Langmuir 1997, 13, 2123-2132
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Langmuir-Adam Trough Studies of Hydrophobicity, Hydrophilicity, and Amphilicity in Small-Molecule and High-Polymeric Phosphazenes Lawrence L. Mack Department of Chemistry, Bloomsburg University, Bloomsburg, Pennsylvania, 17815
Richard J. Fitzpatrick and Harry R. Allcock* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, 16802 Received June 8, 1995. In Final Form: January 21, 1997X Langmuir-Adam trough studies have been carried out to study the behavior of seven cyclic and linear short-chain small-molecule phosphazenes and four high polymers at air-water interfaces. The smallmolecule compounds are models for a variety of phosphazene high polymers. The influence of phenoxy, 1,2-benzenedioxy, OCH2CF3, NH2, NEt2, NBu2, and OCH2CH2CH2CH2OCH3 side groups was studied. The air-water interface behavior of the small molecules depended on (a) the cyclic or linear skeletal structure and (b) the types of side groups linked to the ring or chain. The utilization of these results for understanding the behavior of phosphazene high polymers at air-water interfaces is discussed.
Introduction The surface character of a solid polymeric material determines important properties such as adhesion, biocompatibility, or resistance to swelling in water or organic solvents. It also controls the reaction chemistry of surface units when the solid is brought into contact with chemical reagents. A macromolecular system that we are studying in detail is the 700-plus-member class of polymers known as polyphosphazenes. These polymers have the general structure shown as 1, where the side groups, R, can vary over a wide range from alkoxy and aryloxy to amino acid ester units, organometallic moieties, and alkyl or aryl groups. R N
Experimental Section
P R
n
1
The average degree of polymerization, n, is typically in the region of 15 000, which means that the molecular weights are often as high as 1 × 106 to 4 × 106. The surface character of these polymers is of considerable interest because of their present and prospective uses in several areas of advanced technology and medicine.1-7 A possible source of information about the interfacial characteristics of these polymers is through LangmuirAdam studies8,9 of their behavior at air-water interfaces. X
However, the behavior of macromolecules at air-water interfaces is less well understood than is the response of small molecules under these circumstances.10-15 For this reason we have used a model compound approach to help in the interpretation of the behavior of the high polymers. The model compounds are small molecules that possess the same skeletal elements as the high polymers and bear similar representative side groups. However, the models contain skeletal systems that consist of six-membered rings or short-chain linear structures. The model systems studied are shown in Chart 1 as 2-8. Following these studies, the behavior of four phosphazene high polymers, 9-12, at an air-water interface was examined in order to evaluate the added complexities encountered for the high-molecular-weight species.
Abstract published in Advance ACS Abstracts, March 1, 1997.
(1) Allcock, H. R.; Rutt, J. S.; Fitzpatrick, R. J. Chem. Mater. 1991, 3, 442-449. (2) Allcock, H. R.; Fitzpatrick, R. J. Chem. Mater. 1991, 3, 450-454. (3) Allcock, H. R.; Fitzpatrick, R. J. Chem. Mater. 1991, 3, 11201132. (4) Allcock, H. R.; Fitzpatrick, R. J.; Salvati, L. Chem. Mater. 1992, 4, 769-775. (5) Allcock, H. R.; Fitzpatrick, R. J.; Visscher, K. B. Chem. Mater. 1992, 4, 775-780. (6) Allcock, H. R.; Nelson, C. J.; Coggio, W. D. Chem. Mater. 1994, 6, 516-524. (7) Allcock, H. R.; Smith, D. E. Chem. Mater. 1995, 7, 1469-1474. (8) Gaines, G. L. Insoluble Monolayers at the Liquid-Gas Interface; Wiley-Interscience: New York, 1966.
S0743-7463(95)00453-7 CCC: $14.00
Synthesis of Phosphazenes. The small-molecular compounds 2 and 4 were synthesized by the well-known reaction between hexachlorocyclotriphosphazene, (NPCl2)3, and sodium phenoxide or sodium trifluoroethoxide.16 Species 3 was prepared by a process already reported in the literature.17-19 Compounds 5 and 6 were prepared by the following procedure, with the preparation of 5 given as an illustration of both. Hexachlorocyclotriphosphazene (3.0 g) was dissolved in diethyl ether. Anhydrous ammonia was bubbled slowly through this (9) Roberts, G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (10) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (11) Runge, F. E.; Yu, H. Langmuir 1993, 9, 3191-3199. (12) Gau, C.-S.; Yu, H.; Zografi, G. Macromolecules 1993, 26, 25242529. (13) Kumaki, J. Macromolecules 1988, 21, 749-755. (14) Ries, E.; Matsumoto, M.; Uyeda, N.; Suito, E. Monolayers; Advanced Chemistry Series American Chemical Society: Washington, DC, 1975; pp 286-293. (15) Granick, S. Macromolecules 1985, 18, 1597-1602. (16) Allcock, H. R. Phosphorus-Nitrogen Compounds; Academic Press: New York, 1972. (17) Allcock, H. R.; Siegel, L. A. J. Am. Chem. Soc. 1964, 86, 51405144. (18) Allcock, H. R.; Dudley, G. K.; Silverberg, E. N. Macromolecules 1994, 27, 1039-1044. (19) Allcock, H. R.; Silverberg, E. N.; Dudley, G. K. Macromolecules 1994, 27, 1033-1038.
