Langmuir 1993,9, 491-496
491
Surface Properties of Semiamphiphilic Dithiolium-TCNQ Salts at the Air-Water Interface 0. Fichet,+D. Ducharme,* V. Gionis,r P. Delha&s,**+ and R. M. Lebland Centre de Recherche en Photobiophysique, Universitb du Qukbec b Trois-RiviBres, C.P. 500, Trois-RiviBres, Qubbec G9A 5H7,Canada, and Centre de Recherche Paul Pascal, CNRS, Avenue A. Schweitzer, 33600 Pessac, France Received May 4, 1992. In Final Form: October 13, 1992
The process of molecular organization during the film compressionof dithiolium-anion salta has been studied by determining the surface pressure-area and surface potential-area isotherm together with ellipsometricmeasurementa. These experimentshave been carriedout as a functionof differentparameters such as the presence of salts in the subphase, temperature, and pH. We have demonstrated that the ellipsometric angle is sensitive to the involved polar heads. In particular, the dithiolium-TCNQ salt exhibita both a peculiar monolayer organization and a multilayer formation associated with a Ztype transfer which ie due to a special anion-cation molecular arrangement.
Introduction The control of supramolecular organization in the condensed state for obtaining valuable or new physical properties of materiale is currently a general aim.' The LangmukBlodgett (LB) technique is one of the most powerful tools to obtain a supramolecular architecture.2 Recently, there has been a considerable interest in using the LB technique to match donor and/or acceptor molecules with the intention of producing conducting or optoelectronicmolecular films.3 A variety of papers have been published on conducting LB films incorporating electroactive molecules as either electron acceptors, e.g. tetracyanoquinodimethane (TCNQ), or electron donors, e.g. tetrathiofulvalene (TTF) derivative~.~15 Different strategies have been developed to give rise to a mixed valence system with the ion-radical salts and charge transfer Among the different series of amphiphilic or eemiamphiphilic electroactive molecules, those based on TCNQ anion-radical aalts constitute currently the largest class of compounds. It appears nevertheless that only a very few series of these semiamphiphilicsalts give rise to both a stable Langmuir monolayer and good quality LB films.7 To controlthe building up of the LB films,it is essential to analyze and to control the monolayer organization at the gas-water interface. This point has been the primary goal of our investigations, even if we have to remember that in a second step, both the transfer conditions and a possible molecular reorganization have to be taken into + Centre de Recherche Paul Pascal,
CNRS.
* Centre deRecherche en Photobiophysique,UniveraitsduQu&bec
B Trois-RiviBres. 8 Permanent address: Theor. Phys. Chem. Institute, NHRF, 48, Vaseileoe Conetantinou Avenue, Athens 116/35, Greece. (1)See for instance Lower Dimensional Systems and Molecular Electronics; NATO Advanced Study Institute; Metzger, R. M., Day, P., Papavmiliou, G. C., Ede.; Plenum Prm: New York, 1991; Vol. B248. (2) MBbiue, D. Acc. Chem. Res. 1981,63,14. (3)Tieke, B.Adu. Mater. 1990,2,222. (4) Nakamura, T.; Tachibana, H.; Matsumoto, M.; Tanaka, M.;
Kanabata, Y . In LowerDimenaiowlSystemsand Molecular Electronics; NATO Advanced Study Imtitutq Metzger, R. M., Ed.; Plenum Press: New York, 1991; voL B248, p 619. (6) Vandevyver, M. In Lower Dimensiowl Systems and Molecular E[ectronics;NATO Advanced Study Institute; Metzger, R. M., Day, P., Papavwiliou, G. C., Ede.;Plenum Press: New York, 1991;Vol. B248, p 603. (6)Delhab, P. In Mixed Valency Systems: applications in chemietry, physics and biology; Praesidea, K., Ed.; Klu Academic Publishers: btsrdam, 1991; Vol. (2343, pp 247-271. (7) Richard, J.; DelhaBe, P.;Vandevyver, M. New J. Chem. 1991,15, 137.
