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Langmuir 1997, 13, 796-800
Monolayer Properties of a New Family of Amphiphiles with an Unusual Head-Group Topology Jo¨rg Andreas Schro¨ter, Rene Plehnert, and Carsten Tschierske* Institute of Organic Chemistry, Martin-Luther-University, Halle-Wittenberg, Kurt-Mothes-Strasse 2, D-06120 Halle, Germany
Stefan Katholy, Dietmar Janietz, Florencio Penacorada, and Ludwig Brehmer Potsdam University, Research Group Thin Organic Films, Kantstrasse 55, D-14513 Teltow, Germany Received August 14, 1996. In Final Form: November 6, 1996X Several rodlike 4,4′′-bis(decyloxy)-p-terphenyl derivatives incorporating nonionic hydrophilic groups in the lateral 2′-position (2-oxa-4,5-dihydroxypentyl, 2,5-dioxa-7,8-dihydroxyoctyl, 2,5,8-trioxa-10,11-dihydroxyundecyl, and 2,5,8,11-tetraoxa-13,14-tetradecyl groups) and 2′-(2-oxa-4,5-dihydroxypentyl)-4,4′′diundecyl-p-terphenyl form well-ordered thin films when spread at the air-water interface. One observes two sharp breaks in the pressure/area isotherms separated by a large plateau. The first break occurs at an area of ca. 0.90 nm2/molecule, an area which corresponds to a side-on arrangement of the terphenyl units at the interface. The plateau corresponds to a first-order phase transition. The surface pressure related to this transition significantly rises with an increasing number of oxyethylene units in the hydrophilic lateral groups. Brewster angle microscopic investigations indicate the formation of fluid domains in this region. In some cases these domains coalesce to a homogenous layer. The surface potential is nearly constant in the region of the plateau, which can be explained by a defined collapse due to the formation of a triple layer consisting of a bilayer on top of the monolayer.
1. Introduction The aggregation of amphiphiles in aqueous media and at interfaces and the organization of anisometric compounds with formation of liquid crystalline phases are important examples of molecular self organization, and therefore they are intensively investigated. More recently both structural concepts have been combined and, for example, the influence of anisometric (rod-shaped1-8 or disk-shaped9-11) units in amphiphiles on their monolayers was studied. Those compounds with rodlike mesogenic units usually carry the polar substituent at one of the termini of their rigid cores (structure A; Figure 1). In this way the two driving forces of their molecular self organizationsthe amphiphilic pattern and the parallel arrangement provided by rigid coresscooperate and thus can reinforce the aggregation tendency. The question arises, what happens if a hydrophilic group is not terminally but laterally attached to a rigid core (structure B; Figure 1). Then, the two different organizing forces would be perpendicularly directed to each other. Such structures are of interest also because they can in some respect * To whom correspondence should be addressed. Fax: +49 (345) 5527030. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Albrecht, O.; Cumming, W.; Kreuder, W.; Laschewsky, A.; Ringsdorf, H. Colloid Polym. Sci. 1986, 264, 659. (2) Daniel, M. F.; Lettington, O. C.; Small, S. M. Mol. Cryst. Liq. Cryst. 1983, 96, 373. (3) Fuller, S.; Hopwood, J.; Rahman, A.; Shinde, N.; Tiddy, G. J.; Attard, G. S.; Howell, O.; Sproston, S. Liq. Cryst. 1992, 12, 521. (4) Decher, G.; Ringsdorf, H. Liq. Cryst. 1993, 13, 57. (5) Everaars, M. C.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Thin Solid Films 1994, 242, 78. (6) Joachimi, D.; O ¨ hlmann, A.; Rettig, W.; Tschierske, C. J. Chem. Soc., Perkin Trans. 1994, 2011. (7) Joachimi, D.; Tschierske, C.; O ¨ hlmann, A.; Rettig, W. J. Mater. Chem. 1994, 4, 1021. (8) Woolley, M.; Tredgold, R. H.; Hodge, P. Langmuir 1995, 11, 683. (9) Laschewsky, A. Adv. Mater. 1989, 392. (10) Maliszewskyj, N. C.; Heiny, P. A.; Blasie, J. K.; McCauley, J. P.; Smith, A. B., III. J. Phys. II 1992, 75.
