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Raman spectroscopic studies on the dynamics of a molecular “probe” (methyl iodide) in hydrocarbon solvents
Chemical Physics 78 (1983) 319-322 North-Holland Pubhshmg Company
319
RAMAN SPECTROSCOPIC STUDIES ON THE DYNAMICS (METHYL IODIDE) IN HYDROCARBON SOLVENTS W-F. PACYNKO, Departmenr
OF A IMOLECULAR
“PROBE”
J. YARWOOD
of Chemrsrv.
Unicersrr) of Durham, Durham Cq,
DHI
3LE,
UK
and D-J. GARDINER Deportment
of Chemrsny
Recetved 21 December
and Apphed Chemrstr). Newcastle Po&lechmc
Neucade-upon
-Twe
DK
1982
We report the rates of rotational and vibrational relaxation as a funcnon of cham length m a senes of model lubncanrs (n-alkanes from C, to C,,) and in an mdustnally Important traction fluid (Santotrac-40) It IS sho\xx that the “probe * molecule (CH,I) dynamics reflect the changes of microstructure of these flwds as the chain length mcreases The merh>l iodide molecules can rotate rather freely between the hydrocarbon chains (for relatwel> short chams) bur tend to form more contmuous regions of “pure” hquld for the longest chams The dqnanuc properties do not follou the macroscopic \lscoslty of the solvent but can be con-related Instead wth the effectIke “free’ Lolume axalable 111 the fhud It 1s suggested that such measuremenrs could lead IO valuable information about ihe dsslrsd properties of rhssc mdusrnalf xnponanr fhds
1. Introduction
about the structural cants.
Raman band-shape measurements provide a convenient means of studying the molecular rotation and dynamic environment of a “probe” molecule [1,2] in a fluid whose vibrational spectrum may be too complicated for the technique to be easily used. Previous results on CH,I in polystyrene ([3], fig 1) have shown that the 72R values obtained from Raman spectroscopy are virtually independent of macroscopic viscosity. This is expected [4] when the solvent molecules are considerably larger than the solute molecules_ The CH,I molecules appear to be able to rotate between the polymer chains. These results lead us to beheve that one can mfer details of the microstructure of large flexible molecules (on which lubricant and traction properties must ultimately depend) from the “probe” dynamics and environmental fluctuations. If sufficient information of this kind is available, one ought to be able to make predictions 0301-0104/83/0000-0000/$03.00
@ North-Holland
properties
of suitable lubn-
1
Fl_e 1. T=~ values for CHJ (+) &sohed III pol>st>rent Tbc brohen hne shous the viscosiq as a function of concentration (Reproduced wth pemusslon from unpublished results of Dbge and Amdt [3] )
320
IV F Paqnho er al /
The dynanucs of a molecular “‘probe” m hydrocarbonsoloenls
2. Experimental
3. Results
$Q~(I) and (p”(t) measurementshave been made on the vJ(C-I) band of CH,I (5% by volume) in
Some typlcal reorientational correlation functions are shown in fig 3 whtle the values of -r2R and T” are shown in fig. 4. The results show that rzR values in hydrocarbon solutions are considerably lower than the values obtained for pure liquid CH,I (1.4 ps at 298 K). However, as the macroscopic viscosity increases with chain length 72R rises until it is close to the liquid value in hexadecane. Furthermore, the data appear to fall on two curves (fig. 5) as was observed for OCS in similar solvents [l]. The value of 7rR for Santotrac-40 solutions is higher but m no way reflects the high macroscopic shear viscosity of this matenal. It is well-known [4] that hydrodynamic models do not work when the solvent molecules are much larger than those of the solute. It may, however, be appropriate to apply the modified hydrodynamic model of Kivelson and co-workers [4]. The modified expression for the rotational correlation time is,
a range of n-alkanes and in Santotrac-40 (a wellknown synthetic traction fluid whose major con-
stituent (95%) is a dlcyclohexyl hydrocarbon). I,,,(w) and IVH(a) Raman data (fig 2) have been analysed iii the usual way to yield the necessary correlation functions assunung statistical mdependence of reorientatlonal and non-reorientational processes A gaussian slit profde (= 3 cm-‘) was removed in the time domain Companson of the I VH band profdes (from which ~~~ is obtained) for liquid and solution spectra of yj of CH,I are shown in fig. 2
T,=Tp+
V$JK,/f(z+
l)kz-,
(1)
where V,, is the hydrodynarmc volume ($a3 for a spherical molecule) and K, 1s a factor which turns out
cm-’
x107
cm-’
x10’
[4] to be
a ratio
acting
on
the
Fig 3. Rotauonal correlation funcuons &(I) denved from vj of CH,I m the hqurd (A) and in n-octane (5% by volume) (B)
Fig 2 Comparison of Ihe IVH spectra of yj of CH,I m the pure bqutd (top) and in n-octane (5% by volume) (bottom)
Curve C shows kR(r)
(Norice the difference in cm-
tiOIL
1 scales)
of torques
expected III the “free” rotor appronma-
I ‘,
60
g
‘.