consideration. We have therefore used ellipsometry, an optical nondestructive method based on the fact that the optical properties of the sample surface cause the polarization form of the reflected beam to differ from that of the incident one.8 In that regard, ellipsometry due to its high sensitivity is used for in situ monolayer structural investigation;from thesemeasurements,informationabout the film thickness and the refractive indices can be obtained. We have ala0 associated surface potential experiments, which inform about the surface density of dipoles, interpreted as the reorientation of the polar head of the amphiphilic molecules. Both measurements are complementary since the ellipsometric data give information on the organization of the aliphatic chains and the surface potential on the distribution of polar groups. Indeed, these properties and the surface pressure-area isotherms allow us to investigate the monolayer behavior, i.e. stability and phase transitions, at the gas-water interfacein order to understand the subsequentmultilayer organizationcrucial for the desired physical properties to be observed. In the present paper, we describe the behavior of a new series of semiamphiphilic Langmuir films,the l,a-dithiolium-X (with X = I-, TCNQ-, TCNQF4-),9 varying the compression rate of the monolayer, pH and temperature of the subphase, and the nature of the salts dissolved in the subphase, in order to fully characterize the Langmuir films before LB film fabrication.
Experimental Section Materials. The different compounds based on the 4,5-bis(octadecylthio)-3-methylthio-l,2-dithiolium cation salts (stoichiometry 1:l) were prepared as described previou~ly.~ The polar head group was easily modified by interchanging the involved anion which can be either diamagnetic (I-) or paramagnetic (TCNQ-,TCNQFd-) a~ shown. cia H37
1.2 dithiolium
TCNP'
TCNQh-
Deionized and prepurified water (Nanopure filter system, Barnstead,Boeton,MA)wastwicedietilledinaquartzstill(Mode1 (8)Azzam,R.M. A.;Bashara,N.M.EllipsometryandPolarizedL,ight, 1st ed.;North Holland: New York, 1977. (9) Gionis, V.; Fichet, 0.; Izumi, M.; Amiell,J.; Garrigou-Lngrange, C.; Papavassiliou, G.; Delhab, P. Chem. Lett. 1991, 871.
Q 1993 American
Chemical Society
492 Langmuir, Vol. 9, No.2, 1993
Bi-18, Amerisil Inc., Sayreville, NJ). The water has a specific resistivity of 18 M k m , and a surface tension higher than 71 mN-m-'. Reagent grade chloroform was distilled before use as spreading solvent. Sodium chloride from Fisher Scientific Ltd. (Montreal, Quebec) was purified as described previously.1° The spreading solvent and all the salts used were considered free of impurities if the dry residue from 5 mL of CHCla (or lo-' mol-L-l salt solution) redissolved in 0.1 mL and deposition at the airwater interface caused no movement of the float (vide infra) when the moving barrier was 1 cm away from it. Langmuir Trough. The monolayer trough sat on a micro g isolation type A table (Ealing Scientific Ltd., Montreal, Quebec). The trough was 110 X 15.5 X 0.5 cm3, and coated with Teflon, 0.8mm thick (Chemical Fabrics Corp., USA). Surface pressure and surface potential isotherms were automatically recorded by computer as previously reported." The Teflon moving barrier was ushed by an electric motor at the speed varying from 0.5 to 9H2-molecule-1.min-1. Surface pressure and surface potential isotherms were obtained by a continuous compression at a rate of 5 A*.molecule-l.min-l, whereas the ellipsometric isotherma were measured step by step at a compression rate of 0.5 A,-molecule-'.mi+. Surface pressure and surface potential isotherms are the same within the experimental error (vide infra) whatever the compression rate used. The trough and the room were temperature controlled with an accuracy of f 1 OC. The solutions were spread drop by drop with a 150-rL syringe over the aqueous surface having a spreading molecular area of over 200 A2-molecule-'. Prior to compression the monolayer was allowed to stand 10 min to let the solvent evaporate. All the isotherms were repeated at least 3 times with two different solutions in order to check their reproducibility. Surface potentials are measured by using an ionizing %lAmelectrode placed 1-2 mm about the surface and located a t approximately 5 cm from the float while the Pt reference is dipped in the subphaseell Deviations between surface pressure-area and surface potential-area isotherms were approximately f l A2.molecule-1 and f10 mV, respectively. Ellipsometer. The apparatus used for ellipsometric measurements at the ail-water interface has been previously described;12it is based on a fixed angle technique (& = 60.00 f 0.05O) working with an H e N e laser as a light source (A = 632.8 nm). Measurements were taken in two extinction zones of polarizer (P)and analyzer (A) azimuth angles which permit two ellipsometric angles A and $ to be defined by the following expression^:^^
Fichet et 01.
