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Influence of surface interaction on the reorientational dynamics of pentylcyanobiphenyl confined in a porous glass
]OURNA
ELSEVIER
L OF
Journal of Non-Crystalline Solids 172 174 (1994) 348 352
Influence of surface interaction on the reorientational dynamics of pentylcyanobiphenyl confined in a porous glass G. Schwalb, F.W. Deeg*, C. Brfiuchle lnstitut fiJr Physikalische Chemie, Universitiit Miinchen, Sophienstrasse 11, 80333 Munich, Germany
Abstract Time-resolved transient grating optical Kerr effect experiments were employed to investigate the reorientational dynamics of pentylcyanobiphenyl confined in the nanometer lengthscale pores of silica glasses. Temperature and pore-size-dependent measurements show the drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition, a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction to 25/~, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is attributed to the modification of the surface layer structure by geometrical factors, e.g., the pore curvature.
1. Introduction In recent years a series of studies has investigated the influence of confining geometry and surface interaction on the structural and dynamic properties of liquids [1]. Optical birefringence [2] and nuclear magnetic resonance [3] measurements of the molecular reorientational motion of strongly interacting simple liquids in porous silica glasses have revealed the existence of a more viscous surface layer beside the bulk component. These experiments have focused on single particle correlation times. In our investigation the isotropic phase of confined pentylcyanobiphenyl (5CB), a well-known
* Corresponding author. Tel: +49-89 5902 328. Telefax: +49895902 602.
nematogenic liquid, was studied. Due to the strong anisotropic intermolecular interaction of the mesogens, there exists a short range orientationai order among the molecules leading to critical pretransitional effects [4] and to cooperative reorientational dynamics even at temperatures 80 K above the nematic-isotropic phase transition [5]. Most of the experimental and theoretical work on confined liquid crystals so far has been devoted to static properties like a temperature shift of the phase transition [6,7] and surface-induced orientational order [8]. In this study the reorientational dynamics of 5CB confined in porous silica glass were investigated for a wide temperature range from the bulk phase transition up to 80 K above the phase transition. This paper will focus on the high temperature behavior between 50 and 80 K above the phase transition, which reveals the contribution of a distinct surface layer.
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 1 3 6 - B
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348 352 30
2. E x p e r i m e n t a l procedures
i
3. Results
As was shown by Deeg et al. [5] the collective reorientational dynamics of the bulk 5CB are characterized by a monoexponential decay with a single reorientational correlation time, rb, for the whole temperature range 35-120°C. On approaching the phase transition TN~ from higher temperatures the reorientation time diverges as predicted by the Landau~leGennes theory [12-1, a consequence of the growing orientational correlation length between the mesogens. To get an overview on the collective dynamics of the nematogenic substance inside the pores, the temperature dependence of the mean relaxation time is depicted in Fig. 1. From
%
~,
20 0
i
i
i
\
25
Sol-gel prepared porous glasses represent an excellent host material to study the behavior of liquids in confined geometries. The highly transparent glasses, purchased from Geltech Inc., are characterized by high specific surface areas (400-700mE/g) and high specific pore volumes (0.4q3.9 cm3/g) with a well-defined narrow pore size distribution [9]. The surface of the unmodified pores is covered with polar OH-groups. Chemical treatment with dichlorodimethylsilane allows one to replace these polar OH-groups with less polar groups of-OSi(CH3)2OCzH 5 and systematically alter the surface interaction with the polar 5CB molecules. In this study three non-treated sol-gel glasses with mean pore diameters of 85 ~ (SG 85), 50~ (SG 50) and 25/~ (SG 25) as well as a surface-modified sample with a mean pore diameter of 85/~ (SG 85M) were employed. 5CB from Merck was used without further purification. The nematic-isotropic phase transition temperature of the bulk is TN~ = 35.2°C. The disk-shaped glasses (10 mm diameter, 5 mm thickness) were immersed in a 5CB filled spectroscopic cuvette filled with 5CB which was inserted into a variable temperature cell. The cell temperature could be controlled within _+ 0.1 K. A transient grating optical Kerr effect setup [10] was used for the investigations described here. A more detailed description of the laser apparatus can be found in Ref. [11].