S0743-7463(96)00809-8 CCC: $14.00
Figure 1. Two structural types of amphiphiles incorporating rigid cores.
resemble the amphiphilic pattern of the naturally occurring cholic acid salts and facial amphiphilic peptides.12 Herein we report on the investigation of the monolayer behavior of some calamitic 4,4′′-disubstituted p-terphenyl derivatives incorporating different nonionic hydrophilic groups laterally attached at the 2′-position to the rigid rodlike 4,4′′-disubstituted p-terphenyl core. 2. Synthesis The synthesis of the diol 1 has recently been described.13 The preparation of compounds 2-5 will be reported in a separate paper together with a detailed discussion of their liquid crystalline properties as bulk materials (Table 1).14 The thermal transition temperatures of compounds 1-5 are collected in Table 1.
3. Experimental Section Pressure/area isotherms were recorded using a film balance (Riegler & Kirstein GmbH) equipped with a Teflon-coated (11) Reiche, J.; Dietel, R.; Janietz, D.; Lemmetyinen, H.; Brehmer, L. Thin Solid Films 1992, 226, 265. Janietz, D.; Hofmann, D.; Reiche, J. Thin Solid Films 1994, 244, 794. (12) Carey, M. C.; Small, D. M. Arch. Intern. Med. 1972, 130, 506, 7319. Stein, T. M.; Gellman, S. H. J. Am. Chem. Soc. 1992, 114, 3943. Barrett, D. G.; Gellman, S. H. J. Am. Chem. Soc. 1993, 115, 9343. (13) Hildebrandt, F.; Schro¨ter, J. A.; Tschierske, C.; Festag, R.; Kleppinger, R.; Wendorff, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 1631. (14) Schro¨ter, J. A.; Hildebrandt, F.; Tschierske, C.; Wittenberg, M.; Festag, R.; Wendorff, J.-H. Adv. Mater., submitted.
© 1997 American Chemical Society
Amphiphiles with an Unusual Head-Group Topology
Figure 2. Surface pressure/molecular area isotherms of monolayers of compound 1 at the air-water interface at 5, 20, and 35 °C (compression rate: 0.1 nm2 molecule-1 min-1). Table 1. Transition Temperatures (°C) of Compounds 113 and 2-514 (Abbreviations: cr ) Crystalline Solid; SA ) Smectic A-Phase; Colr ) Rectangular Columnar Mesophase; is ) Isotropic Liquid)
comp.
X
n
transition temperatures/°C
1 2 3 4 5
O CH2 O O O
0 0 1 2 3
cr 83 SA 114 is cr 63 SA 78 is cr 66 SA 80 is cr 54 (Colr 40 SA 48) is cr 45 (Colr 40) is
Langmuir trough and a continuous Wilhelmy type measuring system. The substances were dissolved in chloroform (1.0 mM). The measurements were started 10 min after spreading. Water of the subphase was of Millipore quality. Brewster angle microscopy was carried out with a MINI-BAM and a BAM 1+ (NFT) microscope, respectively, the latter one in conjunction with a NIMA-601 Langmuir trough. The lateral resolution was 10 µm. Surface potential measurements were performed on a KSV 5000 Langmuir-Blodgett trough with a Wilhelmy plate system and a commercial Kelvin probe15 (SPM, KSV Instuments) with a platinum bottom electrode.
4. Results 4.1. Investigation of 4,4′′-Bis(decyloxy)-2′-(4,5dihydroxy-2-oxapentyl)-p-terphenyl (1). The glycerol ether 113 was the first member of the group of compounds with the molecular structure presented in Table 1. Its π/A isotherms in dependence on the temperature are displayed in Figure 2. One observes two sharp breaks in the isotherm slopes. At 20 °C the surface pressure starts to increase at 1.25 nm2/molecule. A large plateau is reached at π ) 16 mN/m corresponding to A ) 0.90 nm2/ molecule. On further compression a second rise occurs, which is followed by a decrease in the surface pressure. The area at this second kink increases with increasing compression (15) Penacorada, F.; Reiche, J.; Katholy, S.; Brehmer, L.; RoutriegezMendez, M. L. Langmuir 1995, 11, 4025.
