Langmuir 1992,8, 1619-1626
1619
Surface Potential Studies of Langmuir Monolayers and Langmuir-Blodgett Deposited Films of
p-MeC6H4S(O)(CH%)llS(CH2)lOCOOH 0. N. Oliveira, Jr.,+D. M. Taylor,' C. J. M. Stirling,*S. Tripathi, and B. Z. Guot Institute of Molecular and Biomolecular Electronics, University of Wales, Dean Street, Bangor, Gwynedd LL57 1 UT, U.K. Received December 9,1991. In Final Form: March 26, 1992
The behavior of the bipolar molecule p-MeCaHS(O)(CH2)1lS(CH2)loCOzH @TSA) at the aidwater interface has been investigated by using surface pressure and surface potential isotherms. It is shown that in the condensed phase, the molecules pack vertically with the carboxyl moiety in the water surface. Strong hysteresis in the pressure isotherm is related to interactions between the horizontally directed SO dipoles in the distal function. The surface potential measurements were extended to Langmuir-Blodgett (LB) deposited monolayers of pTSA from which it was deduced that, upon deposition,molecules undergo a slight tilting. It is also shown that surface potential measurements can be utilized for determining macroscopic film homogeneity and for studying aging effects in deposited films and provide a noncontacting method for measuring the pyroelectric coefficient of LB films. Finally, it is demonstrated that surface potential is a sensitiveindicator of chemical reactions and interactions at the surface of thin fiis. 1. Introduction The surface potential technique has been extensively applied to monolayers at the aidwater interface for many decades and approaches for the quantitative analysis of monolayer surface potential data have developed rapidly in recent Apart from a series of studies by Tredgold and his collaborators:-6 the same does not appear to be true for deposited Langmuir-Blodgett (LB) films, even though the first surface potential measurements on LB films were reported by Porter and Wyman as long ago as 1938.718 One of the reasons for this apparent lack of interest in surface potential studiesis perhaps the difficulty in interpreting the experimental data. Nevertheless, as we show in this paper, such measurements can prove particularly useful not only for understanding the surface behavior of monolayersbut also in characterizing dewsited LB films. As a means of demonstrating the power of the method, we describe the results of an investigation into the filmforming behavior of a bipolar compound in which the two polar moieties reside at the extreme ends of a long alkanoic chain. Langmuir monolayers and deposited LB films of such bifunctional compounds have received considerable attention over manyyears.4-21 In these systemsone function, e.g. a carboxyl moiety, is used to anchor the molecule on To whom correspondence should be addressed. e Quimica de Silo Carloa, USP, Brazil. t Present address: Department of Chemistry, University of Sheffield, Sheffield S3 7HF, England. (1) Taylor, D. M.; Oliveira, 0.N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990,139,508-519. (2) Vogel, V.; Mtibius, D. J. Colloid Interface Sci. 1988,126,408-420. (3) Oliveira, 0. N.; Taylor, D. M.; Morgan, H. Thin Solid F i l m 1992, + Permanent address: Instituto de Fisica
2101211,1518. (4) Tredgold, R. H.; Smith, G. W. J. Phys. D: Appl. Phys. 1981,14, L193-L195. (5) Tredgold, R. H.; Smith, G. W. Thin Solid F i l m 1983,99,215-220. (6) Jones, R.; Tredgold, R. H.; Hoorfar,A. Thin Solid F i l m 1985,123, 307-314. (7) Porter, E. F.; Wyman, J., Jr. J. Am. Chem. SOC.1938,60, 10831094. ( 8 ) Porter, E. F.; Wyman, J., Jr. J. Am. Chem. SOC.1938, 60, 28552869. (9) Legre, J. P.; Albinet, G.; Caill6, A. Can. J . Phys. 1982,60,893-900. (10) Vogel, V.; MBbius, D. Thin Solid Films 1986,132, 205-219.
