Langmuir 1993,9, 2119-2127
2119
Influence of the Spreading Solvent on the Properties of Monolayers at the Air/Water Interface Arne Gericke, Johannes Simon-Kutscher,and Heinrich Huhnerfuss' Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King Platz 6, 0-20146 Hamburg, FRG Received March 29,1993. In Final Form: May 20,1993 The influence of the spreading solvents ethanol, hexane, and chloroform on the properties of monolayers of long-chainalcohols,hexadecanoicacid, hexadecanoicacid esters, and L-a-dipalmitoylphosphatidylcholine is investigated by surface pressure/area and surface potentiallarea isotherms, gas chromatography, surface viscosity measurements, spreading velocity measurements, and external infrared reflection-absorption spectroscopy. It is shown that the spreading solvent ethanol and mixtures containing ethanol cause film loss into the subphase, but on the other hand enforce a higher conformational order of the film-forming molecules of 1-hexadecanolmonolayers and thus a higher surface viscosity. Ethanol solved in the subphase, however, does not influence the properties of the monolayer up to a concentration of about 1 mL/L. Different surface Viscosity values are observed for monolayers spread from chloroform and hexane. The differences between the three spreading solvents are attributed to different spreading kinetics, which result in a different morphology of the monolayers. Infrared spectroscopic investigations do not supply any evidence of solvent molecules embedded in the monolayer.
Introduction Investigations of monomolecular films at the gas/water interface are usually performed by spreading the insoluble surface-active compounds with the help of a so-called "spreading solvent".l The spreading solvent has to fulfill several requirements in order to achieve a well-defined state of the monolayer: It must spread the surface-active material homogeneously over the whole water surface, disappear completely from the surface layer, and be chemically inert. While it is not difficult to achieve the last point, the first two requirements are subject to considerable debate in the literature. Basically, the spreading solvents may be classified as "insoluble volatile solvents" and "water-soluble volatile solvents". The question most often discussed in connection with water-insoluble spreading solvents is as to whether or not solvent molecules remain in the film to a certain extent. Archer and La Mer2 were the first to suggest an influence of different volatile, water-insoluble spreading solvents on the properties of monolayers at the gas/water interface, but the observed differences were later mainly attributed to wax which had been dissolved from the sides of the trough by the solvent during spreadingnu Poskanzer and Goodrich6 also reported a strong dependence of the surface film characteristics on the spreading solvent by applying the method of %uccessive additions" of spreading solution, in order to measure a surface pressure isotherm. In this context one must admit that, by using this procedure, small impurities of the spreading solvent may exert a strong impact upon the film properties, if the total quantity of the solvent is so high that a sufficient amount of the impurities may be accumulated in the surface layer. In addition, Abraham et aL7showed that the evaporation
* To whom correspondence should be addressed.
(1) Gaines,G.L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Intarscience: New York, 1966. (2) Archer, R. J.; La Mer, V. K. J. Phys. Chem. 1955, 59, 200. (3) Barnes, G.T.;Elliot,A. I.; Grigg, E. C. M. J. Colloid Interface Sci. 1968,26,230. (4) Walker, D. C.; Ries, H.E., Jr. Nature 1964,203,292. (5) Gainea, G.L. J. Phys. Chem. 1961,65,382. (6)Poekanzer, A.; Goodrich, F. C. J. Colloid Interface Sci. 1975,52, 213. (7) Abraham, B. M.; Miyano, K.; Ketterson,J. B. In Ordering in two Dimensions; Sinha, Ed.;Eleevier: North-Holland, 1980.
of hexane and chloroform is strongly reduced in the presence of a compressed surface film. On the other hand, Pallas and Pethica8 observed only extremely small effects for pentadecanoic and hexadecanoic acids in comparing a so-called "single shot" method with successive additions of hexane solutions of extremely pure samples. Similar conclusions were recently drawn by Hifeda and Rayfield9 who used a "continuous compression" method for investigating the transition from the liquid-expanded to the liquid-condensed phase of pentadecanoic acid surface films. However, it should be noted that surface pressure/ area measurements exhibit an insufficient sensitivity for the detection of small amounts of impurities in the surface filmlOJ1 (aswell as small amounts of solvent remaining in the film). Despite this problem Gabrielli et al.12 observed differences in the II/A isotherms for poly(viny1 acetate) and stated that solubilization in the bulk and spreading are competitive processes. If the solubility of the filmforming material in the spreading solvent is low ("poor" solvent according to Gabrielli et al.,'2 e.g., methanol), the gas/water interface behaves as a "good" solvent, which in turn leads to a less compact distribution of the macromolecular segments in comparison with "better solvents" (chloroform and benzene). Ries13 investigated pressure/ area isotherms of valinomycin and observed differences between monolayers spread from benzene, chloroform, n-hexane, and cyclohexane. Sugawara et al." were not able to confirm these findings, but their results showed a bad reproducibility. While it was generally accepted for a long time that in the so-called gaseous phase (the region which is characterized by a surface pressure of zero) the surface-active material is distributed homogeneously over the entire surface, in the last years several publications appeared showing a more or less pronounced island structure in (8) Pallas, N. R.; Pethica, B. A. Langmuir 1986,1,509. (9) Hifeda,Y. F.; Rayfield, G.W. Langmuir 1992,8, 197. (10) Albrecht, 0. Thin Solid Film 1989,178,663. (11) Smaby,J. M.;Brock", H.L. Chem. Phys. Lipids 1991,68,249. (12) Gabrielli, G.;Baglioni, P.; Ferroni, E. Colloid Polym. Sci. 1979, 257, 121. (13) Ries, H.E., Jr. Langmuir 1990,6, 883. (14) Sugawara, M.; Sazawa, H.;Umezawa, Y. Langmuir 1992,8,609.
0743-746319312409-2119$04.0010 0 1993 American Chemical Society
Gericke et al.
