Langmuir 1993,9,766770
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Ring Substitution Position and the Monolayer and Langmuir-Blodgett Multilayer Properties of Heneicosa-2,4-diynyl Carboxybenzoates D. Neil Furlong,*yt David Scoberg,? Julianne Davy,$ and Rolf H. Pragert CSIRO Division of Chemicals and Polymers, Private Bag 10, Clayton, 3168 Victoria, Australia, and Department of Chemistry, Flinders University, Bedford Park, 5042 South Australia, Australia Received May 11,1992. In Final Form: November 12,1992
The ortho isomer of heneicosa-2,4-diynyl carboxybenzoate exhibits very different monolayer and Langmuir-Blodgett (LB)behavior from that d t h e meta and para isomers. Ita %/Aisotherm can be more expanded and exhibits a twnover, signaling a significantreorientation in the monolayer state. Moreover, the monolayer(andLB film)of ortho is readily polymerizedby UV irradiation,even when highly expanded. Various monolayerconformationscan be proposed for each isomer to explain the turnover in the isotherm for ortho, compared to the condensed isotherms given by meta and para. Lateral attractive interactions involving the aromatic ring cause domain formationin the monolayers. In the case of ortho only, packing within the domains is conduciveto polymerization of the diacetylenemoieties. With meta and para lateral bridging by subphasecadmium ions is likely, as is the formationof interfacialdimers and liquid crystalline phases. The monolayer behavior of each isomer is mirrored by ita LB characteristics. The differences in lateral and cross-layer interactions are obviously responsible for the spontaneous formation of Z-type films for ortho and Y type films for meta and para. Introduction There is much interest in relationships between surfactant molecular structure and the type, X,Y, or Z,' of film formed by Langmuir-Blodgett (LB) dipping procedures. Most molecules, if they in fact give any LB transfer, do so on both the upward and downward passes forming Y type films. Reports have appeared sporadically of various surfactants, such as some with large zwitterionic headgroups? which form Z-type films spontaneously. Recently Popovitz-Biroand co-workers3have shown how deposition can be switched from Y type to either X or Z type by the incorporation of hydrophilic moieties like amide into the aliphatic chain of the surfactant. These workers attempted to correlate film structure with the wettability of the substrate, although it is unlikely that this is always reliable. We have recently shown4that the ortho-substituted 2,4-diyneester of dicarboxybenzenewill spontaneously form Z-type Langmuir-Blodgett films on hydrophobic substrates. This has particular advantage when the aim is to fabricate films which display enhanced second order optical properties. This present paper examinesthe behavior of the ortho isomer in more detail, as well as describingthe very different characteristicsfound for the meta and para-substituted analogue surfactants. Materials and Methods All salts, acids, and bases were BDH analytical grade reagents and were used as supplied. The water was triply
* Author to whom correspondence should be addressed. t CSIRO Division of Chemicals and Polymers.
Flinders University. (1) Furlong, D. N.; Grieser, F. Chem. Aust. 1992, 617. (2) Ashwell, G. J.; Dawney, E. J. C.; Kuczynski, A. P.; Martin, P. J. SPZE Phyaical Concepts of Materials for Novel Optoelectronic Device Applications I 1990, 589.
(3) (a) Popovitz-Biro, R.;Hill, K.; Hung, D. J.;Lahev, M.; Leiserowitz, L.; Sagiv, J.; Hsuing, H.; Meredith, G. R.;Vanherzeele, H. J.Am. Chem. SOC.1990,112,2498. (bjPopovitz-Biro,R.;Hung,D.J.;Shavit,E.;Lahav, M.; Leiserowitz, L. Thin Solid Films 1990,178,203. (c) Popovitz-Biro, R.;Hill, K.; Landau, E. M.; Lahav, M.; Leiserowitz, L.; Sagiv, J. J.Am. Chem. SOC.1988,110,2676. (4) Scoberg, D. J.; Furlong, D. N.; Drummond, C. J.; Grieser, F.; Davy, J.; Prager, R. H. Colloids Surf. 1991,58, 409.