50 LQ-
-+-_
“1
8 20. 10.
r-k
0.
\” E 30Ir’
Table 1
0-c” q
70
_-e-D--
-d--_b___-_j____
_-_+__
1
r3
0 32
0 35(0 26) a’
045+005 OS&O I
007=005
cs-Go c,,-CM 2
L\-r~i~C
RRE Ll0.m
Cl’ P___m____D___---E-------
a)
0 30to
10
An estimate from rnertial ratios [S]
m_
as a function hydrocarbon
molecule and forces acting within the solvent. It may be interpretted as a coupling parameter linking rotational and translation motions since it is also a measure of the anisotropic intermolecular mteractions. In the Debye “stick” approximation, K, = 1 and in the ‘slip’ approximation, K, = 0. Goulay-Bize et al. [l] have introduced a correction factor for non-spherical solute molecules. For OCS this factor is found to be F,/F,,, = 1.4 where F, 1s the friction coefficent 877~~71. Values of ~2 have been obtained for CH,I in the solvents studied and are listed in table 1 (V, was taken to be 449 x 1O-24 cm3 mol- * based on
13
G(PS)
CH,I malkanes
Fig 4 72R and rv values obtamed for v3 of CH,I of macroscopic vlscoslty (7) in (C,,H,,+,) solvents
1‘
Sohent
.RElJxm
t
14
Fig. 5. 7zR vah~es plotted as a fun&on of viswslty. that the data fall on two hnes with &fferent &_
I
shoamg
a = 475 A). Values of K? obtained using a nonspherical correction factor are within the experimental error of those reported here. For shorter cham n-alkanes the ~2”value is quite close to that of the classical free rotor (72FR = 0.3 ps). Tins apparent correlation is absent for solvents with longer chains. It has been pre\lously observed [4,5] that K, ValUeS are a function of solvent size and anisotropy for a given probe. In particular K, values have been found to increase (p) with Increasmg solvent anisotropy and solute-solcenr IIIteruction and decrease (1) wth increasing solvent srze
4. Discussion For short chain lengths (C,--Cic) the CH,I molecules appear to be able to rotate relatively2 )_ Interactions between individual freely (7: = 7FR molecules are reduced by the hydrocarbon environment and the corresponding 7&-values (fig. 4) get very long (Iv, band narrowing occurs). The value of K2 indicates that the solvent molecules environment is relatively anisotropic presumably due to the large amounts of -CH, and -CH2m the “probe” soltatron shell. Short chain n-alkanes are apparently able to (more efficiently) isolate the probe molecules from molecules of it’s own species As the chain length gets longer (C12-C16) the relaxation parameters show a distmct trend towards values for the pure hquid. Rotation gets slower and vibrational relaxation gets considerably faster (fig 4). In addition, the value of K-, is now very close to the “shp” limit. The methyl iodide molecules can now interact more easily with each other since it is more difficult for the long chain hydrocarbons to prevent such CHJ diffusional
Table 2 Some propemrs
of n-alkane (C,,H,,,,)
n
0 187 0.252 0 334 0 433 0 697 I 066 I 566
6 7
5
10 12 14 16 San101rac-40 a)
,;
=
r’;-
(298 K)
V(cm’ mol-‘)
?(CP) 5
solutrons of CH,I
0.377 0 34s 0 320 0 301 0.272 0 257 0244 0 235 -
116 1 1316 147 5 163 5 195 9 228 6 261 3 294 1 -
22
37
lb where p IS the molar volume calculated from the dens+
contact. However. the sobent molecules are veq large and are expected to have angular veloctttes much lower than those of the sohent molecules. Hence the almost complete lack of \iscosit> dependence (IC, ven small). As e\platned before it IS clear that modtfted hydrodynamtc equattons do not suitably descrtbe molecular rotation tn these systems and that more sophtstrcated models must be sought The reortentational motton of a probe molecule shows a distmct correlation ~7th the relative free molar volume (table 2 and fig 6). Indeed =e find the same linear relauonship betueen relaxanon time (rzR) and trnerse free volume (&,/L/i) as found pre\?ously [l] It IS posstble. hov.e\er_ that
TV(PS)
TZR(PS)
64+_03 55f02 -
05L-0 06&O -
1 1
5 4rtO.2 49&02 4610 2 455+02
08+0 09+0 095x0 lo*01 llrol 25+05
1 I
3 I+_02
1
and 1%1s the compact packmg volume (data from ref [I])
our results (fig. 6) shou different B values. l/n = BQ’J$.