2120 Langmuir, Vol. 9, No. 8,1993 that region [e.g., refs 15-171. Htinig et al.18 stated that fatty acids with chain lengths of greater than 15 C atoms exhibit a coexistence of liquid-crystalline domains in a gaseous ambient phase. The size of these domains with uniform molecular orientation decreases with increasing chain length, and the initial size of these domains depends on the spreading solvent: Slowly evaporating spreading solvents like hexane cause larger domain sizes than chloroform. In contradiction to these findings Moore et al.19 observed no dependence of the domain size on the nature of the spreading solvent for tetradecanoic acid monolayers when applying the fluorescence technique. Miyano and Tamadam who used a cyanine dye as filmforming material showed that the domain size is larger directly after spreading than after a compression to a few millinewtons per meter and a subsequent reexpansion. This reexpanded monolayer showed a film elastic modulus of zero up to a surface coverage of 90 5% (onset of the surface pressure). In contrast, Sauer et found relatively large choline elastic constants for L-a-dipalmitoylphosphatidyl (DPPC) when applying the single shot method in the liquid/gas coexistence region. Miyano and Tamada pointed out that one possible explanation of this discrepancy is the different degrees of inhomogeneity for reexpanded and “used as spread” monolayers. In principal, water-soluble solvents (e.g., ethanol) may alter the properties of the spread film in different ways: Solvent molecules may remain in the hydrophobic region of the surface film, or influence the adjacent water layer, and/or the headgroup structure. In fact, ethanol is known to influence the properties of micellar solutions,22 the foaming of protein solutions,23 and phospholipid phase transitions.24 In aqueous solutions the hydrophobic chain of ethanol induces a new structure of the adjacent water layer exhibiting less entropy (hydrophobic effect2% Joos and Serrien26 have shown that for alcohols of low surface activity the adsorption kinetics is a mixed process of diffusion to the subsurface and the transfer from the subsurface to the surface, and that it is merely entropic and not temperature controlled. Consequently, it is not surprising that ethanol reduces the surface tension of pure water a t least slightly a t alcohol concentrations down to 0.8 mmol/dm3 (-30 P L / L ) . ~For ~ the interpretation of surface pressure/area (IIlA) and surface potential/area (AVIA) isotherms, which represent a relation of the filmfree and the film-covered surface, one must keep in mind that for higher ethanol concentrations (>0.05mol/L) the surface properties of the film-free surface are remarkably changed with respect to ethanol-free water. But even after compensation of this effect Cadenhead and Osonka28 observed strong influences on tetradecanoic acid IIIA isotherms (expanded monolayer) for different ethanol concentrations 20.1 mol/L. On the other hand, the modifications of octadecanoic acid II/A isotherms (con(16) Hhig, D.; MBbius, D. Thin Solid Films 1992,210, 64. (16) Knobler, C. M. Science 1990,249,870. (17) MBhwald, H. Annu. Rev. Phys. Chem. 1990,41,441. (18) HBnig, D.; Overbeck, G. H.; MBbius, D. Adu. Mater. 1992,4,419. (19) Moore, B. G.; Knobler, C. M.; Akamatsu, S.;Rondelez, F. J.Phys. Chem. 1990,94,4588. (20) Miyano, K.; Tamada, K. Langmuir 1992, 8, 160. Miyano, K.; Tamada, K. Langmuir 1993,9,508. (21) Sauer, B. B.; Chen, Y. L.; Zografi, G.; Yu, H. Langmuir 1988,4, 111.
(22) Durga Prasad, Ch.; Singh, H. N. Colloids Surf. 1990,50, 37. (23) Ahmed, M.; Dickinson, E. Colloids Surf. 1990,47, 363. (24) Lohner, K. Chem. Phys. Lipids 1991,57,341. (25) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1973. P.; Serrien, G. J. Colloid Interface Sci. 1989, 127, 97. (26) JOOS, (27) Janczuk, B.; Bialopiotrowicz,T.; Wojcir, W. Colloids Surf. 1989, 36, 391. (28) Cadenhead, D. A.; Osonka, 1. E. J. Colloid Interface Sei. 1970,33, 188.
densed monolayer) due to the presence of ethanol could be attributed to a change of yo and not to an ethanol/ surface film interaction. Furthermore, for high ethanol concentrations (21mol/L) they observed instabilities of the film due to an enhanced film solubility. In this context we have to emphasize that film loss into the subphase is one of the most serious problems for the use of ethanol or mixtures with ethanol as spreading solvents.lpW In conclusion, the contradictory results thus far published in the literature mainly suffer from the fact that they can hardly be compared because of different spreading techniques, different and partly impure solvents and substances, and different methods which in part are not sufficiently sensitive. In order to overcome this basic obstacle, this work aims a t producing a consistent and comprehensive data set by using the same highly purified solvents and substances for various experimental approaches which include determination of Langmuir curves, surface potentials, surface viscosities, and spreading velocities, as well as external infrared reflection-absorption spectroscopy and gas chromatography. The data set thus obtained is expected to allow answers to the following questions: Is the morphologyof the surface film influenced by the spreading solvent? Is the orientation (tilt angle) of the film-forming molecules influenced by the spreading solvent? Are molecules of the spreading solvent remaining in the surface film? Is ethanol as spreading solvent inducing film loss and altering the properties of the system gas/monolayer/water even for low subphase concentrations? The experiments were carried out for the spreading solvents ethanol, hexane, and chloroform because these solvents as well as mixtures of them are most often used for the investigation of monolayers at the gas/water interface. The film-forming substances are the saturated alcohols (214, c16, C18, and (220, (Z)-9-hexadecen-l-ol, hexadecanoic acid, hexadecanoic acid methyl and ethyl esters, and L-a-dipalmitoylphosphatidylcholine(DPPC). We shall discuss neither specific spreading solvent/ surface film interactions as recently discussed by Munger et al.30 for chlorophyll a nor the differences between the spreading of l i p o s ~ m e s ~and l r ~the ~ spreading techniques normally used in monolayer studies.
Experimental Section Materials. The spreading solvents ethanol and hexane, respectively,of analyticalreagent grade (Merck,Darmstadt, FRG) were distilled with the help of a 1.60-m bubble-cap column. Chloroform of analytical reagent grade and licrosolve (Merck, Darmstadt, FRG) were used as supplied (in order to prevent polymerization stabilized by ethanol or 2-methyl-2-butene, respectively). The purity of all solvents was checked by gas chromatography using a flame ionization detector (FID). The water was deionized and purified by a Seralpur Pro 9OC (Seral, Ransbach, FRG) apparatus. The water quality was checked by fluid-fluid extraction with hexane followed by gas chromatographic analysis. The following surface-active compounds were recrystallized from pentane/ethanol (955 v/v) and checked by their melting point and gas chromatographic analyses as well: tetradecanol, hexadecanol, octadecanol, eicosanol, and hexadecanoic acid ethyl ester, all of analytical reagent grade (Merck p.a., Darmstadt, FRG). (Z)-O-Hexadecen-l-ol(99.9 %) (Alltech, FRG), hexadecanoic acid methyl ester (99.9%) (Sigma, FRG), (29) Cadenhead, D. A.; Kellner, B. M. J. J. Colloid Interface Sci. 1974, 49, 143. (30) Munger, G.; Leblanc, R. M.; Zelent, B., Volkov, A. G.; Guge-
shashvili, M. I.; Gallant, J.; Tajmir-Riahi, H. A.; Aghion, J. Thin Solid Films 1992,210/211, 739. (31) Launois-Surpas, M. A.; Ivanova, T.; Panaiotov, I.; Proust, J. E.; Puisieux, F.; Georgiev, G. Colloid Polym. Sci. 1992,270, 901. (32) Heyn, S. P.; Egger, M.; Gaub, H. E. J. Phys. Chem. 1990,94,5062.