a suitable substrate while the second function provides the property or characteristic required in the deposited LB film. Of interest are films with (a) polymerizable osubstituted groups such as alkenesand alkynes (b)moieties for chemical sensing and molecular recognition, and (c) optical and pyroelectric activity. In a recent paper22we described a simple route to the synthesis of a,o-functionalized long-chain alkanoic acids using sulfur to link two shorter chain compounds. One of the bifunctional molecules synthesized by this route, @TSA) namely, ~-M~C~H~S(O)(CH~)~IS(CH~)~OCOZH showed promising pyroelectric activity when deposited as a LB film. The preliminary results presented at the time showed also that the behavior of pTSA at the aidwater interface was complex. In this paper we show how surface potential measurements were used to elucidate the processes occurring in the monolayer. We further show that the technique may be used for (i) assessingthe macroscopic uniformity of deposited films, (ii) studying aging effects in such films, (iii) measuring the pyroelectric coefficient of the films,and (iv)monitoring surface chemicalreactions. 2. Materials and Methods The synthesis of the enantiomerically purepTSA ([UD]= 6 4 O ) has been described elsewhere.22Monolayers were prepared by introducing an aliquot of the spreading solution (1 mg/mL of pTSA in HPLC grade chloroform)onto the surface of an aqueous subphase in a Langmuir trough and allowing 5 min for the solvent (11) Adam, N. K.; Danielli, J. F.; Harding, J. B. Proc. R. Soe. London, Ser. A 1934,147,491-499. (12) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63,452-460. (13) Fosbinder, R. J.; Rideal, E. K. Proc. R. SOC.London, Ser. A. 1933, 143,61-75. (14) Cadenhead, D. A.; Muller-Landau, F. Biochim. Biophys. Acta 1973,307, 279-286. (15) Dervichian, D.; Joly, M. J. Phys. (Paris) 1939,8, 375-384. (16) Sims, B.; Zografi, G . J . Colloid Interface Sci. 1972,41,35-46. (17) Adam, N. K.; Jessop, G . Proc. R. SOC.London, Ser. A 1926,112, 376-380. (18) Albinet, G.; Legre, J. P.; Firpo, J. L.; Caill6,A. Can. J . Phys. 1981, 59,863-870. (19) Goddard, E. D.; Alexander, A. E. Biochem. J . 1960,47,331-334. (20) Ueno, M.; Kawanabe, M.; Meguro, K. J. Colloid Interface Sci. 1976,51, 32-35. (21) Jeffers, P. M.; Daen, J. J. Phys. Chem. 1966, 69, 2368-2373. (22) Taylor, D. M.; Oliveira, 0. N., Jr.; Stirling, C. J. M.; Guo, B. 2.; Tripathi, S. Thin Solid Films 1989,178, 27-35.
0743-7463/92/2408-1619$03.00/00 1992 American Chemical Society
Oliveira et al.