(2)
for the shorter and longer chain alkanes (as predicted by Htldebrand [6]) The CH,I molecule dynamics clearl? probe the mrcrostructure of the model lubricant fluid. We now plan to make sirrular measurements as the lubricant IS subyected to htgh pressures and shear stress In this ua> ne can attempt to correlate traction properttes and dynannc microstructure and
hence
mahe
reasonable
susesnons
desirable structural propsrues and related matenals
about
the
of future lubncanrs
Achnowiedgement Thanhs are due to SERC for eqmpment pants should also like to record our thanks to Professor G Doge and Dr R. Arndt uho sent us the data m fig I pnor to pubhcation. We
References 111.4 U Goula>-Btze. E Dcxwl
RELAnvE
Ftg 6 72R tdues functions
FRE
WUME
ly.nLl
as a functton of the relatnc
free volume
and J Vmcenr-Gas=. Chem Ph>s Letters 69 (1980) 319. [2] D ILchon. D Patterson and G Turrell. Chem Phls 16 (1976) 61 131 R Amdt and G Dbge. unpubllshcd data 141 D IG\elson, Farada) Symposm Yo 11 (1977) (Xix Chcnucal Socxty. London, 1978) p 7 151 M. Fury and J Jonas. J Chem Phls 63 (1976) 2206 (61 J J. Htldcbrand. Science 174 (1971) 490
319
RAMAN SPECTROSCOPIC STUDIES ON THE DYNAMICS (METHYL IODIDE) IN HYDROCARBON SOLVENTS W-F. PACYNKO, Departmenr
OF A IMOLECULAR
“PROBE”
J. YARWOOD
of Chemrsrv.
Unicersrr) of Durham, Durham Cq,
DHI
3LE,
UK
and D-J. GARDINER Deportment
of Chemrsny
Recetved 21 December
and Apphed Chemrstr). Newcastle Po&lechmc
Neucade-upon
-Twe
DK
1982
We report the rates of rotational and vibrational relaxation as a funcnon of cham length m a senes of model lubncanrs (n-alkanes from C, to C,,) and in an mdustnally Important traction fluid (Santotrac-40) It IS sho\xx that the “probe * molecule (CH,I) dynamics reflect the changes of microstructure of these flwds as the chain length mcreases The merh>l iodide molecules can rotate rather freely between the hydrocarbon chains (for relatwel> short chams) bur tend to form more contmuous regions of “pure” hquld for the longest chams The dqnanuc properties do not follou the macroscopic \lscoslty of the solvent but can be con-related Instead wth the effectIke “free’ Lolume axalable 111 the fhud It 1s suggested that such measuremenrs could lead IO valuable information about ihe dsslrsd properties of rhssc mdusrnalf xnponanr fhds
1. Introduction
about the structural cants.