Langmuir, Vol. 9, No. 8, 1993 2121
Properties of Monolayers a t the AirlWater Interface
Table I. Liquid-Condensed/Solid Phase Transitions of Saturated Long-chain Alcohol Monolayers for Different Spreading Solvents phase transitions LC/S (nmZ/molecule) surface-activecompound hexane chloroform ethanol" ethanolb 1-tetradecanol 0.199(k0.001) 0.195(k0.003) 0.168(*0.003) 116%IC 0.177(*0.003) 111%I 1-hexadecanol 0.201(&0.003) 0.198(k0.003) 0.177(*0.008) [12%3 0.197(*0.004) [2%3 1-octadecanol 0.197(&0.001) 0.202(*0.002) 0.162(*0.005) [18%1 not measured 0.199(*0.002) [0.5%] 0.164(*0.003) 118%I 0.180(*0.002) [11%3 1-eicoeanol 0.200(*0.003) 4 250-pL syringe. b 5-pL syringe, c In brackets the differences between the values for the respective spreading solvent and hexane in percent is given. and L-a-dipalmitoylphoephatidylcholine (DPPC) (99.0+ % (Fluka, Buchs, Switzerland) were used as received. The DPPC solutions were stored at 263 K until they were used. DPPC solutions were used only once after allowing them to adapt to room temperature. Hexadecanoic acid-dal was supplied by IC Chemikalien (Miinchen, FRG) with 99% deuterium. The solutions for all surface-active compounds were prepared in the concentration range (2.5 10.15) X 1W moVL. Methods. The n/A isotherm were recorded with the help of a Lauda FW-2 Langmuir trough (Lauda, FRG) and were temperature controlled in the limits of 10.1 K. The compression velocity was (1-2) x 10-9 nmV(mo1ecule min). Thesurfacepotentialmeasurementswere performed by means of the ionization method using a detector. The detector was thermostated to 10 K above the water temperature, in order to prevent water condensation at the surface of the detector, and it was placed 1.5 cm above the water surface. The reference electrode was Ag/KCl. The surface viscosity measurements were performed by a canal viscosimeter consisting of a film balance Lauda FW-1 (Lauda, FRG) equipped with a canal barrier. The canal barrier is constructed of two halves of a PTFE block held together by an adjustable guide rail. The walls of the canal are made of glass slides which were silylated for 30 min with 1.5% trichloroctadecylsilane (Merck, Darmstadt, FRG) in hexane,3aand which are attached on each of the PTFE halves by screws above the water surface. The completeness of the silylation was controlled by measuring the contact angle of water drops at the silylated surface by a contact angle meter (Erma Optical, Japan). Usually contact angles of 105-llOo were achieved, with a variation of &lo across the surface. The length of the canal is 76 mm and the width 1.4 mm. At the beginning of the experiment the canal was closed by a small removable PTFE plate. The compression started 30 min after spreading with a velocity of 0.9 cm/min until the desired film pressure was reached. Then, the canal was opened, and the surface pressure was held constant by a continuous movement of the barrier compensating for the area loss, and the flow rate was measured. Usually 3-5 runs were necessary, in order to obtain mean values varying by 2-5%. The measurements of thespreadingvelocities were performed in a 2-m-long and 5-cm-wide canal by placing one drop of the respective solution (with equal concentrations) at the water surface. The spreading front was marked by a small amount of PTFE powder which was carefullypurified by hexane in a Soxhlet apparatus. The external infrared reflection-absorptionspectroscopy was performed by a Bruker IFS 66 (Karlsruhe, FRG) equipped with a MCT detector and using a modified external reflection attachment of Specac (Orpington, Great Britain) for monolayers at the air/water interface that allowed thermostating the trough (10.5 K). For some experiments a KRS-5 wire-grid polarizer (>95% polarization grade) was placed just above the water surface. If not specified, an incidence angle of 30' was applied. A Happ-Genzel apodization function with a resolution of 8 cm-' waa used, and the spectra were taken by coaddition of 2000 scans. The reflection-absorption is defined as -log(R/Ro),where ROand R are the reflectivities of the pure and the film-covered surfaces, respectively. For details about the infrared reflection-absorption experiment at the &/water interface, refer to refs 34-36. The peak positions of the specific bands were determined by the (33) Naito, K.; Iwakiri, T.; Miura, A.; Azuma, M. Langmuir 1990,6, 1309. (34) Dluhy, R. A. J. Phys. Chem. 1986,90,1373. (35)Fina, L. I.; Tung, Y.S. Appl. Spectrosc. 1991, 45, 986.
"center of gravity" method.37 During the infrared reflectionabsorption experiments the discontinuous compression method had to be used because of constructive limitations. Before starting any experiment, the barrier was moved to a small area, and potential surface-active pollutants were sucked off. We applied either a 5-pL SGE syringe (Weiterstadt, FRG) or a 250-pLUnimetrics syringe (Shorewood,IL). All experiments were carried out at 294 K. Spreading Technique. The spreading solution was applied from a syringe by allowing the droplets to "touch" the water surface; i.e., the distance between drops and the water surface was as small as possible. The droplets were distributed over the entire trough area, and the solvent was allowed to evaporate for 30min before starting the experiment. The spreading procedures were the same for all solvents used in the present investigation.