distilled with a surface tension greater than 71.9 mN/m and a conductivity of less than 0.1 pSlcm at 25 "C. The cationicpolyelectrolyte,poly(dially1dimethylammonium) chloride (PDDAC,molecular weight 25 OOO) was obtained from Monomer-Polymerand DajacLaboratories (Trevose, PA) and was also used as supplied. Chloroform (Ajax, "spectrosol" grade) was used to make up the spreading solutions of the surfactants. The three phthalate diacetylene surfactants (the structure of the ortho derivative is shown in Figure 1)were synthesizedaccordingto literature method^.^^^ They will henceforth be designated ortho (mp 88-89 "C), meta (mp 150-151 "C), and para (mp 186187 "C), referring to the relative positions of the carboxyl and ester moieties. Prior to their use any polymer residue was removed by recrystallization from diethyl ether (distilled AR grade, Ajax). Monolayer, spectroscopic, multilayer, and polymerization procedures were as described previo u s l ~ .Solid ~ substrates for Langmuir-Blodgett transfer were either Pyrex or quartz plates which had been hydrophobed by treatment with trimethylchlorosilaneand by Langmuir-Blodgett deposition of four base layers of cadmium arachidate.4 Rssults We have previously reported4 on some monolayer and Langmuir-Blodgett properties of ortho. Key characteristics are now expanded upon in parts a-c below and are contrasted to the behavior of meta and para. (a) The * / A isotherm for ortho on water and aqueous CdCL showed a strong dependence on the speed of compression. A turnover (curve 1, Figure 2) occurred unless the monolayer was compressed in the very slow (less than 0.001 (nm2/molecule)/min) stepwise mode (dashed curve, Figure 2). The turnover was less dramatic as the pH of the subphase was increased from 2 to 10,and its pressure increased as the temperature was raised from 15 to 25 0C.4 It was anticipated that the positive charge along each molecule of PDDAC would provide some stabilization to the monolayer, because of its electrostatic (5) Davy, J. PhD Thesis, Flinders University, South Australia, 1992.
0743-7463/9312409-0166$04.00/0 0 1993 American Chemical Society
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0
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Area/Molecde ( n d ) Figure 3. * / A isotherms of phthalate diacetylene surfactanta under continuous compression: temperature, 20 OC; subphase, 103 mol/dm3PDDAC; compression rate, 0.017 (nm2/molecule)/ min; curve 1, ortho; curve 2, m e w curve 3, para. 0.1
0.2
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AredMolecule (nmg) Figure 2. r / A isotherms of phthalate diacetylene surfactants under continuous compression: temperature, 20 OC; subphase, mol/dm3 CdClz (pH 6.9); compression rate, 0.017 (nm2/ molecule)/min; curve 1, ortho (dashed line under stepwise compression4);curve 2, mew, curve 3, para.
anchoring to ortho. When PDDAC was added to the subphase, the isotherm was indeed uniformly expanded (Figure 3) and without evidence of a turnover. By contrast, meta and para gave condensed ulA isotherms on water and cadmium chloride, as well as on PDDAC. These isotherms showed no turnover (Figures 2 and 3) and were unaffected by changes in subphase pH (up to 11)or in the speed of compression (insignificant change when the rate of compression was reduced by 10). (b) Under continuous compression, a monolayer of ortho on water or CdC12 was unstable at surface pressures above ca 5 mN/m. If compression was interrupted at any stage the surface pressure would rapidly decay to less than 5 mN/m. However, after slow, stepped compression on aqueous CdCl2, the monolayer was relatively stable at all surface pressures up to collapse. For example, when the monolayer was held at a constant area/molecule, the preeeuredecayedby less than 10%over 30min. Therefore, provided stepwise compression is used, the LangmuirBlodgett (LB) procedure can be performed at constant pressure (using a pressure feedback procedure). At the low surface pressure of 5 mN/m some LB transfer took place (transfer ratios6 of 0.6 to 0.8), but only on the upstroke. At 15 or 20 mN/m (Figure 4) there was complete transfer (ratio around 1)on the upstroke and again very little transfer on the downstroke (ratio 0.2 or less). At 25 mN/m significantly more transfer occurred on the downstroke and the resultant film was much more "Y like". (6) The transfer ratio is defined as area of monolayer transferred/area of substrate exposed.
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Fluorescence microscopy revealed a higher degree of homogeneity across the glass substrate when the film was formed at 15 or 20 mN/m compared to 5 mN/m. Transfer from a PDDAC subphase after continuous compression (the use of PDDAC givesa stable monolayer) also produced hybrid Y/Z type deposition (downstroke transfer ratios ca. 0.5 to 0.6). By contrast, meta and para monolayers on CdCl2 (and PDDAC) were relatively stable at all pressures during continuous compression. Y-type LB films were formed
Furlong et al.
