RAMAN SPECTROSCOPY OF S, S-Se,
AND
S-As MIXTURES
4133
Raman Spectroscopy of Sulfur, Sulfur-Selenium, and
SulfurArsenic Mixtures by A. T. Ward Irerox Research Laboratories, Rochester, New York
(Received M a g 1, 1968)
The molecular complexity of sulfur, sulfur-selenium, and sulfur-arsenic mixtures in the crystalline, glassy, and liquid phases has been investigated by Raman spectroscopyusing a continuous-wave (cw)He-Ne laser as excitation source. The spectra of pure sulfur and of sulfur-selenium mixtures (up to 33% Se) are dominated by the vibrations of discrete molecules (eight-atom rings) present in both the solid and liquid phases. In the sulfurselenium mixtures, these molecules have been identified as mixed rings, S2Ses-2,where z can take the value 4-8. However, the spectra of solid and liquid phases of sulfur-arsenic mixtures, containing as little as 15% As, have been shown to be dominated by the vibrations of an arsenic-sulfur polymer in equilibrium with free eight-membered rings. Increasing the arsenic content of the mixture decreases the proportion of free sulfur, until at 40% As no free sulfur is detectable. In all cases the relative intensities of the spectral features show a much greater dependence on composition than on temperature. The absolute intensitiesare strongly temperature dependent, primarily because of the increase in absorption of the laser light which occurs with increasing temperature.
Introduction The temperature dependence of the molecular complexity of sulfur, particularly in the liquid phase, has received considerable attention. Viscosity, static susceptibility,2 optical absorption,6 heat capacity,=J and quenching experimentss all support the concept of the existence of an equilibrium between discrete Ss molecules and the long-chain sulfur polymer S, in the liquid phase. Sulfur-selenium and sulfur-arsenic mixtures have become the subject of increasing investigation on account of their close relationship to the commercially important selenium-based photoconductors. Evidence for polymer formation in these systems is derived mainly from viscosityg and thermal measurements.1° Direct spectroscopic evidence for the formation of polymer in any of these systems is minimal. Gerding and Westrik" claim to have identified vibrations due to sulfur polymer in a Raman spectroscopic investigation of liquid sulfur at 190". However, they used conventional mercury arc excitation under conditions in which the exciting and scattered radiation were very strongly absorbed. The availability of a more suitable excitation source, V$L, the He-Ne laser with continuous output at 6328 A, a wavelength which does not suffer the prohibitive absorption of the mercury lines used by Gerding and Westrik, has made it possible to perform a reinvestigation of the sulfur system and to extend the study t o the sulfur-selenium and sulfur-arsenic mixtures.
Experimental Section Raman spectra were excited by 6328-A radiation from a Spectra Physics Model 125 helium-neon laser having an output power in excess of 50 mW. The laser
light was mechanically chopped at 600 cps and then focused onto the sample. Light scattered at 90" to the incident beam direction was focused onto the entrance slit of a double monochromator (Spex 1400) consisting of tandem 0.75-m grating monochromators. The resulting spectrum was detected by a photomultiplier (EM1 9558B) and a lock-in amplifier (Princeton Applied Research Model HR-8). Output was displayed on a conventional strip-chart recorder. The sulfur and sulfur-selenium single crystals used were grown from carbon disulfide solutions to dimensions of at least 1 em3, The arsenic-sulfur crystals studied were cleaved from natural specimens of orpiment (Ask&) and realgar (AS&), respectively. Glassy samples were obtained by in vacuo quenching of the appropriate melt prepared by direct synthesis from the high-purity elemental components. The glass boules SO produced were cut into cylindrical ingots -2 cm long which were then polished and mounted for axial propagation of the laser beam. Liquid samples were (1) R. F. Bacon and R. Fanelli, J . Amer. Chem. SOC.,65, 639 (1943). (2) J. A. Poulis, C. H. Massen, and D. V. D. Leeden, Trans. Faraday Soc., 58, 474 (1962). (3) D. M. Gardner and G. K. Fraenkel, J. Amer. Chem. SOC.,78, 3279 (1956). (4) J. A. Poulis and W. Derbyshire, Trans. Faraday Soc., 59, 559 (1962). (5) M. Fukuda, Chem. News, 125, 209 (1922). (6) H. Braune and 0. Moller, Z. Naturforsch., 9a, 210 (1954). (7) E. D. West, J . Amer. Chem. rSoc., 81, 29 (1959). (8) D. L. Hammick, W. R. Cousins, and E. J. Langford, J . Chem. SOC.,797 (1928). (9) A. V. Tobolsky, G. D . T. Owen, and A. Eisenberg, J . Colloid Sci., 17, 717 (1962). (IO) M. B. Myers and E. J. Felty, Mater. Res. Bull., 2, 535 (1967). (11) H . Gerding and R. Westrik, Ree. Trav. Chim. Pays-Bas., 62, 68 (1943).
