A neutron reflection study of the effect of water on the surface of float glass

Journal of Non-Crystalline Solids 292 (2001) 93±107

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A neutron re¯ection study of the e€ect of water on the surface of ¯oat glass R.M. Richardson *, R.M. Dalgliesh, T. Brennan, M.R. Lovell, A.C. Barnes H. H. Wills Physics Laboratory, School of Chemistry, University of Bristol, Tyndall Avenue, Cantocks Close, Bristol BS8 1TL, UK Received 18 July 2001

Abstract Specular neutron re¯ection has been used to investigate the incorporation of water into the surface of ¯oat glass in a number of di€erent environments. For ¯oat glass soaked in water for up to 6 months, two di€erent layers were  contained water at about 40% of identi®ed. A surface layer, whose thickness remained constant at approximately 30 A, the density of pure water. A second layer contained water at about 10% that had penetrated deeper into the sample as  in 6 months. An isotope exchange experiment indicated the immersion time increased reaching approximately 500 A that the lifetime of a water molecule in this layer was about one day. There was a reduction of the glass density in these layers due to the removal of sodium and evidence for gel formation at the surface. When the glass was exposed to a saturated water vapour at temperature up to 80 °C, the water was also found to penetrate into the glass surface over a period of about an hour, but there was no loss of material from the glass. The speed of water penetration was a strong function of temperature. On application of high temperature (150 °C) and pressure (120 bar) the glass underwent rapid ageing when in contact with water. The formation of a visible gel layer was observed, however the penetration depth of the water apparently reduced as the treatment time increased because of dissolution of the gel layer at the surface. Float glass with higher levels of alumina shows a small reduction in the water penetration suggesting that its increased durability resulted from stabilising the sodium ions rather than preventing ingress of water. Ó 2001 Published by Elsevier Science B.V.

1. Introduction The surface of glass continues to be important for technical and scienti®c reasons. It is well established, for instance, that in atmospheric conditions the surface of soda glass corrodes over time as the sodium ions leach out and react with atmospheric carbon dioxide and water. An opales-

* Corresponding author. Tel.: +44-117 928 7666; fax: +44-117 925 0612. E-mail address: [email protected] (R.M. Richardson).

cent coating of sodium hydrogen carbonate dihydrate (Trona) is formed [1]. The reaction of water with the glass is a key step in this process and is also generally believed to be an important in¯uence in chemical durability and fracture strength of glasses. The exposure of new glass to water vapour also e€ects the adhesion and stability of subsequent coatings. The chemical durability of glass has been extensively studied by following the reaction between aqueous solutions and glass [2,3] and the following process of leaching is generally established. In the ®rst step of this process, alkali metal ions such as sodium are replaced by hydrogen ions from the

0022-3093/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 8 5 - 7

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solution. Two mechanisms have been suggested for this process. One is that the hydrogen ions (H‡ or H3 O‡ ) di€use in from the surface and simply exchange with the sodium so that Na‡ ions di€use out of the glass [4]. The other is that molecular water di€uses from the surface and reacts to replace the sodium with H3 O‡ so that Na‡ and OH di€use out of the glass [5,6]. The net result in either case is that sodium ions are removed from the glass and replaced by hydroxonium ions but the amount and state of the additional molecular water that remains in the exchanged layer is still the subject of discussion [7]. In the second step of the process, the siloxane bonds are disrupted by reaction with hydroxyl ions. This leads to the formation of a `gel' layer [3] containing substantially more water than expected from simple substitution of the alkali metal and possibly the eventual breakdown and dissolution of the silica network. The resistance of glass to this leaching process is greatly in¯uenced by the composition of the glass. For instance, the substantial fraction of lime in normal ¯oat glass increases its stability relative to pure sodium silicate. However, it is also known that additions of trace amounts of some compounds can have a signi®cant e€ect on the durability of the glass surface. One such example is alumina [8,9] which may be added to ¯oat glass to improve durability. In a silicate glass the sodium is associated with the non-bridging oxygen atoms in the structure (i.e., BSiAOANa) and are easily exchanged for hydrogen by one or both of the mechanisms described above. It has been shown [10] that addition of alumina provides alternative sites for sodium in the glass structure. Sodium bound to AlO4 tetrahedra (i.e., O3 AAlAOANa) is much more dicult to exchange. The penetration of water and hydrogen ions into the surface of glass has been investigated using several techniques. The resonant nuclear reaction between 15 N-ion and 1 H has been successful in following water penetration up to several mi [11]. crons deep with a resolution of about 40 A Secondary ion mass spectroscopy (SIMS) [12] is also sensitive to the distribution of hydrogen near the surface. The drawback for both these techniques is that the glass sample is in a vacuum and

so some loss of water from the near surface region may occur. For this reason they have tended to be used on glass surfaces that have had exposure to water at elevated temperatures so that rather deep penetration of the water occurs. In this work we have used neutron re¯ection to study the early stages of the penetration of water into glass under various conditions. The technique is capable of determining the distribution of hy although drogen with a resolution of about 30 A more detailed information is determined by ®tting models. An important advantage is that the technique is non-invasive so the glass samples may be studied in ambient conditions. Any water-containing layers should be in quasi-equilibrium with the surroundings. The experimental methods that we have used to measure and analyse the re¯ectivity are described in Section 2 below. Several aspects of water penetration into ¯oat glass have been addressed in this work. We have measured the time evolution of the water-rich layers in glass in contact with water at room temperature. We have also looked at the penetration of water from a saturated humid atmosphere as a function of temperature, and the more dramatic e€ects observed when the ¯oat glass is exposed to a combination of elevated pressure (up to 120 bar) and temperature (150 °C). The e€ect of alumina on the penetration of water into the surface has also been examined by comparing ¯oat glasses with di€erent aluminium contents. Finally the time resolved exchange of D2 O and H2 O on the surface of ¯oat glass has been investigated. 2. Experimental method 2.1. Neutron re¯ection instruments The neutron re¯ection measurements reported here were performed on the re¯ectometers CRISP [13] and SURF [14] at the ISIS facility of the Rutherford Appleton Laboratory (Oxfordshire, UK). In these experiments a well-collimated neutron beam is brought onto the ¯at surface of a sample at a glancing angle of less that 2° and re¯ected into a detector. The beam is polychromatic, containing a range of neutron wavelengths from

