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Langmuir 1993,9, 1036-1039
Organized Collapse of Fatty Acid Monolayers Craig McFate, Douglas Ward, and John Olmsted 111' Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92634 Received August 17, 1992. In Final Form: December 17, 1992 We observe that films of some fatty acids, including stearic acid and heptacosa-10,12-diynoicacid, display a 'spike" in their compression isotherms,followed by a trough of constant surface pressure, as they are compressed beyond monolayers. Studies of pH variations show that this behavior occurs only when all carboxylate groups in the monolayer are neutral. We observe "spikes" fo; C16, Cia, and CX,alkanoic acids, but not for tetracosanoic acid or alkenoic acids. We interpret the isotherms as follows. The 'spike" is due to buckling of the monolayer, followed by a cooperative folding over to generate an organized trilayer. Growth of the trilayer is facile and semireversible once the first folding occurs, but further compression of a fully-formed trilayer leads to irreversible collapse into three-dimensional structures.
Introduction When long-chain hydrocarbons with polar headgroups are spread on an aqueous surface, they can be compressed into close-packed monolayers. When the area of the surfaces matches the space-filling area of the close-packed molecules, the monolayer strongly resists further compression. This leads to a steep increase in resistance to further compression which can eventually be overcome by exerting a force sufficient to drive molecules out of the monolayer. The well-ordered,two-dimensional monolayer is then converted into a less ordered ("fractured") threedimensional multilayerd structure. While this standard view of conversion from mono- to multilayers involves significant disordering, there is evidence that fatty acid monolayers may be converted from a close-packed monolayer into a multilayered structure that retains a high degree of order. Ries first proposed that some film collapse entails the rise of a portion of a monolayer that then "falls over" to form bilayers.' Later, he refined this mechanism for the collapse of 2-hydroxytetracosanoic acid, proposing that a head-to-head column forms under compression, subsequently toppling to form a trilayere2These proposals, based on observed regularities in electron micrographs of partially-collapsed layers, have not been explored further. In contrast with the Ries proposal for conversion of monolayers into bi- or trilayers, the standard view of monolayer collapse is that it involves transformation from a two-dimensional to a three-dimensional (bulk) phase. Thus, Smith and Berg interpreted collapse phenomena of a range of surfactants in terms of a model of homogeneous nucleation and subsequent growth of bulk surfactant fragmer~ts.~ Their theoretical outline has been elaborated by DeKeyser and 5008, who showed that much experimental data are consistent with the nucleation-bulk growth model.4 As a monolayer film is compressed on the aqueous surface, it shows the two-dimensional analog of P-V behavior. As total surface area A decreases, the surface pressure ll exerted by the layer increases. Compression isotherms (ll vs A plots) of fatty acids typically display four relatively well-defined regions, which are illustrated by the isotherm of tetracosanoic acid shown in Figure 1. As long as open space remains on the aqueous surface, ll (1) Ries, H. E., Jr.; Kimball, W.A. J. Phys. Chem. 1955, 59, 94. (2) Ries, H. E., Jr. Nature 1979, 281, 287. (3) Smith, R. D.; Berg, J. C. J. Colloid Interface Sci. 1980, 74, 273. (4) De Keyser, P.; Joos, P. J. Phys Chem. 1984, 88, 274.
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Figure 1. Typical compression isotherm behavior of a fatty acid. These isotherms are of tetracosanoicacid at 25 O C , pH = 3. The same film was compressed, expanded, and then re-compressed. E 0 and dIIldA = 0. At molecular area A1 in the figure, the aqueous surface is fully covered but monolayer molecules are not vertically aligned on the surface. Surface pressure rises smoothly on further compression and dII/ dA < 0, as force must be exerted to drive molecular orientations toward the vertical. At molecular area A2 in the figure, all monolayer molecules are oriented vertically in a close-packed array. Surface pressure rises sharply on further compression and dII/dA 0 but dll/dA = 0. In special cases, compression isotherms showingregions of positive slope have been reported. When the polymerizable surfactant, heptacosa-10,12-diynoic acid [H3C(CH2)1&=C-C=C(CH2)&OOH], is compressed, ita surface pressure rises, then falls, and finally rises again.5 Remarkably, one need not use functionalized surfactanta
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 4, 1993 1037
Organized Collapse of Fatty Acid Monolayers like 2-hydroxytetracosanoic acid or heptacosa-10,12diynoic acid to observe atypical compression isotherms. We have found that some straight-chain alkanoic acids, exemplified by stearic acid (octadecanoic acid), display reproducible positive-slope compression isotherms under the appropriate conditions. Similar behavior was reported recently in the course of a calorimetric study of stearic acid compression? The characterization of these isotherms and their explanation in terms of collapse into trilayers are the subject of this paper.
Experimental Section Materials. Commercial samples of octadecanoic acid and tetracosanoicacid were purified by recrystallizationfrom 100% ethanol. Heptacosa-10-12-diynoicacid was synthesized following literature procedure^.^ It was stored protected from light in a freezer to prevent polymerization. Solutions for spreading on aqueous substrate were prepared in freshly-distilledchloroform. Deionized water, buffered to the appropriate pH (3-5 range) with 0.010 M potassium acid phthalate/HCl or NaOH was found to be a satisfactorysubstrate,giving identicalisothermsas doublydistilled water. Bicarbonate buffer was used for pH adjustment in the 5-7 range,and divalentcationswere added as their chloride salts. Instruments. All compression isotherms were determined using a Brinkman-Laudafilm balance, whose output was fed to a Hewlett-Packard Model 7015B X-Y recorder. The film balance was thermostated using a Brinkman RC-3 constant-temperature circulating bath. An Orion 701A digital Ionalyzer with a combination electrode was used to monitor pH. Methods. Monolayers were formed by slowly syringing chloroform solutions onto the aqueous surface using a 100-pL Hamilton glass syringe. After allowing the solvent to evaporate, thesurface was allowed to anneal for approximately5 min before beginning compression. The compression isotherms that are reproduced here were obtained at a compression speed of 4 x nm2/(molecules). Film collapse under constant pressure compression was achieved using a Brinkman constant-pressure plug-in unit.