© 1997 American Chemical Society
2124 Langmuir, Vol. 13, No. 7, 1997 Chart 1
solution for 1 h at 25 °C. The reaction yielded a solution of gemN3P3(NH2)2Cl4. The 31P-NMR spectrum consisted of an AB2 pattern with the P(NH2)2 resonance centered at 9.0 ppm and the PCl2 resonance at 15.0 ppm. Sodium trifluoroethoxide was then prepared from trifluoroethanol and a 25% excess of sodium in warm THF, and the alkoxide solution was added to the solution of the phosphazene in an amount sufficient to ensure a 100% molar excess. The reaction mixtures were stirred overnight at room temperature and then filtered, and the filtrate was extracted with CH2Cl2 and deionized water. The CH2Cl2 extract was concentrated until an oil was isolated. Compound 3 crystallized from the oil. Characterization data: 31P-NMR, AB2 centered at 15.3 ppm; 1H-NMR, 4.2 (m) (OCH2CF3) and 2.9 (s) (NH2); 13CNMR, 77.5 (t) and 63.0 (q). Mass spec: calc, 563; found, 563. The geminal substitution pattern was also confirmed by single-crystal X-ray crystallography. Compounds 7 and 8 were prepared by a method described previously,20 as were polymers 9 and 10.21-23 Polymers 11 and 12 were synthesized in the following manner. Poly(dichloro(20) Allcock, H. R.; Tollefson, N. M.; Arcus, R. A.; Whittle, R. R. J. Am. Chem. Soc. 1985, 107, 5166-5177. (21) Allcock, H. R.; Kugel, R. L. J. Am. Chem. Soc. 1965, 87, 42164217. (22) Allcock, H. R.; Kugel, R. L.; Valan, K. J. Inorg. Chem. 1966, 5, 1709-1715.
Mack et al. phosphazene) (5.0 g) was dissolved in warm dioxane (200 mL). To this solution was added 8 equiv per P-Cl unit of the appropriate secondary amine (diethyl- or dibutylamine). The reaction mixture was refluxed for 72 h or until the 31P NMR spectrum no longer contained a PCl2 resonance at -18 ppm. 2-(2-Methoxyethoxy)ethanol (8 equiv per remaining P-Cl bond) was treated with sodium (8.5 equiv) in warm diglyme (400 mL). The salt solution was then added to the polymer solution. The reaction mixture was boiled at reflux, and the low-boiling fraction was collected in a side flask until the reflux temperature had risen to 160 oC. The mixtures were then refluxed for 30 h at 160 oC. The products were concentrated and dialyzed against deionized water (5 days) and methanol (3 days). Characterization data for compound 10: 31P-NMR, -10.0 and -7.5 ppm; 1H-NMR, 4.0, 3.6, 3.4, 3.1, and 1.1 ppm; mol wt (GPC), 1.7 × 105. Characterization data for compound 12: 31P-NMR, -8.5 and -7.0 ppm; 1H-NMR, 4.0, 3.6, 3.5, 3.3, 2.9, 1.4, 1.2, and 0.9; mol wt (GPC), 1.3 × 105. Langmuir-Adam Film Balance Technique. The surface film balance technique compares the surface tension of the pure subphase, γ0, to the surface tension of the water altered by the interaction of surfactant materials with the water subphase, γi.8,9 This difference in surface tension, γo - γi, is the film pressure or surface energy per area, π. The Langmuir-Adam technique monitors the interaction of a compound on the surface of a water subphase by measuring π during the contraction of a barrier that sweeps together the surface materials into a decreasing confining area. Such π-surface area (π-A) studies are carried out at a fixed temperature. From the shape of the π-A curve during the contraction of the barrier, information can be inferred about the orientation of the molecules and the nature of the surface-subphase interactions. By changing the nature of the side groups of the compounds placed on the surface, this technique can provide information about the hydrophobicity or hydrophilicity of these substances. Monolayer behavior can be demonstrated with the film balance by comparing the π versus area per molecular unit curves for different initial surface concentrations on the fully expanded surface. If the insoluble surface material forms true monolayers, the π-A versus area per molecule curves should be superimposable at different loading concentrations. By contrast, systems that form small insoluble floating aggregates, so-called expanded liquid structures or “lakes”, will demonstrate π-molecular area curves that vary with concentration. The higher surface-loaded samples should give curves shifted to smaller surface area per molecule because a substantial fraction of the molecules or, in the case of polymers, chain length repeating units, are not in contact with the surface and have no effect on the surface energy. Similarly, compounds that form separate units on the surface may be compressed together by the barrier as it collapses. If sufficient interaction exists with the surface and minimal interaction between particles, thermal energy should allow the particles to spread out over the surface after the mobile barrier has expanded back to the open position. Such “respreading experiments” give a further indication of surface molecular behavior. Surface pressure versus area (π-A) curves and deposition data were obtained with the aid of a Joyce-Loebl 4 film balance unit equipped with a thermoregulated, poly(tetrafluoroethylene) (PTFE)-coated trough with a central dipping well and a Wilhelmymicrobalance detection device. The entire trough apparatus was located away from any synthetic work inside a vibration-damped chamber accessed through glass doors, to keep the trough surface relatively dust free. The Wilhelmy plate consisted of a 1-cm × 4-cm piece of Whatman #1 filter paper. The Joyce-Loebl trough employed a PTFE-coated barrier tape of the constant perimeter design. The surface area inside the barrier at full expansion was typically 950 cm2 and 150 cm2 at full contraction. The microbalance was calibrated daily using a standard 20 mg weight. A Bio-Rad 4850 refrigerated circulation bath regulated the trough temperature over the range 9-40 °C, and the temperature of the subphase was measured using a Kane-May 3003 thermocouple. With the exception of initial trial experiments with penta(23) Allcock, H. R. In Inorganic and Organometallic Polymers; Mark, J. E., Allcock, H. R., West, R., Eds.; Prentice Hall: Englewood Cliffs, NJ, 1992; Chapter 3.
