1233

DISSOLUTION OF TETRAETHYLORTHOSILICATE COATINGS ON QUARTZ GRAINS IN ACID SOLUTION.

FROGNER, PAUL and SJOBERGt, LENNART

Dept. of Geology and Geochemistry , Stockholm University, S-106 91 Stockholm, Sweden

t deceased

SUMMARY

The polymerization of Tetraethylorthosilicate (TEOS) was investigated using Laser Raman Spectroscopy. Samples of pure TEOS were kept for three months at the relative humidities of 50 and 95% respectively.

The formation of ethanol during the hydrolysis of TEOS and the condensation of siloxane bonds were

studied and found to be considerably faster at the higher humidity.

The rate of dissolution of polymerized TEOS was investigated at pH 1 to 5. Quartz grains coated with

TEOS were placed as a stationary bed in a reactor through which a reactant solution was pumped continuously. The dissolution rates of TEOS in aqueous HCI - solutions are independent of pH in the

investigated pH range.

The addition of electrolytes to the reactant solution did not affect the dissolution rate at pH < 3. In contrast,

at higher pH - values, the rates increased with increasing electrolyte concentration. The effect of

electrolytes also increased with increasing pH and the increase in dissolution rate is an effect of the cation

concentration. At pH 4, 3.0 x 10-2 M NaCl, KCI and NH4CI produce the highest weathering rates, 150 nmole

I g I h. At the same concentration, CaCl2 and LiCI give 80 nmole I g I h and 55 nmole I g I h. The cation

enhanced dissolution rates may be modelled as an adsorption of cations on reactive sites on the surface on the TEOS coated quartz grains. The model resembles a Freundlich adsorption isotherm, R I Ro= kCn

where R is the actual rate, Ro , the rate in HCI, C, the electrolyte concentration, and n, the reaction order.

The reaction order, n, varies with pH for the electrolytes where the monovalent cations have the highest reaction order, NH4 +. 0.57, ~. 0.54, Na+, 0.54 and L(, 0.46 and the divalent cation Ca2+, somewhat lower,

0.32. The TEOS coating on the quartz grains was completely dissolved after a few weeks in 0.1 M NaCl

solution at pH 5. SEM-studies show that the material weathered in pure HCI - solutions were still coated with TEOS, while

only a few flakes of TEOS were found after experiments performed at pH 5 in 0.1 M NaCl - solution.

INTRODUCTION

Different consolidating materials have been used for the replacement of the calcite cement in deteriorated

monumental sandstone, the most frequently used being Steinfestiger OH, a commercial produced

tetraethylorthosilicate (TEOS). Most work done have focused on physical changes of sandstones after

application of Steinfestiger-OH, such as uptake of water, permeability and resistance to internal stress. The

stability and resistance against weathering of consolidating materials become even more interesting for the

conservators of stone as additional consolidating products appear. High concentration of salts in the pores is expected in sandstones due to weathering of mineral grains and

the calcite cement. Inside a calcite cemented sandstone the pore fluids probably are saturated with respect

to calcite and pH values of 8 - 9 are expected. These aspects therefore make it urgent to study the

influence of electrolytes on the stability of the consolidant. Stone surfaces that are exposed to rain could obtain pH values as low as 4. The weathering reactions may

take place over a broad pH interval. Furthermore, oscillations in relative humidity (RH) probably occur on

the surface of the stone. These facts make it important to study the effects of relative humidity (RH) on the

polymerization and pH of the stability of consolidants. In this study we use a micro-Raman equipment to

follow these polymerization reactions.

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METHOD AND SAMPLES

The Laser Raman spectrometer setup consists of a 2.5W-argon laser using a light source with a

wavelength of 514.5 nm (green light) coupled to a Dilor X'f spectrometer (Delhaye et al. , 1982, and

Barbillat et al., 1984). To focus the light on the samples a microscope with 10X and 100X objective is used

which give focus on objects from 1 mm to ca. 1 mm. The effect of the laser is set to 300 mW at the

entrance of the microscope. Analysis is done with detection accumulated 20 times with a detection time of

two seconds. The Raman instrument is wavenumber-calibrated using a neon lamp and a silicon standard

and the measurements are done at 400, 900, 1300, 1600, 2900 cm-1. The TEOS samples for the Raman

microspectroscopic study were prepared in the fonn of pellets, 5 mm in thickness with a diameter of 8 mm

that were placed in humidity chambers with 50 and 95% relative humidity respectively.