~
349
• 5CB bulk ~ SG 85M
\
o sG 85
,~
o SG 50
\
o so 25
15
~o
I0 5 0
o o
40
surface
[] ~ . ^ _ ~
confinement I
20
° ~; ~~
effects
effects
~
........ ~
I
I
60
80
temperature
I
100
120
(°C)
Fig. 1. Temperature dependence of the mean relaxation time, obtained by a monoexponential fit of the transient signal, for the 5CB bulk and the 5CB filled porous glass samples SG 85, SG 85M, SG 50 and SG 25. Dotted lines provide a guideline to the eye.
this figure the strong influence of the pore size on the reorientational dynamics is highly evident. Compared to the bulk data, two temperature ranges with very different behaviors can be distinguished. At temperatures between the phase transition and 70°C the confined liquid exhibits faster collective reorientational dynamics. A closer look on the decays reveals a non-exponential orientational decay of the liquid inside the pores. The analysis of this low temperature part of the reorientational dynamics which is dominated by the limitation of the bulk correlation length through the size of the pores will be presented elsewhere [13]. In this paper, we discuss data in the temperature range 85-120°C where the reorientational dynamics of the confined liquid is obviously slowed down in comparison to the bulk. In this temperature range the decay data also reveal a significant deviation from a monoexponential decay. The reason for this deviation can be seen in Fig. 2, where the measured decays of the two 85,~ glasses - the non-treated SG 85 sample and the surface-modified SG 85M sample - and the bulk data are presented. Clearly, the surface-modified sample and the bulk exhibit essentially the same monoexponential decay, whereas the non-treated sample shows a second, distinctly slower response beside the bulk component. The fact that the slow part of the response is negligible in the surface-modified sample strongly indicates that there is a distinct layer of
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348-352
350
10 4
~
S
G
10 4
T = 106°C
SG 25 T = 115.5"C
10 3
10 3
85 .~ 10 2
\~._~:----~
85M
SG 50
10 2
vl
101
101
10°
i0 °
10 - 1
-1
10 - 1
o
i
2
3
4
5
6
-1
0
time (nsec)
2
3
4
6
5
time (nsec)
Fig. 2. Measured transient grating signal at T = 106°C for bulk 5CB and the porous glass samples SG 85 (untreated) and SG 85M (surface-modified).
5CB .molecules near the pore walls with a slower collective reorientation. This is in agreement with single particle reorientation studies of Warnock et al. [2] and Liu et al. [3] on nitrobenzene, pyridine [3,1 and other polar liquids in porous silica glasses. As mentioned above they also have found evidence for a more viscous surface layer. However, in contrast to their results the relaxation time of the surface component is pore size dependent as can be seen from the slope of the slow component in Fig. 3. In order to get a more quantitative picture a fit of the data was carried out. There is a small oscillatory contribution to the data, especially evident in the case of the SG 25 sample, which can be attributed to the generation of shear waves in the glass samples by impulsive stimulated Brillouinscattering [14,1. Assuming an exponential reorientational decay for the surface layer with a relaxation time z~ and a shear wave signal contribution described by a damped sine function (frequency o~, attenuation ~) [14-1, the following form of the fit function is obtained:
I(t) = [Aexp(--t/Zb)
1
Fig. 3. Measured transient grating signal at T = 115.5°C for bulk 5CB and the porous glass samples SG 85, SG 50 and SG 25.
12 • o o o
"~ lO m
5CB bulk SG 85 SG 5 0 SG 25
8
tl o
11
4 M
i 2
90
I00 temperature
ii0
120
(*C)
Fig. 4. Temperature and porc size dependence of the surface layer relaxation timc r~, obtained from a fit of the date according to Eq. (1).The bulk data are also included. Dotted lincs provide a guideline to the eye.
wave component will be presented in a further publication. The results of the least-squares data fitting according to Eq. (1) are shown in Fig. 4 as well as Table 1.