Small-Molecule and High-Polymeric Phosphazenes phenoxyphosphorylphosphazene and cadmium stearate, the rate of barrier contraction or opening was 2.0 cm2/min. The entire trough operation was controlled through a Joyce-Loebl computerinterface unit connected to an IBM-compatible microcomputer. The computer system monitored the barrier position, dipping head position, time, and surface pressure readings of the microbalance. All the glassware was new and was soaked in oxidizing baths of either K2Cr2O7/H2SO4 or Nochromix/H2SO4 before use. The glass apparatus was then rinsed in distilled/deionized water. The barrier tape and PTFE rollers were cleaned using a 30-min rinsing in an ultrasonic bath with Micro detergent and by subsequent rinsings in an ultrasonic bath with distilled/deionized water. Finally, the tape was placed in a large Soxhlet extractor and was exposed to boiling 2-propanol or acetone overnight. Acetone was particularly useful for the removal of residues of 10. The trough was rinsed several times before use with spectroscopic grade (E & M Omnisolv cx1054-3) chloroform and was then dried with filter paper. It was then rinsed twice with 1 L of distilled/deionized water and, using a Pasteur pipet connected to an aspirator, the surface was “vacuumed” to remove trace surfactants. The barrier tape was then installed, and 3 L of distilled/deionized water (pH ) 5.0) was added to the trough. The Wilhelmy detection paper was suspended on the surface of the water and was allowed to equilibrate with the medium. The surface pressure with the barrier fully extended was set to zero. The barrier was then closed and the surface pressure read again. If the surface pressure was above 0.5 mN/m (the tolerance recommended by the manufacturer), the surface was “vacuumed” with the Pasteur pipet system. In all experiments, no material was deposited on the surface until the difference in surface pressure of the pure water from the open to the closed position was less than 0.5 mN/m. As a control system, stearic acid (80 µL of 3.5 × 10-3 M) was deposited on the surface. The subphase used was 2.5 × 10-4 M CdCl2, pH 5.6. π-A isotherms were the same as the standard curve supplied by the manufacturer. The requirements for a suitable spreading solvent include not only the solubility of the phosphazene compound but also high purity, insolubility in the water subphase, a density less than that of the subphase, and a high vapor pressure. All compounds studied except 3 and 10 were spread on the water subphase using spectroscopic grade (E & M Omnisolv cx1054-3) chloroform. Compounds 9 and 3 were deposited using spectroscopic grade benzene as the solvent. The spreading time was 15-20 min for chloroform or benzene solutions. Compound 10 was insoluble in benzene and chloroform, but a solution of equal volumes of diethyl ether and ethyl acetate met most of the above criteria for an acceptable spreading solvent. Because this solvent system was partially soluble in water, an extra precaution was taken to allow a long spreading time (90+ min), to ensure that the solvent had evaporated from the surface. The spreading solution was deposited onto the water surface using a Hamilton, gas-tight, 100-µL syringe. The transference was made by depositing the typical 40-100 µL of test solution through many depositions of 2-3 µL droplets using an even pattern across the clean water surface with the barrier set at full extension. Contact Angle Measurements. For most of the deposition experiments on glass slides, each slide was first soaked in Nochromix/H2SO4 and was then placed in a Soxhlet extractor and rinsed with refluxing spectroscopic grade acetone followed by a rinse in distilled/deionized water. The slides used in the deposition of 10 at preset surface pressure were treated differently, with a method that seemed to be effective and reproducible for cleaning the surface. Eighteen slides were soaked in Nochromix/H2SO4, rinsed in distilled/ deionized water, rinsed in a Soxhlet extractor with refluxing spectroscopic grade acetone, rinsed in distilled/deionized water, dipped in the acid solution again, and finally rinsed in distilled/ deionized water one last time. Gloves were worn by the operator during the rinsing with water, and the slides were manipulated in and out of the containers using forceps. In each case, after the rinsing with distilled water, each slide was dried in an oven and stored in a separate glass jar until use. Seven slides were successfully used in the surface deposition experiments. Contact angles were determined with use of distilled/deionized water as 1-µL sessile drops with a Rame-Hart contact angle goniometer, model 100-
Langmuir, Vol. 13, No. 7, 1997 2125 00, fitted with an environmental chamber. Humidity was maintained at 100% with the temperature between 18 and 25 °C. Several (8-9) water droplets were deposited in a hexagonal pattern on the surface of each slide while it was in place on the platform of the contact angle goniometer. To minimize the variability of the surface of each slide, the average contact angle on the part of the slide that was dipped into the water was compared with that of the part of the same slide that did not come in contact with the water interface. Each surface contact angle measurement listed is the average of four or five droplet measurements on the affected (or still clean) surface of each microscope slide. The error listed after each average value is the calculation of the sample standard deviation.
