Proton resonance in natural glasses

Journal of Non-Crystalline Solids 67 (1984) 119-126 North-Holland, Amsterdam

119

P R O T O N R E S O N A N C E IN N A T U R A L G L A S S E S * P.J. B R A Y and E.J. H O L U P K A Department of Physics, Brown Umverstty, Prowdence, R 1 02912, USA

Nuclear magnetic resonance can be used to detect protons m natural glasses and determine the amounts of "water" (i.e. hydroxyl groups, molecular water, hydronlum ions) in the glasses After presentation m thus paper of basic concepts and equations governing NMR, and a dtcusslon of water in glasses, initial results for NMR studies of some natural glasses are reported. I. Nuclear magnetic resonance

Nuclear magnetic resonance ( N M R ) has been found to be a valuable tool for studying the microstructural properties of a variety of glasses. A l t h o u g h the theory and experimental methods for N M R studies have been presented elsewhere, a brief s u m m a r y will be given here. The nuclei of most atoms possess a magnetic dipole m o m e n t it which can be expressed as it = g/~01 , 1

where I is the dimensionless spin vector of magnitude [ I ( I + 1)] ~ (the intrinsic spin angular m o m e n t u m L of the nucleus is I h / 2 ~ r ) . T h e scalar I, which is called the " s p i n " of the nucleus, can take on integral or half-integral values, #0 is the Bohr nuclear m a g n e t o n and g is the nuclear g-factor. W h e n the nucleus is placed in a magnetic field H , the interaction between the field and the magnetic dipole m o m e n t creates a set of possible energy levels (the Zeeman energy levels) for the system. The energy of the m t h state, characterized by the magnetic q u a n t u m n u m b e r m, is E m = -g#oHm.

Here m m a y range from - I to I in integral steps, so that there exist 21 + 1 equally spaced energy levels as shown in fig. 1. The energy level diagram shown is for a nucleus with spin I = ½, such as 1H (the proton). Transitions between adjacent energy levels can be effected by electromagnetic radiation whose frequency v0 satisfies the Bohr resonance condition: vo = g l t o H / h .

Only one frequency, v0, called the L a r m o r frequency, is observed in the unperturbed system. In the absence of interactions between neighboring nuclei a narrow reso* Research supported by the NaUonal Science Foundation through grant no. DMR-80-04488. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V

(North-Holland Physics Pubhshung Division)

120

P J Bray, E J Holupka / Proton resonance in natural glasses m

-

-I/2

I AE ffH m-

I/2

Fig 1 Energy level diagram for a spin ~2 system.

nance occurs centered around the resonant frequency. However, two nuclei possessing magnetic dipole moments can interact through the dipole interaction. The interaction has the form H = [p,, "P,2 - 3(Ix," 1"12)(P2" r,2)] r~ 3, where !~1 and 1~2 are the magnetic dipole moments of nuclei 1 and 2 respectively. ~12 denotes the unit vector joining the two nuclei and r~2 is the distance between the nuclei. This interaction will cause a shift in the local magnetic field around the nuclei resulting in a shift of the resonant frequency from v0 to v0 + A %. This frequency shift wdl in turn cause a broading of the resonant hne shape. The detads of the resultant lineshape will be given later on in the paper. In addition to a magnetic dipole moment, nuclei with spin I > 1 possess an electric quadrupole moment, but for ~H nuclei (protons) I = ½. Hence this will not be of concern in this paper.

2. Water in glasses The diffusion of water into natural glass occurs during two different phases of the glass history; (1) during the formation of the glass while in the melt stage and (2) after the glass is formed, absorbing water from its local environment, i.e., atmospheric water or water present in the soil. However, the majority of water present in the glass is due to the melt stage. The diffusion constant after the glass is formed is very small (the diffusion constant characteristic of obsidian glass is 1 × 10 4 microns 2 / 1 0 3 y) [1]. The diffusion of water during this period of the glass history wdl not be considered. Stolper has proposed a model in which the concentrations of molecular water and hydroxyl groups can be approximated from knowledge of the melt environment from which it was quenched [2]. This model is based on the assumptions that all the species considered are energetically equivalent and indistinguishable, and there is ideal mixing of the species. The interchange energy resulting from interchanging species between two lattice sites Is zero, and on the average all species have the same coordination number (all species have on the average the same number of nearest neighbors). The last condition is the quasicrystalline approximation of Guggenheim [3]. The melt species involved are molecular water, H2OmoI (melt), bridging oxygens, O (melt) and hydroxyl groups, OH(melt). The assumption that all species are energetically

P J Bray, E J Holupka / Proton resonance m natural glasses

~0.2

I

I

I

I

I

I

!

