characterizing the infrared bands of aqueous soluble silicates

Characterizing the Infrared Bands of Aqueous Soluble Silicates

James S. Falcone Jr.,*,† Jonathan L. Bass,‡ Paul H. Krumrine,§ Karen Brensinger,† andEmily R. Schenk†,|

West Chester UniVersity of PennsylVania, Department of Chemistry, Schmucker Science Center South,West Chester, PennsylVania 19382

ReceiVed: August 22, 2009; ReVised Manuscript ReceiVed: December 29, 2009

A systematic study of the transition in silicate solutions from a solution containing a highly complex mixtureof silicate species to one dominated by a single symmetric cubic octamer has been completed. Infrared andNMR results have been analyzed and compared with each other and literature values. The FT-IR band locationsare dependent on many factors, particularly the dominant band near 1000 cm-1. The analysis supports DentGlasser’s hypothesis that silica polymerization results from changes in distribution between the larger colloidalsilica and intermediate sized anionic fraction rather than the continuous stepwise growth seen with organicpolymerization. A constant value of silica monomer seen in all solutions independent of the complexity ofthe species or their distribution suggests equilibrium between the monomeric form and larger anions andpolymers that is independent of their structure. No evidence is uncovered for specific silicate species dependentIR band assignments.

Introduction

An understanding of the structural complexity of solutionscontaining soluble silicate ions has evolved over the past halfcentury. Lesley Dent Glasser has called them the “Cinderellaanions” because their chemistry is so intractable that they hadlittle appeal to the classical inorganic chemist.1 The distributionof silicate species in sodium silicate solutions is of interestbecause of the variations in properties that these solutions exhibitwith different Si:Na ratios.2-5 Historical, but mostly anecdotal,evidence exists of variations in application performance ofcommercial products made from silicates of supposedly the samevalue of the elemental ratio, often expressed as the modulus,m, or molar ratio of SiO2 to the alkali metal oxide, Na2O forexample. The value of m is 2 times the value of the Si:Na ratio.The strongest evidence comes from work in the 1950s onstrength variations of phosphor-silicate films in black and whiteTV picture tubes.6-8 They were found to be due to variationsin potassium silicate manufacturing methods. The variationswere observed to be much larger in the potassium silicates. Thesodium silicates, on the other hand, were found to exhibit moresubtle variability, potentially sensitive to several factors. Thissubtlety combined with the extreme complexity of the silicateswith modulus values greater than 2 make exact and quantitativestructural inferences more difficult. Thus, it appears the exactnature of the silicate speciation remains unclear9,10 and willcontinue to be subject to a probabilistic interpretation similarto the manner in which organic polymers are understood, thatis, broadly. However, the importance of these solutions asbuilding blocks for commercially valuable materials (sols, gels,precipitated silicates and zeolites) demands a clear understanding

of the value and limitations of methods that can be effectivelyemployed to characterize these systems.

The chemistry of aqueous silica is basically a study ofmetastability due to the minimal energy difference between thevarious polymorphs.11 Hazel12 suggested more than 40 yearsago that hydrogen bond supported structuring was the likelybasis for polymer formation because the energy of activation islow and entropy considerations are lessened due to fewergeometric restrictions. It is convenient to represent soluble silicausing phase diagrams13 that depict regions for true solutions,precipitated silica, and a so-called “stable multimeric domain(MD)”. As shown in Figure 1, this domain extends roughly frompH values of 11-13 at a silica concentration of about 8 molalto a point near the solubility of amorphous silica (roughly 120ppm) at a pH value of about 9.

Iler14 concluded on the basis of reactions with molybdic acidthat solutions of sodium silicate were composed primarily ofparticles ranging in size from 0.8 to 2.0 nm in the MD region.Krumrine and Falcone,15 following up on ideas of Dent Glasser16

and considering the time dependent silica precipitation resultsof Katsanis,17 found it practical to view this MD region asbiphasic, that is, a metastable very high surface area condensed/aggregated silica phase dispersed in a solution phase containingsoluble mono/oligomeric silica. Commercial silicate solutionsare found in the MD region at very high silica concentrations,6-8 molal. The complex behavior of this dispersed silica phaseas a function of concentration, modulus, alkali species, pH,impurities, and temperature appears to be the source of much,if not all, of the variability in performance properties. Complica-tions in analysis occur because many observations summarizedin the extensive literature are drawn from experiments carriedout under insufficiently controlled conditions. The high potentialfor artifacts even in the simplest systems was well characterizedby Hazel,7 Debye and Nauman,18 and Iler,19 who made it clearthat one had to be aware of the purity and/or preparation historyof the silicate, two factors that are not independent.

