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Monolayers on a glycerol subphase
Thin Solid Films, 133 (1985) 113 116 PREPARATIONAND CHARACTERIZATION
113
MONOLAYERS ON A GLYCEROL SUBPHASE* A. BARRAUD,J. LELOUPAND P. LESIEUR CEA-IRDI-DESICP-DOpartement de Physico-Chimie, Service de Chimie MolOculaire, Centre d'Etudes Nuclkaires de Saclay, 91191 Gif-sur- Yvette COdex (France)
(ReceivedJune 4, 1985;accepted August 16, 1985)
As only a small amount of data is available on non-aqueous subphases employed in the Langmuir technique, it was decided to test glycerol as a filmforming subphase. Glycerol offers the advantage of being immiscible with acetonitrile which can hence be used as a spreading solvent. Behenic acid and three amphiphilic charge transfer complexes (docosylpyridinium-TCNQ (I:1), benzot h i a z o l i u m - T C N Q - i o d i n e (2:1:2), and docosylpyridinium-TCNQ-iodine (3:2: 3)) were successfully spread. The properties of these films on glycerol are presented and discussed. One of these compounds gives rise to a highly conducting monomolecular film on the trough (a few ohms per centimetre). Its high sensitivity to mechanical perturbations is discussed.
1. INTRODUCTION Acetonitrile is a polar aprotic solvent in which several charge transfer complexes may be synthesized and are stable x. Unfortunately acetonitrile, being miscible with water, is unfit for the standard Langmuir-Blodgett technique. Spreading from acetonitrile thus needs a change of subphase from water to another polar solvent immiscible with acetonitrite. Glycerol and mercury are possible choices. Few data are available on non-aqueous subphases for the Langmuir technique 2'3. Amphiphilic molecules have been shown to lie fiat on mercury 3, showing that this subphase is unfit for the fabrication of compact Langmuir films. Since no data at all are available on pure glycerol, we decided to explore the filmforming properties of this new subphase (for glycerol used as an additive to water with biological molecules see refs. 4-7). The aim of this set of experiments using acetonitrile as a spreading solvent was to explore the properties of monomolecular films of amphiphilic charge transfer complexes, among which a few have been found to conduct electricity when in powder form. * Paper presented at the Second International Conference on Langmuir-Blodgett Films, Schenectady, NY, U.S.A.,July 1-4, 1985. 0040-6090/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
1 14
A. BARRAUD, J. LELOUP, P. LESIEUR
2. EXPERIMENTAL The trough (75 cm × 7 cm) is classical, with a mobile barrier. It was made from bulk P T F E and its cabinet was flushed with nitrogen. The surface pressure was monitored by a Wilhelmy probe made of filter paper and connected to a magnetic transducer. The measured pressure was compared to the pre-set value and this pressure difference was used to drive the mobile barrier. Glycerol was purchased from Prolabo and was used without further purification. Tetracyanoquinodimethane amphiphilic complexes were obtained in the form of powders and then dissolved in commercial acetonitrile at a concentration of 5 x 10 -4 M. Commercial behenic acid (Fluka) was dissolved in a chloroformacetonitrile mixture (1: 2) for easy dissolution. Areas on the trough were recorded within 1 rain after equilibrium was reached. Resistance measurements at the subphase surface were made with a Keithley 616 A instrument, used as an ohmmeter, and a two-terminal probe (1 m m in diameter, 3 m m apart), gently brought in contact with the film at the subphase surface. 3. RESULTS AND DISCUSSION Figure 1 shows the compression isotherms (pressure n v e r s u s area per molecule A) for behenic acid (A), docosylpyridinium-TCNQ (1:1) complex (B), benzothiazol i u m - T C N Q - i o d i n e (2:1 : 2) c o m p l e x (C) and docosylpyridinium-TCNQ-iodine (3:2:3) complex (D). Table I gives the collapse pressures for each of these compounds on glycerol. Table II shows the results of resistance measurements. 1 7C
I
~B
I A
I
I
I
i
50
qo
30
20
10 30 I
A (A2/~LECULE) Fig. 1. Compression isotherms (surface pressure rc v s . area per molecule A, at 20 °C for behenic acid (A), docosylpyridinium-TCNQ (1 : 1) complex (B), benzothiazolium-TCNQ-iodine (2:1 : 2) complex (C), and docosylpyridinium-TCNQ-iodine (3 : 2: 3) complex (D).