+ Bexp(-t/Zs)
+ Csin(~ot)exp(-ctt)] 2 ,
(1)
where Zb was set equal to the measured reorientation time of the bulk 5CB. The amplitude C of the shear wave component has always been small (C ~< 0.03) and has not shown any significant temperature dependence. More details about the shear
4. Discussion In Fig. 4 the temperature and pore size dependence of the surface layer relaxation time zs is depicted. From our experimental data a significant difference in zs between the SG 50 and the SG 85 sample cannot be inferred. However, the SG 25
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348-352 Table 1 Reorientation time, zs, relative signal contribution, B/(A + B), and calculated layer thickness, l, of the surface layer as well as the bulk reorientation time for 5CB confined in porous glass of different pore sizes. Values were obtained by a fit of the data according to Eq. (1). For more details refer to the text T = 90°C
T = 106°C
T = 115.5°C
SG25
zs(ns): B/(A + B): I(A)
10.1 +_ 1.2 0.34 6.6
5.6 + 0.5 0.34 6.5
4.2 + 0.2 0.31 6.1
SG50
zs(ns): B/(A + B): l(A)
7.0 + 0.7 0.23 8.9
2.9 _+ 0.3 0.31 9.4
3.1 _+ 0.7 0.24 9.1
SG85
zs(ns): B/{A + B): l(A,)
6.3 + 0.3 0.12 8.9
2.7 + 0.1 0.18 9.9
2.5 + 0.5 0.15 9.5
bulk
zb(ns):
1.46
0.84
0.66
sample obviously shows a slower surface relaxation time than the other two samples. Assuming the validity of the Debye-Stoke-Einstein equation, which relates the reorientational relaxation to the viscosity, we may conclude that, compared to the bulk viscosity, the surface layer viscosity is more than six times higher for the SG 25 sample and 3-5 times higher for the SG 50 and SG 85 samples. However, there should be kept in mind that even at this temperature reorientational dynamics of 5CB have some cooperative character [5]. Therefore, a direct translation of reorientational times into viscosities through the Debye-Stoke-Einstein equation, which contains an effective volume of the rotating entity, is difficult. This pore size dependence of z~ is a surprising fact, and to our knowledge has not yet been reported. The studies of Jonas et al. and Warnock et al. have not seen this feature. However, the smallest pore diameters they have investigated were only 36 and 44 .~, respectively. As the data presented here suggest, the pore size effect on the surface relaxation time becomes significant only for the 25/~ sample. In our opinion the slower surface relaxation time inside the smaller pores can be attributed to the geometric effect of higher pore curvature, leading to a stronger local surface interaction of the 5CB molecules and to a higher effec-
351
tive viscosity. In the considered temperature range the ratio Zs/rb is essentially constant for each sample making it reasonable to assume that the surface and the bulk relaxation time are characterized by the same temperature dependence. This also implies that the cooperative character of the reorientational motion is the same in the surface layer and in the bulk. This temperature independence of Z~/Zb has also been found by Jonas et al. The relative signal contribution B/(A + B) of the surface layer for various temperatures and pore sizes is shown in Table 1. There is no strong temperature dependence of B/(A + B) perceptible. The obtained pore size dependence is expected due to geometrical reasons: if the surface layer thickness I is constant, the ratio R of the surface layer volume to the bulk volume Vbwill increase with decreasing pore radius r. With increasing ratio R the relative response B/(A + B) of the surface layer also will increase. Using the following simple model, the thickness I of the surface layer can be estimated. Assuming cylindrical pores with radius r one obtains for the ratio l/r = 1 - ( x ~ - + 1)-l. The anisotropy induced by the excitation pulses decreases with increased viscosity of the sample. Therefore, one cannot calculate the volume ratio R of the two phases with different viscosities directly from the measured signal ratio. Assuming that the response per molecule scales with its relaxation time [15], the ratio B/A of the signal amplitudes is given by B/A = ZbV~/zsVb= (rb/Z~)R. Using the experimental values of B/A and rb/r~ for each temperature and mean pore radius r, the layer thickness I has been calculated. The results are given in Table 1. The observed temperature independence of l is not unexpected because of the temperature independence of B/A and Zb/Z~. The calculated layer thickness I is almost identical for the SG 50 and SG 85 samples, whereas for the 25 ~ sample a significantly smaller layer thickness is obtained. This is consistent with earlier observations by Hench and West [16] of pore-size-dependent hydration layers in porous glasses. As in the case of the relaxation time z~, this is evidence for a modification of the surface layer through the increasing geometrical restriction in the pore.