For the dissolution study TEOS coated quartz grains with a size of 125 - 250 mm, were kept at 70% relative

humidity for two months while the polymerization occurred. The dissolution rate of TEOS was measured on

1.0 g of polymerized material placed as a stationary bed in a reactor through which the reactant solution

was pumped continuously (Fig. 1). The tetraethylorthosilicate coated quartz grains were dissolved using

pure HCI and the eluents NaCl, KCI, NH4CI, CaCl2 and LiCI within the concentration interval 0.01 M-0.1 M

and in the pH range 1-5. Dissolution experiments were also done with the eluents 0.01 M Na2S04 and

NaN03. The temperature was 25°C and the duration time of the experiments two weeks. The samples were

analysed for silica on a Beckman, Du-5, spectrophotometer using the ammonia heptamolybdate blue

method described by Koroleff (1983). It was assumed that all dissolved silica was derived from TEOS since

the solubility of quartz under these conditions can be considered negligible (Berger et al. 1994). The dissolution rates were detennined after achievement of steady state.

RESULTS

RAMAN ANALYSIS

The liquid Steinfestiger-OH displays three peaks in the wavenumber range around 2900 cm-1, namely at

2976 (very strong), 2945 (strong) and 2902 (medium strong), but after 72 hours at 95% RH the peak at

2902 reaches a higher intensity than the 2974 peak (Fig 2). For an equivalent sample, but at 50% RH, 2902

is still after 336 hours (two weeks) weaker than 2974 (Fig 3). These lines are confined to C-H stretching

vibrations of ethyl groups bonded to the orthosilicate. In the wavenumber range 1800 to 1150 cm-1, liquid

Steinfestiger-OH shows four lines at 1716, 1483, 1455 and 1294 cm-1 assigned to C-H bonds (Baranska et al. 1987). These lines become weaker during the polymerization.

The liquid Steinfestiger OH, displays eight lines in the wavenumber range 1150 - 200 cm-1, namely at 1085,

956, 810, 750, 650, 589, 527 and 405 cm-1

. Three corresponding to Si-0-C in TEOS, 1085, 956 and 810

cm-1

(Fig. 4 and 5; Baranska et al. , 1987). The peaks at 1085 and 956 cm-1 are assigned as antisymmetric

and that at 810 cm-1

as symmetric nonnal modes. The lines 750, 650, 527 and 405 cm-1 are caused by the

solvent methyl-ethyl ketone and acetone. These peaks disappear within 27 hours at 50% RH (Fig. 6)

except for that at 650 cm-1

, which probably is the C=O bond of the ketone. The first peak to disappear, 750

cm-1, is assigned to the C-C-C bonds in the highly volatile acetone (Baranska et al. 1987). The three peaks

at 1085, 956 and 810 cm-1

, are assigned to the C-0 bond in TEOS and disappear due to the hydrolysis of

ethyl groups and the succeeding fonnation of siloxane bonds, Si-0-Si, which is represented by the lines

589 and 505 cm-1

(Fig. 6 and 7, Baranska et al. 1987). These two lines display their highest intensity after

two months for the sample polymerized at 50% (Fig. 6). At 95% RH, these lines become strong already

within 48 hours (Fig. 7). The three lines 1094, 1051 and 882 cm-1 which become more intense during the

polymerization are judged to be ethanol lines. During the polymerization of TEOS, ethanol appears as an

intennediate reaction product after the hydrolysis of the ethyl groups. For the pellet of Steinfestiger OH

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polymerized at 95% relative humidity these lines become strong after only 48 hours (Fig. 4). but takes 2160

hours (three months) at 50% relative humidity (Fig. 5).

DISSOLUTION RESULTS

The dissolution of TEOS coatings on quartz grains in HCI in the pH range 1-5 resulted in a constant rate of

30 nmole I g I h, thus no variation depending on pH was observed (Fig. 8). Variation depending on pH is

however found by the addition of electrolytes (Fig 8). Addition of 0.1 M NaCl did not affect the dissolution

rate at pH 2, but raised the rate at pH 3 to 60 nmole/ g I h, at pH 4 to 200 nmole I g I hand to 1000 nmole I g I hat pH 5 (Fig. 8). The concentration of the electrolyte is found to influence the dissolution rate. Addition

of 0.01M NaCl produce a dissolution rate at pH 4 and 5 of 70 respectively 100 nmole I g I h, but the

dissolution rate is raised for the 0.1 M NaCl solution at pH 4 and 5 to 200, respective 1000, nmole I g I h

(Fig. 8).