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 As ISIS is a pulsed source, the 0.5 to 6.5 A. wavelengths of the re¯ected neutrons are determined by their time-of-¯ight from source to detector. The magnitude of the scattering vector, Q, may then be calculated from the glancing angle of incidence, #: Qˆ

4p sin # ; k

…1†

where k is the wavelength. In practice three incident angles were used in this present study (0.3°, 0.8° and 1.6°) to cover a Q  1 . It generally took range from 0.01 to 0:83 A about 2±3 h to cover this range and the re¯ection was dominated by the incoherent background  1 . The re¯ected intensity as a above Q ˆ 0:25 A function of time-of-arrival were normalised for the incident beam spectrum and converted to re¯ectivity vs Q using a standard data reduction routine (QUICK) provided by the ISIS laboratory. The measurements at the three separate angles were subsequently combined into a single re¯ectivity pro®le using a standard program (COMBINE). 2.2. Theoretical background In this work we wished to demonstrate that we had determined uniquely composition pro®les from the re¯ectivity data. This has involved several extensions of the usual data analysis practice and so the necessary background is outlined in the following sections. The neutron re¯ectivity from a macroscopic interface is determined by the composition of the two bulk phases near the surface. The re¯ection of neutrons is closely analogous to the re¯ection of visible light from a surface and the standard expressions from optics may be used to describe the variation of neutron re¯ectivity with Q. However the refractive index, l, for neutrons is generally slightly less than unity, and is given by the formula lˆ1

k2 q ; 2p

…2†

where q is the scattering length density of the material. The scattering length density is deter-

95

mined by the composition of the phase (de®ned by the number density nj of the di€erent atom types j) and the neutron scattering lengths bj of the nuclei. The number density can be calculated if the fraction by weight wj of each atom type and the bulk density, D, of the material are known: qˆ

X j

nj bj ˆ DNA

X w j bj ; Aj j

…3†

where NA is the Avogadro number and Aj is the relative atomic mass of the constituent atoms. At low scattering vectors there is (generally) total external re¯ection of the neutron beam with the critical Q value (QC ) determined by the di€erence in the scattering length density of the two bulk phases: p QC ˆ 4 pDq: …4† Above the critical edge, the re¯ectivity R…Q† decays rapidly with increasing Q as predicted by the Fresnel equation:  2 Q p Q2 Q2C 16p2 Dq2 p  R…Q† ˆ : …5† 2 Q ‡ Q2 QC Q4 The intensity decays with approximately Q 4 dependence. However this decay is modi®ed by variations in the scattering length density with depth, in the interfacial region. In order to observe these details in the re¯ectivity it is convenient to plot the data as R…Q†Q4 , hence allowing these features to be separated from the steep decay. In the following paragraphs the methods used to extract structural information from the neutron re¯ectivities are outlined. 2.3. Isotopic substitution method A single re¯ectivity measurement does not normally provide a unique determination of the scattering length density pro®le of an interface. This is due to the loss of phase information inherent in all scattering experiments. However, it is well established that the number density distribution of di€erent components near an interface can be established uniquely using the method of isotopic substitution to determine partial structure

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factors [15]. This technique utilises the variation of neutron scattering length from di€erent isotopes of the same atom and enables the contribution to the re¯ectivity from one (or more) of the elements within the sample to be changed. The scattering length density q of a two-component system (such as `water' and `glass') is determined by their number density distributions as a function of distance from the interface (nW …z† and nG …z†) and the scattering lengths of the two components (bW and bG ). The scattering length of air is zero, hence it can be omitted. q…z† ˆ bW nW …z† ‡ bG nG …z†:

…6†

This two component structure can be described in terms of three partial structure factors hij : 2 hWW …Q† ˆ n~W …Q† ; 2 hGG …Q† ˆ n~G …Q† and h i nG …Q† ; …7† hWG …Q† ˆ Re n~W …Q†~ where n~i …Q† is the Fourier transform of the number density distribution of species i. The two `selfterms' are determined solely by the number distribution of a single species and the `cross-term' is determined by both number distributions and their relative displacement. In the kinematic approximation the re¯ectivity of this two-component system may be expressed as follows: R…Q† 

16p2 2 bW hWW …Q† ‡ b2G hGG …Q† Q2
‡ 2bW bG hWG …Q† :

…8†

A common and very successful strategy to unambiguously determine the structure of the surface, is to isolate the di€erent partial structure factors by making three re¯ectivity measurements using three isotopic substitutions that give di€erent contrasts between the components. For instance if bW ˆ 0, the re¯ectivity measured would determine hGG …Q† and measurements of identical samples with two other values of bW would allow hWW …Q† and hWG …Q† to be determined by simple arithmetic. These three partial structure factors would de®ne the structure uniquely.