1 -
I5 ' C
20 'C
2
e
0 75
2
io,, 4
40-
8
30m
2o
IO
t
4 0 30
1
\ \I
10.I5
\ ,
0 100
50
0
Moltrular area
0
(a2)
0
150
Figure 1. Temperature effects on surface pressure (r)- and surface potential (Am-molecular area (A) curves for 1,2dithiolium-TCNQ. Subphase was a t pH 5.8: (7) 20 OC; (---) 15 "C; (- - -) 6 O C .
2o
t
lot
,
0
!I
'..'\ \
8
-0.10
+
A = 180 - (Pl P2) $ = 90
+ (A, - A1)/2
For a thin uniaxial nonabsorbing and homogeneous film with the optical axis normal to the interface, the $angle has to be constant as found for the dithiolium salts studied in this work. -The ellipsometric data measured as 6A = A - A, where A and A are respectively the filmed and bare substrate values, against the area per molecule present a deviation in the order of $0.05" between each ellipsometric isotherm. For pure water, the A value was +0.Mo at 20 f 1 OC.
Results The effects of temperature and pH of the subphase on the surfacepressure and surfacepotential versus area (*-A and AV-A) isotherms of l,2-dithiolium-TCNQ are presented in Figures 1and 2, respectively. Figures 3-5 show the surface pressure and ellipsometric (6A-A) and surface potential isotherms of the 1,2-dithiolium-anion salts measured at 20 f 1 "C. We also show the variations of (10) Lamarche,F. Ph.D. Thesis, Universit4du QuBbeca Trois-RiviBree, QuBbec, Canada, 1988. J.;Leblanc,R.M. Can. J. Chem. (11) Dickman,H.;Munger,G.;Aghion, 1981, 59, 328. (12) Ducharme, D.;Tessier,A.; Leblanc,R.M.Reu. Sci. Instrum. 1987, 58, 571. (13) Ducharme, D.; Max, J. J.; Saleese, C.; Leblanc, R. M. J. Phys. Chem. 1990, 94,1925.
O
l
0
A
50
100
J
O 150
Mokular area, (62/molrculr)
Figure 3. (a, top) Surface potential (Am-molecular area (A) and normal dipole moment (@&A isotherms. (b, bottom) Surface pressure (%)-A and ellipsometric (-bA)-A isotherma measured for 1,2-dithiolium-TCNQ. The subphase was at pH 5.8, and temperature 20 f 1"C. The ellipsometric measurement was made at &I = 60' and A = 632.8 nm.
the dipole moment normal to the interface ( p L ) directly calculated from the surface potential multiplied by the molecular area.
Langmuir, Vol. 9, No. 2, 1993 493
Propettie8 of Dithiolium-TCNQ Salts
-.
o0
~
100
50 Mnkcubr
J
~
, J(12/molrculc)
Figure 4. (a, top) Surface potential (An-A and normal dipole moment (pl)-A isotherms. (b, bottom) Surface pressure (+A and ellipsometric(-8A)-A isotherms measuredfor 1,2-dithioliumTCNQF,. Experimental conditions are identical with those in Figure 3.
0
-
0
i
201
I \ -
m
0
50
1.5
1
150
Mdecubr area, %/mokcuk)
Figure 5. (a, top) Surfacepotential (Am-A and dipole moment (pl)-A isotherms. (b, bottom) Surface pressure (r)-A and ellipsometric(4A)-A isotherms measured for 1,2-dithiolium-I. Experimental conditions are identical with those in Figure 3.