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Figure 3. Reversibility studies of the π/A isotherms of monolayers of compound 1 at the air-water interface at 20 °C: (a) first compression; (b) expansion; (c) second compression after 20 min (compression and expansion rates of v ) 0.5 nm2 molecule-1 min-1).
rates and reaches a limit value of 0.26 nm2/molecule at high compression rates (0.5 nm2 molecule-1 min-1). Extremely slow compression can give a straight line without the second break. No significant change in the shape of the isotherms in dependence on the temperature can be detected. However the area at the transition to the plateau increases with increasing temperature whereas the film pressure in the plateau region only slightly decreases. Compressionexpansion cycles indicate a complete reversibility before the plateau is reached. In the region of the plateau a drop in pressure in the expansion curves is observed. A second compression immediately after the first cycle does not lead to the same curve again. However, the hysteresis is a relaxation process: a second compression after a certain period produces the original curve again (Figure 3). The structure of thin layers at the air-water interface was monitored by means of Brewster angle microscopy (BAM).16,17 Immediately after spreading the film is organized homogenous at zero surface pressure. Compression of the film leads to the formation of small domains at the transition to the plateau region of the surface pressure/area isotherm. Soon after the domains have been formed a partial three-dimensional crystallization occurs. The onset of the crystallization and the amount and the size of crystals formed depend on the compression rate. Since material is consumed to the third dimension during the crystallization process, it is obvious that the shape of the π/A isotherm in the small-area region depends on the compression rate. 4.2. Investigation of 2′-(4,5-Dihydroxy-2-oxapentyl)-4,4′′-diundecyl-p-terphenyl (2). In a next step we studied the influence of the terminal flexible substituents on the monolayer properties of the terphenyl amphiphiles. Replacing the terminal ether oxygens which connect the alkyl chains to the terphenyl core in compound 1 by methylene groups has no significant influence on the shape of the π/A isotherms (Figure 4). Almost identical isotherms were observed for both compounds 1 and 2. However, for the alkyl-substituted terphenyl derivative 2 the appearance of the second kink is independent of the compression rate and takes place at an area of 0.26 nm2/molecule. This area corresponds to (16) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (17) Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936.
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Figure 4. π/A isotherm (a) and corresponding surface potential/ molecular area isotherm (b) of compound 2 on water at T ) 25 °C (compression rate: 0.04 nm2 molecule-1 min-1). Figure 6. Surface pressure/molecular area isotherms of monolayers of compounds 1 and 3-5 at the air-water interface at 20 °C.
Figure 5. BAM images of the monolayers and multilayers of 2 during compression (compression rate: v ) 0.04 nm2 molecule-1 min-1) at 20 °C: (a) at A ) 0.80 nm2; (b) at A ) 0.29 nm2. A description of the images is given in the text. Brighther areas are more dense. The brightness of each image was separately scaled.