Results and Discussion The small-molecule phosphazenes studied in this work fall into several different classes based on the types of side groups present and the hydrophobic-hydrophilic balance between different components of the skeletal and side group structures. Symmetrically Substituted Phosphazene Cyclic Trimers. The series of experiments with cyclic phosphazenes 2-4 compares the π-A isotherms for singlesubstituent cyclotriphosphazenes that bear phenoxy, 1,2phenylenedioxy, or trifluoroethoxy side groups and contrasts the behavior of species with bulky, rigid hydrophobic side groups with that of species with more flexible hydrophobic side groups. Both surface concentration and respreading studies were conducted using a minimum of two scans per compound. These results will be contrasted with those from compounds that bear four phenoxy or trifluoroethoxy side groups and two gem-diamino side units. Hexaphenoxycyclotriphosphazene (2). Experiments were conducted at 20 °C using a chloroform solution of 2 with a loading of 47.0, 74.4, or 167.4 µg and using 47.0 or 67 µg benzene solutions of 2. As seen in Figure 1, π-A isotherms showed surface pressure contributions due to both spreading solvents but with no contributions from 2. Apparently, the disk shape of the cyclotriphosphazene molecule and the large hydrophobic phenoxy groups prevent an interaction of 2 with the polar liquid surface. Compound 2 probably forms “lakes” of material that do not spread spontaneously over the surface. This result is noteworthy because, as discussed later, a hydrophobic analogue, hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (4), forms structures that not only aggregate but also spread and interact (albeit weakly) with the surface. Tris(1,2-phenylenedioxy)cyclotriphosphazene (3). Compound 3 was studied in three different experiments to probe the influence of its rigid, triptych-like shape.17-19,24,25 It was also of interest to compare the π-A isotherm behavior of 3 with that of hexaphenoxycyclotriphosphazene (2). The phenylenedioxy side groups in 3 are positioned rigidly and radially at right angles to the phosphazene ring, and this allows a greater exposure of the oxygen atoms to water than in the case of 2. On a water subphase two logical choices exist for the orientation of 3: either flat (horizontal to the surface) or on-end with a vertical orientation. The oxygen atoms are closer to the water surface in the horizontal orientation but are still shielded by the phenyl ring. This weak hydrophilic interaction should be stronger than the analogous interaction with the hexaphenoxy compound (2). However, the interaction might be sufficiently weak that the molecule could be positioned vertically on the surface with little change in the energy. (24) Siegel, L. A.; van den Hende, J. H. J. Chem. Soc. A 1967, 817. (25) Allcock, H. R. In Inclusion Compounds; Atwood, J. L., Davies, J. E., MacNicol, D. D. M., Eds.; Academic Press: New York, 1984; Vol. 1, pp 351-374.
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Figure 1. π-area isotherms at 20 °C of 47 µg of hexaphenoxycyclotriphosphazene (2) in chloroform spreading solvent. 81.6 µg of tris(1,2-phenylenedioxy)cyclotriphosphazene (3).
Experiments were conducted at 20 °C using a chloroform spreading solvent with a loading of 33.6 or 81.6 µg of 3. Only the higher loading value showed weak surface activity near full compression of the barrier. Figure 1 shows π-A isotherms for 3 with a steep vertical rise reminiscent of stearic acid π-A curves. The vertical rise begins at 30-32 Å2/molecule. The initial vertical rise in π-A is too small to be due to molecules of 3 lying horizontally on the surface when the barrier has contracted to small areas. The van der Waals “diameter” of this molecule is roughly 16 Å.17-19,24 An equilateral triangle with a side of 16 Å would have an internal surface area of 110 Å2. Taking into account a surface area of 110 Å2 per particle with a molar mass of 459 g/mol, a surface deposition of 33.6 µg would generate a surface area near 485 cm2, assuming perfect packing and no spaces between particles. Again, this type of rough calculation shows that the observed vertical rise area of less than 20 Å2/molecule is too small to suggest that 3 lies horizontally on the surface under tight surface compression, and it supports the assumption that the tris(1,2-phenylenedioxy)cyclotriphosphazene molecules must be stacked on end when crowded under strong surface contraction pressures before there is appreciable surface interaction. Presumably, a minimum surface concentration of molecules is needed before sufficient intermolecular surface pressure exists to force the molecules into a vertical configuration following compression of the barrier. Hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (4). Trifluoroethoxy-substituted phosphazene high polymers are strongly hydrophobic. Trifluoroethoxy side groups are also more flexible than the phenoxy groups in compounds 2 and 5 or the rigid phenylenedioxy groups of 3. Compound 4 could in theory interact with the surface through the oxygen and the nitrogen lone pair electrons, but this interaction would presumably be weak. Experiments were conducted at 20 °C using a chloroform solution of 4 with a loading of 47.0, 75.2, 79.2, or 94.0 µg. The π-area isotherms obtained for this compound at differing concentrations cannot be superimposed when normalized to area per molecule, and this suggests the existence of surface aggregation rather than monolayer behavior. The curves show a reverse-s shape with the π-area curves shifted further to the left as the surface loading concentration is increased. A final break in slope to the horizontal is seen at around 18-19 mN/m that is
independent of concentration. Respreading experiments indicated a tendency of the surface active compound to redistribute itself over the surface once the external pressure was removed. In our experiments contraction of the surface was immediately followed by an expansion step which, after a 15 min delay, was followed by a second compression. The shape of the second contraction isotherm is similar to that of the initial contraction but is shifted to the left to lower (higher loading) values of area. Asymmetric Phosphazene Cyclic Trimers. A good surfactant material should have a hydrophilic “head”, which interacts strongly with the water subphase, and a hydrophobic “tail” that will be positioned away from the water surface. From this viewpoint the gem-diamino compounds 5 and 6 with phenoxy or trifluoroethoxy cosubstituent groups are interesting test molecules. The gem-diamino groups should interact with the subphase either through hydrogen bonding or as charged species. It might be expected that, after deposition, and particularly after compression, compounds 5 and 6 should be positioned on the surface with the hydrophobic groups oriented away from the surface and the amine groups at the water subphase. 1,1-Diamino-3,3,5,5-tetraphenoxycyclotriphosphazene (5). π-A isotherms for 5 were measured with four different loadings from 62 to 125 µg of the phosphazene, all at 20 °C. Experiments measuring π versus area showed curves that could be roughly superimposed when normalized to area per molecule. Plots of the data are shown in Figure 2. A gently climbing π-A isotherm was seen as the barrier contracted, and a steady increase in slope was observed under a variety of conditions. The fact that the curves in Figure 2 can be superimposed suggests a behavior that is independent of concentration. This is evidence of monolayer formation. Separate molecules float on the surface and reach a stage of intermolecular contact when the area available on the surface approaches 60-70 Å2/molecule. This surface area is similar to the value of 65 Å2/molecule to be discussed later for 7 and is reasonable considering the similar dimensions of the bulky phenoxy groups in the proposed configurations on the surface. The absence of an abrupt change in slope at moderate surface pressures indicates that a gradual increase in the crowding of the molecules on the surface takes place as the barrier contracts. For surface areas less than 60-70 Å/molecule, the molecules
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Figure 2. π-area isotherms at 20 °C, normalized to area per molecule for 5 (1,1-diamino-3,3,5,5-tetraphenoxycyclotriphosphazene) showing evidence of monolayer formation.