MONOLAYERS
TABLE
ON A GLYCEROL
115
SUBPHASE
I
COLLAPSE PRESSURES ON GLYCEROL FOR THE COMPOUNDS STUDIED
Compound Collapse pressure ( m N m - 1)
TABLE
A
B
C
D
55
70
56
52
II
RESISTANCE MEASUREMENTS FOR FILMS ON GLYCEROL OF THE COMPOUNDS STUDIED
Parameter
Film on glycerol No film
M e a s u r e d r e s i s t a n c e (f2) A v e r a g e v a l u e (f~)
4 - 1 2 × 10 8 7 x 10 s
A 4-12 x l0 s 7 × 10 8
B 4-12 x l0 s 7 × 10 s
C 4-12 x l0 s 7 x 10 s
D 0 . 8 - 4 x 10 8 1.5 x 10 s
S l i g h t t o u c h v a l u e (f~)
12 x 10 8
1 2 x 10 8
1 2 x 10 s
1 2 x 10 a
0.8 × 10 s
Nature of the film
--
Insulating
Insulating
Insulating
Conducting
Although glycerol behaves differently from water, the first obvious result is that workable Langmuir films can be obtained on a glycerol subphase. A difference between glycerol and water is the absence of film-forming molecule desorption from glycerol. Molecules are presumably present in glycerol but they do not desorb since glycerol is a better solvent than water. These better solvent capabilities are confirmed by the film behaviour at long times: whatever the pressure the mobile barrier moves slowly forwards, indicating barrier leakage, collapse or dissolution. Collapse can be ruled out since it is easily detected on coloured films. Barrier leakage can be also easily ruled out since no film is found behind the barrier on backward compression. Dissolution slowly removes film from the surface and gives glycerol a faint blue tinge in the dipping well after a few experiments on the same subphase. This effect limits film lifetime to a few hours on glycerol. For the same reason, areas per molecule are only indicative and cannot be taken as absolute values since they depend on the drying time. Another drawback of glycerol lies in its high viscosity. Moving the mobile barrier gives rise to difference in levels between the front and the rear part of the trough. Hence the barrier has to be operated at a very low speed (typically 0.1 cm s-l). The behaviour of the film on glycerol is similar but not identical to that on water. For instance, the compression isotherm for behenic acid shows the same two characteristic regions (a higher-compressibility, low-pressure region and a lowercompressibility, high-pressure region) but the transition pressure is 25 m N m - 1 instead of 35 m N m t. Also the collapse pressure is lower on glycerol: 55 m N m - 1. The opposite behaviour is observed with compound B, which shows a collapse pressure much higher on glycerol (70 m N m - 1) than on water (40 m N m - 1). Film structure may also be different on glycerol since compound C shows a phase transition when on glycerol whereas its compression isotherm shows no such transition when on water. In situ conductivity measurements are an attractive feature of glycerol. Water, especially when stored in air, is too conductive to show any difference when topped
116
A. BARRAUD, J. LELOUP, P. LESIEUR
by a conducting monolayer. On the other hand, glycerol exhibits low conductivity and allows film conductivity measurements. Pure glycerol shows a resistance between probe electrodes ranging from 0.4 to 1.2 x 109 fl (depending on dipping depth). As expected, films of behenic acid and of compound B showed no extra conductance. Compound C, which is highly conducting in the powder form, surprisingly showed no measurable conductance in any conditions when spread on glycerol. An explanation for this can be found in the fast evolution of the solution with a partial destruction of the charge transfer complex. On the other