Debye and Nauman observed aging/turbidity effects over aperiod of 3 years and saw that at a given concentration these

* Corresponding author. E-mail: [email protected].† West Chester University of Pennsylvania.‡ Bass Chemical Consulting, 39 Lee Road, Audubon, PA 19403.§ BAE Systems, US Army Environmental Center, Aberdeen Proving

Ground, MD 21010.| Currently, Department of Chemistry and Biochemistry, Florida Inter-

national University, 11200 SW 8th Street, Miami, FL 33199.

J. Phys. Chem. A 2010, 114, 2438–24462438

10.1021/jp908113s 2010 American Chemical SocietyPublished on Web 01/29/2010

effects were greater for higher ratio silicates. Also, concentratedsolutions appeared more stable, but once initiated turbidityeffects at higher ratios were more profound. O’Connor20

following on Alexander’s21 molybdic acid kinetic studiessuggested that above a modulus of 2 the soluble silica is presentmostly as three-dimensional ions. Subsequent studies, usingvolatile chlorotrimethylsilane22-26 derivatives, yielded indirectevidence for the presence of a variety of structures with siliconnumbers ranging from 1 to 8, even in highly alkaline, dilutesilicate solutions. Mixtures of highly polymerized species indynamic equilibrium were detected, especially in solutions withcompositions close to the solubility limit of amorphous silica.Dent Glasser et al.27,28 suggested that the amount of monomerand dimer in solution was not an indicator of the overall degreeof condensation of silica, i.e., the nature of the dispersed phase.Polymerization was a result of changes in distribution betweenthe larger colloidal silica, mostly based on four member rings,and the intermediate sized anionic fraction, i.e., the sizedependent aggregation of medium weight species, rather thanthe continuous stepwise growth seen with organic polymeriza-tion. Model-building showed that anions based on such cyclicspecies were compatible with the observed properties of largersolution-phase silicate ions. Depolymerization was thought tobe very rapid in highly alkaline solutions, since crystallineNa2SiO3 (pyroxene or linear silicate chain) and Na2H2SiO4 ·8H2O (monomeric silicate anion) upon dissolution rapidlyyielded equivalent distributions of silicate species.

Laser Raman spectroscopy and 29Si FT-NMR (Fouriertransform-nuclear magnetic resonance) spectroscopy moredirectly examine the structure of silicate species in solution.29,30

Early laser Raman spectroscopic studies concluded that dilute,0.3 M sodium silicate solutions of varying ratios (m ) 0.33-3.3)contain an equilibrium mixture of monomeric silicate species,SiO(OH)3

- and SiO2(OH)22-, and polymeric anions, regardless

of their histories and methods of preparation. Subsequent workshowed that monomeric and dimeric silicate species polymerizeto form cyclic polysilicate anions, especially under alkalineconditions (pH g 11), and that the distribution of ionic specieschanges upon aging of the solution.31-33 Bass and Turner29 foundthat FT-IR (Fourier transform-infrared) spectroscopy bandproperties in the wavenumber range 850-1300 cm-1 followedtrends in species distribution due to changes in modulus andconcentration observed by 29Si NMR.

Identification of specific silica species under idealized condi-tions34 has been greatly improved as a result of the use of NMR.The NMR spectra of specially prepared alkali silicate solutionsof varying ratios have been measured and the types of Sienvironments as a function of their relative concentrations havebeen estimated.35-39 Originally, 18 individual ionic species in29Si enriched potassium silicate solutions had been proposedby Harris and co-workers, but more recent studies40-42 usingmore advanced techniques have characterized almost 50 possibleaqueous species with the likelihood of more to be identified.These spectroscopic methods have been applied over the past20 years to commercial solutions. Specific species have beenidentified in the more alkaline solutions and specially preparedsolutions, e.g., quaternary ammonium silicates. A few highlysymmetric structures, such as the dimer and cyclic trimer, canbe specifically identified in commercial solutions using the NMRmethods. However, complexity and impurities generally limitone to the identification of the relative quantity of the Qn siliconsites where the n value, called the connectivity, represents thenumber of oxygen bridges in a silicate species. Also, NMR dataacquisition is relatively slow and limited primarily to “steadystate” systems. FT-IR on the other hand quickly yields informa-tion that correlates to NMR results over a range of silicateratios.29

When alkali metal ion is replaced by tetramethylammoniumion (TMA+) in a solution with constant silica concentration andmodulus, the silicate speciation shifts toward greater connectiv-ity.43,44 In pure TMA silicate solution NMR results show thepredominant species is the symmetric double four ring anion,Q3

8 often called the D4R anion.43,45,46 ATR (attenuated totalreflection) FT-IR results on similar solutions show the broad,relatively unstructured, vibrational bands in the region 1300-800cm-1 for the alkali silicates sharpening to two well separatedbands near 1100 and 1000 cm-1 as the Na+ is replaced by theTMA+. These bands likely reflect asymmetric cross-linkedSi-O-Si and non-cross-linked (i.e., at the “corners” of the cubicstructure) Si-O- stretching vibrations,47 respectively. The NMRresults show a fairly ordered progression of the changes in thepolymer distribution in going from pure sodium silicate to pureTMA silicate, but the literature reports no systematic comparisonof the two methods on the same solutions. In this study wehave prepared a series of Na:TMA silicate solutions with varyingmole fraction TMA at a constant Si:(Na:TMA) ratio of 1 and

Figure 1. Silica solubility diagram after Stumm and Morgan (ref 13) including results for time dependent growth of amorphous silica fromKatsanis, Krumrine, and Falcone (ref 17).