Figure 3. π-area isotherms of compound 6 [1,1-diamino-3,3,5,5-tetrakis(2,2,2-trifluoroethoxy)cyclotriphosphazene] normalized to area per molecule.
are presumed to be forced progressively into an orientation where the phenoxy groups rise above the water subphase and the gem-diamino groups interact with the water surface. This provides an obvious contrast with the behavior of compound 3 and with the linear short-chain species to be discussed later. 1,1-Diamino-3,3,5,5-tetrakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (6). Nine experiments were conducted at 20 °C using a chloroform solution of 6 with a loading that varied from 80 to 178 µg. The isotherms consisted of a gently climbing π-A curve as the barrier contracted, with no abrupt break in the slope detected under a variety of conditions, again showing the same pattern as with compound 5 (see Figure 3). Compound 6 has a hydrophobic grouping comprised of the more flexible trifluoroethoxy groups and a hydrophilic head composed of the amino units. Unlike the case for the gem-diamino-phenoxy compound 5, the surface concentration effects seen with π-A isotherms suggest that the spreading behavior of 6 is a group phenomenon and not the result of molecular monolayers. Note that in Figure 3 the position in Å2/molecule of first rise decreases
as the loading increases. A similar behavior is seen with the hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene 4. For both 4 and 6, the surface area per molecule decreases as the concentration increases. The surface aggregates grow in mass as they decrease relatively in surface area. This would be the case if 4 and 6 form some type of threedimensional structure on the surface, such as a surface micelle or “lake.” The micelles would grow in thickness as they adsorb more molecules, giving a smaller water surface contact area per unit volume. Double compression of 6 indicated some spreading ability similar to that of compound 4. In this experiment π-A data were obtained in one experiment by the threestep process of closing, opening, and closing the barrier. Except for a shift to smaller surface areas, the return isotherm could be superimposed rather well onto the original contraction isotherm. The respreading behavior of 4 and 6 can be explained by the collapse of a thin micellar structure into a thicker structure several molecules deep. It is assumed that, if the surfactant material behaved as very large separate aggregations, respreading would be unlikely because the hydrophobic surfactant-surfactant
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Figure 4. π-area isotherms at 20 °C for short-chain phosphazenes 7 and 8 normalized to area per molecule.