Langmuir, Vol. 9, No. 3, 1993
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(Figure 4, complete transfer on both up and downstrokes through the monolayer) at the various pressures. Z-type films of meta and para could be fabricated by using a cyclic dipping mode, in which only the downstroke passes through the monolayer while the upstroke passes through an adjacent water only surface. Moreover, Y-type films could then be subsequently deposited onto these Z-type films by reverting to the two-way passage of the substrate through the monolayer. (c) The monolayer of ortho showed visible signs of spontaneous polymerization (a pale red color to the eye) when held at a surface pressure of 25 mN/m or greater on a CdCl2 subphase (aredmolecule < 0.29 nm2). At lower pressures, the monolayer (and its LB films) could be polymerized by UV irradiation and heat (ca. 50 "C) to give blue or pale red films; a typical absorption spectrum for the red polymer film (absorption peak around 550 nm) is shown in Figure 5a. The spectra did not exhibit a shoulder on the 550-nm absorption peak, as sometimes seen with polydiacetylenes. Of course it is known that the form of polydiacetylene absorption spectra often depends on the medium (for example the solvent). Polymer could be formed (as seen from the absorption spectrum) even at the very low surface pressure of 5 mN/m (area/molecule ca. 0.42 nm2),although fluorescence microscopy indicated a very heterogeneouslayer. The intensity of polymer color was greater for LB films (of the same number of layers) deposited at 25 mN/m (area/molecule 0.29 nm2)than at 15 mN/m (area/molecule0.34 nm2). However, this increase
was not directly proportional to the number of surfactant molecules per unit area in the LB film. Colored polymer was not detected after the irradiation of an ortho monolayer on a PDDAC subphase at areas per molecule of 0.29 or 0.34 nm2. This indicates that some anchoring of ortho to PDDAC has occurred. However, the monolayer did contract during irradiation and gain in strength (the collapse pressure increased by ca. 10mN/m after irradiation); some polymerization may have in fact taken place. While the blue and red forms of diacetylene polymers are the most often discussed, there is the suggestion in recent work by Mino et al.7 that in some instances polydiacetylenes can show very little absorption in the visible spectrum. The red films of ortho faded considerably on standing in air at room temperature. This could be due to the formation of a noncolored form of the polymer, in this case from reorientation within the film. It may also result from oxidation within the film. By contrast, polymerization of meta or para monolayers or LB films was much less apparent, spectra are shown in Figure 5b,c. In the case of meta there was essentially no indication of color in the irradiated monolayer or LB film and no evidence of packing change in the monolayer during irradiation. In the case of para an absorption band appears after irradiation in the 450-550 nm range (note the spectra for para in Figure 5 are from a film with twice the number of layers as the ortho and meta films). The intensity of the para absorption is very much less than that of ortho, particularly if the general rise in background absorption is considered in both cases.
Discussion r f A Isotherms. Collapse. Bearing in mind that the mechanism of collapse and the associated pressures may be dependent on the relative rates of compreesion,8 we will compare only the collapse areas. Regardless of compression mode or nature of subphase additive (water at pH 2 to 10, divalent Cd2+or cationic polyelectrolyte PDDAC), the area per monolayer molecule at collapse is between ca. 0.22 and 0.25 nm2 (see Table I, only ortho on PDDAC is slightly less well packed). This implies that for each isomer the aromatic ring, diacetylene link, and alkyl chain are out of the water and approximatelynormal to the water surface (shown schematically in Figure 6). The diacetylene link would be adjacent to the aromatic ring with ortho, but extended away from it with meta and para. Hydrogen bonding will no doubt occur on the water subphaseat pH values (below ca. 6) where the acid moiety is not dissociated. However, on CdC12, the subphaseCd2+ are likely to interact with both the carboxylate and the adjacent ester moieties of ortho and thereby inhibit hydrogen bonding. Other possible configurations for ortho, in particular those where the aromatic ring is orthogonal to the diacetylene link, are clearly overridden under high compression. The ?r/A isotherms for metaand para did not change with pH over the range 2-11, even when Cd2+ was in the subphase. This suggests that hydrogen bonding is effective, even in the presence of subphase Cd2+. Expanded Monolayers. The area per molecule at which a measurable surface pressure is found (Ainitid) decreases from ortho to meta to para (Table I). Hence it is clear that the three isomers pack differently when not substantially compressed. Values on a PDDAC subphaseare larger than on CdC12, particularly for ortho, suggesting that the binding of the surfactant anion to the cationic (7) Mino,N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (8) Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1991, 7, 2208.