Results Surface Pressure and Surface Potential Measurements. Surface pressure/area ( W A )isotherms are being widely used for the characterization of monolayers at the gas/water interface, in particular, as a reference for other techniques. Consequently, it is useful to start the discussion with the description of the phase behavior of the gas/monolayer/water system with respect to different spreading solvents: Table I shows experimental values of the area per molecule for the phase transition from the liquid-condensed to the solid state for the saturated alcohols C14, (216, Cia, and C20 as inferred from IIIA curves. (The discussion of monolayer phases is complicated by the different systems of nomenclature for the phases. We shall use the Harkins-Stenhagen nomenclature; for the discussion of ita relation to other nomenclatures, refer to ref 38). The isotherms of the respective monolayers spread from a chloroform or hexane solution are very close to each other. Merely for 1-tetradecanol a difference is observed just at the limit of the experimental error. The observed phase transitions and the shape of the isotherms basically agree with the isotherms presented by Kuchhal et al.39(spreading solvent petroleum ether, middle boiling fraction). They are only slightly shifted to higher areas per molecule (-0.005 nm2/molecule) which presumably can be attributed to the improvement of modern Langmuir troughs. However, strong deviations from these n / A curves with regard to the position of the phase transition are observed for the isotherms, if ethanol was applied as the spreading solvent. In Table I, the fourth column shows the values for the phase transitions using a 250-pL syringe for spreading 100-130 pL of the spreading solution, while in the fifth column the corresponding data for monolayers spread with the help of a 5-pL syringe are given (the latter syringe had to be applied repeatedly, in order to obtain the necessary volume of the spreading solution). The main (36) Gericke,A.; Michailov,A. V.; Hherfuse, H. Vib.Spectrosc. 1993, 4 , 335. (37) Cameron, D. G.; Kauppinen, J. K.; D o u g h , J. M.; Mantach, H. Appl. Spectrosc. 1982,36,245. (38) Bibo, A. M.; Knobler, C. M.; Peterson, 1. R. J.Phys. Chem. 1991, 95.. 5591. ~~
~
(39) Kuchhal, Y.K.; Katti, S. 5.;Biswas, A. B. J. ColloidlnterfaceSci. 1969, 29, 521.
Gericke et 01.
2122 Langmuir, Vol. 9, No. 8,1993
0.0 0.10
0.1s
0.20
0.25
0.30
ueaInm*/molrml~l
Figure 1. W Aisotherms for l-hexadecanolmonolayers at the
airlwater interface spread from different spreading solvents, applying different syringes (temperature294 K): (- -) spreading solventhexane; (-) (1)spreadingsolventethanol,250-pL syringe; (2) spreading solvent ethanol, 5-pL syringe.
-
difference between these syringes is the thickness of the needle and, as a consequence, the size of the droplets formed by the syringe. Presumably, the discrepancies between the monolayers spread from chloroform or hexane and the monolayers spread from ethanol can be largely attributed to film loss due to the drag of surface-active material by ethanol into the subphase. This effect can be reduced by the application of a syringe with a very thin needle (see Figure 1). In order to check this supposition, 200 mL of the water was extracted by hexane after compression of the monolayer and subsequently analyzed by FID gas chromatography (the sample was taken behind the movable barrier, in order to avoid interference with the monolayer). While no surface-active material was found in the water when applying hexane as the spreading solvent, between 10% and 20% (depending on the filmforming material) of the substance was found, if the monolayer was spread by the large syringe and ethanol was applied as the spreading solvent. It is worth noting that applicationof the smaller syringereduced this amount remarkably. Although the shapes of the surface pressure/area isotherms for all spreading solvents are comparable (not shown) and the shift of the isotherms for the monolayers spread from ethanol solution can be largely attributed to film loss, the surface potentiallarea isotherms of l-hexadecanol monolayers spread from ethanol deviate strongly from the curvesof l-hexadecanolfilmsspread from hexane and chloroform, respectively (Figure 2). The same holds for l-tetradecanol (Figure3): The shape of the AVIA curve for ethanol as the spreadingsolvent is completelydifferent from those of the other curves. In addition, the AVIA curve for l-hexadecanol spread from chloroform is shifted to higher areas per molecule in comparison with hexane. These shifts are significant and much larger than those observed for the IIlA isotherms of l-tetradecanol and l-hexadecanol and for the AVIA isotherms of l-tetradecanol. Figure 4 shows the IIlA isotherms for (2))-S-hexadecen1-01and different spreading solvents. In addition to the shift of the curve for ethanol as the spreading solvent to lower areas per molecule, a significantshift of the isotherm for chloroformas the spreadingsolvent in comparison with the isotherm for hexane is observed. The isotherms were reproduced three times by the use of three different spreading solutions each for hexane and chloroform, in
400 300 -
-
200 -
100 -
0
2o
IO
I
1
I
I
i .
0.10
0.20
0.30
0.40
0.50
0.60
area [am2 1 molrcdrl
Figure 4. n / A isotherms for (2)-9-hexadecen-l-o1monolayers
at the air/water interface spread from different spreading solvents (temperature 294 K): (-)
spreading solvent ethanol; ( - - - )
Spreading solvent hexane; (- - -1 spreading solvent chloroform.
order to rule out impurities and potential insufficient weighing accuracy. In order to verify these conclusions by a different method, surface potential measurements were performed in the presence of (Z)-9-hexadecen-l-ol monolayers. At first glance, the AVIA c w e s displayed in Figure 5a suggest shifts for the respective monolayers spread from ethanol,hexane, and chloroform,which appear to compare with the shifts found by means of Langmuir curves. However, it cannot be excluded that the shifts of the AV/A curves are caused by film losses. In order to compensate for this effect, the surface potential values were normalized to the surface pressure instead of the area per molecule. The results which are given in Figure
Langmuir, Vol. 9, No.8, 1993 2123
Properties of Monolayers at the AirlWater Interface CmVl
CmVl
250
r . =, so
.
. \ \
\ .'\
1
\ \
501 01 0
I
S
I
I
I
I
I
10 15 20 25 30 surface pressure CmN/ml
1
35
Figure 6. AV/IIisothermsfor (Z)-9-hexadecen-l-olmonolayers at the air/water interface spread without the help of a spreading solvent on subphases containing different concentrations of ethanol (temperature 294 K): (-) pure water s u b p b , (- -) 1 mL/L ethanol; 2mL/L ethanol.