352
G. Sct, wal~, et al. / Journal o f Non-Crystalline Solids 172-174 (1994) 348-352
5. Conclusions
This study of pentylcyanobiphenyl confined in the nanometer lengthscale pores of silica glasses has revealed the existence of a distinct surface layer besides the bulk phase. The structure of this surface layer was shown to be modified by the geometrical restriction of the pores leading to a pore size dependence of the surface layer thickness and of the surface layer relaxation time.
References [1] J. Klafter and J.M. Drake, eds. Molecular Dynamics in Restricted Geometries (Wiley, New York, 1989). [2] J. Warnock, D.D. Awschalom and M.W. Shafer, Phys. Rev. B34 (1986) 475. [3] G. Liu, Y. Li and J. Jonas, J. Chem. Phys. 95 (1991) 6892.
[4] P.G.de Gennes, The Physics of Liquid Crystals (Clarendon, Oxford, 1974). [5] F.W. Deeg, S.R. Greenfield, J.J. Stankus, V.J. Newell and M.D. Fayer, J. Chem. Phys. 93 (1990) 3503. [6] S. Kralj, S. Zumer and D.W. Allender, Phys. Rev. A43 (1991) 2943. [7] G.S. Iannacchione and D. Finotello, Phys. Rev. Lett. 69 (1992) 2094. [8] G.P. Crawford, R. Stannarius and J.W. Doane, Phys. Rev. A44 (1991) 2558. [9] M.W. Shafer, D.D. Awschalom and J. Warnock, J. Appl. Phys. 61 (1987) 5438. [10] F.W. Deeg and M.D. Fayer, J. Chem. Phys. 91 (1989) 2269. [11] F.W. Deeg, K. Diercksen, G. Schwalb, C. Br~iuchle and H. Reinecke, Phys. Rev. B44 (1991) 2830. [12] P.G. de Gennes, Molec. Cryst. Liq. Cryst. 12 (1971) 193. [13] G. Schwalb, F.W. Deeg and C. Br~iuche, in preparation. [14] K.A. Nelson, J. Appl. Phys. 53 (1982) 6060. [15] K. Sala and M.C. Richardson, Phys. Rev. A12 (1975) 1036. [16] L.L. Hench and J.K. West, Chem. Rev. 90 (1990) 33 (see p. 54).
ELSEVIER
L OF
Journal of Non-Crystalline Solids 172 174 (1994) 348 352
Influence of surface interaction on the reorientational dynamics of pentylcyanobiphenyl confined in a porous glass G. Schwalb, F.W. Deeg*, C. Brfiuchle lnstitut fiJr Physikalische Chemie, Universitiit Miinchen, Sophienstrasse 11, 80333 Munich, Germany
Abstract Time-resolved transient grating optical Kerr effect experiments were employed to investigate the reorientational dynamics of pentylcyanobiphenyl confined in the nanometer lengthscale pores of silica glasses. Temperature and pore-size-dependent measurements show the drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition, a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction to 25/~, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is attributed to the modification of the surface layer structure by geometrical factors, e.g., the pore curvature.