The kinematic approximation is only valid for re¯ectivity values less than approximately 0.1, hence it cannot be used to analyse re¯ectivity pro®les which have important features close to the critical edge. Although there are methods to correct re¯ectivity data near the edge [16], it did not prove possible to use them in this work because the e€ects of instrumental resolution are also most apparent in the region just above the critical edge. For these reasons, the standard analysis route of determining partial structure factors has been avoided. However isotopic substitution, combined with the data analysis strategy described below, has been used to determine a unique structure from the re¯ectivity measurements in this work. 2.4. Data analysis strategy In this work the re¯ectivity has been calculated directly from the scattering length density using the optical formalism originally derived by Abeles [17]. The normal approach is to model the surface as a series of strata of di€erent scattering length density. However, since the aim was to calculate the re¯ectivity from several di€erently labelled versions of the same water and glass distributions some modi®cation of the calculation was made. It was generally found possible to model all the re¯ectivity pro®les by using three layers to represent the water number density distribution and three to describe the glass near the surface. The near-surface glass distribution is de®ned by the thicknesses of the glass layers, their mean number densities and their interlayer di€usenesses and a similar set of parameters de®ned the water distribution. Hence the scattering length density pro®le near the surface can be de®ned in terms of the glass distribution parameters, the water distribution parameters and one further parameter which is the displacement of the water distribution relative to the glass. Since all these parameters were allowed to vary when ®tting a model to the re¯ectivity data, there is no arti®cial constraint on whether the water distribution falls inside or outside the glass distribution. From the introduction, it appears that a full study of water penetration into glass would involve at least four components: hydrogen, oxygen,

R.M. Richardson et al. / Journal of Non-Crystalline Solids 292 (2001) 93±107

sodium and another to represent the remaining glass components (i.e., calcium, magnesium, and aluminium). Hydrogen and oxygen should be treated separately so it is possible to distinguish between H2 O and H3 O‡ , and the sodium is labelled to ®nd out whether it is the component of the glass being removed. However to determine the four number density distributions, 10 separate measurements of di€erently isotopically labelled samples (including oxygen and sodium isotopes) would be required. This would be prohibitively expensive and numerically unreliable and since there is a strong evidence from other techniques that it is the hydrogen-containing species (i.e., one or more of H‡ ; H2 O; H3 O‡ ) that are replacing the sodium ions, this study was focussed on the penetration of these species. In this work the scattering contribution of water has been varied by substituting D2 O for H2 O or by using a mixture of the two. It is assumed that the two isotopes have the same interactions with glass. The scattering lengths of D2 O and H2 O are very di€erent  bH O ˆ 1:68  10 5 A).  (bD2 O ˆ 19:15  10 5 A; 2 For glass, we used the scattering length of an average formula-unit (e.g., SiO2 or Na2 O). This average scattering length and the corresponding number density were calculated using the formula bglass nglass

, X wk X wk bk ˆ and Ak Ak k k X wk ˆ DNA ; Ak k

…9†

where the sums are over the di€erent components of the glass. For ¯oat glass we found these formulae gave values of  and bglass ˆ 14:7  10 5 A  3: nglass ˆ 0:0257 A

…10†

It can be seen, by comparing the values of b, that the incorporation of H2 O or D2 O into a glass surface will give a signi®cantly di€erent neutron re¯ectivity. The results were initially analysed assuming that the re¯ection was from a two-component system (molecular water and glass) and the implications of di€erent speciation are discussed in

97

the appropriate section. For a two-component system three di€erent contrasts will provide enough information to yield a unique set of number density pro®les. The re¯ectivity pro®les were analysed by simultaneously ®tting the same water and glass number density pro®les to the re¯ectivity data from all the contrasts. Typical contrasts used were H2 O, D2 O, null re¯ecting water (NRW) and `optimum' (OPT). NRW is a mixture of D2 O and H2 O [molar ratio of 11.4:1 H2 O:D2 O] that gives zero scattering length density, hence it does not contribute to the re¯ectivity. The purpose of using NRW was to enable the e€ect of water on the glass itself to be investigated without an additional contribution from water. Optimum water is calculated so as to minimise the errors introduced from uncertainties in the scattering lengths of each of the components when extracting partial structure factors by the use of an inversion matrix method [18]. In the present study D2 O and H2 O are in the ratio 1:1.53. The e€ects of instrumental resolution and incoherent background were also incorporated in the calculated re¯ectivities. In practice, it was generally found that it was sucient to use two contrasts e.g., NRW and D2 O or NRW and H2 O to de®ne the number densities uniquely by ®tting to the re¯ectivity results. The process of ®tting model number density distributions to the two or three re¯ectivities was started by assuming the simplest possible distributions, such as a single water layer and a layer of reduced number density of glass on a substrate with normal bulk number density. This model was then re®ned by least-squares methods. If the weighted sum-of-squares of the residuals was comparable with the number of data points and the ®t and residuals appeared satisfactory, the model was accepted. Otherwise, the model was augmented by adding another layer and the process repeated. In general three layers were required to model the glass and three to model the water pro®le. Since several re¯ectivities were ®tted simultaneously there is no doubt that the correct set of model number density distributions has been found using this method since, in e€ect, the three partial structure factors were determined; an alternative set that gives the same re¯ectivity for all contrasts is extremely unlikely. However the

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question remains as to what minor variations of the number densities could be tolerated without signi®cantly changing the quality of the ®t. We have attempted to address this question using the method outlined in the paragraph below. It is a simple extension of the standard practice for ®tting re¯ectivity that gives number density pro®les with error bars. First the best ®t was found, as described above, by minimising the weighted sum of squares: X  Robs …Q† Rcalc …Q† 2 2 v ˆ ; …11† rR where Robs and rR are the measured re¯ectivity and its error and Rcalc is the calculated model re¯ectivity. The value of v2 was then sampled for parameter values near their best-®t. For each value of v2 the probability of that model being true was calculated from the fractional area under the upper tail of the v2 distribution using the CHISQCDF function in the IMSL Library (IMSL Mathematics library, http://www.vni.com/products/imsl/imslfort.html): Probj ˆ 1:0

CHISQCDF…v2 ; d†;

…12†

where d is the number of data points less the number of variable parameters. The mean and the standard deviation of the number density distribution were then calculated: P j n…z†j Probj ; n…z† ˆ P j Probj …13† P 2  …n…z† n …z†† Prob j j j P r2n…z† ˆ : j Probj The mean distribution should normally coincide with the best-®t distribution and the standard deviations can be used as error bars when presenting the distributions. A similar approach has recently been used [19] to analyse X-ray re¯ection data.