hand, films of compound D decreased the measured resistance by a factor of ten. This shows that the film is highly conductive (a few fl cm if conduction is assumed to be localized in the polar plane). Touching the film gently with the measuring electrodes gave the best results (lowest resistance). Moving the electrodes either further down or sideways along the film plane always made the film more resistive. Even small electrode movements perturbed the film to such an extent that the resistance often went back to the value for pure glycerol. This underlines the very high sensitivity of low dimensional conductivity to defects and cracks. In this respect the high viscosity of glycerol helps to damp vibrations and waves at the subphase surface and makes glycerol a good candidate for the fabrication of the high-quality films needed for high conductivity. 4. CONCLUSIONS Glycerol turns out to be a subphase suitable for Langmuir film fabrication. Films withstand a reasonably high surface pressure and show enough time stability to be workable, in spite of a slow dissolution in the subphase which does not take place in water. The high viscosity of glycerol is both a drawback and an advantage: compression speed has to be kept low but, on the other hand, undesirable waves and vibrations are well damped. Advantage can be taken of glycerol-acetonitrile immiscibility to make Langmuir films ofamphiphilic charge transfer complexes, a few of which are electrically conductive. Another advantage of glycerol lies in its high intrinsic resistivity which allows the electrical properties of the film on the trough to be studied. As a matter of fact, highly conducting Langmuir films can be made and studied on glycerol. As expected, they show up an extreme sensitivity to defects and cracks because of their low dimensionality; in this respect glycerol seems a better subphase than water for conductive film fabrication because of its intrinsic damping properties. REFERENCES 1 L.R. Melby, H. J. Harder, W. R. Hertler, W. Mahler, R. E. Benson and W. E. Mochel, J. Am. Chem. Sot., 84 (1962) 3374. 2 A . H . Ellison and W. A. Zisman, J. Phys. Chem., 60 (1956) 416. 3 A . H . Ellison, J. Phys. Chem., 66 (1962) 1867. 4 D.A. Cadenhead and R. J. Demchak, J. Colloid Interface Sci., 24 (1967) 484. 5 D.A. Cadenhead and R. J. Demchak, J. Colloid Interface Sci., 30 (1969) 76. 6 D . A . Cadenhead and K. E. Bean, Biochim. Biophys. Acta, 290 (1972) 43. 7 D . A . Cadenhead and R. J. Demchak, Biochim. Biophys. Acta, 176 (1969) 849.
113
MONOLAYERS ON A GLYCEROL SUBPHASE* A. BARRAUD,J. LELOUPAND P. LESIEUR CEA-IRDI-DESICP-DOpartement de Physico-Chimie, Service de Chimie MolOculaire, Centre d'Etudes Nuclkaires de Saclay, 91191 Gif-sur- Yvette COdex (France)
(ReceivedJune 4, 1985;accepted August 16, 1985)
As only a small amount of data is available on non-aqueous subphases employed in the Langmuir technique, it was decided to test glycerol as a filmforming subphase. Glycerol offers the advantage of being immiscible with acetonitrile which can hence be used as a spreading solvent. Behenic acid and three amphiphilic charge transfer complexes (docosylpyridinium-TCNQ (I:1), benzot h i a z o l i u m - T C N Q - i o d i n e (2:1:2), and docosylpyridinium-TCNQ-iodine (3:2: 3)) were successfully spread. The properties of these films on glycerol are presented and discussed. One of these compounds gives rise to a highly conducting monomolecular film on the trough (a few ohms per centimetre). Its high sensitivity to mechanical perturbations is discussed.