Infrared Bands of Aqueous Soluble Silicates J. Phys. Chem. A, Vol. 114, No. 7, 2010 2439

silica molality of 2.0 mol kg-1 in 5% D2O in water as solvent.The NMR and FT-IR spectra were measured, analyzed, andcompared to develop a better understanding of the relationshipbetween these complementary methods.

Experimental Methods

Stock solutions of sodium and TMA silicate containing 2.0molal SiO2 in a 5% (w/w) D2O/H2O mixture and a 1:1 molratio of Si to Na:TMA ions were prepared from 30% colloidalsilica (LUDOX silica sol), 25% w/w aqueous TMA hydroxidesolution [electronic grade, from Alfa Aesar (lot L26R003)],sodium hydroxide [pellets from Mallinckrodt AR (lot 7708

N10H11)], ultrapure HPLC grade water [J.T. Baker (lotT04252)], and 99.8% D2O (Aldrich Chemical). These stocksolutions were mixed to make solutions with varying TMA+

mole fraction at constant SiO2 concentration. All solutions wereallowed to equilibrate at room temperature for at least 3 daysbefore use.

The FT-IR spectra were recorded using one drop samples ona Nicolet Protege 460 Fourier transform infrared spectrometer(FT-IR) equipped with a Smiths Detection DuraSamplIR II ATRaccessory with the single reflection Duradisk diamond windowaccessory. OMNIC FT-IR software was used for data acquisitionand analysis. Data acquisition included 32 scans per sample inthe wavenumber range 4000-720 cm-1 at a resolution factorof 4 cm-1 and subtraction of a deionized water backgroundspectrum. The diamond window was cleaned with diluteaqueous NaOH and deionized water between runs. All spectrawere corrected for path length variations with wavenumber usingthe advanced ATR correction function in the OMNIC softwareand solution refractive index values obtained using a FisherScientific refractometer. The peak resolve tool in the OMNICsoftware using a Gaussian-Lorentzian function with a constantbaseline was used to resolve all peaks. The peaks were fit untilreproducible results were achieved with minimum standard error.

All 29Si NMR spectra were acquired using 5 mm quartz NMRtubes (New Era) on a Bruker 400 MHz nuclear magneticresonance (NMR) spectrometer. The software used to attain andanalyze the data was the Bruker TOPSPIN 2.0 and TOPSPINPlot Editor. The resonance frequency for 29Si was 79.49 MHzwith samples requiring 700 scans with a pulse tip angle of 90°,a 1.02 s acquisition time, and a delay cycle of 5 s. Samplingconditions remained constant throughout these experiments.

Results/Discussion

In Figure 2 the FT-IR spectra of the pure sodium and TMAsilicate solutions are shown. These spectra are as expected frompast studies. Figure 3 is a composite of spectra for all mixturesmeasured. In Table 1 are shown the results for the peaklocations, relative areas, and the standard error of the fit for thedeconvolution of the spectra in the 720-1300 cm-1 region usinga Gaussian/Lorentzian peak model and a constant baseline; fulldetails of the deconvolution results for each solution areavailable in the Supporting Information. The average peaklocations for each band will be used as the band identifier.Increasing the fraction of TMA+ in the solution leads toincreasing area for the 776 and 1110 cm-1 band. The areadecreases for the other three bands. In Figures 4 and 5 thedeconvolution results for the pure solutions are shown. Table 1does not include the results for the sharp peak seen at 951 cm-1

Figure 2. FT-IR spectra of the stock TMA and sodium silicatesolutions, expansion 1250-720 cm-1, shown on common scale.

Figure 3. FT-IR spectra of Na:TMA mixtures with TMA mole fractionvalues from 0.24 to 0.89, shown on full scale.

