Journal of Colloid and Interface Science 352 (2010) 309–315

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Journal of Colloid and Interface Science

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Identification of solvated species present in concentrated and dilute sodiumsilicate solutions by combined 29Si NMR and SAXS studies

Monique Tohoué Tognonvi a, Dominique Massiot b, André Lecomte c, Sylvie Rossignol a,⇑,Jean-Pierre Bonnet a

a Groupe d’Etude des Matériaux Hétérogènes (GEMH E.A. 3278), CEC, Ecole Nationale Supérieure de Céramiques Industrielles, 12 rue de l’Atlantis, 87068 Limoges Cedex, Franceb Conditions Extrêmes et Matériaux : Haute Température et Irradiation (CRMHT-UPR3079 CNRS), Université d’Orléans, 1D avenue de la Recherche Scientifique,45071 Orléans Cedex 2, Francec Laboratoire Sciences des Procédés Céramiques et de Traitements de Surface (SPCTS-UMR 6638), Université de Limoges, CEC, Ecole Nationale Supérieure de CéramiquesIndustrielles, 12 rue de l’Atlantis, 87068 Limoges Cedex, France

a r t i c l e i n f o

Article history:Received 26 May 2010Accepted 4 September 2010Available online 15 September 2010

Keywords:Concentrated sodium silicate solutionDilutionNMRSAXSSolvated species

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.09.018

⇑ Corresponding author. Fax: +33 5 87 50 23 01.E-mail address: Sylvie.rossignol@unilim.fr (S. Ross

a b s t r a c t

Both concentrated and diluted sodium silicate solutions have been investigated by combining 29Si NMRspectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceousspecies solubility observed in such alkaline concentrated sodium silicate solutions and their evolutionaccording to dilution have been identified. For the most concentrated solution ([Si] = 7 mol/l;pH = 11.56; Si/Na atomic ratio = 1.71), the results evidence the preponderant presence of neutralSi7O18H4Na4 complexes, which behave like colloids of about 0.6–0.8 nm able to form very small aggre-gates with an average size lower than 3 nm. Addition of distilled water to this initial concentrated solu-tion leads, on one hand, to a doubling of the colloid size, i.e. 1.2–1.5 nm, and, on the other hand, to aprogressive decrease of the aggregate size until their total disappearance. Such a behavior could beexplained by considering, first, the dissociation of the neutral Si7O18H4Na4 complexes present in the con-centrated solution into Na+ ions and charged (Si7O18H4Na4�n)n� complexes (with 1 6 n 6 4) and, second,the condensation of these siliceous charged species in order to form larger (Si7yO18y�zH4y�2zNa(4�n)y)ny�

colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8(Si7O18H4Na4�n)n� groups.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Sodium silicate solutions, also called water glass, are watersolutions of sodium oxide (Na2O) and silicon dioxide (SiO2) com-bined in various ratios. Varying the proportions of SiO2 to Na2Oand the solids content results in solutions with differing propertiesthat have many diversified industrial applications, such as sealants,binders, deflocculants, emulsifiers and buffers in abrasive and cast-ing industries. These solutions are also used as reactants during theformation of geopolymers.

Available commercial solutions have compositions character-ized by two major factors: a Si/Na atomic ratio ranging from 0.9to 1.9 and a SiO2 content between 25 and 32 wt%. The effects ofthese two main chemical parameters as well as the destabilizationof these silico-alkaline solutions promoted by the addition of acidor calcium ions, for instance, leading to gels or precipitates havebeen widely studied [1]. More particularly, the identification ofthe structure and microstructure of involved polysilicate ions has

ll rights reserved.

ignol).

become, therefore, a subject of considerable interest, pursuedthrough the application of various techniques.

First attempts to derive the detailed molecular structure ofthese silicate solutions involved trimethylsilylation followed bychromatographic separation [2] or reactions with molybdenic acid[3]. With the growing development of high resolution nuclearmagnetic resonance spectroscopy, 29Si NMR spectroscopy has beenproved to be particularly well suited to obtain in situ informationon the connectivity of silicon atoms in the solution and helpingto understand how polysilicate ion mixtures change under differ-ent chemical conditions. A number of reports concerning the struc-ture of polysilicate ions present in various alkaline silicatesolutions have been published [4–8]. More than twenty silicatespecies in various forms (linear, cyclic, prismatic. . . forms) havebeen identified in a slightly concentrated solution (e.g. SiO2 = 1.5mol/l, Si/K = 1) [4]. These investigations have evidenced two ex-treme situations corresponding respectively to the predominanceof monomeric species at high pH and low silicon concentration[9] and to three-dimensional reticulated gels [10] at low pH andhigh silicon concentration. At intermediate pH and [Si] concentra-tion, the system should be either molecular (polysilicates ions) or

310 M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315

colloidal (silica sols) with a rather fuzzy borderline between bothdomains [11].

