- Email: [email protected]
Comment on “infrared spectroscopy of surfaces of glasses containing alkali ions” by R.H. Doremus
Journal of Non-Crystalline Solids 45 (1981) 137-139 North-Holland Publishing Company
137
LETTERS TO THE EDITOR COMMENT ON "INFRARED SPECTROSCOPY OF SURFACES OF GLASSES CONTAINING ALKALI IONS" BY R.H. DOREMUS
W. SMIT Laboratory of Colloid Chemisto', Eindhoven Universi O' of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
Received 4 March 1981
In a paper with Lanford et al. [1] and in a comment [2] on a paper by Smit et al. [3] Doremus advocates a mechanism in which hydronium ions exchange for alkali ions in the case of leaching alkali containing silicate glasses with acid or water. Doremus. [4] claims in his paper on "Infrared spectroscopy of surfaces of glasses containing alkali ions" additional evidence for exchange of hydronium ions from the spectra reported by Baucke [5,6]. This author observed that as the reaction with water proceeded in a lithium silicate glass an absorption band at about 3360 cm-i grew. Whereas Baucke claimed that this band showed that the hydrogen diffusing into the glass was in the form of protons, Doremus ascribes this absorption band to hydronium ions. Doremus' conclusion is based on the assumption that the absorption band for the resulting SiOH groups should be similar to that found for pairs of SiOH groups inside, near and on the glass surface, which absorb at about 3650-3660 cm ~, as shown in Doremus' table 1. An alternative interpretation is also possible. Doremus' attribution of the 3360 cm -I absorption band to hydronium ions because this frequency is characteristic of the OH stretching vibration in molecular water seems to be not realistic. According to Gigu6re [7] the OH stretching vibration of the hydronium ion gives an absorption around 2900 cm 1. Its low frequency (compared to 3400 cm 1 in water) is indicative of very strong hydrogen bonding, which should also be expected to be present for hydronium ions in the proximity of non-bridging oxygen ions. Beckmann and Harrick [8] found an absorption peak at 3375 cm ~ in anodically grown silicon dioxide films on silicon by internal reflection spectroscopy in the infrared. Revesz [9] attributed this absorption band to silanol groups between which H-bonding is possible because of their preferential alignment along structural channels. Grigorovich.et al. [10] studied micr0Porous silica layers deposited on the surface of silicon prisms by the hydrolysis of SiC14, also using the method of internal reflection spectroscopy. The initial film gives an intensive absorption 0022-3093/81/0000-0000/$02.50 © 1981 North-Holland
138
W. Smit / Comment on "'Infrared spectroscopy" I~v R.H. Doremus
band for the stretching vibrations of OH for the silanol groups and the adsorbed water with a maximum at 3250 cm 1 and an absorption band for the deformation vibrations of the molecularly adsorbed water at about 1625 cm 1. The latter absorption band disappears completely after evacuation of the silica layer at 35°C to a pressure of 0.13 Pa. Subsequent evacuation to 0.01 Pa for 30 rain at 35°C does not bring about a change in the spectrum in the 3380-3400 cm-1 range, thus molecularly adsorbed water has already completely been removed. Therefore, the remaining absorption band must belong to bound silanol groups. From further analyses by dehydroxylation at higher temperatures, by rehydroxylation and by deutero exchange, Grigorovich could distinguish in the micropores single (3700-3650 cm-~) and paired (3680-3620 and 3550-3500 cm -I) silanol groups, and also silanol groups which form hydrogen bonds with several neighbouring silanol groups. The latter absorb in the 3350-3380 cm -~ range. The profiles of hydrogen and sodium in the surface of a commercial soda-lime silicate glass [1] seems to provide strong evidence for exchange of hydronium ions, because three hydrogen atoms replaced each sodium ion exchanged. However, Scholze et al. [11] concluded from an isotope effect for the leaching process of soda-lime silicate glasses in HC1/H20 and in DC1/D20 that H30 + ions do not co-operate in the leaching process. The basic process is the Na + ~ H + exchange. The silanol groups formed are for the most part condensated. In addition to the water formed from the silanol groups, Scholze et al. found that only about 0.46 water molecules accompany one proton on its diffusion into the investigated glass. The degree of condensation of the silanol groups and the amount of water molecules accompanying the protons depend on the alkali content of the glass and on the nature of the replaceable alkali ion. It appears to the present author therefore to be not justified to draw as far-going conclusions from the behaviour of one single type of glass, as has been done in Doremus' paper. Recently Smitet al. [12] performed tritium exchange experiments on microporous silica layers which were deposited on vitreous silica by the hydrolysis of SiC14. The exchange process can be resolved in three diffusion processes which can be related, in order of decreasing rate, to tritium (i) in the physisorbed water molecules in the micropores; (ii) in the silanol groups in the micropores and (iii) in bulk intra-skeletal silanol groups, respectively, in agreement with the description of the microporous silica layer given by Grigorovich et al. [10]. The fact that the tritium diffusion out of the silica layer cannot be described as a single diffusion process refutes Doremus' [2] assumption that the hydrogen ions in the silica layer exist to a significant fraction as hydronium (H3 O+ ) ions. This is additional evidence against the attribution of the 3360 cm-l absorption band to hydronium ions. Scholze et al. [11] reported a rapid D 2 0 / H 2 0 exchange in the surface layer of the leached glass. The estimated order of magnitude of the diffusion coefficient of this exchange process is equal to the diffusion coefficient of the water molecules ( ~ 10-15 m2 s - i ) in the micropores of the microporous silica
W. Smit / Comment on "Infrared spectroscopy" by R.H. Doremus
139
layers investigated by Smit et al. [12]. Presumably, micropores are also formed in the leached layer of silicate glasses when the silanol groups condensate. The remaining silanol groups are aligned preferentially along these structure channels and can form hydrogen bonds with several neighbouring silanol groups. Such silanols absorb in the 3350-3380 cm -~ range.