1620 Langmuir, Vol. 8, No. 6, 1992 to evaporate before compression was initiated. The experiments were performed in a polypropylene trough fitted with a constant perimeter belt arrangement and mounted on a thermostatically controlled metal base plate located on an antivibration table in a Clean Room. When freshly drawn, the ultrapure water (UPW) used in this work had a pH of 7.0, but this decreased to about 5.6 over a period of several minutes owing to the absorption of COZfrom the atmosphere. To vary subphase pH, appropriate quantities of HC1 or NaOH (AnalaR grade) were added to the water. For LB deposition, monolayers were spread onto a M BaClz (Anal&, Aldrich) subphase. Monolayerpressure (r)was measured with a paper Whilhelmy plate and electrobalance to an absolute accuracy of 0.1 mN/m and a resolution of 0.05 mN/m. The trough was also fitted with a vibrating plate voltmeter (Kelvin probe) with which surface potential, AV, defined as the potential of a monolayer measured relative to that of a clean subphase surface, was measured to an accuracy of f10 mV. Both the vibrating plate located 2 mm above the subphase surface and the reference electrode placed in the subphase were made from platinum foil and were cleaned regularly. During monolayer compression both r and AV were continuouslyrecordedas functions of area/molecule(A). Detailed descriptions of our experimental procedures may be found in several previous publication~.~3-~~ The substrates for LB deposition were clean glass microscope slides onto which a 900 A thick film of aluminum was evaporated in an Edwards E12E4 vacuum evaporator with liquid nitrogen trap to prevent oil back-diffusion. Prior to vertical dipping, the pTSA monolayer was "annealed" by compressing and expanding at least 5 times at a low compression rate of 0.15 (A2/molecule)/s, before finally compressing to the dipping pressure of 30 mN/m. At a dipping speed of 0.2 mm/min, pTSA deposited as a Z-type film, deposition occurring only on the upstroke with deposition ratios in the range 0.9 to 1.1over the bottom 3.6 to 4.0 cm of the microscope slide. We were unable to find conditions conducive to the deposition of Y-type films. Samples were stored in the Clean Room in one of two desiccators. One contained a Pz06desiccant to maintain a relative humidity of ca. 10% while the second contained no desiccant and ita tap was left open to the atmosphere thus maintaining an average relative humidity of 60%. For surface potential measurements, the substrates were placed face up on a specially constructed, temperature-controlled table located under the vibrating plate of a Trek Model 320B surface potential probe. Two geared motors provided x and y movement of the sample table, thus enabling a raster scan of the surface potential to be carried out. The probe had a sensitivity of A10 mV and a spatial resolution of 6 mm. The apparatus could be evacuated with a conventional rotary/diffusion pump arrangement but unless otherwise stated all measurements were conducted with the chamber filled with air a t room temperature (ca. 18 "C). The surface potential, AV, of the deposited LB film is defined as the difference in potential between a film-coated substrate and the bare substrate. Thus, the difference in work function between the gold probe and the oxidized surface of the aluminum film is taken as the reference potential, VRE. Throughout the experiments it was found that VREvaried randomly from sample to sample, and from day to day, in the range +800 to +950 mV in good agreement with the range +900 to +lo00 mV reported by Jones et Despite this large variation, it should be noted that for a particular sample on a particular day the potential across an uncoated A1/A1203film was uniform, showing a variation of less than A10 mV, thus providing a good reference from which to measure AV, (23) Taylor, D. M.; Oliveira, 0. N., Jr.; Morgan, H. Thin Solid Films
1989,173, L141-Ll47. (24) Morgan, H.; Taylor, D. M.; Oliveira, 0. N., Jr. Thin Solid Films 1989, 178, 73-79.
(25) Taylor, D. M.; Oliveira, 0. N., Jr.; Morgan, H. Chem. Phys. Lett. 1989,161, 147-150. (26) Oliveira, 0. N., Jr. Ph.D. Thesis, University of Wales, 1990.
-1
800
I
>
1600 E
L
0
1-
0 area
per
molecule
,
A
I
A2 1
Figure 1. Surface pressure (r) and surface potential (AV) isotherms for pTSA. The data was obtained during the first ( 0 ) and second (x) compressions of a newly spread film. Also shown (0) is the hysteresis observed in the pressure isotherm during the first expansion of the monolayer. (pH = 5.6, T = 20 OC.)