From a microstructural point of view, concentrated sodium sil-icate solutions are usually considered as a dispersion of amorphoussilicate colloids in an alkaline aqueous medium. Several authors,like Iler [12] who extracted the colloids with tetrahydrofuran andperformed static light scattering measurements, consider that thecolloid size ranges between 1 and 5 nm. However, the investiga-tions of size distributions of colloids by ultrafiltration or dynamiclight scattering measurements reported the presence of smalleras well as larger particles. For example, Roggendorf et al. [13] sta-ted a colloid size ranging from 30 to 100 nm. More recently, at leastthree size classes of colloidal particles which exist in parallel in thesolutions were detected [14]. The respective colloid radii are 0.4–0.6 nm, 2.5–13 nm, and 75–85 nm.

A synthesis of all these structural and microstructural results,collected from different characterization techniques, is rather chal-lenging. They have definitely demonstrated the existence of multi-faceted correlations between pH, silicon concentration, Si/Naatomic ratio and lead to conclude to a much more complex chem-istry than a simple Si/Na ratio representation may suggest. A slightvariation of one of these main chemical parameters may inducedrastic changes on the connectivity of silicon atoms and the result-ing distribution of polysilicate ions in silico-alkaline solutions per-tinent to ceramic material synthesis.

In a general prospective purpose concerning the low tempera-ture synthesis of oxide engineering granular as well as compositeceramic materials made from commercially ceramic powders andsodium silicate solutions acting as advancing eco-friendly binders,we have systematically studied the detailed effects of the acidifi-cation of such solutions at unusual high pH, i.e. pH > 9, and ofthe silicon concentration on the formation of these binding gels[15]. As the process always starts by a dilution of the sodium sili-cate mother solution, the knowledge of the precursor sols is impor-tant in order to determine the characteristics of the gelation andthe properties of the resulting gels.

In the present paper, we focus on the determination of thestructure and size distributions of the polysilicate species presentin concentrated and diluted sodium silicate solutions, as quantita-tively as possible, from both silicon-29 nuclear magnetic resonancespectroscopy (29Si NMR) and small-angle X-ray scattering (SAXS)analysis. We show that the very slight variation in pH induced bysuch a simple dilution largely modifies the respective structureand size of the polysilicate species. The major polysilicate speciehas been clearly identified and we propose a model which accountsfor its evolution according to dilution.

2. Sample preparation and experimental techniques

2.1. Preparation of dilute solutions

The starting commercial sodium silicate solution (VWR Prolabo)had the following characteristics: [Si] = 7 mol/l; relative density =1.33; pH = 11.56; Si/Na atomic ratio = 1.71. The dilute solutionswere prepared by adding distilled water to this concentratedsolution at room temperature under magnetic stirring to achievesilicon concentrations ranging from 1.76 to 6 mol/l. The composi-tions and pH values of the studied solutions are reported in Table 1.

Table 1Composition and pH values of the investigated sodium silicate solutions with aconstant Si/Na atomic ratio = 1.71.

[Si] (mol/l) 7 6 5 4 3.6 2.26 1.76pH values 11.56 11.49 11.46 11.39 11.36 11.29 11.25

2.2. Silicon-29 nuclear magnetic resonance spectroscopy (29Si NMR)

29Si NMR spectroscopy using both one and two-dimensionalexperiments at natural abundance has been used in order to char-acterize the initial concentrated solution. NMR experiments wereperformed at CRMHT-UPR3079 CNRS-Orléans using a BrukerAvance 200 spectrometer, operating at a 4.7 T magnetic field(29Si Lamor frequency 39.76 MHz). Despite the liquid nature ofthe initial solution Magic Angle Spinning was necessary to obtainresolved spectra and the 1D experiment was conducted at roomtemperature under magic-angle-spinning (MAS) conditions at5 kHz. Single pulse spectra were obtained using 45� pulse angle(3 ls) and a recycle delay of 30–60 s. Chemical shifts are externallyreferenced to tetramethylsilane (TMS) 29Si at 0 ppm. In order toidentify the existing correlations between Qn species, a two dimen-sions (2D) NMR 29Si refocused INADEQUATE experiment has beenalso investigated leading to a low signal to noise but significantspectrum despite natural abundance.

2.3. Small-angle X-ray scattering (SAXS)

To investigate the size distributions of the polysilicate speciespresent in concentrated and diluted sodium silicate solutions,small-angle X-ray scattering experiments (SAXS) have been ap-plied. Contrary to the previously quoted methods for colloidal sizedetermination which could whether strongly change the chemicalequilibrium of the investigated solutions or require additional data,small angle scattering measurements (X-ray or neutrons) haveproved to be an extremely powerful tool to study in situ colloidal,polymers as well as fractal aggregates suspensions without dis-turbing them in any way [16].