TABLE 1: FT-IR Fit Data: Band Location and Area Results for Gaussian/Lorentzian Deconvolution for Different MoleFraction Na:TMA Silicate Solutionsa

band 1 band 2 band 3 band 4 band 5

TMA mol fraction location, cm-1 area location, cm-1 area location, cm-1 area location, cm-1 area location, cm-1 areafitεstd

0.00 766.9 0.8 869.9 3.6 1008.7 10.3 1056.3 4.9 1111.9 2.0 0.610.24 765.3 1.3 870.8 3.6 1009.0 9.7 1054.2 5.3 1111.7 2.0 0.630.38 761.9 1.3 872.2 3.6 1009.4 9.3 1054.3 4.2 1108.6 3.2 0.590.48 765.2 1.4 874.5 3.4 1007.5 8.1 1052.2 4.7 1109.2 3.0 0.720.58 765.7 1.8 877.0 3.2 1005.5 7.0 1048.9 5.6 1109.7 3.4 0.90.68 765.8 1.8 875.5 3.2 1005.0 7.1 1047.7 5.2 1109.7 4.3 1.010.79 766.1 1.9 875.4 3.0 1004.4 7.3 1047.5 4.1 1109.4 5.1 1.250.89 766.5 2.0 875.9 2.8 1003.9 7.6 1047.3 2.9 1109.3 5.9 1.421.00 767.6 2.1 876.0 2.5 1002.7 7.4 1048.1 1.5 1108.8 6.3 1.73av location: 766 874 1006 1051 1110

a The standard error of each fit is shown in the last column.

2440 J. Phys. Chem. A, Vol. 114, No. 7, 2010 Falcone Jr. et al.

in all solutions containing TMA+. This vibration appears to bedue solely to the presence of TMA+ and has been subtractedfrom all results.

Table 2 is a summary of the extensive literature for thelocations of IR and Raman bands observed for different formsof silicates from structured crystalline solids to aqueous silicatesolutions including our results.29,48-61 For sodium silicate glasseswith compositions from 60% to 100% SiO2, four broadLorentzian bands that vary with network modifier composition(Na2O) dominate the spectra. Their typical midrange values (incm-1) and changes in these values with increasing modifierconcentration (cm-1, %Na2O) are the following: ∼1200, -4.5;

Figure 4. Sodium silicate FT-IR spectrum showing Gaussian-Lorentziandeconvoluted peak fit results.

Figure 5. TMA silicate FT-IR spectrum showing Gaussian-Lorentziandeconvoluted peak fit results.

TABLE 2: Summary of Literature Values for FT-IR and Raman Bands in Amorphous Silicates (Including Silica Sols),Aqueous Silicate Solutions, and Crystalline Silicates with Standard Deviation for Each Band Grouping in Parentheses Followingthe Average Value and All Band Maximum Locations in cm-1

average bandlocation, cm-1

bands in amorphoussilicates

amorphous sourcematerials

bands in aqueoussilicate solutions

bands in crystallinesilicates

crystalline sourcematerials

427(19) 440(9)a 409(13)b anhydrous metasilicate476(11) 478c amorphous silicic acid 475(14)d

527(15) 535(7)e 510f anhydrous metasilicate591(9) 576g glass 601(3)e 590(2)b anhydrous metasilicate712(4) 712(4)b anhydrous and

9-hydrate metasilicate779(21) 798(21)h glass, sol, silicic acid, gel 771(8)i 760(9)b anhydrous and

9-hydrate metasilicate879(21) 877(28)j glass and gel 889(16)k 869(10)l anhydrous and 9-hydrate metasilicate,

orthosilicate,Q2

(4R) and D4R structure941(17) 953(5)m glass, sol, gels 938(8)n 915(16)o anhydrous and

9-hydrate metasilicate978(11) 987(8)p glass, silicic acid 982q 970(4)r anhydrous metasilicate

and orthosilicate1006(7) 1012(2)s glass 1006(8)t 1005(7)u anhydrous and 9-hydrate metasilicate,

Q2(4R) and D4R structures

1047(16) 1052(15)V glass, sol, gels 1040(14)w 1057(21)r anhydrous metasilicateand Q2

(4R) structures1099(9) 1096(8)x glass, sol, gels 1102(10)y 1100z D4R structure1144(13) 1143(14)aa glass, silicic acid, gels 1140(3)bb 1149(19)cc D4R structure and

9-hydrate metasilicate(?)1194(21) 1194(21)dd glass, silicic acid, gels

a References 48, 49, and 52. b References 52 and 61. c Reference 48 (IR only). d References 48, 49, and 59. e References 49 and 59.f Reference 61 (Raman only). g Reference 58 (Raman only). h References 29, 50, 52, 55, and 58. i References 48, 49, 52, 59, 60, and this study.j References 50, 52, and 61. k References 29, 48, 52, 60, and this study. l References 52, 56, 57, and 61. m References 29, 50-52, 55, 58, and61. n References 48, 51, and 52. o Reference 52. p References 48 and 50 (IR only). q References 29, 48, and 52. r References 52, 57, and 61.s References 50 and 61. t References 29, 47, 48, 52, 54, 60, and this study. u References 47, 52, 57, and 61. V References 50-52, 55, and 61.w References 29, 48, 49, 51, 52, 59, 60, and this study. x References 29, 50-52, 58, 61, and this study. y References 29, 47, 51, and this study(IR only). z Reference 47 (IR only). aa References 48, 50, 51, and 61. bb References 48 and 59. cc References 52 and 56. dd References 48, 50,51, and 52.