Volume 70, Number 10 November 1968
A. T. WARD
4134 obtained by melting -4 g of the parent solid under nitrogen in a cylindrical Pyrex Raman tube (10 cm X 1 cm 0.d.) closed at one end by an optical flat. In the case of sulfur the parent solid was Baker Reagent sublimed sulfur and in the case of S-Se and S-As the appropriate glass was used. Sample temperature was controlled by a Variac operating through a glass-walled nichrome-wound resistance heater concentric with the Raman tube. Sample temperature was monitored by a sheathed copper-constantan thermojunction operating through a millivolt potentiometer.
Results A . Xulfur. Figure 1 shows the temperature dependence of the fundamental vibration region of the Raman spectrum of pure sulfur. The spectrum obtained at 25" is that of the rhombic crystal. It shows characteristic fine structure which may be explained in terms of crystal-field effects.12 Spectra of the liquid are shown for temperatures close to the melting point WIDTH 247c (-llSo), near the so-called polymerization transition temperature (-l59"), and at increasing temperatures up to a maximum of -260". Figure 2. Composition dependence of the Raman The basic features of the crystal spectrum may be spectrum of mixed crystals in the sulfur-selenium system: (a) SOssSeo 35; (b) So &ea.os; (e) SI ooSeo0. understood in terms of the assignments of Scott, McCullough, and Kruse13for the Raman-active 2A1 3Ez 2E3 species appropriate to a molecular unit Ss of D B ~ ing E2 mode is accidentally degenerate with the Al mode symmetry. The peaks centered at 84 and 151 cm-l are at 475 cm-l. The peak centered at 188 cm-I is preassigned to the Ez symmetry species; those at 218 and sumably the Raman-forbidden B1 fundamental appear474 cm-I are assigned to the AI species; and those at ing in violation of strict D4dselection rules. 248 and 437 cm-l are assigned to the EBspecies. AcThese same basic features persist in the liquid phase cording to the assignments of Scott, et al., the remainwith either zero or very small changes in frequency and relative intensity. However, as the temperature is raised above 160" significant changes in absolute intensity, relative intensity, and line width occur which begin to obscure the lesser peaks. The broadening and growth of the 474-cm-' peak relative to that of the 151 and 218-cm-l peaks is particularly evident. The splitting of fundamentals observed for rhombic sulfur is not preserved in the liquid phase except in the case of the E3peak centered at 437 cm-*. The splitting of this fundamental increases slightly from 6 cm-l at 25" to 11 cm-I at 122", a separation which is still evident even at 163'. The solid and broken lines in Figure 1 represent traces obtained with the electric vector, E , of the exciting radiation polarized perpendicular and parallel to the direction of observation of the scattered light, respectively. I n the case of the crystal at 25" these traces coincide, but in the case of the liquid the totally symmetric A1 vibrations at 218 and 474 cm-I are greatly attenuated when E is rotated from the perpendicular to the parallel configuration. B. S-Se. In Figure 2 the spectra of So.6eSeo.33
+
Figure 1. Temperature dependence of the Raman spectrum of crystalline and liquid sulfur. The Journal of Physical Chemistry
+
(12) A. T. Ward, J . Phus. Chem., 72, 744 (1968). (13) D. W. Scott, J. P. McCullough, and F. H. Kruse, J. M o l . Spectrosc., 13, 313 (1964).