Comparing electrolytes with respect to dissolution rates is performed at pH 4 (Fig. 9). Electrolytes at pH 4 in

the concentration interval 0.01 - 0.1 M, do not dissolve the polymerized material on quartz grains too fast,

but give concentrations high enough for accurate analyses. The results are presented in figure 9 and show

that the dissolution rates are similar for NH4CI, KCI and NaCl but lower for CaCl2 and LiCI. It is also found

that substitution of 0.01 M NaCl with 0.01 M Na2S04 or 0.01 M NaN03 at the same pH did not affect the

dissolution rate.

SEM results

The SEM study shows that the material weathered in pure HCI solution at pH 1-5 or 0.1 M NaCl at pH 1-4,

175 hours still are coated with TEOS (Fig. 10), but at 0.1 M NaCl, pH 5 only has a few flakes left which did

not adhere to the quartz grains (Fig. 11). This results are in agreement with the dissolution results at pH 5

and 0.1 M NaCl which shows that the dissolution rate decreases after 75 hours.

DISCUSSION

Steinfestiger OH is used as a consolidating material in weathered monumental stones (sandstones or

granites) to protect them from further deterioration. The tetraethylorthosilicate molecule penetrate the stone

with a solvent of a methyl ethyl ketone and acetone when it is not polymerized. It is perhaps important that

the stone is dried before application of TEOS to avoid an all to rapid polymerization of the TEOS molecule

and that the temperature and wind conditions are favourable to not evaporate the highly volatile TEOS

molecule. The method of treatment and the conditions when the application is done like humidity and

temperature are thus of great importance to reach acceptable results. Raman spectrometric analysis of TEOS reveals that there are three different wavenumber ranges of

interest that follow different reaction steps during polymerization. One range where it is possible to study

hydrolysis of the ethyl groups through the decrease in intensity of the lines around 2900 cm -1

(Fig. 2 and 3),

a second range where the production of ethanol due to the hydrolysis of ethyl groups may be monitored

using lines around 1000 cm-1 (Fig. 4 and 5). A third range that permits the study of the process where the

condensation of silanol groups to siloxane bonds around 500 cm -1

occurs (Fig. 6 and 7). In the

wavenumber range around 2900 cm-1, lines assigned to the C-H stretchings of methyl and ethyl groups

were found for liquid TEOS. At the beginning these lines are similar to the spectrum of ethanol with a strong

peak at 2974 cm-1, but after 72 hours at 95% RH the spectrum becomes more like acetone with a higher

intensity for the peak at 2902 cm-1 than at 2974 cm-1 (Fig. 2). For the sample in 50% RH and after 336

hours (two weeks) 2974 cm-1 is still more intense than 2902 cm-1

(Fig. 3). This probably means that the

polymerization not has proceeded with the same rate as for the samples of the higher relative humidity

because addition of water is needed for the hydrolysis to start. The three lines 882, 1051 and 1094 cm-1

which become stronger during the polymerization due to ethanol production also suggest a slower

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polymerization rate for the sample kept at 50% RH (Fig. 5). These peaks become intense very fast in the

sample reacted at 95% RH (Fig. 4) indicating a fast release of ethanol during the hydrolysis.

A relation between the lines at 810 cm-1 and 505 cm-1 seems to exist. The ratio of these two lines (505 I 810) may be used to determine the degree of polymerization of TEOS (Fig. 12a). The line at 810 cm-1 is

related to the bond between the orthosilicate and the ethyl group, Si-0-C and the line at 505 cm-1 to the

siloxane bond Si-0-Si. During the hydrolysis of the ethyl groups, the 810 line becomes weaker and the

resulting condensation of silanol groups to siloxane bonds will make the line 505 even stronger. The 505 I 810 ratio is approximately the same after 48 hours for the sample kept at 95% RH as after 2160 hours in

50% RH. These observations probably indicate that the hydrolysis and the proceeding condensation of

silanol groups to siloxane bonds are much faster for 95% RH than for the sample of 50% RH. It is however

difficult to estimate a quantitative value of how much faster the polymerization proceeds at the higher

humidity. The line at 589 cm-1 is probably also related to the siloxane bond since the ratio 589 I 810 (Fig.

12b) give approximately the same relation between 50 and 95% RH as the 505 I 810 ratio. Another

interpretation was proposed by Weij et al. (1980) that a line around 580 cm-1 could be due to the

condensation of an intermediate dimer, Si-0-Sn where the catalysator tin is involved. The formation of

Si-0-Sn compounds by mixing organotin carboxylates and alkoxysilane is well established (Weij et al. 1980).