Figure 6. 29Si NMR data for stock sodium silicate (top) and TMAsilicate (bottom), enlarged scale -70 to -105 ppm, 700 scans persample, 5 Hz line broadening.


1050, -2.0; 950, -2.5; and 750, -2.5. Earth scientists62 havegenerally assumed that the bands in glasses correspond tospecific mineral silicate classes, and that species with higherconnectivity values absorb at higher energies; thus, the abovevibrations are said to be due to Q4 (SiO2), Q3 (Si2O5

2- orSi4O11

6-), Q2 (Si2O64- or Si4O11

6-), and Q1 (Si2O76-) or Q0

(SiO4-) silicon centers, respectively. They appear to broaden

and their frequencies decrease in intensity as species depoly-merize and the Si-O-Si bond angle decreases. The additionof Al3+ to a glass was found to decrease the intensity of the1000-1100 cm-1 band and increase that of the 950 and 750cm-1 bands. However, it has been noted that in silicates withlong-range order a single band may be generated by vibrationalmodes in different crystalline structures.

Band locations for the nonglassy amorphous solids and sols showa clear trend of increase in energy as the structure of the silicate

increases in complexity. The bands seen in aqueous solution arealso quite similar to those seen for glasses and amorphous andcrystalline solids, and they have been shown to vary systematicallywith changes in silicate modulus and silica concentration.63 In fact,the similarity of band locations even in this wide range ofenvironments is striking. Our bands at 776, 874, 1006, 1051, and1110 cm-1 are in the range of variation seen for silicate species inany environment. It appears that bands between roughly 950 and1020 cm-1 are more sensitive to environment than other observedbands. The vibrational band centered near 980 cm-1 for solidsappears to shift by roughly 30 cm-1, suggesting a strong influenceof hydrogen bonding environment on Si-O stretching, and possibleuse of this band as a marker for changes in hydrogen bonding.The bands, included in the results in Table 2 for the crystallinemonomeric (Q0) Na2H2SiO4 ·8H2O (often called the nine hydratesodium metasilicate), reported recently,52 are suspect, particularly

TABLE 3: NMR Chemical Shifts Relative to TMS for the Different Mole Fraction Na:TMA Silicate Solutions with Averageand Standard Deviations for the 20 Identified Shifts Labeled A-T and Literature Averages for Similar Chemical Shift Values

TMA mole fraction

shift label 0.00 0.24 0.38 0.48 0.58 0.68 0.79 0.89 1.00 av valuea std lit. avb

A -72.06 -72.13 -72.23 -72.27 -72.27 -72.31 -72.45 -72.52 -72.42 -72.30 0.15B -80.17 -80.25 -80.4 -80.4 -80.4 -80.47 -80.52 -80.68 -80.74 8.15 0.18 8.0C -80.5 -80.64 -80.79 -80.79 -80.82 -80.9 -81 -81.15 -81.19 8.57 0.22 8.5D -81.92 -82.01 -82.11 -82.11 -82.13 -82.13 -82.27 -82.29 9.83 0.12 9.8E -82.19 -82.31 -82.4 -82.4 -82.49 -82.51 -82.57 -82.69 -82.7 10.18 0.17 10.4F -86.3 -86.37 -86.47 -86.51 -86.48 14.13 0.09 14.2G -86.66 -86.76 -86.81 -86.86 -86.84 14.49 0.08 14.5H -88.0 -88.0 -88.1 -88.33 -88.36 -88.41 -88.35 -88.4 15.95 0.18 15.9I -88.37 -88.57 -88.57 -88.62 -88.68 -88.74 -88.77 -88.9 16.36 0.16 16.4J -88.55 -88.86 -88.86 -89.01 -88.99 -89.11 -89.19 -89.4 -89.16 16.72 0.24 16.6K -89.24 -89.45 -89.55 -89.45 -89.67 -89.65 -89.74 -89.84 -89.94 17.32 0.22 17.3L -89.75 -89.84 -89.89 -89.89 -90.04 -89.97 17.60 0.10 17.9M -90.26 -90.28 -90.33 -90.38 -90.41 -90.45 -90.4 -90.56 18.09 0.10 18.3N -92.6 -92.78 -92.88 -92.97 20.51 0.16 20.4O -93.53 -93.61 -93.66 -93.61 -93.66 21.32 0.05 21.4P -94.04 -94.15 -94.25 -94.2 -94.23 21.88 0.08 21.9Q -96.25 -96.33 -96.15 -96.01 -96.6 -96.73 24.05 0.27 24.3R -96.98 -96.69 -96.65 -96.59 -97.43 -97.54 -97.62 24.78 0.45 24.8S -97.49 -97.33 -97.5 -97.77 -97.85 -98.08 -98.11 25.44 0.30 25.8T -98.55 -99.29 -99.48 -99.53 -99.69 -99.73 -99.81 -100.02 27.22 0.45 26.7

a The average value of Q0 for the 9 solutions is shown in the first row followed by the average value of the chemical shift relative to thisaverage Q0 value. b From refs 36, 39, 40, and 41.