Experiments were performed using an original experimentalset-up adapted to a 18 kW rotating anode X-ray generator, operat-ing at 45 kV–200 mA with a fine (0.5 � 1 mm2) spot size. TheCu Ka1 incident beam, provided by a double channel cut germa-nium monochromator, is quite parallel and directed on the samplethrough a system of slits with a point like geometry. The scatteredintensity I(q) was recorded using a linear position-sensitive detec-tor placed at a sample–detector distance of 0.5 m. Thus, the valuesof the modulus of the scattering vector q (q = 4pk�1sin h, where k isthe CuKa1 wavelength and 2h the scattering angle), range from0.08 up to 4 nm�1. A vacuum chamber placed on the sample–detector path allows to avoid any scattering or absorption of air.

Experiments were performed on a solution layer of about0.5 mm thickness placed between two parallel kapton windows.The scattered intensity of the studied solutions (concentratedand diluted) was collected at room temperature with a 24 h expo-sure time. Appropriate corrections for background scattering fromslits, kapton cell walls, residual atmosphere and for absorption ef-fects were applied [17].

3. Results

3.1. Silicon-29. nuclear magnetic resonance spectroscopy

3.1.1. Relative amount of Qn speciesDepending on the number of siloxane bridges (Si–O–Si), differ-

ent silicon surroundings can be distinguished by 29Si NMR. Fivemain classes of NMR signals have been identified and usually la-beled Qn, where Q stands for a fourfold coordinated silicon atomand n gives the number of other silicon atoms to which each siliconatom is connected to through an oxygen atom [7]. Thus, Q0 denotesa silicon atom which is not bonded to any other silicons (mono-meric silicate ion) while Q4 is assigned to a silicon atom bondedto four other silicon atoms.

M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315 311

The 29Si NMR spectrum of the initial concentrated sodium sili-cate solution ([Si] = 7 mol/l; Si/Na atomic ratio = 1.71; pH = 11.56)is plotted in Fig. 1. The five signals observed were simulated usingPseudo-Voigt functions by adjusting the position, the intensity andthe full width at half maximum (FWHM). The resulting chemicalshifts, �71.6, �79.8, �88.5, �97.3, and �107.5 ppm, are character-istic of Q0, Q1, Q2, Q3 and Q4 species respectively [18–20]. The rel-ative fraction of each Qn species has been determined by dividingthe integrated intensity of the corresponding signal by the sumof all signal intensities. The obtained relative integrated intensitiesof the different Qn species present in the solution are reported inTable 2. Q3 type surrounding is preponderant (�52%) and the Q3/Q2 and Q3/Q4 ratios are equal to 2 and 3.5 respectively, the amountof monomer Q0 being almost negligible (<1%).

Furthermore it can be remarked that the linewidth is increasingwith the coordination number from a sharp Lorentzian profile forthe Q0 units to a broad gaussian profile (4.24 ppm FWHM) forthe Q4 units. It is also important to note that the obtained resolu-tion, except for Q0 units, is considerably lower than what is usuallyachieved in dilute solutions and does not allow a more detailedattribution.

3.1.2. Spatial correlations between Qn speciesTo go further in the description of the extended molecular enti-

ties present in the concentrated mother solution, it would be use-full to establish the connectivities between the different Qn species.This can be done using J-coupling mediated correlation experi-ments [4] that is characteristic of Si–O–Si chemical bonds. Whenthe experiment is rather easy in 29Si enriched samples, it becomeschallenging in natural abundance materials with sharp resonances[21] and even more difficult when resonances are broadened likein the case of our sample.

We carried out a refocused INADEQUATE experiment that con-sists in a two step experiment generating double quantum29Si–29Si coherences and converting back to an in phase signal. Thisexperiment can be applied to liquids [22] or solids under rapid ma-gic angle spinning [23,24]. The timing of the experiment was opti-mized on the first slice (28 ms pumping time and 28 ms convertingtime) to obtain the better signal to noise ratio and is a compromisebetween the generation of double quantum coherences and relax-ation times.

-60 -80 -100 -120 -140

Q0

Q1

Q2Q3

Q4

(ppm)

Inte

nsit

y (a

.u.)

3

Fig. 1. Experimental and calculated 29Si NMR spectra of the initial concentratedsodium silicate solution ([Si] = 7 mol/l, Si/Na = 1.71, pH = 11.56).

Table 2NMR chemical shifts observed for the initial concentrated sodium silicate solution([Si] = 7 mol/l, Si/Na = 1.71, pH = 11.56) and corresponding relative quantities ofsilicon in each Qn type surrounding.

Qn species Q0 Q1 Q2 Q3 Q4

Chemical shift d (ppm) �71.6 �79.8 �88.5 �97.3 �107.5Qn relative quantity (%) <1 6 26 52 15

The symmetrized refocused INADEQUATE spectrum of the start-ing concentrated solution is presented in Fig. 2 as a Single Quan-tum/Single Quantum correlation where intensities on thediagonal sign chemical binding of like units (Qn–Qn) and of diago-nal intensities sign chemical binding of unlike units (Qn–Qm). Thespectrum clearly establishes that Qn units are preferentially linkedto Qn or Qn+1 units, i.e. (Q3–Q3), (Q3–Q4) and (Q2–Q3).