References [1] W.A. Lanford, K. Davis, P. Lamarche, T. Laursen, R. Groleau and R.H. Doremus, J. Non-CrystaUine Solids 33 (1979) 249. [2] R.H. Doremus, J. Coll. Inter. Sci. 72 (1979) 347. [3] W. Smit, C.L.M Holten, H.N. Stein, J.J.M. de Goeij and H.M.J. Theelen, J. Coll. Inter. Sci. 67 (1978) 397. [4] R.H. Doremus, J. Non-Crystalline Solids 41 (1980) 145. [5] F.G.K. Baucke, in: Mass Transport phenomena in ceramics, eds., A.R. Cooper and A.H. Heuer (Plenum, New York, 1975) p. 337. [6] F.G.K. Baucke, in: The Physics of non-crystalline solids, ed., G.H. Frischat (Trans. Yech., Bey Village, Ohio, 1976) p. 503. [7] P.A. Gigu~re, J. Chem. Education 56 (1979) 571. [8] K.H. Beckmann and N.J. Harrick, J. Electrochem. Soc. 118 ( 1971 ) 614. [9] A.G. Revesz, J. Electrochem. Soc. 124 (1977) 1811. [10] S.L. Grigorovich, A.V. Kiselev and V.I. Lygin, Kolloid Zh. (Engl. Transl.) 38 (1976) 121. [11] H. Scholze, D. Helmreich and I. Bakardjiev, Glastechn. Ber. 48 (1975) 237. [12] W. Smit, C.L.M. Holten and H.N. Stein, J. Non-Crystalline Solids 43 (1981) 279.
137
LETTERS TO THE EDITOR COMMENT ON "INFRARED SPECTROSCOPY OF SURFACES OF GLASSES CONTAINING ALKALI IONS" BY R.H. DOREMUS
W. SMIT Laboratory of Colloid Chemisto', Eindhoven Universi O' of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
Received 4 March 1981
In a paper with Lanford et al. [1] and in a comment [2] on a paper by Smit et al. [3] Doremus advocates a mechanism in which hydronium ions exchange for alkali ions in the case of leaching alkali containing silicate glasses with acid or water. Doremus. [4] claims in his paper on "Infrared spectroscopy of surfaces of glasses containing alkali ions" additional evidence for exchange of hydronium ions from the spectra reported by Baucke [5,6]. This author observed that as the reaction with water proceeded in a lithium silicate glass an absorption band at about 3360 cm-i grew. Whereas Baucke claimed that this band showed that the hydrogen diffusing into the glass was in the form of protons, Doremus ascribes this absorption band to hydronium ions. Doremus' conclusion is based on the assumption that the absorption band for the resulting SiOH groups should be similar to that found for pairs of SiOH groups inside, near and on the glass surface, which absorb at about 3650-3660 cm ~, as shown in Doremus' table 1. An alternative interpretation is also possible. Doremus' attribution of the 3360 cm -I absorption band to hydronium ions because this frequency is characteristic of the OH stretching vibration in molecular water seems to be not realistic. According to Gigu6re [7] the OH stretching vibration of the hydronium ion gives an absorption around 2900 cm 1. Its low frequency (compared to 3400 cm 1 in water) is indicative of very strong hydrogen bonding, which should also be expected to be present for hydronium ions in the proximity of non-bridging oxygen ions. Beckmann and Harrick [8] found an absorption peak at 3375 cm ~ in anodically grown silicon dioxide films on silicon by internal reflection spectroscopy in the infrared. Revesz [9] attributed this absorption band to silanol groups between which H-bonding is possible because of their preferential alignment along structural channels. Grigorovich.et al. [10] studied micr0Porous silica layers deposited on the surface of silicon prisms by the hydrolysis of SiC14, also using the method of internal reflection spectroscopy. The initial film gives an intensive absorption 0022-3093/81/0000-0000/$02.50 © 1981 North-Holland
138
W. Smit / Comment on "'Infrared spectroscopy" I~v R.H. Doremus