Raman band-shape measurements provide a convenient means of studying the molecular rotation and dynamic environment of a “probe” molecule [1,2] in a fluid whose vibrational spectrum may be too complicated for the technique to be easily used. Previous results on CH,I in polystyrene ([3], fig 1) have shown that the 72R values obtained from Raman spectroscopy are virtually independent of macroscopic viscosity. This is expected [4] when the solvent molecules are considerably larger than the solute molecules_ The CH,I molecules appear to be able to rotate between the polymer chains. These results lead us to beheve that one can mfer details of the microstructure of large flexible molecules (on which lubricant and traction properties must ultimately depend) from the “probe” dynamics and environmental fluctuations. If sufficient information of this kind is available, one ought to be able to make predictions 0301-0104/83/0000-0000/$03.00
@ North-Holland
properties
of suitable lubn-
1
Fl_e 1. T=~ values for CHJ (+) &sohed III pol>st>rent Tbc brohen hne shous the viscosiq as a function of concentration (Reproduced wth pemusslon from unpublished results of Dbge and Amdt [3] )
320
IV F Paqnho er al /
The dynanucs of a molecular “‘probe” m hydrocarbonsoloenls
2. Experimental
3. Results
$Q~(I) and (p”(t) measurementshave been made on the vJ(C-I) band of CH,I (5% by volume) in
Some typlcal reorientational correlation functions are shown in fig 3 whtle the values of -r2R and T” are shown in fig. 4. The results show that rzR values in hydrocarbon solutions are considerably lower than the values obtained for pure liquid CH,I (1.4 ps at 298 K). However, as the macroscopic viscosity increases with chain length 72R rises until it is close to the liquid value in hexadecane. Furthermore, the data appear to fall on two curves (fig. 5) as was observed for OCS in similar solvents [l]. The value of 7rR for Santotrac-40 solutions is higher but m no way reflects the high macroscopic shear viscosity of this matenal. It is well-known [4] that hydrodynamic models do not work when the solvent molecules are much larger than those of the solute. It may, however, be appropriate to apply the modified hydrodynamic model of Kivelson and co-workers [4]. The modified expression for the rotational correlation time is,
a range of n-alkanes and in Santotrac-40 (a wellknown synthetic traction fluid whose major con-
stituent (95%) is a dlcyclohexyl hydrocarbon). I,,,(w) and IVH(a) Raman data (fig 2) have been analysed iii the usual way to yield the necessary correlation functions assunung statistical mdependence of reorientatlonal and non-reorientational processes A gaussian slit profde (= 3 cm-‘) was removed in the time domain Companson of the I VH band profdes (from which ~~~ is obtained) for liquid and solution spectra of yj of CH,I are shown in fig. 2
T,=Tp+
V$JK,/f(z+
l)kz-,
(1)
where V,, is the hydrodynarmc volume ($a3 for a spherical molecule) and K, 1s a factor which turns out
cm-’
x107
cm-’
x10’
[4] to be
a ratio
acting
on
the
Fig 3. Rotauonal correlation funcuons &(I) denved from vj of CH,I m the hqurd (A) and in n-octane (5% by volume) (B)
Fig 2 Comparison of Ihe IVH spectra of yj of CH,I m the pure bqutd (top) and in n-octane (5% by volume) (bottom)
Curve C shows kR(r)
(Norice the difference in cm-
tiOIL
1 scales)
of torques
expected III the “free” rotor appronma-
I ‘,
60
g
‘.
50 LQ-
-+-_
“1
8 20. 10.
r-k
0.
\” E 30Ir’
Table 1
0-c” q
70
_-e-D--
-d--_b___-_j____
_-_+__
1
r3
0 32
0 35(0 26) a’
045+005 OS&O I
007=005
cs-Go c,,-CM 2
L\-r~i~C
RRE Ll0.m
Cl’ P___m____D___---E-------
a)
0 30to
10
An estimate from rnertial ratios [S]
m_
as a function hydrocarbon
molecule and forces acting within the solvent. It may be interpretted as a coupling parameter linking rotational and translation motions since it is also a measure of the anisotropic intermolecular mteractions. In the Debye “stick” approximation, K, = 1 and in the ‘slip’ approximation, K, = 0. Goulay-Bize et al. [l] have introduced a correction factor for non-spherical solute molecules. For OCS this factor is found to be F,/F,,, = 1.4 where F, 1s the friction coefficent 877~~71. Values of ~2 have been obtained for CH,I in the solvents studied and are listed in table 1 (V, was taken to be 449 x 1O-24 cm3 mol- * based on