interactions would predominate over the hydrophilic surface-surfactant interactions. Once the surface materials are compressed, they should cluster together and resist separation. Therefore, the ease with which a compound respreads is a test of these interactions. Because the shape of the isotherm is constant after the second contraction, it appears that 4 and 6 have some tendency to respread. This spreading behavior of 4 and 6 suggests that the size of the surface units is relatively small, perhaps containing less than 100 molecules, and may be transported by thermal energy over the surface once the barrier is removed without breaking down into the independent particles that would be found in a monolayer. The shapes of the π-A isotherms are the same for both gem-diamino-containing compounds, 5 and 6, indicating that some of the diamino groups play a significant role in affecting the shape of the π-A isotherm. It can be assumed that these groups are oriented toward the water subphase and cause the observed change in surface energy as the barrier contracts. A study of pH changes in the water subphase was also undertaken. Two π-A experiments were conducted at the same loading concentration of 6 but with different subphase pH values (pH ) 1.8 (using HCl/KCl buffer) and pH ) 5.0). No differences were detected in the shape of the π-A curves or in the numerical values of the π value at maximum compression. The amino groups could already be highly protonated at pH 5.0 and undergo no significant increase in the number of positive charges at pH 1.8. At both pH values the amino groups are unable to interact with the subphase with sufficient energy to cause the surface micelle structure of 6 to break up into separate molecular units of the type found with compound 5. Thus, the evidence indicates that a marked increase occurs in surface activity when the symmetrically substituted cyclotriphosphazenes are replaced by the gemdiamino compounds. The dipole moment is increased and the two hydrophilic side groups are available to interact with the water subphase. The gem-diamino tetraphenoxy cyclophosphazene 5 shows monolayer activity while the symmetric hexaphenoxy cyclic compound 2 does not. This is evidence of the strong influence by the diamino groups to overcome the inability of the triphosphazene ring to interact with the water surface when it is surrounded by the six phenoxy groups in 2. The interaction of the
phosphazene ring in 4 with the water subphase may be the important structural component in causing the surface interaction. The (2,2,2-trifluoroethoxy)cyclotriphosphazenes of types 4 and 6 do interact with the water surface. A surface pressure as large as 19 mN/m is needed to collapse the surface structure for hexakis(2,2,2-trifluoroethoxy)cyclotriphosphazene (4). This value is much larger than the 10 mN/m value found for linear compound 7 (see later), which should be more polar than 4. The greater flexibility and smaller size of the trifluoroethoxy side groups compared to the phenoxy units may facilitate the interaction of both the ring atoms and the pendent oxygen atoms with the water surface. This suggests that linear phosphazenes, which have greater torsional flexibility than the cyclic compounds, might also show surface activity. These observations for compounds 2, 4, 5, and 6 make it reasonable to assume that it is the hydrophobic trifluoroethoxy groups and not the hydrophilic amino groups that dominate the interaction of compounds 4 and 6 with the water surface. Neither 4 nor 6 displays true monolayer behavior. In contrast, the phenoxy compound 5 with the same gem-diamino side group as 6, shows a clear monolayer-like behavior, making the gem-diaminowater interaction appear to be more dominant than the influence of the hydrophobic phenoxy side groups. Linear Short-Chain Phosphazenes. The linear short-chain species provide an opportunity to examine the consequences of replacement of a rigid cyclotriphosphazene skeleton by a flexible, linear counterpart. The lone pair electrons of the backbone nitrogen atoms and the polar phosphoryl end group should give surface interactions that contrast dramatically with those reported above. The following series of experiments with the shortchain compounds compares the π-A isotherms of phenoxyphosphorylphosphazenes that have two different chain lengths. Pentaphenoxyphosphorylphosphazene (7). Concentration and respreading studies of this compound were performed in nine experiments using chloroform as the spreading solvent. Isotherms for the linear short-chain phosphazenes are shown in Figure 4. The shape of these π-A curves can best be explained by the standard model.26 The curves for compound 7 show an initial increase in surface energy starting at approximately 90 Å2/molecule. (26) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley-Interscience: New York, 1990; Chapter 4.
Small-Molecule and High-Polymeric Phosphazenes
Figure 5. Proposed conformation of the short-chain phenoxyphosphorylphosphazenes on the water surface: pentaphenoxyphosphorylphosphazene (7) and heptaphenoxyphosphorylphosphazene (8).
The most surprising characteristic of these π-A studies is the change in slope to an almost horizontal line at 1011 mN/m at around 65 Å2/molecule. The first initial rise in the surface energy isotherm is caused by the reorientation of increasing numbers of phosphazene molecules on the surface in such a way that the polar phosphoryl end unit of the molecule is oriented toward the surface of the aqueous subphase, as is depicted in Figure 5. It is speculated that the sudden change in slope at 10 mN/m in Figure 4 is due to a disruption of the monolayer to form folds in the sheet structure, giving a second layer or multilayers of the compound. Once a multilayer is formed, less energy per molecule is required to compress the barrier. The slope of the isotherm is reduced because the molecular orientation is now more random, and fewer surfactant molecules interact with the subphase. Compound 7 respreads over the water surface after the barrier has been reopened for less than 10 min. Respreading experiments indicated completely superimposable π-area isotherms after periods of 20 min before the second compression. It is surmised that this interaction is enhanced by the relatively strong dipole interaction of the phosphoryl group with the water. Experiments were conducted to demonstrate that the chloroform solvent used in these experiments contained little or no surface active material. Compared to compound 4, the most obvious reason for the differences in π-A curve shape is probably related to the differences in the interaction of the phosphoryl and amino groups with the water surface. Both are hydrophilic, but the amino groups in 4 at pH 5 are charged and they should interact more strongly with the water surface than the phosphoryl groups. This effect probably overshadows the influence of the larger dipole moment of the phosphoryl group. The linear versus cyclic structures of the two types of molecules can also be used to rationalize the π-A differences. The higher conformational freedom of the linear molecule more easily allows the nonbonding electrons of the backbone nitrogen atoms to interact with the water. Moreover, phenoxy groups can rotate away from the water surface more easily in the linear phosphazene than in the rigid cyclic phosphazene. Heptaphenoxyphosphoryldiphosphazene (8). This compound was studied in two experiments to examine the effect of an increase in the length of the hydrophobic component of a linear phosphazene. Several π-A isotherm experiments were conducted with this compound alone and as part of a mixture of 7 with 8. Because both 7 and 8 possess a terminal hydrophilic phosphoryl unit, both compounds should interact through hydrogen bonding with the surface of the water subphase through the phosphoryl oxygen atoms. The only differences in surface pressure should result from the increased volume of the hydrophobic components in 8. In principle, the phenoxyphosphazene portion of 8 could extend away from and along the surface due to weak interactions with water through the phenoxy oxygen atoms and the nitrogen