I

''|

~tJ o. ¢1

-r O

O.3 hydroxyl

0. I

e.,,

O.3

O .-~0.!

0.1

o c-

.2

/~m01eculor

H20

o

o

E 0o

•

I

I

I

I

I

I

0.1

I

0.2

×a F i g 2 Mole fractnons of H 2 0 a n d O H versus X B = N 1 / ( N 1 + rN2) [21

/o

$

Io

5

to

H gaul:

Fig. 3. H y d r o m u m r e s o n a n c e m nitric acid m o n o h y d r a t e at 90 K [4].

121

122

P J Bray, E J Holupka / Proton resonance m natural glasses

equivalent and indistinguishable implies that the number of bridging oxygens refers to all oxygen types ie., free oxygen, bridging and nonbrldging oxygens. The interaction between the melt species is described by the homogeneous equilibrium, n2Omo I (melt) + O(melt) = 2 O n ( m e l t ) , The equihbrium constant

m 2

m

K1 : (~OH) /(aH20,mol Xa~) ) relates the activities, (a~n, a~20,mol, a~), of the respective species. Assuming that the activities are equal to the mole fraction of each species, the number of molecular water molecules, hydroxyl groups and oxygens can be determined in terms of the known constants K 1, N 1, N 2, and r. K 1 is the equilibrium constant defined above, N1 is the number of moles of water, N z is the number of moles of anhydrous silicate melt which contains r moles of oxygen atoms. Plotting the mole fractions of molecular water and hydroxyl groups against X B = N 1 / ( N 1 + r N 2), agreement with experiment is obtained for 0.1 < k a < 0.3 (fig. 2). Hence this model presents a good argument for the development of molecular water and hydroxyl groups in silicate glass. The occurrence of hydronium in silicate glasses has not been widely observed. However Richards and Smith have detected hydronium using N M R in an acid hydrate which formed a glass [4]. The experimental result appears in fig. 3. Therefore the occurrence of hydronium in natural glass is a possibility.

3. NMR results for natural glasses The three different types of proton configurations of interest are hydroxyl groups, molecular water and hydronium. The hydroxyl group (OH) exists as a single hydrogen atom bonded to an oxygen atom. The oxygen nucleus possesses no magnetic moment, so the proton within the hydrogen nucleus will feel no interaction due to the oxygen. However, it is possible that alkali ions possessing magnetic moments can get close enough to the hydrogen nucleus to interact significantly. This interaction

tO

-3

-2

-1

-,.

I

2

3

Fig 4 OH resonance for sodmm slhcate and c,¢slumslhcate [5].

123

P J Bray, E J Holupka / Proton resonance m natural glasses

can lead to a broadening of the OH resonance. Muller-Warmuth observed this effect in several silicate glasses [5]. No temperature dependence of the proton lineshape or linewidth in silicate glasses containing Na, K and Cs was observed. The proton lineshapes were single lines with slowly decreasing tads (fig. 4). Due to the lack of temperature dependence it was deduced that the resonances were due to protons rigidly attached to the glass network, i.e., no motional narrowing occurred. However, the existence of the tail imphed a weak dipole interaction between the protons and alkali ions, such as Na, K or Cs.

The configurations O

I

H

O-Si-O

I

I

O-Si-O,

R

O

O

I

O

where R Is an alkali atom, lead to a dipole interaction between the hydrogen and alkali nuclei. This explains the occurrence of the tail in the proton resonance. On comparing the hneshapes of sodium silicate and cesium silicate it can be seen that the resonance in cesium sdicate is broader (fig. 4). This agrees with the above since the spin is -~ and 3 for the cesium and sodium nucleus respectively. The strength of the dipole interaction is proportional to the spin of the alkali nucleus, leading to a broader line for cesium silicate (see fig. 4). So we expect to see a resonance similar to that in fig. 4. By studying the lineshape and line width of the hydroxyl resonance it is possible to determine the magnetic environment of the OH groups in the natural glasses.

II I! I I I | I_,

J •'c5

-K)

I "S

3P r's FOR

I 0

II II I ! I !