1. Introduction In recent years a series of studies has investigated the influence of confining geometry and surface interaction on the structural and dynamic properties of liquids [1]. Optical birefringence [2] and nuclear magnetic resonance [3] measurements of the molecular reorientational motion of strongly interacting simple liquids in porous silica glasses have revealed the existence of a more viscous surface layer beside the bulk component. These experiments have focused on single particle correlation times. In our investigation the isotropic phase of confined pentylcyanobiphenyl (5CB), a well-known
* Corresponding author. Tel: +49-89 5902 328. Telefax: +49895902 602.
nematogenic liquid, was studied. Due to the strong anisotropic intermolecular interaction of the mesogens, there exists a short range orientationai order among the molecules leading to critical pretransitional effects [4] and to cooperative reorientational dynamics even at temperatures 80 K above the nematic-isotropic phase transition [5]. Most of the experimental and theoretical work on confined liquid crystals so far has been devoted to static properties like a temperature shift of the phase transition [6,7] and surface-induced orientational order [8]. In this study the reorientational dynamics of 5CB confined in porous silica glass were investigated for a wide temperature range from the bulk phase transition up to 80 K above the phase transition. This paper will focus on the high temperature behavior between 50 and 80 K above the phase transition, which reveals the contribution of a distinct surface layer.
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 1 3 6 - B
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348 352 30
2. E x p e r i m e n t a l procedures
i
3. Results
As was shown by Deeg et al. [5] the collective reorientational dynamics of the bulk 5CB are characterized by a monoexponential decay with a single reorientational correlation time, rb, for the whole temperature range 35-120°C. On approaching the phase transition TN~ from higher temperatures the reorientation time diverges as predicted by the Landau~leGennes theory [12-1, a consequence of the growing orientational correlation length between the mesogens. To get an overview on the collective dynamics of the nematogenic substance inside the pores, the temperature dependence of the mean relaxation time is depicted in Fig. 1. From
%
~,
20 0
i
i
i
\
25
Sol-gel prepared porous glasses represent an excellent host material to study the behavior of liquids in confined geometries. The highly transparent glasses, purchased from Geltech Inc., are characterized by high specific surface areas (400-700mE/g) and high specific pore volumes (0.4q3.9 cm3/g) with a well-defined narrow pore size distribution [9]. The surface of the unmodified pores is covered with polar OH-groups. Chemical treatment with dichlorodimethylsilane allows one to replace these polar OH-groups with less polar groups of-OSi(CH3)2OCzH 5 and systematically alter the surface interaction with the polar 5CB molecules. In this study three non-treated sol-gel glasses with mean pore diameters of 85 ~ (SG 85), 50~ (SG 50) and 25/~ (SG 25) as well as a surface-modified sample with a mean pore diameter of 85/~ (SG 85M) were employed. 5CB from Merck was used without further purification. The nematic-isotropic phase transition temperature of the bulk is TN~ = 35.2°C. The disk-shaped glasses (10 mm diameter, 5 mm thickness) were immersed in a 5CB filled spectroscopic cuvette filled with 5CB which was inserted into a variable temperature cell. The cell temperature could be controlled within _+ 0.1 K. A transient grating optical Kerr effect setup [10] was used for the investigations described here. A more detailed description of the laser apparatus can be found in Ref. [11].
~
349
• 5CB bulk ~ SG 85M
\
o sG 85
,~
o SG 50
\
o so 25
15
~o
I0 5 0
o o
40
surface
[] ~ . ^ _ ~
confinement I
20
° ~; ~~
effects
effects
~
........ ~
I
I
60
80
temperature
I
100
120
(°C)
Fig. 1. Temperature dependence of the mean relaxation time, obtained by a monoexponential fit of the transient signal, for the 5CB bulk and the 5CB filled porous glass samples SG 85, SG 85M, SG 50 and SG 25. Dotted lines provide a guideline to the eye.