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lone electron pairs. The phenyl groups would be oriented away from the water surface. Such a model predicts that the π-A isotherm for 8 should be the same shape as for 7, with the same π value of the isotherm at the slope change, but with a larger area per molecule for the heptaphenoxy compound. Figure 4 shows π-A isotherms from both compounds, and the data are compatible with this model. The value of the slope change for 8 is 105 Å2/molecule, and the corresponding value for 7 is 65 Å2/ molecule. The changes in slope occur near 9 mN/m for 8 and 9.5 mN/m for 7. The fact that these two surface area values are so different provides some support for the structures suggested in Figure 5. We speculate that the much larger hydrophobic volume of the heptaphenoxy compound does not protrude vertically above the water surface, as, for example, with stearic acid, but is bent over and occupies a larger proportion of the surface. The phosphazene chain is assumed to be flexible and to have hydrophilic properties sufficient to cause the molecule to bend toward the surface. Preliminary Examination of Selected Phosphazene High Polymers. The extension of these experiments to the behavior of high polymeric phosphazenes at air-water interfaces revealed several problems inherent to the study of macromolecules. This is illustrated by the following observations. Poly[di(phenoxy)phosphazene] (9). This polymer showed no isotherm behavior. This is not surprising in view of the results with compounds 2 and 7. A sheath of bulky, hydrophobic phenoxy groups effectively shields the backbone atoms of 9 from interaction with the water subphase. Poly[bis(2,2,2-trifluoroethoxy)phosphazene] (10). This polymer is known to form highly hydrophobic surfaces.. As discussed, compounds 6 and 4 showed interactions with the water surface. Nineteen experiments with the polymer were conducted at 20 °C using diethyl ether and ethyl acetate solutions with loadings from 49.0 to 121 µg of 10. Figure 6 shows the spreading behavior of 10 at different concentrations and illustrates the abrupt change of slope at 17-18 mN/m. When the data are normalized to surface area per monomer unit, a general superposition of the isotherms is possible, as expected for a simple monolayer behavior. As in the case of compound 6, changes in pH (pH 5.0 and pH 1.8) did not affect the π-A isotherm contour and the break remained near 18 mN/m. A rather sluggish spreading behavior was detected during multiple expansions of the surface films (Figure 7). At high loading concentrations (122 µg) the polymer always respread several hours after compression, but there was a distinct hysteresis. Lower surface loading concentrations (51 µg) showed respreading behavior within 20 min, almost at the speed of the retracting barrier. This result indicates stronger hydrophobic interactions than those found for polymers 11 or 12. The hysteresis behavior may reflect the microstructure of the films. If the etherethyl acetate solution is a “poor” solvent for polymer 10, individual macromolecules would exist as tight polymer coils with small radii of gyration. After deposition, each individual polymer molecule could act as an individual particle at large barrier areas but the coils would still be tightly contracted on the surface. During contraction of the barrier at high loading concentrations, strong polymer-polymer interactions would delay the respreading of the polymer because of chain entanglement. At the low concentrations, polymer chain entanglements may not be encouraged at large compression. These polymer coils may easily respread by thermal energy following the retraction of the barrier. To further study the behavior of polymer 10, an extremely thin film of the polymer was deposited onto a
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Figure 6. π-area isotherms at 20 °C for 10 at different loading concentrations, normalized to area per monomer unit mass. The letters A and B refer to the surface area showing proposed initial chain entanglement on the surface (A) and the area where polymer chains leave the surface (B).
Figure 7. π-area isotherms for polymer 10. Continuous contraction, expansion, and contraction of the barrier demonstrates the spreading ability of the polymer. Surface loaded with 122 µg of 10.
solid surface in the manner perfected by Blodgett.27 Glass microscope slides were used as the substrate. The deposition was accomplished at preset surface pressures controlled by the computer using two methods of deposition. In the first, the polymer was spread on the water surface and the slide was then dipped into and out of the liquid by the apparatus at a fixed linear velocity (method A). In the second (method B), the slide was positioned under the surface of the water subphase before deposition of the polymer, the polymer was deposited onto the surface, and the slide was then raised through the surface material as it was withdrawn from the liquid at a fixed linear velocity. All dipping procedures were conducted at 20 °C. The success of the deposition was measured by the contact angle, θ, of distilled water on the surface of each slide. Table 1 lists the results of these preliminary measurements comparing the average θ on the part of each slide where 10 had been deposited versus that on regions on the same slide where no polymer was deposited. (27) Agarwal, V. K. Phys. Today 1988, June.
Table 1. Contact Angle θ between Water and the Surface of a Glass Slide Coated with 10 θ
surface pressure at deposition (mN/m)
glass
coated
deposition method
0.0 6.0 16.5 16.5 18.2 22.2
20.2 ( 2.4 15.8 ( 4.9 28.3(0.9 24.9 ( 0.6 12.2 ( 2.9 21.1 ( 2.7
20.7 ( 0.4 41.2 ( 5.1 45.0 ( 5.6 43.0 ( 2.8 66.9 ( 1.9 101.9 ( 1.0
A A B A A A (3 times)
No visible polymer surface coating was seen on any of the slides, nor were optical interference effects detected. The results are tabulated in Table 1 as a function of surface pressure at the time of deposition with the standard deviation of four or more measurements. The difference in the contact angle of water with the coated and uncoated portions of each slide is significant. The thickness and surface coverage of the polymer on the glass surface strongly influence the macroscopic surface-liquid surface tension. The contact angle difference between glass and the surface coated with 10, when plotted against the
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Figure 8. π-area isotherms for polymer 11 at various loading concentrations, normalized to area per monomer unit mass.