3.2. Small-angle X-ray scattering

In order to determine the influence of silicon concentration onthe association of the previously identified Qn species and conse-quently on particle size and on their potential aggregation, SAXSexperiments have been performed before and after dilution ofthe initial solution.

3.2.1. Influence of dilution on SAXS diagramsThe experimental scattering intensity diagrams corresponding

to the investigated solutions are plotted on a bi-logarithmic scalein Fig. 3. Whatever the dilution, the scattering curves are similarfor scattering vectors, q, lower than 0.3 nm�1. In this scatteringvector domain, this invariant signal is assigned to a residue ofbackground curve and corresponds to less than 3% of the signal re-corded without sample. It is likely due to small variations of exper-imental conditions during the long exposure time which cannot betaken into account when correcting the raw data. Nevertheless, inthe 0.4–4 nm�1 q range, the intensity of background curve is veryweak and constant and does not alter the quality of the collectedscattering signals. Thus, the evolution of SAXS curves associatedto the progressive dilution of the initial solution observed between0.4 and 4 nm�1 (Fig. 3) is characteristic of scattering entities mod-ifications. A preliminary qualitative analysis of Fig. 3 allows thescattering curves to be classified in two sets with clearly differentfeatures, corresponding to low and high silicon concentrations.

For silicon concentrations lower than 5 mol/l, the log–log plotsshow a plateau which is recognized as the so-called Guinier region[16,17], indicating the absence of large scattering entities and nosignificant interactions between scattering objects. As the scatter-ing vector q increases, this plateau is followed by a concave down-ward curvature indicating the transition to the so-called Porod Lawregion in which the graph is a straight line [16,17]. When the sili-con concentration in the solution decreases from 5 mol/l to1.76 mol/l, the Guinier’s domain increases on the SAXS diagram to-wards the high q scattering vectors while the Porod’s domain wid-ens simultaneously towards low scattering vectors. For thegreatest silicon concentrations, i.e. 5–7 mol/l, we are facing tomore complex SAXS curves. As both Guinier and Porod’s domainsare reduced, the presence of smaller as well as larger scatteringentities than in dilute solutions may all together exist. Such abehavior reflects in a first point of note that at least the size distri-bution of scattering entities largely evolves during dilution.

3.2.2. Characteristics of the scattering entitiesAs sodium silicate solutions can contain several silicate species

with very different size and atomic arrangement [1,25], the ob-served SAXS patterns are therefore produced by a set of differentand somewhat complex scattering entities, especially in concen-trated solutions. Rather than applying the so-called Guinier lawto determine their mean size through the classical radius of gyra-tion which could not accurately describe the investigated sols,the entire scattering curves have been fitted with an appropriatemodel in order to extract microstructural information such as sizedistribution by considering some simplifying assumptions.

The scattering entities have been assumed to behave like spher-ical colloids as suggested by the feature of experimental SAXScurves from dilute solutions. At very low scattering vector, SAXS

δδ (ppm)-110-100-90-80

-110

-100

-90

-80

δδ (p

pm)

(Q3-Q3)

(Q3-Q4)

(Q2-Q3)

Q2 Q3Q4

Q2

Q3

Q4

High Low

Interaction

Fig. 2. 2D NMR diagram of the initial concentrated sodium silicate solution ([Si] = 7 mol/l, Si/Na = 1.71, pH = 11.56). Color code indicates interactions intensity, the maininteractions occurring between Q3–Q3, Q3–Q4, and Q2–Q3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

3.6 mol/l

4 mol/l

5 mol/l

7 mol/l

6 mol/l

Residual background

scattering (<3%)2.26 mol/l1.76 mol/l

PorodGuinier Intermediate

0.1 1 101

10

100

q (nm-1)

Inte

nsit

y (a

.u.)

Fig. 3. SAXS diagrams of initial concentrated ([Si] = 7 mol/l) and dilute sodiumsilicate solutions (Si/Na = 1.71, 11.25 < pH < 11.56). The corresponding Si concen-tration is indicated for each diagram. The curves are arbitrary vertically shifted forclarity.

312 M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315

intensity is more highly sensitive to geometrical parameter such asthe radius of particles than to short range variations in electrondensity, therefore, such an assumption does not influence signifi-cantly the calculated particles size. The two other followingassumptions were also considered for simulation:

(i) the electron density gap between scattering entities and sol-vent is constant whatever the nature of the siliceous species;

(ii) the interactions between scattering entities are smallenough to be neglected, at least, in a first attempt.