3. Experimental methods and results Flat glass samples prepared by the ¯oat glass process [20], and supplied by Pilkington (Lanca-

shire, UK), have been used to study the structural changes which occur at the surface of glass on exposure to aqueous environments. During the ¯oat process one side of the glass is in contact with a molten tin surface, consequently this side becomes enriched in SnO2 ; thus the two surfaces are not equivalent. All the measurements discussed in this paper have been made on the ®re polished surface (i.e., non-tin side). Several sets of samples have been used to elucidate di€erent aspects of the water/glass interaction. For each set, comparison has been made between two or three samples cut from the same piece of glass and treated identically apart from the substitution of isotopically labelled water (e.g., H2 O, D2 O or a D2 O=H2 O mixture). For the neutron re¯ection experiments, the samples were contained in a hermetically sealed stainless steel box with silica windows; the sample box was designed such that both temperature and atmosphere could easily be changed. In this investigation, the scattering length density of the ¯oat glass was calculated using typical compositions determined by Pilkington and the density was found (by weighing blocks of a known size) to be 2:52 g cm 3 . The main constituents of the glass samples used in this investigation are SiO2 (72.1 wt%), Na2 O (13.5 wt%), CaO (8.5 wt%) and MgO (3.9 wt%) which give a calculated scattering length density (qˆnglass bglass ) of 3:77  2 . However values determined experimen10 6 A tally from the position of the critical edge (QC =16p) were always found to 2be in the range of  . We believe this 3:50  10 6 and 3:70  10 6 A discrepancy of up to 7% arises because the critical angle is actually only sensitive to the glass sample within 1 lm of the surface (due of beam attenuation). It suggests that there is a layer of reduced density between 0.1 and 1 lm thick at the surface of the ¯oat glass which reduces the critical angle but is too thick to resolve as fringes in the re¯ectivity data. A likely explanation for the di€erence between the bulk and surface density is that the ®re polishing of the glass surface in a reducing atmosphere is believed to remove material such as sodium from the immediate surface. The number density was allowed to vary during the ®tting procedure; in all cases the re®ned value of nglass was marginally less than the calculated one (Eq. (10)).

R.M. Richardson et al. / Journal of Non-Crystalline Solids 292 (2001) 93±107

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No attempt was made to remove the upper surface layer by abrasion or chemical etching as this would lead to undesirable roughening of the surface and possible chemical contamination. 3.1. The e€ect of immersion time on the penetration of water into the ¯oat glass surface The ingression of water into the glass surface was investigated as a function of immersion time. Four pairs of ¯oat glass samples were prepared for the experiment by soaking one in distilled H2 O and its `twin' in D2 O. The samples were immersed in water in closed polythene containers, making sure the top ®re polished surface did not come into contact with any part of the container. The water was mildly acidic (pH  5±6) due to absorption of atmospheric carbon dioxide. The soak time was varied from 1 week to 6 months, at ambient temperature. During the re¯ectivity measurements the samples were placed into the sealed environmental box containing a few cm3 of the appropriate water so that 100% relative humidity (RH ) could be maintained, and consequently eliminate evaporation losses from the glass surface. A total of ®ve samples were studied, the four described above and a pair of untreated ¯oat glass samples. The neutron re¯ectivity data for the samples soaked in H2 O and D2 O for 1 month are shown in Fig. 1. The ®ts to the data were calculated using a modi®ed standard slab model (Section 2.4). All the samples exhibit similar trends to those displayed in Fig. 1, including the di€erence in the magnitude of the re¯ection between the H2 O and D2 O soaked sample. This is due to the large positive scattering length associated with the D2 O, compared to the small negative contribution from the H2 O. The water and substrate number density pro®les, n…z†, for the 1 month soaked glass shown in Fig. 2 have been extracted from the ®t of the re¯ectivity data in Fig. 1. The error bars in Fig. 2 represent the standard deviation in the calculated ®t (determined using the method described in Section 2.4), and indicate typical levels of uncertainties for all the number density distributions in this paper. Fig. 2 shows the number density pro®le of water in the 1 month soaked sample exhibits two distinct

Fig. 1. Re¯ectivity data (plotted as R…Q†Q4 vs Q) for ¯oat glass immersed in D2 O (closed squares) and H2 O (open circles) for 1 month. Lines are the best ®ts arising from modelling the re¯ectivity data.

Fig. 2. The number density pro®les n…z† for the ¯oat glass immersed in water for 1 month. The error bars represent the standard deviation of the ®tting process.

regions. These trends are seen for all the immersion times as shown in Fig. 3. The ®rst layer starts  thick, and at the origin, is approximately 30 A appears almost constant with immersion time. It is interesting to note that the data analysis procedure described above has determined that this waterrich layer is de®nitely inside rather than outside the glass. In our original communication of these results [21], which used a standard analysis procedure, this was only a reasonable assumption. The second region extends further into the glass; the water penetration distance increases systematically as the soak time becomes longer, after 6 months this can be seen to extend to a total of  as Fig. 4 shows. Since the number density 500 A  3 , the proof molecules in pure water is 0:033 A ®les can be interpreted in terms of the water forming a thin layer of large volume fraction

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deep within the glass and the formation of a low density layer near the surface such as a gel [3,22]. The number densities determined for the pair of untreated glasses were identical within experimental error, which demonstrates the uniformity of the ¯oat glass. 3.2. The e€ect of water vapour at elevated temperatures on the surface of ¯oat glass Fig. 3. The number density pro®les n…z† of ¯oat glass immersed in water for 1 week, 1 month, 2 months, and 6 months, compared with an untreated surface.