band for the stretching vibrations of OH for the silanol groups and the adsorbed water with a maximum at 3250 cm 1 and an absorption band for the deformation vibrations of the molecularly adsorbed water at about 1625 cm 1. The latter absorption band disappears completely after evacuation of the silica layer at 35°C to a pressure of 0.13 Pa. Subsequent evacuation to 0.01 Pa for 30 rain at 35°C does not bring about a change in the spectrum in the 3380-3400 cm-1 range, thus molecularly adsorbed water has already completely been removed. Therefore, the remaining absorption band must belong to bound silanol groups. From further analyses by dehydroxylation at higher temperatures, by rehydroxylation and by deutero exchange, Grigorovich could distinguish in the micropores single (3700-3650 cm-~) and paired (3680-3620 and 3550-3500 cm -I) silanol groups, and also silanol groups which form hydrogen bonds with several neighbouring silanol groups. The latter absorb in the 3350-3380 cm -~ range. The profiles of hydrogen and sodium in the surface of a commercial soda-lime silicate glass [1] seems to provide strong evidence for exchange of hydronium ions, because three hydrogen atoms replaced each sodium ion exchanged. However, Scholze et al. [11] concluded from an isotope effect for the leaching process of soda-lime silicate glasses in HC1/H20 and in DC1/D20 that H30 + ions do not co-operate in the leaching process. The basic process is the Na + ~ H + exchange. The silanol groups formed are for the most part condensated. In addition to the water formed from the silanol groups, Scholze et al. found that only about 0.46 water molecules accompany one proton on its diffusion into the investigated glass. The degree of condensation of the silanol groups and the amount of water molecules accompanying the protons depend on the alkali content of the glass and on the nature of the replaceable alkali ion. It appears to the present author therefore to be not justified to draw as far-going conclusions from the behaviour of one single type of glass, as has been done in Doremus' paper. Recently Smitet al. [12] performed tritium exchange experiments on microporous silica layers which were deposited on vitreous silica by the hydrolysis of SiC14. The exchange process can be resolved in three diffusion processes which can be related, in order of decreasing rate, to tritium (i) in the physisorbed water molecules in the micropores; (ii) in the silanol groups in the micropores and (iii) in bulk intra-skeletal silanol groups, respectively, in agreement with the description of the microporous silica layer given by Grigorovich et al. [10]. The fact that the tritium diffusion out of the silica layer cannot be described as a single diffusion process refutes Doremus' [2] assumption that the hydrogen ions in the silica layer exist to a significant fraction as hydronium (H3 O+ ) ions. This is additional evidence against the attribution of the 3360 cm-l absorption band to hydronium ions. Scholze et al. [11] reported a rapid D 2 0 / H 2 0 exchange in the surface layer of the leached glass. The estimated order of magnitude of the diffusion coefficient of this exchange process is equal to the diffusion coefficient of the water molecules ( ~ 10-15 m2 s - i ) in the micropores of the microporous silica
W. Smit / Comment on "Infrared spectroscopy" by R.H. Doremus
139
layers investigated by Smit et al. [12]. Presumably, micropores are also formed in the leached layer of silicate glasses when the silanol groups condensate. The remaining silanol groups are aligned preferentially along these structure channels and can form hydrogen bonds with several neighbouring silanol groups. Such silanols absorb in the 3350-3380 cm -~ range.
References [1] W.A. Lanford, K. Davis, P. Lamarche, T. Laursen, R. Groleau and R.H. Doremus, J. Non-CrystaUine Solids 33 (1979) 249. [2] R.H. Doremus, J. Coll. Inter. Sci. 72 (1979) 347. [3] W. Smit, C.L.M Holten, H.N. Stein, J.J.M. de Goeij and H.M.J. Theelen, J. Coll. Inter. Sci. 67 (1978) 397. [4] R.H. Doremus, J. Non-Crystalline Solids 41 (1980) 145. [5] F.G.K. Baucke, in: Mass Transport phenomena in ceramics, eds., A.R. Cooper and A.H. Heuer (Plenum, New York, 1975) p. 337. [6] F.G.K. Baucke, in: The Physics of non-crystalline solids, ed., G.H. Frischat (Trans. Yech., Bey Village, Ohio, 1976) p. 503. [7] P.A. Gigu~re, J. Chem. Education 56 (1979) 571. [8] K.H. Beckmann and N.J. Harrick, J. Electrochem. Soc. 118 ( 1971 ) 614. [9] A.G. Revesz, J. Electrochem. Soc. 124 (1977) 1811. [10] S.L. Grigorovich, A.V. Kiselev and V.I. Lygin, Kolloid Zh. (Engl. Transl.) 38 (1976) 121. [11] H. Scholze, D. Helmreich and I. Bakardjiev, Glastechn. Ber. 48 (1975) 237. [12] W. Smit, C.L.M. Holten and H.N. Stein, J. Non-Crystalline Solids 43 (1981) 279.