Figure 7. 29Si NMR data for the mixtures made from the stock solutions: (a) 0.89 mol fraction; (b) 0.79 mol fraction; (c) 0.68 mol fraction; (d)0.58 mol fraction; (e) 0.48 mol fraction; (f) 0.38 mol fraction; (g) 0.24 mol fraction.


the higher wavenumber bands. This hydrate is extremely deliques-cent, and unless very strict precautions are taken to dry and excludeit from air, it is expected to rapidly form a concentrated aqueoussolution, polymerize, and provide anomalous results, those of avery concentrated sodium metasilicate solution.

The NMR spectra run on these same solutions are shown inFigures 6 (pure samples) and 7 (mixtures). The locations andestimated areas for the NMR peaks are shown in Tables 3 and4, respectively. Several calculated areas for shifts identified byTOPSPIN in the busy portions of the spectra such as the region88-90 ppm were partitioned manually. The relatively flatbaseline and sharp nature of the observed shifts, and the absenceof any bands in the region greater than 100 ppm, indicate theabsence of Q4 silicon containing colloidal species. Also, in Table3 we show the average chemical shifts and their standarddeviations relative to monomer for our scans compared withtypical values summarized from past studies. The trend in D4Rappearance is generally similar to that obtained by Engelhardtand Rademacher43 at the same Si:TMA ratio, but at a lowersilica concentration of 1.6 molar. Also, the -0.01 ppm/%Nachange in the chemical shift (usually associated with the D4Rspecies) seen with decreasing TMA mol fraction in our result

was the same as that calculated from their data. This shiftdecrease has been found to be independent of destabilizingcation for Na+, K+, and TBA+ at constant D4R species content.64

Table 4 presents the peak area results as a percentage of totalarea and the average connectivity, n, for each solution calculatedfrom the percent of each silicon center type (Qn composition)estimated from NMR areas. These peak area values should beviewed as qualitatively accurate. Table 5 is a summary of someof the specific species identified in many studies over the years,their representative shifts with respect to monomer, averagevalues of these results, and the peaks in our results that wouldmost closely match the average assignments.

As expected, the addition of TMA+ causes an increase in averagedegree of polymerization or connectivity of the silicate species in

TABLE 5: Summary of Various Species Identified inSilicate Solutions, Chemical Shifts Associated with EachRelative to Q0, Their Averages, and Corresponding Peak inThis Study

species identitya EngelhardtbHarris and

Knightc Chod Haouaseav

valueshiftID

monomer 0 0 0 0 0 Adimer 8.51 8.59 8.7 8.64 8.6 Ctrimer 8.04 8.15 8.3 8.2 8.2 B

16.92 16.7 16.9 16.85 16.8 Jcyclic trimer 10.13 10.17 10.1 10.12 10.1 Elinear tetramer 8.25 8.3 8.3 8.14 8.2 B

16.17 16.4 16.5 16.72 16.5 Icyclic tetramer 15.99 16.09 16.2 16.17 16.1 Hsubcyclic trimer 7.92 8.06 8.2 7.65 8.0 B

9.78 9.86 9.8 9.41 9.7 D18.09 18.22 18.3 17.7 18.1 M

bridged cyclictetramer

14.2 14.22 14.2 14.18 14.2 F21.94 21.94 21.9 21.88 21.9 P

subcyclic tetramer 7.86 7.9 8 7.96 7.9 B15.76 15.84 16 15.99 15.9 H16.08 f 16.4 16.3 I23.99 24.2 24.3 24.11 24.2 Q

bicyclic pentamer 9.86 9.86 9.8 9.79 9.8 D16.28 16.3 16.56 16.49 16.4 I17.11 17.16 17.14 17.12 17.1 K

prismatic hexamer,D3R

17.08 17.22 17.1 17.09 17.1 K

tricyclic hexamer 16.12 16 16.18 16.02 16.1 H16.64 16.6 16.48 16.46 16.6 J17.51 17.46 17.48 17.5 K24.74 24.72 24.8 24.82 24.8 R

cis-tricylic hexamer 10.5 10.36 10.39 10.51 10.4 E16.8 16.6 16.58 16.45 16.6 J

trans-tricylic hexamer 10.81 10.6 10.56 14.21g 10.7 E17.85 17.8 17.84 17.84 17.8 L

doubly bridgedtetramer

14.57 14.52 14.5 14.48 14.5 G21.44 21.43 21.28 21.3 21.4 O

pentacyclic heptamer 16.92 17.63 16.92 17.2 K18.93 18.88 19.11 18.75 18.9 M17.93 17.89 17.89 17.92 17.9 L

tetrahedral tetramer 25.69 25.85 25.8 Scubic octamer,

D4R27.31 25.38 26.74 26.5 T

hexacyclic octamer 17.72 17.8 17.81 17.8 L20.52 20.42 20.39 20.4 N26.71 26.72 26.66 26.7 T