Ring Substitution Position and Film Properties
Langmuir, Vol. 9, No. 3, 1993 769
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Ainitid (nm2) A,-* (nm? subphase/compression ortho meta para ortho meta para
10-3 mol/dm3 0.48 CdClz/continuous 10-3 mol/dm3 0.48 CdClz/stepwise 0.75 10-3 mol/dm3 PDDAC/continuous
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PDDAC provides some restraint on the low level compression of the surfactant molecules. When not compressed ortho can readily orient so that its carboxyl, ester, and diacetylene groups are adjacent to the water, with the aromatic ring and aliphatic chain out of the water (two possible options are shown schematically in Figure 7). It seems that the "switch" between this and the close packed conformation of Figure 6 results in the turnover in the n / A isotherm. By contrast, neither meta nor para can be readily oriented, so that both the oxygencontaining groups and the diacetylene link are adjacent to the water. Moreover,in the nearest optionsthe aromatic ring is also adjacent to the water, suggesting that the transition from expandedto close packing involves energy gain in removing the aromatic ring from the water and energy cost in removing the ester linkage. Hence, with meta and para, various conformers of similar energies are feasible as the monolayer is compressed and no "turnover" in the * / A isotherm is expected or indeed seen (Figure 3).
Figure 7. Schematicrepresentation of expanded conformations of ortho at the air-aqueous CdCll interface.
It is well knownsthat the polymerization of diacetylenes requires close nearest neighbor packing and alignment. While it is not surprisingthat condensed ortho monolayers are readily polymerized, the formation of polymer during UV irradiation of an expanded ortho monolayer (6 mN/m on CdC12, average aredmolecule 0.42 nm2) was not expected. Clearly the required degree of alignment is present even when the monolayer is expanded, suggesting the presence of domains of high internalorder. Moreover, the changesin interfacial conformation during compression seem not to affect this nearest neighbor alignment. Other studieslOJ1indicate that domains of micrometer size are present in the expanded ortho monolayer. Apparently attractive inbractions operate between ortho molecules even in the expanded conformation (such as in Figure 7). It is noted that monolayers of aliphatic diacetylenicacids, such as 16-8 diynoic acid, cannot be polymerized until highly compressed. Therefore, the net attractions between ortho molecules would seem to involvethe aromaticmoiety. The relative inactivity of para and particularly meta toward polymerization indicates that the aromatic ring itself is not sufficient to drive these surfactants to pack appropriately, either when expanded or compressed. While ortho molecules can apparently attract laterally via a mechanism involving the aromatic ring, meta and para can interact via Cd2+ bridging. Bridging is not favored with ortho became each subphase Cd2+ can form a bidentate complex with each ortho molecule in the monolayer. However, even without such bidentate complexation, bridging to form interfacial dimers12or liquid crystal mesophases is more likely with meta and para, their more linear headgroups favoring such structures. Clearly the changes in interfacial packing of meta and para that result from dimer-mesophase formationinhibit polymerization in the monolayer. LB Transfer. Ortho gave near zero transfer on the downstroke onto hydrophobic substrates. This suggests "head to tail" deposition; the surfactant headgroup could be oriented either toward or awayfrom the substrate. Table \
(9) (a) Tieke, B.; Wagner, G. Makromol. Chem. 1978,179,1639. (b) Day,D. R.; Ringsdorf, H. Makromol. Chem. 1979,180,1069. (c) Lopez, E.; O'Brien, D. F.; Whitesides, T. H. J. Am. Chem. SOC.1982,104,305. (d) Wenzel, M.; Atkinson, G. H. J. Am. Chem. Soc. 1989,111,6123. (10)Scoberg, D. J.; Grieser, F.; Furlong, D. N. J. Chem. SOC.,Chem. Commun. 1991,415. (11) Grieser, F.;Furlong, D. N.; Scoberg, D. J.; Ichinoee, I.; Kimizuka, N.; Kunitake, T. J . Chem. SOC.,Foraday Trans. 1 1992,88,2207. (12) Gray, G.W. Molecular Structure and the Roperties of Liquid Crystals; Akademic Press: London, 1962.
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770 Langmuir, Vol. 9, No. 3, 1993 Table 11. Sessile Drop**Contact Angles of Water substratehreatment clean glass" 1. T M C W treated glass 2. Two LB layers of cadmium arachidate 3. four LB layers of cadmium arachidate 4.two LB layers of ortho 5.four LB layers of ortho TMCS treated glass with three LB layers of cadmium arachidate
advancing contact anale (dea)