I 5

Fig 5. Theoretical hneshape for molecular water (sohd line) [6].

iO

GAUSS

124

P J Bray, E J Holupka /

Proton resonance m natural

I

glasses

I

-15

-I0

-5

0

5

I0

15 H-H o

Fig. 6 Proton resonances due to O H groups (narrow hne) and m o l e c u l a r water (broad line) [7]

The water molecule contains two hydrogen nucleL Hence two protons will interact via the dipole interaction. The distance between these protons is approximately 1.58 A, which will lead to a strong dipole interaction. This interaction leads to the "Pake doublet" characteristic of a two spin ½ system [6]. For molecular water at low temperatures, we expect to see something similar to fig 5. At higher temperatures the rotational and translational motion of the molecular water tend to average the dipole interaction to zero, convert-

2O

IO'

/

/

:3 ,::[

~0~ -r

I 20 227

! 25 127

! 30 60

I 3.5 I3

I 40 -23

I 4 5 xlO"LVT -51 TEMP ('C)

Fig 7 Width of m o l e c u l a r water hne as a funcUon of temperature [7].

16©

Fig. 8. Geometry of the h y d r o m u m ion.

P J Bray, E J Holupka / Proton resonance m natural glasses

125

Fig 9 Lme shapes for three spin ~2 nuclei at the corners of an equilateral triangle a, Isolated rigid triangle, b, ltagld triangle broadened by neighbors [9]

ing the Pake doublet to a single narrow line• Bartholemew observed the temperature dependence of the proton resonance lineshape due to the molecular water in silicate glasses containing large amounts of water [7]. Two hnes were observed due to hydroxyl groups and molecular water (fig. 6). The molecular water creates the broad line and the hydroxyl groups create the n'arrow line. The width of the broad line increases at low temperature as expected (see fig. 7). The hydronium ion has been modeled as a tetrahedron, the base being an equilateral triangle with the three hydrogen atoms at each corner (see fig. 8) [8]. This configuration of protons with their hydrogen nuclei should be characteristic of a three spin ½ system. Fig. 9 shows a powder pattern for three spin ½ particles at the corners of an equilateral triangle interacting through the dipolar interaction. The resonance at low temperature for hydronium should m~mic that of fig. 9. Again the hydronium line is subject to the same temperature dependence as that of the moleculer water.

4. Experimental Seven natural glasses were obtained from L.D. Pye of Alfred University. Water contents of the glasses were determined using 1H (proton) N M R . The glasses were first dried in vacuum at 80 o C for four hours in a vacuum oven to remove surface water. The samples were then transferred to 5 m m outer diameter 507-PP N M R tubes. The 1H absorption spectra were obtained using a Bruker WM 250 high field N M R spectrometer operating at 250.133 MHz. All spectra were obtained using the pulse technique with a pulse width of 1.0 #s, a repetition rate of 0.3 s, and 1000 scans. All spectra were taken at room temperature.

•

126

P.J Bray, E.J Holupka / Proton resonance m natural glasses

a.

b.

Fig. 10 Proton absorption hne for a, Thailand tektite; b, Lthyan Desert glass. T = 20 o C.

5. Results There appears in all spectra only one absorption line (fig. 10). This is presumably the narrow line obtained for hydroxyl groups (a similar line can be obtained for molecular water at temperatures suffiently high to activate rotational or translational motion; see fig. 7). It is clear from fig. 10 that the Libyan Desert glass contains significant amounts of hydroxyl groups or molecular water; the response for the Thailand tektite is also reasonably strong. The particular instrument employed for these initial N M R studies does not permit detection of the broad resonance expected for molecular water at ambient and lower temperatures (see figs. 6 and 7). Therefore, it is not possible from these preliminary results to determine the total water and hydroxyl content of the glasses. W o r k is in progress to detect the entire p r o t o n spectrum with other instruments, and to obtain quantitative results for water content by comparison with materials having k n o w n p r o t o n contents in various configurations (i.e. O H and H 2 0 ).

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

I. Friedman, R.L. Smith and W.D Long, Geol. Soc. Amer. Bull. 77 (1966) 323 E. Stolper, Geochim. Cosmoclum. Acta 46 (1982) 2609 E.A. Guggenhelm, Mtxtures (Oxford Univ. Press, 1952) p. 270. R.E. Ra~hards and J.A S. Smith, Trans. Faraday Soc. 47 (1951) 1261. W. Muiler-Warmuth, Z. Naturforsch. 20a (1965) 902. G.E. Pake, J. Chem. Phys. 16, #4 (1948). R.F Bartholomew and J.W.H. Schreurs, J. Non-Crystalhne Sohds, 38-39 (1980) 679. R. Grahn, Arkav Fys. 19, no. 12 (1960). E.R. Andrew and R. Bersohn, J. Chem. Phys. 18 (1950) 159.