relatively insensitive to concentration i n the range studied ( ~ 2 X 1 0 ~ to 6 X 1 0 ~ g/mL). From the values of so , in Table III an effective radius, r can be derived by applying the Svedberg equation 25

3

2

25

s

M =

RTso D(l~u6 )

(3)

w

where M is the particle "molecular weight", R is the gas constant, Τ is absolute temperature, D is the diffusion coefficient, ν is the volume per unit mass (= of the particles, and 20%) of oligomeric species. These species are predominantly associated with the colloidal particle surface. After dialysis, however, these species may be partially released into solution. This process probably results from the reduction of ionic strength and the consequent increase in electrostatic repulsion between the negatively charged surface and the anionic oligomers. The relative proportion of oligomeric silica decreases as the size of the sol particles is increased. Particle growth may thus arise from the condensation of monomers and oligomers at the particle surface in accord with the mechanism proposed by Her (I). Furthermore, the particle core has a density consistent with amorphous silica. The adsorption of oligomers around the core of the particles has implications that may explain the high charge density and exceptional stability of silica sols (9). Enhanced stability may also arise from solvation forces. Such an interfacial structure has important consequences in both determining the mechanism and enhancing the capacity for sorption of other ionic species from solution. Finally, the surface and pore structure of gels obtained after dehydrating sols may be affected by the presence of oligomeric silica. This effect will be particularly evident with sols of small particle size in which partial particle coalescence may occur, together with the generation of small micropores (

The Colloid Chemistry of Silica - American Chemical Society

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