RAMAN SPECTROSCOPY OF S, S-Se,
AND
S-As MIXTURES
4135
221cm-1
I 184cm-I
I
\ /
46cm-I
I22
TEMP (0)
SLIT WIDTH
370
AS4S4
376
25'C
Figure 3. Temperature dependence of the Raman spectrum of crystalline and liquid So.&3eo.o~.
and SO.geSeo,o~ are compared with the spectrum of S~.ooSeo.oo (rhombic sulfur), all samples being in the crystalline phase at room temperature. All three traces show the characteristic Raman-active vibrations of Ss. I n addition the spectra of selenium-containing samples show new structure, the intensity of which increases with selenium content. However, this composition-dependent intensity increase is not the same for all of the new components. I n Figure 3 the spectrum of crystalline So.9sSeo.o~ at 25" is compared with that of the liquid at 120° and 153". There is a significant decrease in the absolute intensity of the spectrum caused by an increase in the absorption of the exciting radiation with increasing temperature. However, most of the structure observed for the crystal is preserved in the liquid phase, with little change in relative intensity except in the case of the lines at 165 and 180 cm-l, which do not appear in the spectra of the liquid. Again, the solid and broken lines demonstrate the result of rotating the plane of polarization of the incident light with respect to the direction of observation of the scattered light. Several peaks are greatly attenuated when E is rotated from the perpendicular to the parallel orientation. C. S-As. The spectra shown in Figure 4 are those of the arsenic-sulfur crystals orpiment, As& (lower trace), and realgar, AS& (upper trace). The broken line superimposed on the Raman spectrum of orpiment is a trace of the Raman spectrum of As2S3 glass prepared as described previously. The dashed line represents the Raman spectrum of a commercial glass (Harshaw Chemical Co.) of the same composition.
Figure 4. Raman spectra of native arsenic-sulfur crystals: (a) As& (realgar); (b) As2& (orpiment).
The spectra of the two glasses are basically similar insofar as they both exhibit a broad asymmetric band between 300 and 400 cm-l accompanied by lesser peaks at 140, 189, and 230 cm-I superimposed on a rising background. Furthermore, the glass spectra are clearly related to the spectrum of the corresponding crystal. The principal features of the former may be approximated by a broadening of the orpiment spectrum accompanied by an over-all shift of about 30 cm-l to higher energies. The composition dependence of the Raman spectrum of As,S1-, glasses for x = 0.05-0.40 is shown in Figure 5 . The peaks associated with As2S3, in particular the broad band between 300 and 400 cm-', appear in all spectra and increase in intensity with increasing x at the expense of the characteristic peaks due to the vibrations of Sg. The latter become appreciably weaker as x is increased and are not detected at all when x reaches 0.40 ie., As2S3. The growth of the predominant 340-cm-' peak characteristic of As& suffers an apparent reversal a t x = 0.40 on account of an increase in absorption of the laser line at this composition. At the most dilute composition investigated, x = 0.05, resolution of the principle band into two components at 337 and 367 cm-1 is clearly evident. This 30-cm-I separation is probably related to the 45-cm-l separaVolume 72,Number 12 November 1968
A. T. WARD
4136 340cm-I
I
1
189cm-I
As0.40 I 4 u '0.60
AsO. I5
'0.65
As0.05
'0.95
Figure 5. Composition dependence of the Raman spectrum of glasses in the sulfur-arsenic system: (a) A~o,aoSo.so;(b) Aso.zsSo.7~;(c) ASo.15So.85; (d) Aso.osSo.sn.
tion of the two most intense lines, vix., 310 and 365 cm-', in the spectrum of orpiment. Although this splitting disappears with increasing arsenic content, it persists with increasing temperature at constant composition x = 0.05 as shown in Figure 6. The form of the spectrum of As0.0jS0.95is virtually independent of temperature up to 175". There is, however, a decrease in absolute intensity on account of the increasing optical absorption of the sample at elevated temperatures, I n addition to the Raman lines at 218 and 474 cm-l due to totally symmetric Ss vibrations, both components of the 300-400-~m-~ As-S band are greatly attenuated under appropriate polarization of the incident light.
Discussion A . Sulfur. The previous investigation of liquid sulfur by Gerding and Westrik indicated that new lines attributable to polymeric sulfur could be expected to appear at 43, 103, 330, and 379 cm-' in the Raman spectrum of the liquid at temperatures above 160". None of these lines was observed in the present work even at the highest temperatures explored (approximately 100" above the polymerization threshold). Since the time dependence of the spectrum reported by Gerding and Westrik was not observed in this work, even after equilibration times of several hours, the new lines reported by these workers are considered to be of spurious origin. The Journal of Physical Chemistry
Figure 6. Temperature dependence of the Raman spectrum of ASO.O~SO,E glass.