the limit value observed for compound 1 at high compression rates. Inspection of the thin films by Brewster angle microscopy reveals that no crystallization occurs even at small compression rates. Again, the film is organized homogenous after spreading, and compression of the film leads to the formation of small domains at the transition to the plateau region (Figure 5a). On further compression the domains coalesce to larger ones at the inflection point of the second rise of the isotherm (Figure 5b). On further
compression these fluid domains are growing and finally form a homogenous layer. Expansion of the monolayer leads to a decrease in the size of the domains, and finally they disappear. A homogenous film appears again. Because no crystallization occurs, compound 2 was used for further investigations by means of surface potential measurements. The surface potential/area isotherm obtained is also displayed in Figure 4. We see first in the gas phase the rapid increase of the surface potential due to the increase of the number of molecules in the measurement area. When the surface pressure starts to increase, a decrease in the slope of the surface potential is observed. When the plateau is reached, the slope of the surface potential curve further decreases. 4.3. 4,4′′-Bis(decyloxy)terphenyl Derivatives with Different Hydrophilic Groups (3-5). Additionally, by keeping the hydrophobic unit constant, we have changed the lateral hydrophilic groups. The π/A isotherms of compounds 3-5 along with the isotherm of 1 are shown in Figure 6. The isotherms, in general, are of the same shape as that of compound 1. The molecular areas at the first kink (ca. 0.90 nm2/molecule) and at the second kink (ca. 0.26 nm2/molecule) are in the same order of magnitude in each surface pressure isotherm, and they are not influenced by the size of the lateral groups.18 The occurrence of the second break in the isotherms of compounds 3-5 is independent of the compression rate. The surface pressure of the transition to the plateau region significantly rises by increasing the number of oxyethylene units in the hydrophilic lateral groups. This indicates that the increased hydrophility gives rise to a stabilization of the monolayers. The monolayer behavior of compounds 3-5 during compression was also investigated by Brewster angle microscopy. The same kind of small domains as in the case of compounds 1 and 2 can be observed in the region of the large plateau. However these domains remain until the collapse sets in and do not coalesce to a homogenous layer. Furthermore, no crystallization can be observed. The surface potential isotherm of compound 4 corresponds to that of compound 2. (18) Only the collapse of the triethylene glycol derivative 5 occurs at lower surface pressure and at a larger area compared with that of the diethylene glycol derivative 4. The reason is not yet clear.
Amphiphiles with an Unusual Head-Group Topology
Figure 7. Schematic presentation of the possible arrangements of the facial amphiphiles 1-5 at the air-water interface: (A) monolayer with a side-on arrangement of the molecules; (B) monolayer consisting of molecules oriented more or less perpendicular to the water surface (only the perpendicular arrangement is displayedsthe molecules could also be tilted in respect to the layer normal); (C) defined triple layer; (D) side view during “roll-over” collapse with formation of a defined triple layer (alkyl chains are not displayed).
5. Discussion 5.1. Surface Pressure Isotherms. In some respect the isotherms of compounds 1-5 are similar to those of monolayers consisting of molecules with two polar head groups at opposite ends of an aliphatic chain (bolaamphiphiles)19-21 and those of certain calamitic liquid crystals.1,8,22-25 From the shape of the isotherms of compound 1 we can draw the following conclusions. At low lateral pressures the amphiphilic pattern should give rise to a side-on arrangement of the molecules at the air-water interface. The first kink appears at an area of 0.90 nm2/molecule, which corresponds to that area which would be required by the flat-lying p-terphenyl cores. Therefore we assume that at this point the surface should be occupied by the dense packed aromatic terphenyl parts of the molecules lying flat on the water surface. As sketched in Figure 7A, the hydrophilic groups should dip into the water subphase whereas the lipophilic alkyl chains are forced out of the interface. The increased molecular area of the films at the transition to the plateau at elevated temperatures (Figure 2) can thus be explained by increased thermal motion of the terphenyl cores. Increasing the hydrophilicity of the lateral groups by incorporation of additional oxyethylene units increases the surface pressure and gives rise to a stabilization of the side-on arrangement (Figure 7). Brewster angle microscopic investigations indicate domain formation in the region of the large plateau. These domains are rather fluid. Only in the case of compound 1, soon after the first break a crystallization process was observed. This crystallization is kinetically controlled and thus depends on the compression rate. On expansion, crystals remain and give rise to a hysteresis in the reversibility. (19) Kellner, B. M.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 152. (20) Hasegawa, T.; Umemura, J.; Takenaha, T. Thin Solid Films 1992, 210/211, 583. (21) Jeffers, P. W.; Daen, J. J. Phys. Chem. 1965, 69, 2368. (22) Diep-Quang, H.; Ueberreiter, K. Colloid Polym. Sci. 1980, 258, 1055. (23) de Mul, M. N. G.; Mann, J. A., Jr. Langmuir 1994, 10, 2311; 1995, 11, 3293. (24) Daniel, M. F.; Lettington, O. C.; Small, S. M. Thin Solid Films 1983, 99, 61. (25) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251.