Fig. 4. Layer thickness plotted as a function of immersion time with lines of best ®t. The surface layer (closed squares) remains  approximately constant at 30 A.

(40%) near the glass surface and a second layer of lower volume fraction (10%) penetrating deeper into the bulk sample. At large depths the number density of `average' glass molecules tended to a constant value that was marginally less than expected from the density and composition of the glass (Eqs. (9) and (10)). However, nearer the surface the water penetration is accompanied by a loss of material from the glass. This is manifested as a reduction in the  3 at depths number density of about 0:003 A corresponding to the maximum water penetration and greater reductions near the surface. The contribution made by sodium to the total number density of glass is nNa ˆ DNA

wNa  3: ˆ 0:006 A ANa

…14†

The observed drop in the glass number density is consistent with an exchange of sodium for water

The aim of this experiment was to compare the e€ects of exposure to water vapour with those of immersion in bulk water. It was anticipated that the build up of leached sodium at the surface might lead to an additional mechanism for attack on the glass but the results showed that the main in¯uence was the temperature of the water vapour. A single piece of ¯oat glass was placed in the sealed environmental box in a 100% humidity D2 O atmosphere. The re¯ectivity from the ®re polished side of the sample was then measured at a temperature of 25 °C. The sample box was then heated and maintained at 40 °C where a further measurement was taken after allowing a period of time for the system to settle. Further re¯ectivity pro®les were obtained for temperatures of 60 and 80 °C after which the temperature was reduce to 60, 40 and 25 °C at which measurements were also taken. The duration of each experiment was approximately 2 h so the cumulative e€ect of exposure of the glass surface to a heated atmosphere was observed. This procedure was then repeated on two further glass samples using 100% RH atmospheres of optimum and null re¯ecting water (Section 2.4). As for the soaked samples, the re¯ectivity data sets for each water contrast have been modelled simultaneously in order to obtain unambiguous number density pro®les for the water and glass components of the system. Typical pro®les for 80 °C are shown in Fig. 5. The water penetration into the glass surface is clearly seen to grow from 130  as time and temperature increased to 350 A (Fig. 6). Little variation in penetration depth was observed on heating up to 60 °C or on cooling from 60 °C as might be expected if rapid ingress of water only occurred at high temperature. This suggests that at 80 °C the system may have been changing throughout the course of the two-hour

R.M. Richardson et al. / Journal of Non-Crystalline Solids 292 (2001) 93±107

Fig. 5. Re¯ectivity data (plotted as R…Q†Q4 vs Q) for ¯oat glass in a 100% RH atmosphere at 80 ° C. Lines are the best ®ts arising from modelling the re¯ectivity data.

Fig. 6. The cumulative e€ect of temperature and time on the number density pro®les of ¯oat glass exposed to a heated atmosphere of 100% RH water. The sample temperature had been stepped up and down (25, 40, 60, 80, 60, 40, 25 °C) as described in the text and the ®gure shows results from the sample at 40 ° C (1) during the upward steps, at 80 °C and at 40 °C (2) during the downward steps.

experimental1 run. However the re¯ectivity at low  ) is collected quickly at the beginning Q (<0.1 A of a measurement, with most of the run time recording high Q data, where relatively little information was gained. The deep penetrating layer in Fig. 6 corresponds to data measured at low Q, so the accuracy of this result should not be e€ected by changes over time. The scattering length pro®le of the glass did not vary with temperature and time within experimental error.

101

a combination of high temperature and pressure. Di€erent heating cycles were used in which the time the sample was maintained under pressure at 150 °C was varied. The total times used for the heat cycle from ambient temperature to 150 °C (corresponding to a water vapour pressure of 120 bar) and back again were 40, 55, 75, 99 and 150 min. Heating was carried out in D2 O on separate pieces of ¯oat glass cut from the same sheet. After heating the samples were brie¯y washed with D2 O to remove any loose material which may have accumulated on the surface as a result of the treatment. The samples were subsequently placed into a 100% RH D2 O atmosphere in the sealed sample box. The re¯ectivity of the ®re polished side of the glass was measured as in previous experiments. This procedure was then repeated for samples heated in H2 O. During the 150 min heating cycle a visible layer of soft gel like substance formed at the sample surface which was easily washed away. This is likely to be a result of the destruction of the silica network and the formation of a loosely bound silica gel as observed by Rudd et al. [22]. The modelled number density pro®les of glass and water are shown in Fig. 7 for the samples heated for 55, 75 and 150 min. A considerable change in the number density pro®les can be seen between 150 and 75 min. The thickness of the layer of low scattering length density material at the surface of the glass is reduced in size from 700 to  This probably corresponds with the ob300 A. served loss of material at the surface immediately after heating. The depth of penetration of the

3.3. The e€ect of water at elevated pressure and temperature on the surface of ¯oat glass A Parr 251M 4250 series titanium reactor vessel was used to expose a series of ¯oat glass samples to

Fig. 7. The number density pro®les n…z† of water and glass for samples heated for 55, 75 and 150 min up to a temperature of 150 °C and a pressure of 120 bar.

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Fig. 8. Re¯ectivity (plotted as R…Q†Q4 vs Q) for ¯oat glass heated for 55 min (open symbols) and 150 min (closed symbols) in D2 O (squares) and H2 O (circles). Lines are the best ®ts arising from modelling the re¯ectivity data.