prismatic decamer,D5R

27.15 26.06 26.6 T

bicyclic hexamer 16.2 16.36 16.35 16.3 I24.1 24.17 24.23 24.2 Q

pentacyclic nonamer 15.7 15.7 15.7 H16.1 16.54 16.6 J18.1 18.23 18.2 M24.9 25.03 25.0 R

tricyclic octamer 15.5 15.57 15.5 H24.5 24.32 24.4 Q

a Species names generally following ref 36. b Reference 39.c Reference 36 and ref 6 in that paper. d Reference 40. e Reference41. f This shift was not indentified. g This shift was not used incalculating average.

TABLE 4: NMR Relative Peak Areas (Percent of TotalArea) for Each of the 20 Chemical Shifts in Table 3 LabeledA-T, Average Connectivitya, n, and the Estimated Qn

Distributionb of the Silicon Centers in the Nine DifferentMole Fraction Na:TMA Silicate Solutionsc

TMA mole fraction

shift label 0.00 0.24 0.38 0.48 0.58 0.68 0.79 0.89 1.00

A 6% 6% 5% 6% 5% 5% 5% 5% 6%B 8% 8% 7% 5% 4% 5% 4% 3% *C 5% 5% 4% 7% 4% 4% 3% 4% *D 4% 3% 4% 3% 3% 2% 4% 3% *E 6% 5% 5% 7% 4% 4% 3% 3% *F 2% 1% 2% 2% 2%G 2% 1% 2% 1% 2%H 9% 8% 7% 5% 4% 5% 4% 4%I 7% 8% 5% 7% 6% 6% 4% 4%J 11% 10% 6% 7% 6% 6% 4% 4% *K 6% 7% 8% 2% 6% 6% 7% 5% 6%L 4% 4% 3% 4% 4% 3%M 7% 7% 7% 7% 6% 6% 6% 3%N 0% 1% 2% 2%O 3% 2% 2% 3% 3%P 3% 2% 3% 3% 3%Q 6% 5% 3% 2% 4% 4%R 8% 3% 4% 7% 6% 5% 3%S 2% 9% 10% 6% 4% 4% 2%T 0% 5% 12% 16% 25% 36% 50% 60% 88%

symboln 2.13 2.19 2.30 2.31 2.41 2.43 2.48 2.53 2.81Q3 40% 43% 51% 53% 59% 62% 65% 71% 94%Q2 41% 38% 33% 30% 28% 24% 22% 17% 0%Q1 13% 13% 11% 11% 9% 9% 8% 7% 0%Q0 6% 6% 5% 6% 5% 5% 5% 5% 6%

a Average connectivity ) ∑0nn % Qn. b The values of %Qn were

calculated on the basis of a balance of location and peak assignmentto species. The following rules were used: % Q0 ) shift A in allcases; % Q1 ) shifts B and C in all cases; % Q2 ) 100% - sum(Q0, Q1, Q3); % Q3 was assigned on mole fraction of TMA. Thefollowing rules also apply: % Q3 ) sum(shifts L-T) + 33% ofshifts J and K for TMA mole fractions up to 0.89; % Q3 )sum(shifts K-T) + 50% of shifts J for TMA mole fraction ) 0.89;% Q3 ) sum(shifts J-T) for TMA mole fractions ) 1.0. c Anasterisk indicates a very weak signal with area likely less than 1%in this region. Several shift areas had to be estimated due to greatoverlap. These are shown in italics.


solution as shown at the bottom of Table 4. It is expected thatTMA+ induced changes in structure would result in correlatedchanges in NMR chemical shifts for species and their amountsconsequently increase or decrease. For example, if growth of thesubstituted cyclic trimer was a key intermediate in any polymer-ization process, we would expect a strong positive correlationbetween the signal areas of shifts B and D, B and M, and D andM. We calculated Pearson correlation values for all pairs of data

(including the TMA mol fraction and each shift) and their statisticalsignificance using Minitab 14 statistical software (Minitab is atrademark of Minitab, Inc., State College, PA). Correlation valuesrange between +1 and -1 and measure the strength of the linearcorrelation between data sets.65 Table 6 shows the matrix of resultsof this correlation analysis for pairs with p-values 0.005 or less.For example, there is a strong correlation between the appearanceof chemical shift T and an increase in TMA mol fraction, but shiftT appearance occurs with disappearance of many of the other shifts.Even considering the complexity of the NMR results, severalrelevant chemical shifts indicating specific species were highlycorrelated. In Table 7, species most consistent with the results ofthe correlation analysis were used to estimate the most likelycomposition of the 0.89 mol fraction TMA solution. Extendingthis analysis to lower mole fractions is problematic due to theincreased complexity; however, the correlation analysis stronglysuggested that singly and doubly bridged cyclic tetramers wereimportant structures. The associated shifts F, G, O, and P also canbe clearly seen disappearing together in Figures 6 and 7 as theTMA mole fraction increases. No other significant nega-tive correlations for chemical shifts between pairs of shifts wereobserved except for the appearance of the D4R anion at the expenseof other species. This observation seems to support the growth byaggregation model suggested by Dent Glasser. Additionally, therelative Q0 concentration remained constant within error which issuggestive of some type of equilibrium between the monomer andmore complex species regardless of their distribution.