Previous combined 29Si NMR and SAXS studies conducted onsimilar systems [25] had shown that these simplifying hypothesesare quite reasonable and allow size distribution of scattering enti-ties to be extract from experimental SAXS. Therefore, according tothe above-mentioned assumptions, the calculated scattered inten-sity, I(q), is expressed by [17]:

IðqÞ � A1 �Z 1

0Pðq; rÞ � DnðrÞ � dr þ A2 � IbðqÞ ð1Þ

where P(q, r) is the form factor of the scattering entities which isgiven by the well-known Rayleigh function [17] established for a

spherical-shape particle (radius a, volume V) with smooth interfaceand uniform density:

Pðq; aÞ ¼ 9 � V2 � sinðq � aÞ � q � a � cosðq � aÞðq � aÞ3

!2

ð2Þ

Dn(r) is a log-normal size distribution introduced to parametrise thescattering entities size distribution, with:

DnðrÞ ¼ 1ffiffiffiffiffiffiffi2pp 1

r � L exp�ðln r � ln RoÞ2

2L2

!ð3Þ

where Ro and L are the center and the width of the size distribution,respectively. The last term of Eq. (1) is of less importance and onlyaccounts for the residual background scattering, Ib(q) being thescattering intensity collected without sample. Finally, A1 and A2are scaling factor.

For the highest silicon concentrations, the presence of bothsmaller and larger scattering entities than in dilute solutions is ex-pected. Thus in these cases, a third term, rigorously analogous tothe first one, was added in Eq. (1) in order to simulate the experi-mental scattering curves by calculating the intensity scattered by aset of colloidal particles with a bimodal size distribution. The twolog-normal laws should describe respectively the size distributionof the smallest (typically monomers, dimers. . .) and of the largest(polymers or aggregates) scattering entities. The latest should re-sult from condensation (polymers) or aggregation (aggregates) ofthe previous siliceous species. Based on the size of equivalentspheres proposed by Gaboriaud et al. [25], the starting log-normallaws for simulation were respectively centered on 0.3 and 0.8 nm,the dispersion setting at 20% (average standard deviation/size).

The curves corresponding to the best fit (non linear least-squares refinement procedure) of the theoretical function I(q)(Eq. (1)) to the experimental patterns are displayed by the contin-uous smooth line in Fig. 4 on a bi-logarithmic scale. The excellentsuperposition between calculated and experimental curves ob-served for [Si] > 2.26 mol/l suggests that the selected model withsimplifying assumptions is appropriate for describing the scatter-ing behavior of the initial concentrated and dilute solutions.

However, for the lowest silicon concentrations, i.e. 2.26 and1.76 mol/l, the decrease of colloid size distribution associated withthe best fit (yet slightly improvable) is too important to be not

3.6 mol/l4 mol/l

5 mol/l

7 mol/l

6 mol/l

2.26 mol/l1.76 mol/l

Experimental

Calculated

0.1 1 101

10

100

Inte

nsit

y (a

.u.)

q (nm-1)

Fig. 4. Experimental and calculated SAXS diagrams of initial concentrated anddilute sodium silicate solutions. The corresponding Si concentration is indicated foreach diagram. The simulation has been performed assuming that silicate speciesbehave as spherical colloids which size distribution obeys to a bimodal law. Thecurves are arbitrary vertically shifted for clarity.

0

0.5

1

0 1 2 3 4 5 6 7

Rad

ius

(nm

)

Concentration [Si] (mol/l)

Without interaction

With interactions

a

ξ

Fig. 6. Mean radius of the scattering entities resulting from simulation byconsidering silicates species as a mixture of small (a) and large (n) sphericalcolloids as a function of silicon concentration.

M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315 313

questionable. Simulations with various colloid form factors andsize distribution functions have been unsuccessfully tested. Onlythe introduction in the modeling of a structure factor taking intoaccount the inter-particle interactions (hitherto neglected) haslead to a significant improvement of the capability of the simula-tion to reproduce these experimental scattering curves with moremeaningful resulting size distributions (Fig. 5). The used structurefactor, based on a potential of an inter-particle interaction of ‘‘hardsphere” type was calculated by using the approximation of Percus–Yevick and introduced in Eq. (1) by means of a ‘‘local monodisperseapproximation” [26].

The mean radius of the size distributions derived from the fit-ting procedure are plotted in Fig. 6 as a function of silicon concen-tration. The initial concentrated solution should contain both verysmall and large scattering entities and larger ones with a radius ofabout 0.3–0.4 nm and 1.2–1.3 nm respectively.

As long as the silicon concentration remains higher than3.6 mol/l, the progressive addition of distilled water into the initialconcentrated mother solution leads to a slight increase of the smallscattering entities mean size and to a significant decrease of theaverage size of the largest ones.