this figure the strong influence of the pore size on the reorientational dynamics is highly evident. Compared to the bulk data, two temperature ranges with very different behaviors can be distinguished. At temperatures between the phase transition and 70°C the confined liquid exhibits faster collective reorientational dynamics. A closer look on the decays reveals a non-exponential orientational decay of the liquid inside the pores. The analysis of this low temperature part of the reorientational dynamics which is dominated by the limitation of the bulk correlation length through the size of the pores will be presented elsewhere [13]. In this paper, we discuss data in the temperature range 85-120°C where the reorientational dynamics of the confined liquid is obviously slowed down in comparison to the bulk. In this temperature range the decay data also reveal a significant deviation from a monoexponential decay. The reason for this deviation can be seen in Fig. 2, where the measured decays of the two 85,~ glasses - the non-treated SG 85 sample and the surface-modified SG 85M sample - and the bulk data are presented. Clearly, the surface-modified sample and the bulk exhibit essentially the same monoexponential decay, whereas the non-treated sample shows a second, distinctly slower response beside the bulk component. The fact that the slow part of the response is negligible in the surface-modified sample strongly indicates that there is a distinct layer of
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348-352
350
10 4
~
S
G
10 4
T = 106°C
SG 25 T = 115.5"C
10 3
10 3
85 .~ 10 2
\~._~:----~
85M
SG 50
10 2
vl
101
101
10°
i0 °
10 - 1
-1
10 - 1
o
i
2
3
4
5
6
-1
0
time (nsec)
2
3
4
6
5
time (nsec)
Fig. 2. Measured transient grating signal at T = 106°C for bulk 5CB and the porous glass samples SG 85 (untreated) and SG 85M (surface-modified).
5CB .molecules near the pore walls with a slower collective reorientation. This is in agreement with single particle reorientation studies of Warnock et al. [2] and Liu et al. [3] on nitrobenzene, pyridine [3,1 and other polar liquids in porous silica glasses. As mentioned above they also have found evidence for a more viscous surface layer. However, in contrast to their results the relaxation time of the surface component is pore size dependent as can be seen from the slope of the slow component in Fig. 3. In order to get a more quantitative picture a fit of the data was carried out. There is a small oscillatory contribution to the data, especially evident in the case of the SG 25 sample, which can be attributed to the generation of shear waves in the glass samples by impulsive stimulated Brillouinscattering [14,1. Assuming an exponential reorientational decay for the surface layer with a relaxation time z~ and a shear wave signal contribution described by a damped sine function (frequency o~, attenuation ~) [14-1, the following form of the fit function is obtained:
I(t) = [Aexp(--t/Zb)
1
Fig. 3. Measured transient grating signal at T = 115.5°C for bulk 5CB and the porous glass samples SG 85, SG 50 and SG 25.
12 • o o o
"~ lO m
5CB bulk SG 85 SG 5 0 SG 25
8
tl o
11
4 M
i 2
90
I00 temperature
ii0
120
(*C)
Fig. 4. Temperature and porc size dependence of the surface layer relaxation timc r~, obtained from a fit of the date according to Eq. (1).The bulk data are also included. Dotted lincs provide a guideline to the eye.
wave component will be presented in a further publication. The results of the least-squares data fitting according to Eq. (1) are shown in Fig. 4 as well as Table 1.
+ Bexp(-t/Zs)
+ Csin(~ot)exp(-ctt)] 2 ,
(1)
where Zb was set equal to the measured reorientation time of the bulk 5CB. The amplitude C of the shear wave component has always been small (C ~< 0.03) and has not shown any significant temperature dependence. More details about the shear
4. Discussion In Fig. 4 the temperature and pore size dependence of the surface layer relaxation time zs is depicted. From our experimental data a significant difference in zs between the SG 50 and the SG 85 sample cannot be inferred. However, the SG 25
G. Schwalb et al. / Journal of Non-Crystalline Solids 172-174 (1994) 348-352 Table 1 Reorientation time, zs, relative signal contribution, B/(A + B), and calculated layer thickness, l, of the surface layer as well as the bulk reorientation time for 5CB confined in porous glass of different pore sizes. Values were obtained by a fit of the data according to Eq. (1). For more details refer to the text T = 90°C
T = 106°C
T = 115.5°C
SG25
zs(ns): B/(A + B): I(A)
10.1 +_ 1.2 0.34 6.6
5.6 + 0.5 0.34 6.5
4.2 + 0.2 0.31 6.1