As seen in Figure 1, the rise in surface pressure at 20 "C occurs at a molecular area of 90 A2-molecule-1,followed by a sharp increase of the surface pressure during compression. The limiting molecular area of this film is approximately 57 A2-molecule-' and the PA c w e does not indicate any apparent collapse. The collapse pressure of this film occurs at about 45 "am-'. Variations in subphase temperature between 6 and 20 "C ( f l"C) do not cause any drastic change in the behavior of the monolayer. The slope d?r/dA around 30 "am-' is the same at 20 and 15 "C, but is weaker at 6 "C. Simultaneously, the molecular area is increased for surfacepressure below 15"am-'. The collapaesurfacepressure decreases with temperature: 33 mN0m-l at 6 "C as compared to 45 mN-m-1 at 15 or 20 "C. Between 15 and 20 "C, the monolayer is in a liquid condensed state for surface pressures ranging from 40 to 20 mN.m-1: a variation of 1 A2-molecule-' on the PA isotherm results in a surface pressure change of 8 mN-m-1 instead of AT = 15 "am-' for a solid phase. The surface potential area isotherms (6, 15, and 20 "C) of the monomolecular films show a surface
potential jump, followed by a constant increase of the AV valuea up to the end of the compression. All of them exhibit a plateau region between 0.63 and 0.75 (fO.O1) V without any large decreaseafter the collapse. The jumps of surface potential, which are defined as the surface potential difference between the appearance of the liquid phase and the collapse point, are about 0.23 f 0.02 V during the film compression. The effect of pH on surface pressure and surface potential-area isotherms of 1,2-dithiolium-TCNQ is shown in Figure 2. Both PA and AV-A isotherms are pH dependent. The PA isotherms are shifted down by about 10 A2smolecule-' in the presence of an acid or a basic subphase. This result shows that there is not only polarization but a configuration change of the polar head group because the ions react directly with the 1,2dithiolium-TCNQ monolayer. It may be a nucleophilic p attack by OH- that opens the ring 1,a-dithiolium of the cation, and a new compound results; alternatively, a protonation by H+ of the TCNQ- radical should occur in the acid subphasa.l4 These observations point out the necessity to check the chemical stability of these new compounds at the gas-water interface. As shown in Figure 3, the PA isotherm of 1,2dithiolium-TCNQ presents an onset of surface pressure at 90 &*molecule-' with a collapsing surface pressure at 45 "em-' which corresponds to 57 A2Omolecule-1. The AV and 6A isotherms show the onset of a stable surface potential and ellipsometricangle which occurs at the same molecular area, i.e. 110A2-molecule-'. A small increase in surfacepotential and a linear decrease of the ellipsometric angle are observed as the molecular area decreases from 110to 87 Az*molecule-'. The variations in AVand 6A are much more important during compression from 87 to 72 A2-molecule-'. The 6A-A isotherm exhibits a plateau region during compression from 72 to 61 A20molecule-l, but a linear decrease for compression below 61 A2-molecule-'. The AV values change much from 72 A2-mo1ecule-' to attain a maximum value (750 f 10 mV) at 61 A2*molecule-'. For the area smaller than 61 A2*molecule-',we o b r v e a slightdecreaseof AV amociated with a strong increase of the ellipsometric angle. These remarks lead us to propose the formation of multilayer8 on the water surface as already evidenced for several other TCNQ salts.lS In Figure 4, the PA isotherm of 1,2-dithiolium-TCNQF4 presents an onset of the surface pressure at 75 A2-molecule-', and a collapse pressure at 65 mN-m-' or 44 A2-molecule-'. A slope change is observed on the isotherm at around 28 "am-' (58 A2*molecule-'). At 92 A2-molecule-', an ellipsometricjump takes place, and the ellipsometric angle decreases from -2.0 to -2.17'. A surface potential jump (AV = 390 f 10 mV) occurs at 81 A2-molecule-' and AV increases linearly during compression to attain a maximum value (AV = 440 f 10 mV) at 64 A2.molecule-1. At this area per molecule, the 6A-A isotherm shows a change in the slope and the 6A value varies from -2.3 to -2.6O. For an area less than 64 &molecule-', AVvalues decreased, but 6A values increase in absolute values. The PA, AV-A, and bA-A isotherms for 1,2-dithiolium-I are shown in Figure 5. The onset of the surface pressure starta at 65A2*molecule-', and the collapse surface pressure occurs at 48 mN*m-l(41A2-molecule-'). For an area per molecule above 77 A2-molecule-', a gaseous state ~~
(14) Inzett, G.; Chambers, J. Q. J. Electroonal. Chem. 1989,266,266. (15) Barraud,A.; Florsheimer, M.;Mohwald, M.;Richard,J.; RuaudelTeixier, A.; Vandevyver, M. J. Colloid Interface Scr. 1988, 121,491.