The results from the dissolution in pure HCI, pH 1-5, show that there are probably no pH-dependency which is usually found for uncoated minerals, as the reaction rate for the dissolution of TEOS was the same over

the whole range. On the other hand, a strong pH dependency is found with the addition of electrolytes. For

the electrolytes investigated, pH dependency is strongest for the cations NH/, Na· and i<. It is also found

that substitution of NaCl, with Na2S04 or NaN03 does not effect the dissolution rate, which probably makes

the type of anion unimportant in the weathering mechanism. The dissolution rates in the concentration

interval 0.01M-0.1M were similar for the cations NH/, Na· and i< which have approximately the same

hydratisation energy (300-400 kJ I mole). The rates were lower for ca2• and u·. A Freundlich adsorption

isotherm shows that the reaction order is approximative 0.55 for the monovalent cations NH/, Na· and I< and 0.31 for the divalent cation Ca2•. The monovalent cation L( has the reaction order, 0.46 which is

somewhat lower than for the other monovalent cations (Fig. 13). This could be due to the high hydratisation

energy of Lt , 506 kJ I mole, which make it less reactive. The divalent cation ca2• which has the lowest reaction order, is also the ion with the highest hydratisation energy, 1510 kJ I mole.

CONCLUSIONS

This report shows that it is possible with Raman microspectroscopy to follow the polymerization and the

intermediate reactions products like ethanol of the solid solutions of tetraethylorthosilicate. The Raman investigation of TEOS indicates that the hydrolysis and condensation reaction of stored samples at 95% relative humidity is much faster than for samples stored at 50% RH.

The study reveals that no pH dependency is observed for samples dissolved with pure HCI solutions. It is however found that addition of electrolytes gives a pH dependency for solution with a pH < 2. Among the

investigated electrolytes it is found that, NH4CI, NaCl and KCI produce approximately the same dissolution

rates but higher dissolution rates than for CaCl2 and LiCI. An exchange for a solution of NaCl with solutions

of Na2S04 and NaN03 shows that the anions take no active part in the weathering mechanism as no

change in dissolution rate is observed. A description of the dissolution reaction by a Freunlich adsorption isotherm makes a distinction between monovalent and divalent cations. NH4 •, i< and Na• have

approximately the same reaction order around 0.55 but 0.46 for L(. The divalent cation investigated, ca2•

gives a reaction order of 0.31. It is found that the hydratisation energy increases in the same order NH4

+ <

I< < Na• < Lt < Ca2• as the reaction order decrease. The TEOS coating of the quartz grain is found to be

completely dissolved after a few weeks in 0.1 M NaCl, pH 5. The SEM study shows that the material

weathered in pure HCI are still coated while the material ran for 175 hours in 0.1 M NaCl at pH 5 only have a few flakes left which does not adhere to the quartz grains.

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ACKNOWLEDGEMENTS

Especially I would like to thank Runo Lowendahl at the SWedish Central Board of National Antiquites for his

optimism and encouraging support during this work. I also would like to express my gratitude to Professor

Kurt Bostrom, Dr. Curt Broman, Dr. Peter Schweda and Dr. Sten Lindblom for their support with the study

after my supeNisor Dr. Lennart Sjobergs decease. I want to thank Lilly Johansson for her help with analysis

of silica and Kent Nelson for the photos. This study was supported financially by the Swedish Central Board of National Antiquites and Stockholm University.

REFERENCES

Baranska, H., Labudzinka, A. , Terpinski, J., 1987, Laser Raman Spectroscopy, Analytical Applications, Ellis Horwood Series. In Analytical Chemistry, John Wiley & Sons.

Barbillat, J. , Dhamelincourt, P. & Delhaye, M., 1984: Le Microdil 28: microsonde Raman a detection multicanale. Journal of Phys. 45 (Colloq. C2), 255-256.

Berger G., Cadore E. , Schott J., and Dove P., M., 1994, Dissolution rate of quartz in lead and sodium

electrolyte solutions between 25 and 300 °c: Effect of the nature of surface complexes and

reaction affinity, Geochim. Cosmochim. Acta 58, 541-51 .

Delhaye, M., Bridoux, M. Dhamelincourt, P. Barbillat, J., De Silva, E. & Roussel, B., 1982: A new

generation of laser micro-spectrometers: Micromars. In K.F.J. Heinrich (editor), Microbeam

Analysis - 1982. San Francisco Press San Francisco, Calif., 275-278.