The persistence of the splitting of the E3 fundamental at 437 cm-l a t temperatures sQfficientto erase all other evidence of nearest neighbor effects raises the possibility of an error in the assignments of Scott, McCullough, and Kruse. One of the components of this doublet could conceivably be assigned as the missing E:, fundamental previously assumed to be degenerate with the AI fundamental at 474 cm-l. However, comparison of the spectrum of liquid sulfur at 122" (Le., of s8) with that reported by Lucovsky, et ul.,14 for monoclinic selenium (ie., for Seg) does not support this hypothesis. The corresponding region of the spectrum of Ses has peaks at 249 and 254 cm-l assigned by Lucovsky, et ul., to the species A1 and Ez, respectively. According to these workers, applying a "frequency scale factor," approximately equal to 1.9, to these Raman shifts for Seg leads to the expectation of Raman-active fundamentals at 473 cm-l (AI) and 475 cm-l (E,) for the S g system, thus supporting the accidental degeneracy assumed by Scott, et al. The temperature-dependent broadening and growth of the spectral features in the 400-500-cm-1 region relative to the rest of the spectrum may be explained in two ways. The enhancement may be caused by an in(14) G. Lucovsky, A. Mooradian, W. Taylor, G. B. Wright, and 5, 113 (1967).
R.C . Keeser, Sol. State Commun.,
RAMAN SPECTE~OSCOPY OF S, S-Se,
AND
S-As MIXTURES
crease in the activity of unresolved Raman-forbidden fundamentals at 411 cm-’ (B1) and 471 cm-l (El). These fundamentals could become Raman active, in violation of strict D q d selection rules, in the event of thermally induced distortion of the SS ring symmetry. Alternatively, the apparent growth may be attributed to the formation of polymeric species (S,) for which Raman-active fundamentals, by analogy with trigonal selenium (Se,), could be anticipated at -420 cm-’ (E) and -450 cm-l (A). Here again the scaling factor of Lucovsky, et al., has been used ta convert the selenium Raman frequencies to the corresponding values for sulfur. Polarization measurements support the latter explanation insofar as the polarization dependence of the 400-500-cm-’ region does not change significantly with temperature. This requires that the principal contribution to the increased scattering intensity be derived from a totally symmetric A-type vibration, e.g., the 450-cm-1 vibration of S,. Further evidence for the possibility of coexistence of SS rings and S, chains is given in Figure 7. The spectra shown are Raman spectra of pellets of sublimed sulfur and “crystex” sulfur, a sulfur modification purported to contain polymeric S, chains stabilized by organic substituents. Raman spectra of the pellets, which were prepared by subjecting powdered samples to pressures of 1.33 kbars for 1 min, were obtained by mounting the samples in a configuration allowing grazing incidence of the laser light and perpendicular observation of‘ the scattered light. New lines appear
86crn1-1
li 27
: n-’
m-I
6cm-I
i \
SLIT WIDTH
ik
i Is:
Figure 7. Ranian spectra of compressed sulfur pellets: (a) “crystex,” (b) sublimed sulfur.
4137
in both spectra in addition to the well-characterized lines due to SS. The Raman shifts of the principal new lines are 273, 416, and 456 cm-l, in good accord with the values 273,420, and 450 cm-1 obtained by applying the -1.90 scale factor to the corresponding frequencies of Se, as observed in trigonal selenium. The high intensity of the new lines relative to those due to SSis considered to preclude the possibility of their originating by transgression of selection rules appropriate to the SSmolecule. B. #-fie. Raman shifts and polarization characteristics of those lines appearing in the spectra of sulfurselenium mixtures but not appearing in the spectra of pure sulfur or pure selenium are summarized in Table I. Close inspection shows that these lines occur in groups which bear a definite frequency and symmetry relationship to the dominant Raman lines of pure sulfur and pure selenium. The empirical groupings and frequency ratios are indicated in Table 11. Only one accidental degeneracy (at 151 cm-l) has to be assumed in order to complete the groupings. It is suggested that the frequencies of a particular group correspond to the dominant Raman-active fundamentals of discrete Table I: Raman Shifts Due to the Presence of Selenium in Mixed S-Se Crystals and S0.&3eo 05 Liquid So.aeSeo.aa
(crystal)
105 122 128 138 165 180 202 344 360 380 -435
-So.
esSeo.o6--
Crystal
Liquid
122 129 138
122 128 138
Polarization
...