When the silicon concentration reaches a value lower than3.6 mol/l, simulations of the scattering curves suggest the total dis-appearance of the largest scattering entities. These solutions con-tain only one kind of scattering species larger than the smallestcolloids and smaller than the largest entities present in the concen-trated solutions. Whether the inter-particle interactions are taken

2.26 mol/l

1.76 mol/l

with interactions

Experimental

Calculated

0.1 1 101

10

100

Inte

nsit

y (a

.u.)

without interaction

q (nm-1)

Fig. 5. Influence of the structure factor on the fit between calculated andexperimental SAXS curves collected for the most diluted solutions (siliconconcentration = 2.26 or 1.76 mol/l). The curves are arbitrary vertically shifted forclarity.

into account or not, simulations lead for the more diluted solutionsto a mono-modal size distribution of scattering entities with a dif-ference of mean radius lower than 5% (Fig. 6). However, the intro-duction of structure factor slightly reduces the range of error (�4%for [Si] = 1.76 mol/l). Therefore, whatever the hypotheses, the cal-culated mean diameters of the scattering entities in the more di-luted solutions are in a 1.2–1.5 nm range (Fig. 6).

In summary, the more the initial solution is diluted, the morethe size distribution of the scattering entities is mono-modal andnarrow and the more the scattering entities interact together asevidenced by the appearance of interference effects (attractive orrepulsive) on the scattering curves revealing their spatialcorrelations.

4. Discussion

4.1. Nature of the predominant entities in concentrated sodium silicatesolution

Considering values given in literature [1,27] for the equilibriumconstants of the different polymeric species (neutral or ionized)resulting from Si(OH)4 monomers condensation and the solubilityconstant of silica (KS = 2.98) associated with the following reaction:

SiO2 þ 2H2O ¢ SiðOHÞ4 KS ¼ ½SiðOHÞ4� ð4Þ

the concentration of silicon atoms contained in the whole dissolvedsilica-derived species has been calculated [15]. For the pH value ofthe concentrated sodium silicate solution, i.e. pH = 11.56, this Siconcentration would be less than 0.07 mol/l. Therefore, solvatedspecies containing only silicon, oxygen and hydrogen cannotexplain alone the high silica concentration of the initial concen-trated mother solution (7 mol/l). Thus, the predominance of sol-vated complex species containing at once silicon and sodiummust be considered.

The analysis of SAXS results leads to consider that the concen-trated solution contains isolated scattering entities (colloids) ofabout 0.6–0.8 nm diameter which can also be more or less gath-ered together through weak bonds to form aggregates slightly lar-ger than 2 nm. The presence of aggregates suggests that the smallentities which behave like colloids are probably neutral.

Furthermore, according to the NMR analysis, the initial concen-trated sodium silicate solution contains silicon atoms mainly in-volved, in decreasing order, in three (Q3 = 52%), two (Q2 = 26%)and four (Q4 = 15%) siloxane bridges (Table 2). A further assigne-ment of the 1D NMR resonance lines to specific polysilicate struc-tures is rather challenging. Actually, a number of studies on many

314 M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315

alkaline silicate sols [2,4,5,7] have provided a nomenclature for ref-fering to the chemical shifts of each Si nucleus of about 20 differentpolysilicate species (monomer, dimer, linear and cyclic trimer, cu-bic octomer. . .) which could exist in these solutions. Theoretically,once peaks are assigned, the respective concentrations of thesespecific polysilicates species may be deduced. Nevertheless, byonly considering a 1D NMR spectrum the solution may not be un-ique. Thus, 2D NMR experiment provides an useful insight into thedetermination of the predominant polysilicate species by consider-ing the evidenced Q3–Q3, Q3–Q2 or Q3–Q4 correlations (Fig. 2).Those Si–O–Si bonds link together one Si involved in three siloxanebridges with another silicon atom involved in three (Q3–Q3 corre-lations), two (Q3–Q2 correlations) or four (Q3–Q4 correlations)siloxane bridges.

From these above-mentioned considerations, different struc-tures of complex molecules have been considered. Among all thepossible complex molecules containing silicon, sodium, oxygenand hydrogen, only two appear to be in accordance with theNMR and SAXS results while satisfying the rules of chemistry.These two proposed molecules displayed in Fig. 7 have the samechemical composition (Si7O18H4Na4) and a very similar structure.Among the seven silicon atoms, only one is implicated in four Si–O–Si bonds (Q4 species). The four associated silicon atoms arethemselves involved in three Si–O–Si bonds (Q3 species and Q3–Q4 correlations). The last two silicon atoms, unrelated to the cen-tral silicon atom, are linked to two of the previous Si (Q2 and Q3–Q2 correlations). Among the eight oxygen atoms which are not in-volved in siloxane bridges, four belong to Si–O–H groups and fourare implicated in Si–O–Na bonds. The two variants shown in Fig. 7are neutral (likely to aggregation) and respect the rules of correla-tions revealed by 2D NMR analysis (Fig. 2). The relative amounts ofQ4, Q2, and Q3 species in these two molecules are in good agree-ment with the values deduced from NMR results (Table 2): to the52% of Q3 deduced from experiment should be associated 26% ofQ2 (experimental value = 26%) and 13% of Q4 (experimentalvalue = 15%).