Fig. 9. Re¯ectivity (plotted as R…Q†Q4 vs Q) for ¯oat glasses with 1.67 wt% (squares, not shifted), 1.02 wt% (circles, shifted by 0.05) and 0.17 wt% (triangles, shifted by 0.1) alumina (Al2 O3 ) immersed in D2 O and null re¯ecting water for 1 month. Lines are the best ®ts arising from modelling the re¯ectivity data.

adsorbed water layer is also reduced by the same amount. This change results in a signi®cant difference in the observed re¯ectivity, especially at Q  1 as shown in Fig. 8. values of less than 0:016 A The modelled number density pro®les of the data for samples heat treated for 40 and 99 min are similar to those obtained for 55 and 75 min, the most signi®cant change being a systematic reduction in the thickness of the low scattering length density layer at the surface of the glass. 3.4. The e€ect of alumina content on the incorporation of water at the ¯oat glass surface In order to investigate the e€ect of alumina content on the durability of the ¯oat glass surface, pairs of ¯oat glass samples containing 0.17, 1.02 and 1.76 wt% alumina (Al2 O3 ) were immersed in NRW and D2 O in sealed polyethylene containers for 1 month. For each re¯ection measurement the samples were placed in a 100% RH atmosphere of NRW or D2 O as appropriate in the sealed environmental box in order to prevent evaporation from the surface. The data were ®tted simultaneously as in previous experiments. The ®ts are shown in Fig. 9 with resulting corresponding number density pro®les in Fig. 10. It can be seen in Fig. 9 that a fringe appears in the data at low  1 for the samples immersed in D2 O, Q  0:03 A this feature moves to lower Q as alumina content decreases. This suggests the presence of a thick D2 O layer getting thicker.

Fig. 10. The number density pro®les n…z† of water and glass for ¯oat glass samples with varying alumina content immersed in water for 1 month.

As with previous experiments the number density pro®les of the water and glass exhibit two  thick at distinct water layers, a thin layer 30 A the glass surface and a second, deeper, more penetrating layer. The thickness of the deep layer increases with decreasing alumina content, this corresponds to the reducing Q position of the fringe with decreasing aluminium content, observed in the re¯ectivity data in Fig. 9. 3.5. Kinetics of exchange of D2 O for H2 O on a sample immersed in H2 O for 6 months The mobility of the adsorbed water at the glass surface has been investigated by substituting H2 O with D2 O in a pre-treated sample. This was achieved by placing a sample immersed in H2 O for

R.M. Richardson et al. / Journal of Non-Crystalline Solids 292 (2001) 93±107

6 months into a 100% RH atmosphere of D2 O and monitoring the change in the re¯ectivity with time. In order to increase the time resolution of the experiment, the time for which data was collected in each measurement was reduced from the standard duration of 120 to 20 min by reducing the counting time and the Q range. As a result the statistics of each measurement were signi®cantly poorer than might be expected from longer run times. The re¯ectivity was measured initially in an H2 O atmosphere and then exposed to a D2 O atmosphere. After 2 h its re¯ectivity was monitored at four 20 min intervals. The re¯ectivity had changed dramatically from that in the H2 O atmosphere but the changes were relatively small during this period of monitoring. The experiment was therefore completed by making an `in®nite time' measurement by immersing the sample in D2 O for 30 h and then measuring its re¯ectivity in a D2 O atmosphere. Some of the resulting re¯ectivity data are shown in Fig. 11. A large initial increase was observed in the overall re¯ectivity as D2 O exchange occurs because of the greater scattering power of the deuterium. The most signi® 1 , where the cant changes occur at low Q, <0.1 A feature resulting from the deeply penetrating water layer is observed. This suggests that the D2 O is exchanging with the H2 O in the deep layer as well as the near surface and that the lifetime of a water molecule in the glass is a few hours. However, it is not possible to extract the number density pro®les of the water and glass components from these measurements as only one contrast was used. This

Fig. 11. The time resolved re¯ectivity (plotted as R…Q†Q4 vs Q) for ¯oat glass soaked in H2 O for 6 months (closed squares), then placed in a saturated atmosphere of D2 O for 176 min (open circles) and 2069 min (closed triangles).

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is further complicated as a scattering length cannot be assigned to the water as it was initially in two separated parts, the D2 O in the atmosphere above the glass and the mixture of D2 O and H2 O within the glass. 4. Discussion It is well established that the rate at which the water attacks the surface of glass depends on the composition of the glass and the aqueous environment. Furthermore, the interpretation of experimental results is complicated as the process can involve both composition and structural changes. Most existing work on the ageing of glass surfaces has been made on samples subjected to non-ambient environments or measurement conditions (e.g., high temperatures or vacuum). In this work, we aimed to investigate the e€ects of less extreme conditions that are applicable to normal conditions of storage and usage of glass. In this section we will interpret our results from neutron re¯ection experiments in the light of experiments using other methods. The neutron re¯ection results have con®rmed that water readily di€uses into the surface of the glass when immersed in water and this is accompanied by a signi®cant decrease in the density of the glass. Water was also found to penetrate into the surface of ¯oat glass after a sample was placed into an environment of 100% saturated relative humidity but there was relatively little decrease in the glass density. Many experiments have established that the interaction of water with a glass surface produces an exchange of cations in the silica network with hydrogen ions in the water. For instance, Koenderink et al. [23] studied the leaching dependence of RO=Na2 O=SiO2 glasses where R is an alkali earth ion. Barium was found to be the most reactive towards water, whereas calcium and magnesium the least; they observed no depletion of Ca or Mg after ageing. This implies that any leaching from the ¯oat glass surface in the experiments described here will be due to the removal of (predominantly) sodium ions. However there is a second process that can take place as water enters the