...
...
180 201 347 363 382 434
Depolarized Depolarized Depolarized
...
...
... ...
202 344 360 380
Polarized Depolarized (?) Polarized Polarized
...
..*
Table I1 : Empirical Groupings and Frequency Ratios for Raman Shifts in the Sulfur-Selenium System Assignment
Sea
E2 (depolarized) AI (polarized) AI (polarized)
84 114 249
Group 1 Group 2 Group 3 Group 4
105 (151) 344
122 165 360
128 180 380
138 202 435
Sa
151 218 474
species of the type SxSe8-x,where each group is characteristic of a different value of 2. Semiempirical’ estimates of x can now be made by comparing the observed frequency ratio for each group with the frequency ratio relating Ses and SS. As pointed,out by Lucovsky, et al., this relationship does not obey the simple formula Volume Y8*Number 18 November 1068
A. T. WARD
4138 Table I11 : Comparison of Observed and Calculated Frequency Ratios for S,Se8, Rings
Mol wt ( M ) MSelMs,se-z
Vibr freq ratio (Ez) Freq ratio (obsd) (Ez) Vibr freq ratio (AI) Freq ratio (obsd) (AI) Vibr freq ratio (AI) Freq (obsd) (Al)
632 1.0 1.0
585 1.04 1.06
...
...
538 1.08 1.12
...
...
1.0
1.07
1.14
...
...
1.23
1.0
1.07
1.14
1.23
...
...
...
...
491 1.135 1.20
...
where M is the molecular weight, which would be expected to hold, albeit approximately, if the force constants of the two systems were equal. Presumably, a better approximation would be
where IC2 is the ratio of the force constants for Ss and Ses making the greatest contribution to the potential energy distributions for the normal vibrations of frequency vs, and mestrespectively. Assuming a monotonic variation of the force constants with the degree of substitution of sulfur for selenium leads to the more general approximation
where a is a constant. The constant a is calculated from the frequency ratio and the molecular weight ratio at x = 0 and x = 8. Frequency ratios for intermediate values of x may now be calculated and compared with the observed values. The results are shown in Table 111. The close agreement between observed and calculated values, especially for the totally symmetric vibrations (for which approximation 3 should hold best), is considered good evidence for the existence of molecular rings SxSe8-, for x = 4-8 in the sulfurselenium mixtures examined. Further inspection of the relative intensities of the Raman lines due to these species suggests that, as might be expected, the abundance of rings with high Se content increases with the total Se content of the mixture. C. S-As. The complexity of the Raman spectra of orpiment and realgar precludes a reliable analysis at this stage. This in turn complicates the problem of making assignments for the arsenic-sulfur glasses. The possibility of interference from lines in the fluorescence spectrum of the laser source is an added consideration because of the rather wide spectroscopic slit width (-7 cm-l) necessary for investigating the arsenic-sulThe Journal of Phusical Chemistry
...
444 1.19 1.28 1.25 1.32 1.325 1.32 1.38
397 1.26 1.38 1.45 1.44 1.45 1.43 1.45
350 1.34 1.49 1.54 1.565 1.57 1.56 1.53
303 1.44 1.62 1.64 1.725 1.77 1.71 1.75
256 1.57 1.80 1.80 1.92 1.92 1.91 1.91
fur glasses (cf. a maximum slit width of 4 cm-l for the investigations of sulfur and sulfur-selenium mixtures). At least two of the weaker lines (140 and 189 cm-l) in the spectra of the arsenic-sulfur glasses coincide with lines in the spectrum of neon. However, the apparent growth of these lines with increasing arsenic concentration (all other experimental factors constant) suggests that they are, indeed, due to the addition of arsenic and are not due to spurious neon fluorescence. The broad, near-featureless band centered at 340 cm-' in the spectra of the arsenic-sulfur glasses is clearly attributable to a polymeric species probably related to the layer lattice existing in Ask33 crystal. The weaker peaks at 140, 189, 230, and 490 cm-1 are probably also associated with this polymeric lattice. The gradual disappearance of peaks due to s8 vibrations with increasing arsenic concentration infers the existence of an equilibrium between monomeric sulfur rings and arsenic-sulfur polymer at the temperature of formation of the glasses. A three-component equilibyium between sulfur rings, latticelike arsenic sulfur polymer, and long-chain sulfur polymer is also conceivable. An attempt has been made to demonstrate more clearly the relationship between the spectrum of glassy AszSa and crystalline As2S3using a computer method. I
Figure 8. Comparison of the Raman spectrum of As&& glass with computer-modified spectrum of As& crystal: (a) As& (glass); (b) computer-synthesized curve superimposed on an arbitrarily chosen background; (c) computer-synthesized curve with zero brackground. (N.B., horizontal scale is linear in wave number.)