Figure 8 shows the FT-IR areas under the bands at 766, 874,1006, 1051, and 1110 cm-1 in the spectrum for each solutionplotted versus the average connectivity of the silicate speciesdetermined using NMR. The bands at 766 and 1110 cm-1 bothincrease with connectivity, while the other three are generallydecreasing. Figure 9 shows the relationships between % Q2

silicon, % Q3 silicon, sum of 874, 1006, and 1051 cm-1 bandareas, and the sum of the 766 and 1110 cm-1 band areas andconnectivity for the solutions. This correlation is more clearlyshown in Figure 10 where FT-IR areas are plotted versus NMR

TABLE 6: NMR Shift Pearson Correlations with p e 0.005a

increasingTMA mol

fraction B C D E F G H I J L M O Q

B -92D 83E 96G 99H -91 98 84I 87 91 93J -95 94 83 94 89M 88 85 89 92 87O 98 96 86P 97 96 84 99Q -88 84 88T 94 -94 -92 -90 -92 -92 -88 -90 -85

a Results are correlation coefficients times 100.

TABLE 7: Probable Silicate Species Composition of 0.89Mol Fraction Na:TMA Silicate Solution Based on NMR ShiftAreas and Correlation Analysis

possible silicate species in solution NMR shifts %

monomer A 5linear polymers C; B, J; B, I 10cyclic trimer E 3cyclic tetramer H 4subcyclic trimer B, D, M 5cis-tricyclic hexamer E, J 3other Q3 containing structures 4D3R K 5D4R T 60

Figure 8. Correlation between the average connectivity, n, of silicate species in each solution and the percent of the area associated with each peakin the FT-IR of the solution: 0, 766 cm-1; ∆, 874 cm-1; O, 1006 cm-1; +, 1051 cm-1; ×, 1110 cm-1.


areas for each correlated pair along with the equations for eachpair. At 100% Q3 the FT-IR bands areas are almost split evenlybetween those associated with the 766 and 1110 cm-1 vibrationsand those due to the three midfrequency vibrations. Most simplystated, on the basis of our results and an interpretation of thewide range of assignments found in the literature, the latter twobands appear to be due to atom movements within theframework of cages and the three midfrequency bands appear

to be due to Si-O stretches at different Qn sites. Given thevariety of complex species known to be present in aqueoussilicates, the known strong influence of environmental factors(such as concentration and composition) on peak location,documented concerns for reliability of assignments,42,66 and thetrends observed in this systematic study, it seems unjustified toapply species specific assignments in all but the simplest cases.

Figure 9. Correlations of area associated Q2 and Q3 from NMR and the sums of the correlated FT-IR band areas.

Figure 10. Correlation of NMR peak area due to the following: O, Q2 silicon and the area under FT-IR bands at 874, 1006, and 1051 cm-1; ∆,Q3 silicon and bands at 766 and 1110 cm-1. Values shown in parentheses are 95% confidence values of the last digits.


Conclusions

FT-IR bands at 776, 874, 1006, 1051, and 1110 cm-1 were foundto fit sodium silicate solutions containing a wide range of speciesproduced as a result of the systematic replacement of sodium ionwith tetramethylammonium ion. Bands at these locations are alsoobserved for silicate species in glasses and crystalline andamorphous solids. Bands in the range 950-1020 cm-1 appear moresensitive to environment. A correlation analysis on NMR resultsstrongly suggested a growth by aggregation model with singly anddoubly bridged cyclic tetramers as important structures in thetransition from the more disordered pure sodium silicate solutionto the more ordered tetramethylammonium silicate solution. Theconstant relative Q0 concentration suggests some type of equilib-rium between the monomer and more complex species regardlessof their distribution.

Acknowledgment. This work was made possible by thefinancial assistance provided by Rhodia Silcea for laboratoryequipment, software, supplies, and student assistance; NSF andCephalon for funds to purchase the NMR; and West ChesterUniversity of Pennsylvania for providing the FT-IR and thelaboratory space for this research. The authors also wish toacknowledge the assistance of Professors Felix Goodson andJoel Ressner in the acquisition of the NMR data.

Supporting Information Available: Results from the OM-NIC peak analysis software including more detailed peakinformation, statistics, and a graphical representation of the fullfit for each solution measured. This information is available freeof charge via the Internet at http://pubs.acs.org.

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characterizing the infrared bands of aqueous soluble silicates

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