Koroleff, F., 1983, Determination of silicon. Method of seawater analysis (ed. Kremling K.) 2nd ed. Ver1ag

Chemie. pp 174-180. Weij, F., W ., 1980, The Action of Tin Compounds in Condensation- type RTV Silicone Rubbers, Macromol.

Chem., 181, 2541- 2548.

e eluent

sampling bottle

Fig. 1. Schematic drawing of the flow-through reactor.

.0 "tii c: <II -.5 <II ...

'.Q ~

Ci CZ::

Hydrolysis of ethyl groups at 95 % RH

2800 3000 Wavenumber (cm-1

)

840 h

72h

liquid TEOS

3200

Fig. 2. Raman spectra of the 2900 cm·1 range showing hydrolysis of ethyl groups at 95% RH.

Release of ethanol at 95 % RH

48h

;;.... 2lh .... ·v;

i:: ~ c: .... Cll ... ·.c ~

Ci CZ:: liquid

TEOS

ethanol

600 800 1000 1200 Wavenumber (cm-1

)

Fig. 4. Raman spectra of the 1000 cm·1 range showing the release of ethanol at 95% RH.

1238

.0 ·v; c: <II .... .5 <II ...

'.Q ~

Ci CZ::

Hydrolysis of ethyl groups at 50 % RH

2800 3000 Wavenumber (cm ·1

)

840h

336h

liquid TEOS

3200

Fig. 3. Raman spectra of the 2900 cm·1 range showing hydrolysis of ethyl groups at 50% RH.

Release of ethanol at 50 % RH

2160 h

0 456h ·v;

c: <II .... . 5 Cll

~· ...

'..:I ~ Ci c::::

liquid TEOS

ethanol

600 800 1000 1200 1400 Wavenumber (cm-1

)

Fig. 5. Raman spectra of the 1000 cm·1 range. showing the release of ethanol at 50% RH.

1239

Polymerization at 50% RH Polymerization at 95 % RH

..... -·;;; = ~ -= ... ~ .. ; ~

"a:i ~

2160 h

456 h

27 h

liquid TEOS

200 400 600 800 1000 . J

Wavenumber (cm )

Fig. 6. Raman spectra of the 500 cm·1 range showing condensation of silanol groups to siloxane bonds (505 cm-1) and hydrolysis of ethyl groups (810 cm·1 )

at 50% RH.

1200 -0- 10-1M NaCl

---er- 10-2M NaCl 800

..c -b- HCI -OJ) -0 400 e =

0

0 2 4

pH

Fig. 8. pH-dependency of the dissolution rates for HCI and 0.01 and 0.1 M NaCl.

6

48h

liquid TEOS

200 400 600 800 .}

Wavenumber (cm )

Fig. 7. Raman spectra of the 500 cm·1 range showing condensation of silanol groups to siloxane bonds (505 cm-1) and hydrolysis of ethyl groups (81 O cm·1 )

at 95% RH.

300 KCI

~NaCl ---r;;r NH4CI --0- CaCI

..c 200 ---0-- LiCI -OJ) --0 e 100 =

1000

0 -I----.----,---~

0.00 0.04 0.08

Cone.

Fig.9. Dissolution rates for the electrolytes at pH 4. The dissolution rate increases with concentration.

0. 12

1240

Fig. 10. SEM picture of an unreacted TEOS coated quartz grain, 250 mm in size.

Fig. 11. SEM picture of a TEOS coated quartz grain, 250 mm in size, after 175 hours in 0.1 M NaCl at pH 5. Only a few flakes of TEOS is left.

1241

• • 95 RH • 50 RH ~ 2 -,--------------~

• 95 RH 50 RH

. . 1

0

. ···· · · ·· · · · ··· ·· ··· · ----.····· ····· ····

500 1000 1500 2000 2500

hours

----··········· ·· ·· ··· ··· ···· ······ ··· 1

•

0-;----.----.-----,...----,------l

0 500 1000 1500 2000 2500

hours

Fig. 12. Ratios of the Raman lines, a tool for polymerization of TEOS at 50 and 95% RH: a) 505 I 810, b) 589 I 810, both ratios show a faster polymerization at 95% RH.

1.0 0 KCI

0.8 D NH4Cl 6. NaCl -- \l CaCl 2 0

~ 0.6 0 LiCI -~ -- 0.4 OJ)

0 -0.2

0.0

-3 -2 -1 log C

Fig. 13. Freundlich adsorption isotherm describing the adsorption of cations on reactive sites

during the reaction. The isotherms in the diagram are for pH 4.

0