surface spreading pressure of deposition, roughly outlines the sigmoidal shape of the surface pressure isotherm of 10. As seen in Figure 6, the break in slope for a surface pressure isotherm of 10 is around 17-18 mN/m. The 18.2 mN/m reading is assumed to be beyond the break in slope. Our assumption is that the polymer chains begin to pack on top of each other beyond the break in slope. This explains why the glass slide is more efficiently coated at higher surface pressures of 18.2 mN/m in comparison to 16.5 mN/m. The slide acquires a thicker coating of 10 under these high surface pressure conditions. No appreciable difference in contact angle was detected by either of the two deposition procedures at 16.5 mN/m. This is evidence that polymer 10 has no preferred orientation on the water subphase or on the glass slide. The hydrophilic groups in the polymer are presumed to be the phosphazene backbone and oxygen atoms on the fluoroethoxy side groups. These groups are assumed to orient themselves by bond rotation toward the polar surface of the glass slide. During deposition, hydrophilic groups could easily pivot to reach available hydrophilic sites on the glass surface. Thus, multiple dipping of the substrate may simply deposit more and more surface active material. This very likely is the case for a triple-dipped slide coated at 22.2 mN/m. The hydrophobic surface of a slide coated at 22.2 mN/m displayed the same contact angle with water 3 weeks after deposition, which indicated that the polymer coating was stable over this time period. Poly[(diethylamino)(methoxyethoxyethoxy)phosphazene] (11) and Poly[(dibutylamino)(methoxyethoxyethoxy)phosphazene] (12). Polymers 11 and 12 were selected specifically as macromolecules that might form monolayers on a water surface. The single-substituent polymer, poly[bis(methoxyethoxyethoxy)phosphazene] (“MEEP”), is water-soluble, and this is consistent with the view that the methoxyethoxyethoxy group is strongly hydrophilic. On the basis of the results from 4 and 8, it is assumed that the amino groups interact weakly with the water subphase. The diethylamino groups are, therefore, considered to be weakly hydrophobic. The dibutylamino group can be considered to be strongly hydrophobic relative to the diethylamino group. The steric bulk of the dibutylamino group may also reduce the flexibility of the polymer chains. Polymers 11 and 12 should be truly amphiphilic, with properties that would allow them to spread to monolayer thickness on the surface of the water.
π-A evidence for both of these polymers indicated that polymeric monolayers were indeed formed. Isotherm data from concentration and spreading studies all indicate that compounds 11 and 12 behave at low surface compression as separate particles on the water surface. The molecular origins of the curves are seen in Figure 8. A steep rise occurs up to the point of 27-32 mN/m, which is followed by a rapid change to a less steep but nonzero slope. Three different loading concentrations superimpose at the same temperature when each isotherm is normalized to area per monomer unit. The spreading behavior indicates that this polymer is free to move on the surface. After the first π-A isotherm experiment was conducted, the barrier was opened rapidly, and a second isotherm run was started 16 min later. The fact that the two isotherms can be superimposed indicates that the polymer coils can rebound from their compressed state and will again separate into individual particles. The dibutylamino polymer, 12, differs from 11 by the fact that the dibutylamino group is more hydrophobic and bulkier than the diethylamino unit. The 100-µg and 50µg isotherm curves can be superimposed. Compound 12 behaves like 11. The break pressure is approximately the same (30 mN/m for 11 and 32 mN/m for 12) as is the area per monomer unit for the initial rise in surface pressure (Figure 9). The slope after the break is noticeably steeper for the dibutylamino compound, 12, an effect that may reflect the greater bulk and lower compressibility of the dibutylamino side groups. A model to explain the π-A curves for polymers 10-12 is proposed. As the barrier begins to close, the polymer molecules can act as separate, two-dimensional random coils on the surface, each unit with its own average radius of gyration that is determined by the structure of the repeat unit, the degree of polymerization, and the temperature of the chain. Compression of polymer chains on the water due to barrier closing causes the two-dimensional random coils to interpenetrate (presumably at position A in Figure 6), and this forces each polymer molecule into conformations that are further removed from random-coil equilibrium. This stress is relieved by polymer coils being forced up from the water surface as the barrier closes (position B in Figure 6). At small surface areas all monolayer properties are lost and the polymer can be considered as a random network of coils several layers thick with no significant order.
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Figure 9. π-area isotherms for polymer 11 and 12, normalized to area per monomer unit mass.
Conclusions This study has demonstrated that the Langmuir-Adam trough can be used to ascertain the relative hydrophobicity and hydrophilicity of side groups attached to a phosphazene ring or chain. Trifluoroethoxy side groups showed greater hydrophobic interactions than did phenoxy side groups. Both the rigidity of the phosphazene structure and the nature of the side groups determine whether the compound forms a molecular monolayer film or collective aggregates on the air-water interface. In these experiments, the phosphoryl end group in linear short chain molecules showed a stronger hydrophilicity than the gemdiamino side groups on a cyclic compound. The high polymeric phosphazenes formed separated macromolecules on the surface, and these polymers can be deposited
onto a solid substrate to modify the hydrophobichydrophilic balance of the surface. Thus, in spite of the difficulties encountered with the study of the macromolecules at air-water interfaces, the small-molecule results provide valuable information that can be used to interpret the behavior of the polymers. Acknowledgment. L.L.M. thanks Bloomsburg University for the sabbatical leave at The Pennsylvania State University, which made this work possible. R.J.F. and H.R.A. thank the Office of Naval Research for support. We also thank Dr. Dennis Ngo for the synthesis of several compounds. LA950453V