3. Results and Discussion
3.1. pTSA Monolayer at the Air/Water Interface. 3.1.1. P A and AV-A Isotherms. Figure 1 shows the rather complex surface behavior of pTSA. The isotherm obtained during the first compression of a newly spread monolayer was invariably different from subsequent ones. In particular, the first compression-expansion cycle showed strong hysteresis, whereas in subsequent cycles it was significantlyless. It can be seen also that the small pressure maximum which occurs at around 120 A2 in the first compression, is absent both in the expansion curve and in subsequent compression-expansion cycles. In most experiments, the second and subsequent cycles were identical, but there were some cases where cycles became identical only after the third compression of the same monolayer. The collapse pressure of pTSA was sensitive to pH, temperature, and compression rate. The maximum collapse pressure, in excess of 60 mN/m, was observed at T = 20 "C and pH = 5.6. For areas below about 120 A2/molecule in the first compression, a large change in area occurred with little change in pressure. Below ca. 30 A2/moleculethe pressure rose rapidly, yielding a limiting area/molecule of ca. 22 A2 in the fully condensed phase. This compares well with our previous results for octadecyl p-tolyl sulfoxide (ca. 20 A2/molecule at 25 mN/m27)and with stearic acid, confirming that the molecules are stacked vertically. Figure 1shows also that the surface potential isotherms obtained during the first two compressions increased monotonically throughout the range with no special features corresponding to the phase changes in the pressure isotherm. Applying the Helmholtz equation' where p I is the apparent dipole moment, eo the permittivity of free space, and A the average area per molecule, it is readily deduced from the potential isotherms that decreases from ca. 1.40 D in a freshly spread monolayer occupying 160 A2/molecule to ca. 0.40 D in the condensed phase.22 The hysteresis observed in the first pressure isotherm is accompanied by a permanent reduction in AV with p L only recovering to 1.10 D at the end of the first expansion. However, on recompression the condensed phase value of ca. 0.40 D was reproduced. (27) Oliveira,O. N., Jr.;Taylor,D. M.; Lewis, T. J.;Salvagno, S.;Stirling, C. J . M. J . Chem. SOC.,Faraday Trans. 1 1989,85, 1009-1018.
Surface Potential Studies of Langmuir Monolayers
Langmuir, Vol. 8, No. 6,1992 1621
It should be noted that if the maximum area per molecule during monolayer spreading is restricted to less than 100 A2/molecule, then the subsequent isotherm follows that observed in the second compression in Figure 1. This is true even if, after spreading, the area/molecule is initially increased to 160 A2 prior to the first compression. Thus, the permanent changes seen between the first and second compression isotherms in Figure 1 can also arise during spreading. The limiting area/molecule in the condensed phase decreased slightly with increasing quantity of material spread onto the water surface. 3.1.2. Effect of pH. Apart from a slight decrease in the limiting area/molecule in the condensed phase, increasing subphase pH over the range 1.8 to 9.1 had relatively little effect on monolayer behavior which remained generally similar to that in Figure 1. For pH greater than ca. 10,the small maximum at 120 A2/molecule and extended plateau observed at lower pH is replaced by a much shorter plateau commencing at ca. 70 A2/molecule. Significantly, isotherms for pH >10 showed very little hysteresis even in the first compression-expansion cycle and subsequent isotherms were always identical to the first. It is also significant that for pH
-E > 400 a
-m c (Y c
c
200
z L
L
m 3
0 0
025
05 distance
(
075 10 a r b i t r a r y units 1
Figure 8. Surface potential profiles of a LB-deposited monolayer of pTSA (a) 2 days after deposition, (b) after exposure to 3-butyn-1-01 for 15 h, and (c) after exposing the region between the bars to UV light for 40 min. Table I. Effect of the Surface Potential, A V, of Exposing LB Monolayers of pTSA to the Vapor of 3-Butyn-1-01or Acetone for 15 h followed by 1 h under Vacuum To Remove Any Condensate from the Surface. surface Dotential ( A m , mV samde A samde B samde C sample as deposited 400 460 365 after exposure to alcohol vapor 285 340 and 1h under vacuum after exposure to acetone vapor 345 and 1 h under vacuum after UV irradiation 210 4 SamplesA andB were deposited 4 and 3 days, respectively,before the experimenta started. (Sample C age was 5 days.) Exposure to the solvent vapors also affects the reference voltage, VRE,but this recovers ita initial value after placing the samples under vacuum for an hour. Sample A was exposed to UV irradiation following alcohol exposure. The measurements were taken at a fixed position on the sample surface. ~