The size of these two molecules has been estimated consideringthe distance d(Si–Si) between two Si equal to 0.306 nm, the Si–Oand O–H bond lengths equal to 0.162 and 0.097 nm respectively[28] and the length of the O–Na bond as inferior to 0.23 nm(sum of ionic radii). The values of Si–O–Si and O–Si–O angles weretaken equal to 144� and 109� respectively [15,28]. The size thuscalculated, <0.73 nm, is in agreement with the value, 0.6–0.8 nm,deduced from SAXS measurement. Such a size is also in accor-dance with previous studies conducted on analogous systems[14,25].

The atomic Si/Na ratio equal to 1.75 for such molecules, verysimilar to the 1.71 value measured in the initial solution, is alsoin accordance with the predominant presence of Si7O18H4Na4 spe-cies in the concentrated sodium silicate mother solution.

Si

Si

Si

Si

Si

Si

Si

O

O O

OO

ONa

O

O

OO

O

HOHO

OH

OHNaO

NaO

ONa

Si

SiO

O

O

O

NaONaO

HO

HO

Fig. 7. Structure of two Si7O18H4Na4 molecules compatible with the 29Si NMR and SA([Si] = 7 mol/l, Si/Na = 1.71, pH = 11.56).

4.2. Influence of dilution on the nature of predominant species

SAXS analysis has shown that distilled water addition affectsthe solvated entities present in the alkaline concentrated sodiumsilicate solution (Fig. 3). This is mainly reflected by a colloid meansize increase and an aggregate average diameter decrease (Fig. 6).When the silicon concentration becomes lower than 3.6 mol/l, theonly scattering entities identified by simulation of SAXS spectra areinteracting colloids of about 1.2–1.5 nm diameter. Dilution is thusassociated with a colloids growth from 0.6–0.8 to about 1.2–1.5 nmaverage diameter. This evolution suggests a polycondensationreaction, which usually occurs in the case of a concentration in-crease but not during a dilution.

Na2O and SiO2 represent more than 40 wt% of the aqueous com-mercial solution used in this study. The solvation of such an highquantity of silica should result from the formation of Si7O18H4Na4

solvated molecules (Fig. 7). As the dilution leads to the disappear-ance of the scattering entities of 0.6–0.8 nm diameter (Fig. 6) towhich they are assigned, addition of distilled water to the concen-trated mother solution must affect these predominant species.

During dilution Si7O18H4Na4 like neutral complexes could bedissociated with release of Na+ ions and formation of charged(Si7O18H4Na4�n)n� species (with 1 6 n 6 4), as described by the fol-lowing equations:

Si7O18H4Na4 ¢ ðSi7O18H4Na3Þ� þ Naþ ð5ÞðSi7O18H4Na3Þ� ¢ ðSi7O18H4Na2Þ2� þ Naþ ð6ÞðSi7O18H4Na2Þ2� ¢ ðSi7O18H4NaÞ3� þ Naþ ð7ÞðSi7O18H4NaÞ3� ¢ ðSi7O18H4Þ4� þ Naþ ð8Þ

None of these ionized species can constitute the predominantcolloids of 1.2–1.5 nm evidenced when the silicon concentrationis lower than 3.6 mol/l (Fig. 6). However, a condensation reactionbetween (Si7O18H4Na4�n)n� groups could be considered:

yðSi7O18H4Na4�nÞn� ¢ ðSi7yOð18y�zÞHð4y�2zÞNað4�nÞyÞny� þ zH2O ð9Þ

with 1 6 n 6 4. Colloids of 1.2–1.5 nm mean size would correspondto y value between 2 and 8 depending respectively on the consid-ered linear or spherical geometry in which these primary(Si7O18H4Na4�n)n� species are bonded. The presence of charges onthe species resulting from such condensation is in accordance withthe absence of aggregation process in the more diluted solutionsand with the appearance of interference effects on the scatteringcurves revealing the scattering entities interactions (repulsive inter-actions) which ensue and consequently their spatial correlations(Fig. 5).

According to that description, the evolution induced by dilutionand detected by SAXS experiments would be summarized by thefollowing global equilibrium:

Q2

Q3

Q4Si

Si

Si

Si

Si

O

O

OO

OH

O

O

ONa

ONa

OH

XS results obtained for the initial concentrated sodium silicate mother solution

M.T. Tognonvi et al. / Journal of Colloid and Interface Science 352 (2010) 309–315 315

ySi7O18H4Na4 ¢ ðSi7yOð18y�zÞHð4y�2zÞNað4�nÞyÞny� þ nyNaþ

þ zH2O ð10Þ

The predominance of the charged condensated species can oc-cur only if the constant of equilibrium (10) is not too low, i.e. thecharged colloids must correspond to a relatively stable configura-tion.Their formation during dilution suggests that the activity ratio

anyNaþ

aySi7O18H4Na4

decreases with the Si7O18H4Na4 concentration. Due to lake

of information concerning activity coefficients and equilibriumconstant, it is not possible to check that interpretation. However,if the activity coefficients are not too much sensitive to dilution,the formation of the charged polymers would be favoured forn > 1. An example of possible condensed species is the dimer(Si14O35H6Na4)4� with y = 2 and n = 2.