C mV 1 250
1
-
1
(-a)
1IuNh 1
,[I:'
I
,
I
I
I
,
0 0
5
10 15 20 25 30 s u r f a c e p r e s s u r e CmN/ml
35
Figure 5. (a, top) AVIA isotherms for (Z)-g-hexadecen-l-ol monolayers at the aidwater interface spread from different spreading solvents (temperature 294 K): (-) spreading solvent ethanol; (- - -1 spreadingsolventhexane; (- - -) spreadingsolvent chloroform. (b, bottom) AV/II isotherms for (Zl-g-hexadecen1-01monolayers at the aidwater interface spread from different spreading solvents (temperature 294 K): (-) spreading solvent ethanol; (- -1 spreadingsolventhexane; (- - -) spreading solvent chloroform.
-
5b clearly show that the shifts of the AV/II curves, which are compensated for the f i i loss, are smaller than those of the AV/A curves. However, the remaining shifts,which are still large and significant,reveal that additional effects have to be taken into account, in order to explain these shifts. In particular, the drastic shift of the AV/II curve of the "ethanol" curve encouraged us to perform some additional measurements, in order to investigate the influence of ethanol on the properties of monolayers. Therefore, we spread pure (ZJ-9-hexadecen-l-olon subphases containing 0, 1, and 2 mL/L ethanol and measured the surface potential during the compression with respect to the surface pressure. The results are shown in Figure 6: While the addition of 2 mL/L (0.034 moVL) changes the properties of the monolayer quite strongly,the AV/IIcurve for 1mL/L (0.017 moVL) is largely identical with the curve for ethanol-free water. It should be noted that the influence observed for ethanol occurs at lower concentrations as reported by Cadenhead and Osonka2e for tetradecanoic acid. In general, these results support conclusions drawn by Lim and B e r p who showed that solute effects are generally more pronounced for monolayers in the expanded surface states. Another notable result can be inferred from Figures 5b and 6 The curves for (2)-9-hexadecen-1-01spread from chloroform (Figure 5b) and spread without a spreading solvent on a pure water subphase (Figure 6) show the same AV/n characteristics. Basically, this coincidence bears interesting perspectives. (40)Lim, Y.C.; Berg, J. C. J . Colloid Interface Sci. 1975,51, 162.
i
lo
0
' 0.1s
0.10
0.20
0.25
0.30
area [ am* I molecule I
Figure 7. II/A isotherms for hexadecanoic acid methyl ester monolayers at the aidwater interface spread from different spreading solvents (temperature 294 K): (-) spreading solvent ethanol; (- -) spreadingsolventhexane; (- - -) spreading solvent chloroform.
-
However, additional experiments with other pure monolayers and with monolayers spread from chloroform must show to whether or not these findings can be generalized. The influence of different spreading solvents on the II/A isotherms of hexadecanoic acid methyl ester monolayers is shown in Figure 7: In comparison with the curves measured for hexane and chloroform as spreadingsolvents, the curve for ethanol as the spreading solvent is shifted to lower areas per molecule (phase transition liquidcondensed (LC) to solid (S),0.144 f 0.001 nm2/molecule), and we were not able to change this result by applying the 5-pL syringe. Furthermore, we measured a slight difference between the curves obtained by applying the spreading solvents chloroform (LC/S transition 0.175 f 0.003 nm2/molecule)and hexane (LC/S transition0.182 i 0.004 nm/molecule); i.e., the difference is just at the limit of the experimentalerror. In this context it should be mentioned that the chloroform used herein was stabilized by 1% ethanol. Therefore, it cannot be excluded that the shift of the LC/S transition for the chloroform curve (taking into account the strong shift of the curve for ethanol as the spreading solvent) may in part be attributedto a small film loss into the subphase. Table I1 shows the areas per molecule for the phase transitions from the liquid-expanded (LE) phase to the LE/LC transition region for hexadecanoic acid ethyl ester
Gericke et al.
2124 Langmuir, Vol. 9, No. 8,1993 Table 11. Transition from the Liquid-Expanded Phase (LE) to the Liquid-Expanded/Liquid-Condensed Transition Region (LE/LC) for DPPC and Hexadecanoic Acid Ethyl Ester Monolayers Dhase transitions surface-active comDound
LE/ (I;E/Lc) (nm2/molecule) hexanea
chloroform
ethanolb
~~~~~~
DPPC
0.704(~0.002) 0.766(*0.008) 0.516(*0.002) [8%Ic [33%1 hexadecanoic 0.425(*0.001) 0.429(*0.001) 0.370(*0.001) [13%1 acid ethyl ester [1%3
For DPPC hexane/ethanol (91) was used. b The 5-pL syringe was used. c In brackets the difference between the values for the respective spreading solvent and chloroform in percent are given. 0
and L-a-dipalmitoylphosphatidylcholine (DPPC). In the case of ethyl ester the II/A curve for chloroform as the spreading solvent is slightly shifted to higher areas per molecule than the curve for hexane, which corresponds to a shift of the transition from the liquid-expanded to the LE/LC transition region (Table 11). This effect is more pronounced for DPPC, but in this case we used a mixture of 9:l hexane/ethanol as the spreading solvent instead of pure hexane, and therefore, this difference may be mainly attributed to film loss. It should be noted that both chloroform and hexane/ethanol are widely used as spreading solvents for the investigation of DPPC monolayers and that one obtains a strong deviation of the H/A curves for these two spreading solvents. Surface Viscosity Measurements. With the help of the canal method one is in a first approximation able to measure the surface shear viscosity,41 which will be abbreviated in the following as %urface viscosity". The disadvantages of the canal method in the determination of a "pure" surface shear viscosity, e.g., the need of a surface pressure gradient along the length of the canal, are of minor importance for our measurements, because the aim of this work is not the determination of absolute surface viscosity values but the comparison of surface viscosities of monolayers spread from different spreading solvents. The surface viscosity vs is often calculated from the observed rate of film flow through the canal (Q) using a formula proposed by Harkins and K i r k w o ~ d : ~ ~
where W is the width, L is the length of the canal, and IIz and IIl are the surface pressures in front of the canal and a t its outlet, respectively. The second term in eq 1 is a correction for the viscous drag of the subphase liquid (70 is bulk viscosity) by the monolayer. However, this correction term does not account for the structuring of the adjacent water layer by the monolayer, which depends on the nature of the film-forming substance. Therefore, we disregard the second term in eq 1, and we calculate the surface viscosities for the coupled system monolayer/ adjacent water layer according to eq 2 (for details see refs 43-45).