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glass. The penetrating water is sometimes accompanied by expansion of the network which also results in a surface layer with reduced density because the formation of some SiAOAH groups requires some motion or relaxation of the silica network [24]. This rearrangement of the surface layer can allow for the uptake of additional water, which can produce a swollen surface layer often referred to as the `gel layer'. We have identi®ed evidence for both processes (simple leaching and gel layer formation) in our results from glass immersed in water. The penetration depth for each `soak time' is approximately equal to the thickness of the reduced density glass (e.g., as shown in Fig. 3). The experimentally observed reduction in the number density of glass in the deep layer (Dnglass   3 ) is rather less than the reduction that 0:003 A would be expected if all the sodium were removed  3 ) which suggests a from the glass (nNa  0:006 A partial removal of sodium. The number density for water in this region is also of the same magnitude  3 ) which strongly suggests that (nwater  0:003 A the water is replacing the sodium. However such an exchange would takes place by replacing sodium with hydroxonium and sodium and hydroxyl would di€use out: network-Na‡ ‡ H2 O ‡ ˆ network-OH‡ 3 ‡ Na ‡ OH

…15†

which suggests that the water exists as hydroxonium in the deep layer. We therefore repeated the analysis of the re¯ectivity data assuming that the two components were glass and hydroxonium. The quality of ®t and the resulting number density pro®les were very similar except that the number density of hydroxonium was 2/3 3that of molec † which is also ular water (Dnhydroxonium  0:002 A consistent with partial substitution of sodium in the deep layer. However near the surface the reduction in nglass is greater than the sodium density and this suggests that the glass network has expanded to form a gel. Many investigations have been made into the kinetics of extraction of alkali metal from di€erent types of glass. Doremus [4], Lanford et al. [11],

March and Raunch [25] found that the alkali ions are removed from the glass as the square root of time. In comparison the ¯oat glass soaked in water  was formed showed that a layer of about 200 A rapidly then the thickness of the layer increased linearly with immersion time (Fig. 4). In order to compare the thickness measured in this work with those found by previous workers at higher temperatures we have constructed an Arrhenius plot of water penetration depth against temperature for a constant immersion time. This is shown in  is obFig. 12. At 298 K, a di€erence of 100 A served between the measured neutron re¯ection data and the penetration depth extrapolated from the high temperature data. The slope of the line including the neutron re¯ectivity data indicates that the activation energy is about half that calculated from high temperature data alone. This is consistent with the penetration being controlled by more than one process. The time dependence of the water penetration are consistent with existing work on the extraction of alkalis reviewed by Paul [8]. A square root of time leaching dependence generally occurs for short times (<200 h) and low temperatures (<90 °C) but a linear relationship is found between Na‡ extraction and time, for long times and/or high temperatures. Leaching is not just con®ned to the ion exchange between the alkali and hydrogen; corrosion of glass is due to the simultaneous extraction of both sodium and

Fig. 12. An Arrhenius plot of water di€usion depth against the inverse of hydration temperature for a constant immersion time. The open circles are data recorded by Lanford et al. [11] for 14 h immersion. The solid square is the result from neutron re¯ectivity at 298 K extrapolated to 14 h immersion. The lines are best ®ts to the data including (thick line) and not including the neutron re¯ectivity point.

R.M. Richardson et al. / Journal of Non-Crystalline Solids 292 (2001) 93±107

silicon (as silicate groups) ions. Helmich and Raunch [26] used nuclear reaction analysis to measure the hydration of silica glass on exposure to water vapour and found a constant hydrogen concentration with varying treatment time at the surface, which suggests saturation, and at the greater depths the H content slowly increased with time. A similar, but much more dramatic, trend was observed in the number density pro®le of water for the ¯oat glass as shown in Fig. 3. The high water content at the surface probably corresponds to the formation of a gel layer. External factors such as temperature and pressure play an important role in the chemical stability of the glass surface. Upon exposing the surface of ¯oat glass to water vapour at di€erent temperatures, the number density pro®les in Fig. 6 exhibited similar trends to the soaked sample. The main di€erence being that although water penetration increased with temperature the number density of the glass remained constant with very little density reduction even near the surface. This may be a consequence of the glass not being in direct contact with a reservoir of water. Hence ion exchange cannot take place freely. However, the neutron re¯ection results showed that there was not a signi®cant build up of leached ions at the surface. Raising the temperature of the water vapour increases the penetration depth of the water. This penetration becomes more rapid as the temperature increases above 60 °C; the relationship between the temperature and the depth of water ingression is not linear. It is most likely that in the glass exposed to hot water vapour, most of the water in the deep layer is in the form of molecular water rather than the H3 O‡ since no cation exchange has taken place to remove the sodium. When the samples are exposed to more extreme combinations of pressure and temperature, the reaction is more rapid (Fig. 7). The high water number density very near the surface (i.e., within  corresponds to a nearly saturated the ®rst 50 A) water layer: a gel layer. This layer has a number density almost twice that of the soaked samples. After a treatment time of 150 min the water content of this initial layer is seen to reduce, however this can be accounted for as some of the layer was seen to be washed o€ prior to the re¯ection

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experiment. The water penetration of the deep layer reduces with treatment time. Lanford et al. [11] suggested that in addition to the di€usion of water into glass there is also dissolution (etching) of the surface which at long hydration times continuously diminishes the thickness of the leached layer. Our results con®rm this scheme. By increasing the temperature and pressure we have e€ectively rapidly aged this sample, so the ¯oat glass is being removed by the harsh environment. It is well known that the chemical durability of glass can be improved by lowering its alkali content, decreasing the ionic radius of the alkali ion, or by adding elements such as divalent oxides and CaO [8]. The addition of alumina (Al2 O3 ) to the ¯oat glass samples reduces the hydration of the surface (Fig. 10). In Scholzes' [3] survey on glass± water interactions, he summarised that the amount of water that enters the surface depends on the space available in the glass network. As water adversely e€ects the chemical durability a tight glass structure is required. The results in section 3.4 have suggested that by increasing the alumina content in the ¯oat glass there is indeed a reduction in the penetration of water as would be expected from a tighter structure. However the magnitude of the e€ect is rather small and seems unlikely to account for the large di€erences in durability that have been reported [2,8]. The stabilising e€ect of alumina is most probably due to another mechanism which is the binding of sodium ions to AlO4 ions, making leaching more dicult [10]. The time resolved isotopic exchange between H2 O and D2 O, on the surface of ¯oat glass soaked in H2 O for 6 months, occurs rapidly implying that the water is very mobile throughout the leached layer (Fig. 11). The substantial uptake of D2 O happened quickly (within few hours) but the e€ect had not saturated within this period. The high mobility of water suggests that leaching is not limited by the transport of water within the leached layer. 5. Conclusion The e€ect of water on the surface of ¯oat glass has been investigated using the technique of neutron