INTERFACIAL TENSION OF
4139
TWO-PHASE TERNARY LIQUID SYSTEMS
The treatment involved artificially broadening the individual features of the spectrum by a factor of 5 while allowing the total area under each peak and the position of each peak to remain ynchanged. The spectrum was then reconstructed and the result, shown by the broken line in Figure 8, is compared with a composite spectruim of the two As& glass samples (solid line). Superposition of the computer-synthesized curve upon an arbitrary background (dotted line) indicates more clearly the nature of the energy shifts required to give an exact fit to the experimental curve. I n view of the fact th,st the orpiment spectrum was obtained
from a polycrystalline sample, for which orientation effects were not considered, the similarity in the form of the spectra is close enough to be advanced as evidence in support of the polymeric distorted layer-lattice-type structure proposed for AS& glass.
Acknowledgment. The author wishes to thank M. B. Myers and R. C. Keezer for making available the samples of As-S glass and S-Se crystal, respectively. M. Slade is thanked for computer programming and operation, Helpful discussions with G. Lucovsky, M. B. Myers, D. Olechna, and R. Zallen are also gratefully acknowledged.
A Theory of Interfacial Tension of Two-Phase Ternary Liquid Systems by Donald J. Cotton iVavaC Ship Research and Development Center, Annapolis, Maryland
81408
(Receiued Mag 6, 1068)
A theoretical equation has been derived which describes the interfacial tension and stability of a two-phase ternary liquid system in terms of the concentrations and physical properties of its components. The resulting linear relationship is re-plied to experimental data. The equation has been successfully tested with existing experimental data and has been used to determine interfacial region structure.
Introduction
Theoretical Development
A number of theoretical equations have been derived t o describe thle interfacial tension of binary solutions, but few equations have been developed for multicomponent, two-phase systems. Shain and Prausnitzl have derived an equation with which the interfacial tension gradient of one of the components can be obtained for multicomponent systems. Their equation, however, is derived for regular solutions and is based on an assumed surface region equivalent lattice structure. Plisken and Treyba12 have derived a relationship applicable to nonregular, multicomponent systems which is complimentary to the Shain and Prausnitz equation. Their relationship, however, is cumbersome and fails for aqueous systems. Shendalman and O1Tooie3have recently delineated a theory of interfacial tension for random ternary mixtures. Their equation has not been tested extensively because of insufficient data with which to calculate necessary parameters. The purpose of this paper is to present the development of a theory to describe the interfacial tension of a two-phase, three-component system in terms of known physical properties of its components. The resulting linear equation is easily applied to experimental data.
Consider a classical thermodynamic liquid threecomponent system consisting of two nearly immiscible components and a third component that is miscible in both. Such a system will contain two phases: one will be predominately of one component; the other will be predominately of a second component. The mutually miscible component will be distributed between the phases. The surface tension of nearly ideal liquid mixtures can be expressed to a close approximation by the equation n
u =
CutN: i
where n is the surface tension of solution, ut is the surface tension of the pure ith component, N S ais the mole fraction of the ith component in surface region, and n is the number of component^.^ Assuming that the interfacial tension of a phase can be described by a (1) 5 . A. Shain and J. M. Prausnitz, A.I.Ch.E. J . , 10, 766 (1964). (2) I. Pliskin and R . E. Treybal, ibid., 12, 795 (1966). (3) L. H. Shendalman and J. T. O'Toole, J . Phys. Chem., 71, 4222 (1967). Volume 79,Number l b November 1968