The surface viscosities for five film-forming substances shown in Table I11 exhibit differences for the three (41) Adamson,A. Physical Chemistry of Surfaces, 5th ed.;Wiley: New York, 1990. (42) Harkins, W. D.; Kirkwood, J. G. J. Chem. Phys. 1988,6,53. (43) Hiherfuss, H. J. CoZZotd Interface Sci. 1985, 107, 84. (44)Htherfuss, H. J. Colloid Interface Sci. 1988,126, 384. (45) Hiherfuss, H. J. Colloid Interface Sci. 1987, 120, 281.
Table 111. Surface Viscosity for A l l = 5 mN/m and Different Spreading Solvents surface-active compound 1-tetradecanol 1-hexadecanol (2)-9-hexadecen-l-oP hexadecanoic acid hexadecanoic acid methyl ester
surface viscosity (le Pa m e) hexane chloroform ethanol 0.64(*0.02) not measured 0.60(*0.05) 1.00(*0.02) 0.69(&0.02) 0.75(*0.04) 0.49(*0.01) 0.57(10.03) 0.56(*0.02) 0.46(*0.01) 0.44(*0.01) 0.45(*0.01) 0.55(*0.01) 0.58(*0.01) 0.52(*0.01)
*
a (Z)-g-Hexadecen-l-ol spread without spreading solvent: (0.50 0.01) x 1o-B Pa m s.
spreading solvents by up to 30%, e.g., in the case of 1-hexadecanol monolayers (in all cases the surface pressures were kept at I I z = 5 mN/m and IIl = 0 mN/m). Monolayers spread from ethanol solutions normally show a higher surface viscosity than those spread from hexane, although this order is reversed in the case of hexadecanoic acid methyl ester. This conclusion is in line with earlier results.43 The situation is more complicated for monolayers spread with the help of chloroform, because in the case of (2)-9-hexadecen-l-ol and hexadecanoic acid m o m layers the surface viscosity is more or less equal to the surface viscosity for the monolayer spread from ethanol, while for 1-hexadecanolthe surface viscosity attains a value between those of the monolayers spread from ethanol and hexane. The value for the hexadecanoic acid methyl ester monolayer spread from chloroform is the lowest in comparison with the spreading solvents ethanol and hexane. A monolayer of (2)-9-hexadecen-l-ol spread without the help of a spreading solvent yields the same value (as = (0.50 f 0.01) X 10-8 Pa) as that spread from hexane. In order to check as to whether or not ethanol solved in the subphase influences the surface viscosity, (2)-9hexadecen-1-01was spread without a spreading solvent on an ethanol/water subphase. It turned out that the surface viscosity values of the monolayer on the pure subphase and on subphases containing 1 and 2 mL/L ethanol are comparable within the error of this method. It should be noted that the surface viscosity measurements are performed at constant surface pressure, and therefore, the observed differences cannot be attributed to film loss. Infrared Reflection-Absorption Spectroscopy (IRAS). The main question which can be answered by the IRAS method is as to whether or not a certain amount of the spreading solvent remains in the monolayer. Unfortunately, the main absorption band of chloroform (C-Cl stretching vibration) is situated around 770 cm-I, where the reflection-absorption spectrum of a monomolecular film on a water substrate shows a steep baseline. Due to this obstacle we cannot answer this question unambiguously. However, the experimental data allow the conclusion that if any chloroform is remaining in the film, this amount must be very small. In order to investigate whether hexane remains in the monolayer, we used perdeuterated hexadecanoic acid and spread it with the help of hexane. After 15 min we were not able to detect any hexane in our system regardless of the compression status of the monolayer; i.e., the concentration of the hexane in the monolayer was below the detection limit of this method (5 mol ?6 hexane). This result supporta the hypothesis that hexane largely disappears out of the monolayer. In order to investigate whether ethanol remains in the monolayer, we spread different film-forming substances with the help of perdeuterated ethanol, but in all cases we were not able to detect any C-D stretching vibration.
Properties of Monolayers at the AirlWater Interface 0.001
Langmuir, Vol. 9, No. 8, 1993 2125 ~ c m - ' ~ 2919.0
2918.5
- 0.00s 4I B 3000
c 2800
2600
warenumber
2400 [ Em-'
2200
2000
1
Figure 8. Infrared reflection-absorption spectrum for a hesadecanoic acid methyl ester monolayer (0.264 nm2/molecule) on a water subnhase containing 2.5 mL/L ethanol-de for the range 3000-2000cm-1 (spreadingsolvent hexane; temperature 294 K).
Furthermore, spreading of perdeuterated hexadecanoic acid with ethanol showed no evidence of a C-H stretching vibration. However, if perdeuterated ethanol is present in the subphase in a relatively high concentration (2.5 mL/L) below a hexadecanoic acid methyl ester monolayer spread from hexane, small deuterium signals indicate that ethanol is present in the headgroup and/or hydrophobic tail region (Figure 8, 2249 and 2214 cm-l; the bands at 2920 and 2850 cm-l are due to the methylene stretching vibration of the film-forming substance; the strong band around 2300 cm-* is due to Cod. The deuterium signals increase, if more perdeuterated ethanol is added to the subphase. In general, for large areas per molecule P0.320 nm2/molecule),no perdeuterated ethanol is observed,while between -0.240 and 0.320 nm2/moleculea signal occurs, and for smaller areas per molecule it disappears again. Obviously, ethanol is able to penetrate into the monolayer until higher surface pressures are reached, and then the solvent molecules are squeezed out again. However, at higher concentrations of perdeuterated ethanol, signals are detectable that indicate a partial inclusion of solvent molecules even at higher film pressures. Furthermore, it has been demonstrated that the frequencies of the symmetric and antisymmetric CH2 stretching vibrations of molecules with long alkyl chains are conformationsensitive, and that they may be empirically correlatedwith the order of the hydrocarbon chains, where the order decreases with increasing f r e q u e n ~ y .In~ the ~ ~ presence ~ of ethanol molecules,as indicated by the deuteriumsignals of perdeuterated ethanol, the wavenumbers of the CH2 stretching vibrations of the film-forming substance are slightly shifted to higher wavenumbers with increasing ethanol concentrations ( 1 2 mL/L), which implies that the conformational order of the film-forming molecules is reduced. This aspect of conformational order has been pursued further by additional I U S measurements with l-hexadecanol monolayers. In Figure 9 the wavenumbers of the antisymmetric methylene stretching vibration vs the area per molecule are plotted for a 1-hexadecanol monolayer and hexane and ethanol as spreading solvents. The curves show that in the case of ethanol as the spreading solvent a higher conformational order, i.e., less kinks in the chain, is encountered in the gas-phase region than in the case of hexane as the spreading solvent. When approaching the (46) Snyder, R. G.; Straues, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86,5146. (47) Mac Phail, R. A.; Straws, H.L.; Snyder, R. G.; Elliger, C. A. J.