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re¯ection. It has been shown that the amount of water adsorbed into ¯oat glass, and its depth of penetration depends on time, temperature and composition. A picture emerges in which several di€erent processes can take place at the glass surface. The overall e€ect of the water±glass interaction depends on the conditions applied. The hot vapour experiments have shown that it is possible for molecular water to penetrate rapidly into the surface (up to a depth of several hundred  Angstroms) without signi®cant leaching of ions from the glass. At room temperature, however, the penetration and leaching proceed at a similar rate. This suggests that at low temperatures, the leaching process is able to keep up with the penetration but at higher temperatures the water penetration is much faster. Further experiments on the exposure of glass to vapour at room temperature for long periods are required to clarify the role of solvent in the removal of the leached ions. It was found that the water molecules (or hydroxonium ions) were highly mobile in the leached layer. Another point requiring experimental determination is whether this high mobility is also a feature of the water in an unleached layer. The addition of alumina to the glass has a rather minor in¯uence on the water penetration so we must conclude that its in¯uence on the glass durability results from stabilising the sodium ions rather than preventing water ingress. There is a clear need for further experiments where the stabilising element is present in the solution rather than as a component of the glass. Such experiments should help to clarify the mechanism by which these ions oppose attack by water. The break-up of the silica network is another process that takes place at glass surfaces even at room temperature on long exposure to water. At ambient temperatures it shows up as a low glass-density  layer at the surface which remains about 30 A thick. It may be that there is a balance between disruption of the network and the dissolution at the surface that keeps the gel layer at a constant thickness. At higher temperatures and pressures we have observed dissolution of the gel layer. Further experiments to determine the e€ect of water temperature on the thickness of this layer would clarify this point.

The neutron re¯ection method together with isotopic substitution has been shown to be a useful method for investigating the interaction of water with ¯oat glass surfaces. Its high precision and its ability to look at surfaces in ambient surroundings has allowed us to establish a detailed picture of the processes that take place under moderate conditions. Further experiments to improve the details in this scheme are planned in the near future.

Acknowledgements The authors would like to thank Dr John Webster at the ISIS Neutron Source, Rutherford Appleton Laboratory, Oxfordshire, UK for help with the re¯ection experiments. We also wish to acknowledge Professor John E. Enderby for many stimulating discussions. R.M.D. would also like to thank Pilkington Plc for a CASE award and much useful advice. EPSRC are acknowledged for ®nancial support. References [1] H. Chen, J.W. Park, Phys. Chem. Glasses 22 (1981) 39. [2] A. Paul, Chemistry of Glasses, 2nd Ed., Chapman and Hall, London, 1990. [3] H. Scholze, J. Non-Cryst. Solids 102 (1988) 1. [4] R.H. Doremus, J. Non-Cryst. Solids 19 (1975) 137. [5] W. Haller, Phys. Chem. Glasses 4 (1963) 217. [6] B.M.J. Smets, T.P.A. Lommen, Phys. Chem. Glasses 24 (1983) 35. [7] D.R. Wolters, H. Verweij, Phys. Chem. Glasses 22 (1981) 55. [8] A. Paul, J. Mater. Sci. 12 (1977) 2246. [9] A. Paul, M.S. Zaman, J. Mater. Sci. 13 (1978) 1499. [10] J.P. Hamilton, C.G. Pantano, J. Non-Cryst. Solids 222 (1997) 167. [11] W.A. Lanford, K. Davis, P. Lamarche, T. Laursen, R. Groleau, R.H. Doremus, J. Non-Cryst. Solids 33 (1979) 249. [12] R.G. Gossink, Glass Technol. 21 (1980) 125. [13] J. Penfold, Physica B 173 (1991) 1. [14] J. Penfold, R.M. Richardson, A. Zarbakhsh, J.R.P. Webster, D.G. Bucknell, A.R. Rennie, R.A.L. Jones, T. Cosgrove, R.K. Thomas, J.S. Higgins, P.D.I. Fletcher, E. Dickinson, S.J. Roser, I.A. McLure, R.W. Richards, E.J. Staples, A.N. Burgess, E.A. Simister, J.W. White, J. Chem. Soc. Faraday Trnas. 93 (1997) 3899.

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[21] M.R. Lovell, R.M. Dalgliesh, R.M. Richardson, A.C. Barnes, J.E. Enderby, B. Evans, J.R.P. Webster, Phys. Chem. Chem. Phys. 1 (1999) 2379. [22] G.I. Rudd, S.H. Garofalini, D.A. Hensley, C.M. Mate, J. Am. Ceram. Soc. 76 (1993) 2555. [23] G.H. Koenderink, R.H. Brzesowsky, A.R. Balkende, J. Non-Cryst. Solids 262 (2000) 80. [24] K.M. Davies, M. Tomozawa, J. Non-Cryst. Solids 203 (1995) 185. [25] P. March, F. Rauch, Glastech. Ber. 63 (1990) 154. [26] M. Helmich, F. Raunch, Glastech. Ber. 66 (1993) 195.