1. INTRODUCTION Acetonitrile is a polar aprotic solvent in which several charge transfer complexes may be synthesized and are stable x. Unfortunately acetonitrile, being miscible with water, is unfit for the standard Langmuir-Blodgett technique. Spreading from acetonitrile thus needs a change of subphase from water to another polar solvent immiscible with acetonitrite. Glycerol and mercury are possible choices. Few data are available on non-aqueous subphases for the Langmuir technique 2'3. Amphiphilic molecules have been shown to lie fiat on mercury 3, showing that this subphase is unfit for the fabrication of compact Langmuir films. Since no data at all are available on pure glycerol, we decided to explore the filmforming properties of this new subphase (for glycerol used as an additive to water with biological molecules see refs. 4-7). The aim of this set of experiments using acetonitrile as a spreading solvent was to explore the properties of monomolecular films of amphiphilic charge transfer complexes, among which a few have been found to conduct electricity when in powder form. * Paper presented at the Second International Conference on Langmuir-Blodgett Films, Schenectady, NY, U.S.A.,July 1-4, 1985. 0040-6090/85/$3.30
© Elsevier Sequoia/Printed in The Netherlands
1 14
A. BARRAUD, J. LELOUP, P. LESIEUR
2. EXPERIMENTAL The trough (75 cm × 7 cm) is classical, with a mobile barrier. It was made from bulk P T F E and its cabinet was flushed with nitrogen. The surface pressure was monitored by a Wilhelmy probe made of filter paper and connected to a magnetic transducer. The measured pressure was compared to the pre-set value and this pressure difference was used to drive the mobile barrier. Glycerol was purchased from Prolabo and was used without further purification. Tetracyanoquinodimethane amphiphilic complexes were obtained in the form of powders and then dissolved in commercial acetonitrile at a concentration of 5 x 10 -4 M. Commercial behenic acid (Fluka) was dissolved in a chloroformacetonitrile mixture (1: 2) for easy dissolution. Areas on the trough were recorded within 1 rain after equilibrium was reached. Resistance measurements at the subphase surface were made with a Keithley 616 A instrument, used as an ohmmeter, and a two-terminal probe (1 m m in diameter, 3 m m apart), gently brought in contact with the film at the subphase surface. 3. RESULTS AND DISCUSSION Figure 1 shows the compression isotherms (pressure n v e r s u s area per molecule A) for behenic acid (A), docosylpyridinium-TCNQ (1:1) complex (B), benzothiazol i u m - T C N Q - i o d i n e (2:1 : 2) c o m p l e x (C) and docosylpyridinium-TCNQ-iodine (3:2:3) complex (D). Table I gives the collapse pressures for each of these compounds on glycerol. Table II shows the results of resistance measurements. 1 7C
I
~B
I A
I
I
I
i
50
qo
30
20
10 30 I
A (A2/~LECULE) Fig. 1. Compression isotherms (surface pressure rc v s . area per molecule A, at 20 °C for behenic acid (A), docosylpyridinium-TCNQ (1 : 1) complex (B), benzothiazolium-TCNQ-iodine (2:1 : 2) complex (C), and docosylpyridinium-TCNQ-iodine (3 : 2: 3) complex (D).
MONOLAYERS
TABLE
ON A GLYCEROL
115
SUBPHASE
I
COLLAPSE PRESSURES ON GLYCEROL FOR THE COMPOUNDS STUDIED
Compound Collapse pressure ( m N m - 1)
TABLE
A
B
C
D
55
70
56
52
II
RESISTANCE MEASUREMENTS FOR FILMS ON GLYCEROL OF THE COMPOUNDS STUDIED
Parameter
Film on glycerol No film
M e a s u r e d r e s i s t a n c e (f2) A v e r a g e v a l u e (f~)
4 - 1 2 × 10 8 7 x 10 s
A 4-12 x l0 s 7 × 10 8
B 4-12 x l0 s 7 × 10 s
C 4-12 x l0 s 7 x 10 s
D 0 . 8 - 4 x 10 8 1.5 x 10 s
S l i g h t t o u c h v a l u e (f~)
12 x 10 8
1 2 x 10 8
1 2 x 10 s
1 2 x 10 a
0.8 × 10 s
Nature of the film
--
Insulating
Insulating
Insulating
Conducting
Although glycerol behaves differently from water, the first obvious result is that workable Langmuir films can be obtained on a glycerol subphase. A difference between glycerol and water is the absence of film-forming molecule desorption from glycerol. Molecules are presumably present in glycerol but they do not desorb since glycerol is a better solvent than water. These better solvent capabilities are confirmed by the film behaviour at long times: whatever the pressure the mobile barrier moves slowly forwards, indicating barrier leakage, collapse or dissolution. Collapse can be ruled out since it is easily detected on coloured films. Barrier leakage can be also easily ruled out since no film is found behind the barrier on backward compression. Dissolution slowly removes film from the surface and gives glycerol a faint blue