Fichet et al.
494 Langmuir, VoI. 9, No.2, 1993 Table I. Optical Parameters of the Different If-Dithiolium-Anions Obtained for a Given Molecular Area Determined in the Presence of Pure Water at T = 20 refractive A d indices area r anions (A*) (mN/m) (deg) (A) nl n~ -1.47 19 1.458 1.476 61 20 TCNQ1.604 -2.93 19 1.609 56 30 TCNQFd-
I
44
28
-2.86
Theee calculations are carried out with 1.332 for water.
22
1.624
1.584
= 1 for air and nz =
of the monolayer is confirmed by a fluctuation on the 6A values. A surface potential jump (AV = 340 i 10 mV) occurs at 77 A2*molecule-land increases linearly from 340 to 560 mV. During the compression from 77 to 44 A2-molecule-', 6A values decrease linearly (-2.10 to -3.0"). Below 44 A2-molecule-', a plateau region appears on the AV-A and 6A-A isotherms which corresponds to the collapse region on the surface pressurearea isotherm. In summary, for most of the 1,2-dithiolium-anion salts, we have observed some similaritiesin the surface potential and ellipsometric angle area isotherms (see Figure 3-5). The AV-A and 6A-A isotherms of all the 1,a-dithioliumanion salts exhibit the following features: (i) the onset of a stable surface potential and ellipsometric angle occurs at a larger moleculararea than the onsetof surfacepressure; (ii) a regular increase in the Surface potential and ellipsometric angle is observed as the molecular area decreases, except for the 1,2-dithiolium-TCNQ salt, a 6A is found constant, -1.43O, in the region 61-72 A2.molecule-l for which a multilayer formationis possible for a molecular arealowerthan 61A2; (iii)in the collapse region, the iodine salt exhibitsa plateau for AV-A and 6A-A isotherms,while there is a sudden decrease in the AV-A isotherms of TCNQ- and TCNQF4-, and a large increase in the 6A-A isothermsin this region for both salts. These observations imply a different collapse process. Diecussion From this set of experiments on surface properties, the organization process during f i i compression can be analyzed. We develop in this discussion a comparative analysis of the three different salts based on two points. In a first part, we apply a model for the ellipsometric isotherms which has been proposed for the study of lipid m01ecules.l~Nevertheless, this geometric model does not appear sufficientfor explainingthese experimentalresults and a critical analysis of it is developed. Secondly, the nature of the different observed phases is tentatively described, based upon the pressure surface area (FA) and surface potential-area (AV-A) isotherms. Starting from this phenomenologicalapproach, a microscopic model is proposed for explainingthe specialfeatures concerning the 1,2-dithiolium-TCNQ salt compared to the other salts. This discussion allows us to propose an explanation for the Z-type transfer onto a substrate which has been observed with this comp~und.~ Ellipsometric Model. In a homologous series of phosphatidylcholines,it has been shown that in the solid state the ellipsometric angle decreases linearly with the chain length.13 To explain these results, the authors proposed a geometric model which assumes that the polar zwitterionic choline groups, due to their high degree of hydration, have a refractive index equal to that of the subphase and, thus, are not detected by ellipsometry. It is worthwhileto compare the ellipsometric isotherms of these different 1,2-dithiolium-anions with the above
model to explore the possible influence of the ionic polar heads. The refractive index of the interphase, the region between the alkyl chains and the aqueous bulk, may differ from that of pure water due to the interactions of water molecules and hydrophilic groups. Consequently, the refractive index of the interphase will be considered a variable. We start from the classical expression valid for a thin uniaxial nonabsorbing and homogeneous film with the optical axis perpendicular to the surface. The experimental ellipsometric angle 6A is equal to
where and n2 are the indices of refraction for air and water, respectively. nl is the refractive index perpendicular to the optic axis (parallel to the euface), and nj is the index of refraction parallel to the optic axis (perpendicular to the surface). d and X are the layer thickness and the incident wavelength (d