5. Conclusion

A combined analysis of results provided by both 29Si NMR andSAXS experiments performed on a concentrated sodium silicatesolution ([Si] = 7 mol/l; Si/Na atomic ratio = 1.71) leads to considerthat the high siliceous species solubility observed in an alkalinemedium (pH = 11.56) results from the formation of a Si7O18H4Na4

neutral complex. Results obtained for the more diluted solutionssuggest that a distilled water addition to the initial concentratedsolution leads to a progressive dissociation of these neutral com-plexes into (Si7O18H4Na4�n)n� charged entities and Na+ ions(1 6 n 6 4). The scattering entities detected by SAXS being twicebigger in the most diluted solutions than in the concentratedone, a polycondensation reaction between a limited number, be-tween 2 and 8, of (Si7O18H4Na4�n)n� groups should occur. The re-sults suggest that besides silicate species already identified insolution of moderate concentration, strongly concentrated solu-tions could include silicate cages containing at the same time Q2,Q3 and Q4 groups.

References

[1] F. Gaboriaud, Etude du rôle de l’ion alcalin au cours de la gélification dessolutions silico-alcalines déstabilisées par addition d’ions calcium, Thèsedoctorat de l’Université de Bourgogne, France, 1999.

[2] R.K. Harris, R.H. Newman, J. Chem. Soc., Faraday Trans. II 73 (1977) 1204.[3] J.G. Vail, Soluble Silicates: Their Properties and Uses, Reinhold, New York,

1952.[4] C.T.G. Knight, J. Chem. Soc., Dalton Trans. 6 (1988) 1457.[5] R.K. Harris, C.T.G. Knight, W.E. Hull, J. Am. Chem. Soc. 103 (1981) 1577.[6] R.K. Harris, C.T.G. Knight, W.E. Hull, J. Am. Chem. Soc. Symp. Ser. 194 (1982) 79.[7] G. Engelhardt, D. Zeigan, H. Jancke, D. Hoebbel, W. Wieker, Z. Anorg. Allg.

Chem. 418 (1975) 17.[8] A.V. McCormick, A.T. Bell, C.J. Radke, Zeolites 7 (1988) 183.[9] C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, J. Wiley and Sons, New York,

1976.[10] M.J. Munoz-Aguado, M. Gregorkiewitz, J. Colloid Interface Sci. 185 (1997) 459.[11] R.K. Iler, The Chemistry of Silica. Solubility, Polymerization. Colloid and Surface

Properties, and Biochemistry, Wiley-Interscience, New York, 1979.[12] R.K. Iler, Soluble Silicates, Symposium Series of American Chemical Society,

Falcone, Washington, DC, 1982.[13] H. Roggendorf, W. Grond, M. Hurbanic, Glastech. Ber. Glass Sci. Technol. 69

(1996) 216.[14] D. Böschel, M. Janish, H. Roggendorf, J. Colloid Interface Sci. 267 (2003) 360.[15] M.T. Tognonvi, Physico-chimie de la gélification du silicate de sodium en

milieu basique, Thèse de doctorat, Université de Limoges, France, 2009.[16] P. Lindner, T. Zemb, Neutrons, X-ray and Light Scattering: Introduction to an

Investigative Tool for Colloidal and Polymeric Systems, Elsevier, Amsterdam, 1991.[17] O. Glatter, O. Kratky, Small Angle X-ray Scattering, Academic Press, London,

1982.[18] S.D. Kinrade, T.S. Swaddle, Inorg. Chem. 27 (1988) 4253.[19] C.T.G. Knight, Zeolite 9 (1989) 448.[20] E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt, A.R. Grimmer, J. Am. Chem.

Soc. 102 (1980) 4889.[21] C.A. Fyfe, H. Grondey, Y. Feng, G.T. Kokotailo, J. Am. Chem. Soc. 112 (1990)

8812.[22] J. Buddrus, in: R.K. Harris, R. Wasylishen (Eds.), INADEQUATE Experiment:

Encyclopedia of Magnetic Resonance, John Wiley, Chichester, 2007.[23] P. Florian, F. Fayon, D. Massiot, J. Phys. Chem. C 113 (2009) 2562.[24] D. Massiot, F. Fayon, M. Deschamps, S. Cadars, P. Florian, V. Montouillout, N.

Pellerin, J. Hiet, A. Rakhmatullin, C. Bessada, C.R. Chim. 13 (2010) 117.[25] F. Gaboriaud, A. Nonat, D. Chaumont, A. Craievich, B. Hanquet, J. Phys. Chem. B

103 (1999) 2091.[26] J.S. Pedersen, Adv. Colloid Interface Sci. 70 (1997) 171.[27] A.R. Felmy, H. Cho, J.R. Rustard, M.J. Mason, J. Solution Chem. 30 (2001) 509.[28] A.F. Wells, Structural Inorganic Chemistry, fifth ed., Clarendon Press, Oxford,

1984.