tinge in the dipping well after a few experiments on the same subphase. This effect limits film lifetime to a few hours on glycerol. For the same reason, areas per molecule are only indicative and cannot be taken as absolute values since they depend on the drying time. Another drawback of glycerol lies in its high viscosity. Moving the mobile barrier gives rise to difference in levels between the front and the rear part of the trough. Hence the barrier has to be operated at a very low speed (typically 0.1 cm s-l). The behaviour of the film on glycerol is similar but not identical to that on water. For instance, the compression isotherm for behenic acid shows the same two characteristic regions (a higher-compressibility, low-pressure region and a lowercompressibility, high-pressure region) but the transition pressure is 25 m N m - 1 instead of 35 m N m t. Also the collapse pressure is lower on glycerol: 55 m N m - 1. The opposite behaviour is observed with compound B, which shows a collapse pressure much higher on glycerol (70 m N m - 1) than on water (40 m N m - 1). Film structure may also be different on glycerol since compound C shows a phase transition when on glycerol whereas its compression isotherm shows no such transition when on water. In situ conductivity measurements are an attractive feature of glycerol. Water, especially when stored in air, is too conductive to show any difference when topped
116
A. BARRAUD, J. LELOUP, P. LESIEUR
by a conducting monolayer. On the other hand, glycerol exhibits low conductivity and allows film conductivity measurements. Pure glycerol shows a resistance between probe electrodes ranging from 0.4 to 1.2 x 109 fl (depending on dipping depth). As expected, films of behenic acid and of compound B showed no extra conductance. Compound C, which is highly conducting in the powder form, surprisingly showed no measurable conductance in any conditions when spread on glycerol. An explanation for this can be found in the fast evolution of the solution with a partial destruction of the charge transfer complex. On the other hand, films of compound D decreased the measured resistance by a factor of ten. This shows that the film is highly conductive (a few fl cm if conduction is assumed to be localized in the polar plane). Touching the film gently with the measuring electrodes gave the best results (lowest resistance). Moving the electrodes either further down or sideways along the film plane always made the film more resistive. Even small electrode movements perturbed the film to such an extent that the resistance often went back to the value for pure glycerol. This underlines the very high sensitivity of low dimensional conductivity to defects and cracks. In this respect the high viscosity of glycerol helps to damp vibrations and waves at the subphase surface and makes glycerol a good candidate for the fabrication of the high-quality films needed for high conductivity. 4. CONCLUSIONS Glycerol turns out to be a subphase suitable for Langmuir film fabrication. Films withstand a reasonably high surface pressure and show enough time stability to be workable, in spite of a slow dissolution in the subphase which does not take place in water. The high viscosity of glycerol is both a drawback and an advantage: compression speed has to be kept low but, on the other hand, undesirable waves and vibrations are well damped. Advantage can be taken of glycerol-acetonitrile immiscibility to make Langmuir films ofamphiphilic charge transfer complexes, a few of which are electrically conductive. Another advantage of glycerol lies in its high intrinsic resistivity which allows the electrical properties of the film on the trough to be studied. As a matter of fact, highly conducting Langmuir films can be made and studied on glycerol. As expected, they show up an extreme sensitivity to defects and cracks because of their low dimensionality; in this respect glycerol seems a better subphase than water for conductive film fabrication because of its intrinsic damping properties. REFERENCES 1 L.R. Melby, H. J. Harder, W. R. Hertler, W. Mahler, R. E. Benson and W. E. Mochel, J. Am. Chem. Sot., 84 (1962) 3374. 2 A . H . Ellison and W. A. Zisman, J. Phys. Chem., 60 (1956) 416. 3 A . H . Ellison, J. Phys. Chem., 66 (1962) 1867. 4 D.A. Cadenhead and R. J. Demchak, J. Colloid Interface Sci., 24 (1967) 484. 5 D.A. Cadenhead and R. J. Demchak, J. Colloid Interface Sci., 30 (1969) 76. 6 D . A . Cadenhead and K. E. Bean, Biochim. Biophys. Acta, 290 (1972) 43. 7 D . A . Cadenhead and R. J. Demchak, Biochim. Biophys. Acta, 176 (1969) 849.