13
G(PS)
CH,I malkanes
Fig 4 72R and rv values obtamed for v3 of CH,I of macroscopic vlscoslty (7) in (C,,H,,+,) solvents
1‘
Sohent
.RElJxm
t
14
Fig. 5. 7zR vah~es plotted as a fun&on of viswslty. that the data fall on two hnes with &fferent &_
I
shoamg
a = 475 A). Values of K? obtained using a nonspherical correction factor are within the experimental error of those reported here. For shorter cham n-alkanes the ~2”value is quite close to that of the classical free rotor (72FR = 0.3 ps). Tins apparent correlation is absent for solvents with longer chains. It has been pre\lously observed [4,5] that K, ValUeS are a function of solvent size and anisotropy for a given probe. In particular K, values have been found to increase (p) with Increasmg solvent anisotropy and solute-solcenr IIIteruction and decrease (1) wth increasing solvent srze
4. Discussion For short chain lengths (C,--Cic) the CH,I molecules appear to be able to rotate relatively2 )_ Interactions between individual freely (7: = 7FR molecules are reduced by the hydrocarbon environment and the corresponding 7&-values (fig. 4) get very long (Iv, band narrowing occurs). The value of K2 indicates that the solvent molecules environment is relatively anisotropic presumably due to the large amounts of -CH, and -CH2m the “probe” soltatron shell. Short chain n-alkanes are apparently able to (more efficiently) isolate the probe molecules from molecules of it’s own species As the chain length gets longer (C12-C16) the relaxation parameters show a distmct trend towards values for the pure hquid. Rotation gets slower and vibrational relaxation gets considerably faster (fig 4). In addition, the value of K-, is now very close to the “shp” limit. The methyl iodide molecules can now interact more easily with each other since it is more difficult for the long chain hydrocarbons to prevent such CHJ diffusional
Table 2 Some propemrs
of n-alkane (C,,H,,,,)
n
0 187 0.252 0 334 0 433 0 697 I 066 I 566
6 7
5
10 12 14 16 San101rac-40 a)
,;
=
r’;-
(298 K)
V(cm’ mol-‘)
?(CP) 5
solutrons of CH,I
0.377 0 34s 0 320 0 301 0.272 0 257 0244 0 235 -
116 1 1316 147 5 163 5 195 9 228 6 261 3 294 1 -
22
37
lb where p IS the molar volume calculated from the dens+
contact. However. the sobent molecules are veq large and are expected to have angular veloctttes much lower than those of the sohent molecules. Hence the almost complete lack of \iscosit> dependence (IC, ven small). As e\platned before it IS clear that modtfted hydrodynamtc equattons do not suitably descrtbe molecular rotation tn these systems and that more sophtstrcated models must be sought The reortentational motton of a probe molecule shows a distmct correlation ~7th the relative free molar volume (table 2 and fig 6). Indeed =e find the same linear relauonship betueen relaxanon time (rzR) and trnerse free volume (&,/L/i) as found pre\?ously [l] It IS posstble. hov.e\er_ that
TV(PS)
TZR(PS)
64+_03 55f02 -
05L-0 06&O -
1 1
5 4rtO.2 49&02 4610 2 455+02
08+0 09+0 095x0 lo*01 llrol 25+05
1 I
3 I+_02
1
and 1%1s the compact packmg volume (data from ref [I])
our results (fig. 6) shou different B values. l/n = BQ’J$.
(2)
for the shorter and longer chain alkanes (as predicted by Htldebrand [6]) The CH,I molecule dynamics clearl? probe the mrcrostructure of the model lubricant fluid. We now plan to make sirrular measurements as the lubricant IS subyected to htgh pressures and shear stress In this ua> ne can attempt to correlate traction properttes and dynannc microstructure and
hence
mahe
reasonable
susesnons
desirable structural propsrues and related matenals
about
the
of future lubncanrs
Achnowiedgement Thanhs are due to SERC for eqmpment pants should also like to record our thanks to Professor G Doge and Dr R. Arndt uho sent us the data m fig I pnor to pubhcation. We
References 111.4 U Goula>-Btze. E Dcxwl
RELAnvE
Ftg 6 72R tdues functions
FRE
WUME
ly.nLl
as a functton of the relatnc
free volume
and J Vmcenr-Gas=. Chem Ph>s Letters 69 (1980) 319. [2] D ILchon. D Patterson and G Turrell. Chem Phls 16 (1976) 61 131 R Amdt and G Dbge. unpubllshcd data 141 D IG\elson, Farada) Symposm Yo 11 (1977) (Xix Chcnucal Socxty. London, 1978) p 7 151 M. Fury and J Jonas. J Chem Phls 63 (1976) 2206 (61 J J. Htldcbrand. Science 174 (1971) 490