SG50
zs(ns): B/(A + B): l(A)
7.0 + 0.7 0.23 8.9
2.9 _+ 0.3 0.31 9.4
3.1 _+ 0.7 0.24 9.1
SG85
zs(ns): B/{A + B): l(A,)
6.3 + 0.3 0.12 8.9
2.7 + 0.1 0.18 9.9
2.5 + 0.5 0.15 9.5
bulk
zb(ns):
1.46
0.84
0.66
sample obviously shows a slower surface relaxation time than the other two samples. Assuming the validity of the Debye-Stoke-Einstein equation, which relates the reorientational relaxation to the viscosity, we may conclude that, compared to the bulk viscosity, the surface layer viscosity is more than six times higher for the SG 25 sample and 3-5 times higher for the SG 50 and SG 85 samples. However, there should be kept in mind that even at this temperature reorientational dynamics of 5CB have some cooperative character [5]. Therefore, a direct translation of reorientational times into viscosities through the Debye-Stoke-Einstein equation, which contains an effective volume of the rotating entity, is difficult. This pore size dependence of z~ is a surprising fact, and to our knowledge has not yet been reported. The studies of Jonas et al. and Warnock et al. have not seen this feature. However, the smallest pore diameters they have investigated were only 36 and 44 .~, respectively. As the data presented here suggest, the pore size effect on the surface relaxation time becomes significant only for the 25/~ sample. In our opinion the slower surface relaxation time inside the smaller pores can be attributed to the geometric effect of higher pore curvature, leading to a stronger local surface interaction of the 5CB molecules and to a higher effec-
351
tive viscosity. In the considered temperature range the ratio Zs/rb is essentially constant for each sample making it reasonable to assume that the surface and the bulk relaxation time are characterized by the same temperature dependence. This also implies that the cooperative character of the reorientational motion is the same in the surface layer and in the bulk. This temperature independence of Z~/Zb has also been found by Jonas et al. The relative signal contribution B/(A + B) of the surface layer for various temperatures and pore sizes is shown in Table 1. There is no strong temperature dependence of B/(A + B) perceptible. The obtained pore size dependence is expected due to geometrical reasons: if the surface layer thickness I is constant, the ratio R of the surface layer volume to the bulk volume Vbwill increase with decreasing pore radius r. With increasing ratio R the relative response B/(A + B) of the surface layer also will increase. Using the following simple model, the thickness I of the surface layer can be estimated. Assuming cylindrical pores with radius r one obtains for the ratio l/r = 1 - ( x ~ - + 1)-l. The anisotropy induced by the excitation pulses decreases with increased viscosity of the sample. Therefore, one cannot calculate the volume ratio R of the two phases with different viscosities directly from the measured signal ratio. Assuming that the response per molecule scales with its relaxation time [15], the ratio B/A of the signal amplitudes is given by B/A = ZbV~/zsVb= (rb/Z~)R. Using the experimental values of B/A and rb/r~ for each temperature and mean pore radius r, the layer thickness I has been calculated. The results are given in Table 1. The observed temperature independence of l is not unexpected because of the temperature independence of B/A and Zb/Z~. The calculated layer thickness I is almost identical for the SG 50 and SG 85 samples, whereas for the 25 ~ sample a significantly smaller layer thickness is obtained. This is consistent with earlier observations by Hench and West [16] of pore-size-dependent hydration layers in porous glasses. As in the case of the relaxation time z~, this is evidence for a modification of the surface layer through the increasing geometrical restriction in the pore.
352
G. Sct, wal~, et al. / Journal o f Non-Crystalline Solids 172-174 (1994) 348-352
5. Conclusions
This study of pentylcyanobiphenyl confined in the nanometer lengthscale pores of silica glasses has revealed the existence of a distinct surface layer besides the bulk phase. The structure of this surface layer was shown to be modified by the geometrical restriction of the pores leading to a pore size dependence of the surface layer thickness and of the surface layer relaxation time.
References [1] J. Klafter and J.M. Drake, eds. Molecular Dynamics in Restricted Geometries (Wiley, New York, 1989). [2] J. Warnock, D.D. Awschalom and M.W. Shafer, Phys. Rev. B34 (1986) 475. [3] G. Liu, Y. Li and J. Jonas, J. Chem. Phys. 95 (1991) 6892.
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