~
~~
In section 3.2.2 we described how aging effects could cause AV for pTSA monolayers to decrease significantly during the first few days after deposition. To ensure that such an artifact was not totally responsible for the lower potential observed here after the long exposure to alcohol vapor, similar experimenta were conducted on two older samples in which the rate of change of AV with age was much slower. Again a significant decrease (ca. 120 mV) in AV occurred as seen in Table I. In a further control experiment, a third sample was exposed to acetone vapor for 15 h following the above procedures. No permanent change in potential was observed (Table I) verifying that film integrity was maintained during the exposure. We may conclude, therefore, that exposure to the alcohol leads to a decrease of ca. 120 mV in AV which is likely to arise from hydrogen bonding of the hydroxyl moiety of the alcohol to the sulfoxide moiety in the surface of the pTSA monolayer. The dipole moment, 0--H+, of the hydroxyl group will be directed toward the substrate, thus making a negative contribution to AV. The adsorption of butynol to the LB film was confirmed by exposing part of the film to UV light (125-W mercury lamp, principal wavelength 254 nm) for 40 min. The exposed area of film, dilineated by the markers on curve c in Figure 8, undergoes a further decrease in AV. We have confirmed in control experiments that identical results are obtained whether or not the Al/A1203 reference surface is simultaneously exposed to the UV. We believe that the decrease in AV is caused by the photopolymerization of the C 4 bonds of the alcohol. The appearance of a new Fourier transform IR band at 1590 cm-',
Oliveira et al.
1626 Langmuir, Vol. 8, No. 6,1992
\I
-I
I , ,
I , , , , I , ,
1800
1100
,,
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1600
,
, I , ,
1500
,,
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Figure 9. FTIR spectra of two monolayers of pTSA plus adsorbed butynol (a)before and (b) after exposure to UV light. The new band at 1590 cm-' is consistent with the formation of a conjugated system.
characteristic of conjugated C=C bonds, following this irradiation provides supporting evidence for this view (Figure 9). 4. Conclusions
By simultaneous measurement of the surface pressure and surface potential ofpTSA monolayers at the aidwater interface, considerable progress can be made in identifying the processes that occur during monolayer compression. For example, conclusive evidence has been obtained that in the condensed phase these bipolar molecules probably stack with their long chains vertical and anchored via the carboxyl moiety to the water surface. Hysteresis in the first compression/expansion cycle and differences between the first and subsequent compressions of the monolayer are believed to arise from the formation of domains or
islands of vertically stacked molecules held together by the attraction between horizontally directed dipoles of the distal p-MeCsHrSO moiety. In the condensed monolayer, the vertical component of the p-tolyl-SO moiety is estimated to be 1.15 D compared with 0.74 f 0.21 D for a LB monolayer. The discrepancy has been tentatively attributed to a small tilting of the chains in the LB monolayer. The AV for floating monolayers is of the same order of magnitude as that for the corresponding LB monolayer may be taken as an indication that AV arises mainly from dipoles associated with the film-forming molecules, confirming the views expressed by Heckl et al.36and Winter and Tredgold.= It has been further shown that surface potential measurements may be used to determine the uniformity of LB films and to investigate aging phenomena in them. We have also used the technique to show that pTSA is pyroelectric with a coefficient of 2.68 X lo4 C cm-2 K-l, one of the highest reported for LB films, though it is unlikely to find practical application in view of ita long response time. Finally we have shown that surface potential is sensitive to chemical interactionsand reactions at the surface of an LB film. The hydrogen bonding of 3-butyn-1-01to a pTSA monolayer and ita subsequent UV polymerization gave easily observed changes in AV.
Acknowledgment. This work was supported by the Science and Engineering Research Council (Grant No. GR/ E3036.02). O.N.O.also wishes to thank FAPESP and CNPq (Brazil) and ORS (U.K.) for financial support. (56) Winter, C. S.; Tredgold, R. H. J . Phys. D Appl. Phys. 1984,17, L123-LI26.