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In the region of the large plateau, starting at the first break either a reorientation of the molecules by lifting the terphenyl units (Figure 7B)26 or a collapse of the horizontal lying molecules with a “roll-over” mechanism (Figure 7C and D) can be discussed. If one assumes a lift-off process, then the molecules should form a monolayer consisting of molecules arranged parallel to each other and oriented more or less perpendicular to the water surface (Figure 7B). In this case the second break should correspond to the collapse of this monolayer. Though the area (0.26 nm2/molecule) could correspond to this arrangement, it cannot be explained that the molecular area at collapse does not significantly depend on the size of the lateral hydrophilic groups. Furthermore, the alkyl chains have to be inserted into the water, which seems rather unlikely. In the case of a defined “roll-over” collapse, not all hydrophilic groups have to be forced out of the water. Instead bilayer domains are formed on top of the monolayer during collapse. Thus a triple layer is built up (Figure 7C and D). In this case the limiting area should be ca. 0.30 nm2/molecule. This value is again in fair agreement with the molecular area at the second kink (0.26 nm2/molecule), which in this case should correspond to the collapse of the triple layer. 5.2. Surface Potential Isotherms. Solely on the basis of the pressure/area isotherms and Brewster angle microscopic investigations, no clear decision can be made between these two possibilities. More information was obtained from the surface potential measurements at the air-water interface during compression of the monolayer. According to eq 1 there are two significant contributions: the number N of molecules in the area of the Kelvin probe and the perpendicular component of the dipole moment to the water surface (µ⊥).27
∆V ) 4ΠNµ⊥
(1)
The surface potential isotherms of the two investigated compounds 2 and 4 are of the same type. In the gas analogous phase the strong increase of the surface potential is due to the increasing number of molecules, i.e. the number of dipoles under the Kelvin electrode. When the surface pressure starts to rise the slope of the curve decreases. It seems that the further increase of the surface potential is partly compensated by the decrease of µ⊥, which could be due to the rearrangement of the alkyl chains during compression. When the plateau is reached, the slope of the surface potential curve is further decreased. We can explain this effect very well with the model of the conversion of the monolayer to a triple layer (Figure 7C and D). As a consequence of a defined roll-over collapse during further compression of the Langmuir film, a bilayer is formed on top of the monolayer. Therefore we have a coexistence of a triple layer (the initial small domains observed with BAM at the beginning of the plateau) and a monolayer (regions between these domains). The perpendicular dipole of the triple layer is nearly the same as that of the monolayer, since the dipoles of the molecules in the second and in the third layer are oriented in opposite directions (centrosymmetric arrangement) and nearly compensate each other. The dipoles of the first layer should have almost the same direction as those in the monolayer and therefore should give the same contribution to the surface potential. In this way we have (26) This arrangement is found in the liquid crystalline smectic layer structure (SA-phase) of the bulk material of compound 1.13 (27) Schulman, J. H.; Rideal, E. K. Proc. Roy. Soc. (London) 1931, A130, 259.
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nearly no change in the number of noncentrosymmetric dipoles of the film during compression. If we would assume the formation of the monofilm as presented in Figure 7B, the aliphatic chains in the domains are oriented in opposite directions (separated by the terphenyl-ring systems). Therefore the perpendicular dipole moment decreases continuously until the whole film is completely converted into the new phase at the end of the plateau. The surface potential would be expected do decrease, but not to slightly increase. 5.3. Conclusions. In summary we have reported the monolayer behavior of novel rodlike terphenyl derivatives with a laterally attached hydrophilic group. Two sharp breaks occur along the π/A isotherms of these compounds. Between these breaks a large plateau occurs, which we could describe as a first-order phase transition.28 From Brewster angle microscopy and from surface potential measurements we conclude that in the region of this phase transition a defined roll-over collapse with
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formation of a triple layer occurs. This kind of collapse was recently reported for cyanobiphenyl derivatives.23 These compounds form a triple layer with an arrangement of the cyanobiphenyl molecules akin to surface-induced smectic layers in bulk liquid crystals and free-standing liquid crystalline films: The molecules in the surface layer are tilted, whereas the double layer on top is formed by antiparallel arranged molecules, which are oriented perpendicular to the surface. In our case however the individual molecules have another orientation. They are arranged parallel to the water surface. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. LA960809I (28) We are hesitant to describe this as a real first-order phase transition as in thermodynamics, because here we have at the end a completely different system than before.