Phys. Chem. 1984,88,334.
1
T
2917.01 0.15
I
I
I
I
I
I
I
I
I
0.20 0.25 0.30 0.35 area ~ n m ~ / m o l e c u l e ~
0.40
Figure 9. Wavenumbers of the antisymmetric stretching vibration vs area per molecule for hexadecanoicacid methyl ester at the &/water interface spread from different spreadingsolvents (temperature 294 K): (-) spreading solvent ethanol; ( - - - ) spreading solvent hexane. Ccml 120
.
80
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/
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/ -/ I -- - - -- - - - - - - - - - - -
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l
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l
,
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Figure 10. Spreading velocities, i.e., spreading distance VB time,
for 1-hesadecanol monolayers at the air/water interface and different Spreadingsolvents (temperature294 K): (-) spreading solvent ethanol; (- - -) spreadingsolvent hexane; (- - -) spreading solvent chloroform.
transition state from the gaseous to the liquid-condensed phase, the differencesbetween the monolayers spread from ethanol and hexane are more and more reduced, and finally the values are identical. In addition, it seems that the tilt angle for the molecules spread from ethanol in the gaseous phase is less than the tile angle of the molecules spread from hexane, but the values show a strong scatter, presumably due to the inhomogeneityin that region (curves not shown). The 1-hexadecanol monolayer spread from chloroform shows a conformational order which is comparable with that of the monolayer spread from ethanol. Spreading Velocity Measurements. The pure spreading solvents show very different spreading kinetics at the air/water interface; e.g., a drop of ethanol is known to spread rapidly and turbulently across the surface, dissolving in less than 1sSa So far, it seems to be interesting to determine the spreading velocities of different systems of surface-active compound/spreading solvent. We used solutions of equal concentrations and placed one drop with equivalent volumes at the surface for each measurement. The results for 1-hexadecanol and (Z)-9-hexadecen-l-ol are shown in Figures 10and 11,respectively. In bothcases, the ethanolic solution exhibits the highest spreading velocity, followed by chloroform and hexane. If it is tentatively assumed that faster spreading processes lead to a better distribution of the surface-active material at the water surface, one can expect that the homogeneity of the monolayer also decreases in the order ethanol > chloroform > hexane. This supposition is supported by (48) Walters, D.A. Langmuir 1990,6, 991.
2126 Langmuir, Vol. 9, No.8,1993
Gericke et al.
Ccm 1 120 1
0
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I
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. '
80 -
-
60
-
40 -
-
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60 90 t i m e [SI
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150
Figure 12. Spreadingvelocities,Le., spreadingdistance vs time, for different monolayers at the aidwater interface and the spreading solvent ethanol (temperature 294 K): (-) (2)-9hexadecen-1-ol; (- - -) 1-hexadecanol; (-) hexadecanoic acid methyl ester.
the results shown in Figure 10,because the spreading front for the monolayer spread from hexane will never reach the point for the spreading solvent ethanol after 130 s. In Figure 12 the spreading velocities of three different filmforming substances dissolved in ethanol are shown. It is evident that the spreading properties of the film-forming molecules are improved by the spreading solvent: While pure hexadecanoic acid methyl ester shows a very small spreading velocity in comparison with l-hexadecanol,49it spreads from ethanolic solution considerably faster than 1-hexadecanol.
Discussion As pointed out in the preceding section, the main problem related to the application of ethanol as the spreading solvent is the film loss into the subphase. Although it is possible to reduce this loss by applying syringes with small needles, one is not able to reduce the loss to zero. In addition, this effect is not minimized with increasing chain length, Le., increasing hydrophobicity, but it is on the contrary slightly enhanced (Table I). In the time scale of the IIlA measurement the surface-active material is obviously not able to adsorb from the subphase to the surface. The comparison between the spreading solvents hexane and chloroform shows only minor differences with the exception of (2)-9-hexadecen-1-01. An unambiguous explanation for this latter case cannot be given yet; however, it cannot be excluded that some (49) OBrien, R. N.;Feher,A. I.; Leja, J. J. Colloid Interface Sci. 1976, 66, 414.
spreading solvent remains in the film. But firstly we were not able to detect any chloroform in the film by infrared reflection-absorption spectroscopy, and secondly, it is at least questionable that a small amount of solvent is able to influence II/A isotherms so strongly.1° Furthermore, impurities in the solvent are hardly responsible for the shift, because we checked the purity of the solvents several times by gas chromatography. As early as 1985,Hiihnerfuss performed surface viscosity measurements with long-chain saturated alcohols, applying ethanol and heptane as spreading solvents.43 On the basis of these earlier investigations the following conclusions were drawn: Application of ethanol as the spreading solvent supplies higher surface viscosity values than spreading from heptane. This effect decreases with decreasing chain length such that the values of monolayers spread from ethanol and heptane approach each other. These observations were tentatively explained by an inclusion of heptane molecules between the hydrophobic chain of the film-forming substances, which is expected to give rise to lower surface viscosity values because of a reduction of the hydrophobic interaction between the film molecules. This conclusion appeared to be supported by a comparison between surface viscosity values of l-octadecanol and (2)-9-octadecen-l-o1, which showed that s t e r i d hindrance-be it intra- or intermolecular-leads to a lowering of the surface viscosity. At first glance, the data summarized in Table I11 seem to support these earlier conclusions: in the case of 1-hexadecanol, higher surface viscosity values were found for the application of ethanol as the spreading solvent than for hexane, and for the shorter chain tetradecanol this solvent effect does not play a role any longer. Furthermore, 1-hexadecanol monolayers exhibit a higher surface viscosity than the sterically hindered (2)-9hexadecen-1-01. On the other hand, the infrared reflection-absorption measurements presented herein do not supply any evidence of spreading solvent molecules in compressed monolayers. This holds at least for the substances and spreading solvents investigated herein within the detection limits of the IRAS method. Therefore, we tend to assume that the differences between the surface viscosity values of monolayers spread from ethanol and hexane as reported by Hiihnerfuss4 and the differences which were partly also found in this work (Table 111)reflect rather different micromorphological structures of the monolayer, which are induced by the respective spreading solvent, than inclusion of solvent molecules. More investigations are necessary, in order to be able to specify the respectively micromorphological structures; however, some first indications of different conformational orders and arrangements of the film molecules can be inferred from the present infrared measurements. Furthermore, a connection between micromorphology and surface viscosity was already found by Naito et al.,33 and Hbnig et al.18 have shown that different spreading solvents may lead to different island sizes in the gas-phase region. The differences in the spreading velocities shown in Figures 10 and 11support this hypothesis, because a more rapid spreading is expected to correspond to a more homogeneous distribution. In addition, we actually observed a higher conformational order for 1-hexadecanol in the gasphase region, and within the experimental limitations mentioned above, a smaller tilt for the monolayer spread from ethanol than for the film spread from hexane, while the values for hexadecanoic acid methyl ester were the same within the experimental error for all three spreading solvents (not shown). Although a higher molecular order is expected to produce a higher viscosity, these conformational differences diminish (Figure 9) for the surface
Langmuir, Vol. 9, No. 8, 1993 2127
Properties of Monolayers at the AirlWater Interface
Table IV. Main Spreading Solvent Effects on the IIesultr of Different Methods spreading eolvent effect p i b l e film lorn by ethanol; isotherms of chloroform and hexane similar (except for (2)-9-hexadecen-1-01) solvent effecte in addition to film lose by ethanol;
methcd
IUA AVID
AVlblorofviscosity
IRAS
-
AV/&I-
dmt
)Isth.nol~v~onn>~o=rMtbwt*t
~~~~
inclusion effect conformational order (co) tilt angle (ta) spreading velocity (sv) homogeneity of monolayer (hm)
ethanol may be included depending on surface pressure and subphaee concentration;chloroform not included or only little; hexane not included C-* c o c h l o " > coheun. for l-hexadecanol, a< t a w ; for hexadecanoic acid methyl ester, U = teChl"- = taha, sv> s v c h h " > 8llmetbd> hmehlo">hmlmIuiO> h m * t h t
Note: This scheme ie a simplification, and a few exceptions have to be taken into account.
pressure region where we carried out the surface viscosity measurement (lI2 = 5 mN/m). In this context one must take into account that the antisymmetric methylene stretching vibration is conformation sensitive only due to the Fermi resonance interfering with the methylene bending vibration. If this bending band is only slightly shifted, the frequency of the methylene stretching vibration is expected to remain more or less constant. In would be of superior interest to investigate the morphology by methods which can visualize the shape of the film islands for different spreading solvents in the region of the surface pressure unequal to zero and to connect the received results with the surface viscosity. If the observed variations in the surface viscosity can be attributed to the different degrees of macroscopical inhomogeneities (islands) in the film, the degree of inhomogeneity depends as well on the spreading solvent as on the film-forming material. In particular, one cannot decide whether chloroform spreads more homogeneously than the other solvents, because sometimes the viscosity is higher than for the other solventsand sometimes smaller (Table 111). Furthermore, it should be noted that for hexadecanoic acid the influence of the spreading solvent is diminished, possibly due to partly ionized headgroups. On the other hand, the same differences of the surface viscosity values between the three spreading solventswere measured for hexadecanoic acid on a subphase of pH 2.
Conclusions The main results are summarized in Table IV. In particular, the following conclusions were drawn. (1)Ethanol as the spreading solvent leads to film lose into the subphase. Thisholds also for mixtures of hexane/ ethanol (91) which are widely used to spread DPPC. The film loss can often be reduced by using syringes with smaller needles. (2) The ethanol which is solved in the water after the spreading process does not influence the properties of the monolayer up to a concentration of S1 mL/L. (3) Neither hexane nor chloroform solvent molecules were detected in the monolayer using infrared reflectionabsorption spectroscopy. IRAS measurements with perdeuterated ethanol showed that ethanol may penetrate into the monolayer, but that it is squeezed out at higher surface pressures. This inclusion effect was observed at concentrations of about 2.5 mL/L. (4) Surface potentiaVarea and IR reflection-absorption measurements show distinct differences in the molecular order in the gaseous and expanded phases for the monolayers spread from different spreading solvents. This is interpreted as a different molecular order of the monolayers induced by the respective spreading solvents.
These differences are diminished upon compression of the monolayers to the condensed states. (5) Surface viscosity measurements carried out at l I 2 = 5 mN/m showed that application of ethanol as the spreading solvent in general supplies higher surface viscosity values than spreading from hexane. Thie effect decreaseswith decreasingchain length such that the valuea of monolayers spread from hexane and etlpnol approach each other. In the case of hexadecanoic acid methyl ester the surface viscosity of the monolayer spread from hexane attained the highest value. If chloroform is used aa the spreading solvent, the surface viscosity values are similar to those of the monolayer spread from ethanol, or the values are smaller. (6) For some specific measurements it may be useful to apply ethanol as the spreading solvent, because the film lose does not influencethe result in thew cases (e.g., surface viscosity measurements at a distinct surface pressure), and it seem that ethanol distributes the surface-active material more homogeneously over the surface. (7) For (2')-9-hexadecen-1-01 the measured surface viscosities for hexane as the spreading solventand without any spreading solvent are equal, while the surface potentials (with regpect to the surface pressure) for chloroform and without spreading solvent are nearly equal. The points mentioned above reveal that the system air/ monolayer/water is strongly influenced by the Spreading solvent and that, consequently, all studies carried out with monolayers at the air/water interface and possibly even the order of LangmuhBlodgett films can be affected. Therefore, we strongly recommend documenting the spreading solvent used in the respective investigation in all publications (unfortunately, this important point is often missing) as well asthe concentrationsof the solutions. Furthermore, the spreading technique can influence the molecular order in the film (this aspect will be pursued further in ref 50). We refrain from giving any recommendation of which spreading solventis the "best",because the results presented herein clearly show that the decision will depend on the film-forming substance and the experimental method to be applied.
Acknowledgment. This work was supported by the Deutsche Forschungsgemeinechaft, FRG (Schwerpunktprogramm Methoden der Fernerkundungvon Atmwphiire und Hydrosphiire, Project OMMI HU 583/1-11, and by the Fraunhofer Gesellschaft, FRG (Contract T/RF35/ L0013/L1309 SAXON-FPN). (60)Gericke, A.; Simon-Kutecher, J.; Hiihnerfuse, H. Langmuir, in press.