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Figure 2. Compression isotherms of stearic acid at 25 O C : (a) (upper curve) pH > 6.0; (b) (lower curve) pH < 4.5; (c) (middle curve) pH = 5.5. On the y-axis zero has been displaced to show the isotherms more clearly.
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Results When a stearic acid monolayer is compressed on aqueous substrate at pH 16.0, its collapse behavior is unremarkable (Figure 2a). When the substrate is made more acidic, pH 5 4.5, the compression isotherm shows a sharp spike, followed by a "trough" of lower surface pressure and then a second sharp rise (Figure 2b). At intermediate pH, the isotherm displays intermediate behavior (Figure 2c). The sharp spike occurs at a molecular area of 0.20 nm2, corresponding to monolayer coverage, as indicated by comparing parts a and c of Figure 1. Normal compression isotherms are either highly irreversible or fully reversible. For example, when stearic acid on high pH substrate or tetracosanoic acid at any pH is compressed beyond the monolayer collapse point and then re-expanded, surface pressure rapidly drops to zero. On recompression, a much smaller surface area is reached before surface pressure increases. Figure 1illustrates this irreversible behavior for tetracosanoic acid on pH = 3 substrate. Liquid surfactants like oleic acid, in contrast, are well-known to compress fully reversibly, with identical compression and expansion isothermse8 (5) Ringedorf, H.; Day, D. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. ( 6 )Kato, T.; Akiyama, H.; Tanaka, T. Chem. Phys. Lett. 1991, 184,
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(7) Tieke, B.; Wegner, C.; Naegele, D.; Ringsdorf, H. Angew. Chem., Int. ed. Engl. 1976, 15, 764. (8)Gaines, C . L., Jr. Insoluble Monolayers at Liquid-Cas Interfaces; Wiley (Interscience): New York, 1966.
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Figure 3. Repetitive compression and expansion isotherms for stearic acid at 25 O C , pH = 3, cycling between areas of 0.30 and 0.08 nm2/molecule: dotted line, initial compression; solid upper line,recompressionwithout annealing;dashed line, recompression after 5 min of annealing. The lower solid line is the reproducible expansion isotherm. Stearic acid on low pH substrate exhibits semireversible behavior that is shown in Figure 3. The dotted line shows the compression isotherm of a fresh monolayer. When compression is interrupted in the "trough" region of the isotherm (surface area between 0.10 and 0.15 nm2/ molecule), surface pressure falls from ca. 25 mN/m to ca. 12 mN/m. Upon expansion, this pressure drops slowly until surface area exceeds that monolayer coverage. It then falls rapidly to zero as expansion continues. Upon immediate recompression, the isotherm is like the original compression isotherm but with a smaller "spike" (solid line in Figure 3). However, if the expanded film is allowed to anneal for 15 min prior to recompression, the new isotherm closely resembles the original compression isotherm (dashed line in Figure 3). As surface area per molecule fallsbelow about 0.08nm2/ molecule, further compression of stearic acid generates a sharp upturn in the surface pressure. This second barrier
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therms, not fully reversible but without a characteristic “spike” at monolayer coverage. Alkenoic acids of chain length 18 (oleic acid) and 22 (erucic acid) are liquid-like. However, the C Z diacetylenic ~ acid, heptacosa-lO-12diynoic acid, has the characteristic spike in its compression behavior at pH = 5.5. This is shown in Figure 6. The visual appearance of compressed stearic acid films and of polymerized compressed diynoicacid films indicates a smooth, unstrained surface for these films. Compressed films of tetracosanoic acid, or stearic acid at pH >4.5, show a filmy, strained appearance and white streaks due to bulk solid. In the “trough”region of compression, stearic acid films on pH = 3 substrate show no signs of strain. Diynoic acid films polymerized by exposure to UV light after being compressed at a surface pressure of 50 mN/m form royal blue, micalike platelets. Temperature variations from 5 to 50 O C have no effect on the shapes of compression isotherms for any of these systems. Compoundsshowing the “spike”behavior at low pH have similar-shaped isotherms throughout this temperature range; those that collapse normally are normal at all these temperatures.
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Figure 4. Compression isotherm for stearic acid at 25 OC, PH = 3,lO mM MgC12 added to the substrate.
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Figure 5. Plots of surface area vs time for stearic acid films at pH = 3, T = 25 O C , subjected to constant surface pressures of 35 mN/m (upper curve) and 40 mN/m (lower curve).
to compression is most sharply defined when the substrate contains 10 mM Zn2+or Mg2+cations. Figure 4 shows the compression isotherm for stearic acid at pH = 3,lO mM Mgz+. Although compression at a constant rate reveals a barrier of between 40 and 50 mN/m at an area corresponding to monolayer coverage,film collapse is also observed without increasing surface pressure above 40-50 mN/m, if the film is subjected to constant pressure for several minutes. Figure 5 showsthe-area plots for stearic acid on pH = 3 substrate held at 35 and 40 mN/m. A much slower collapse with the same sigmoidal time profile is observed under a constant surface pressure of 25 mN/m. Alkanoic acids of similar chain length to stearic acid ala0 displays this unusual collapse behavior. Isotherms for palmitic acid (cl6) and eicosanoic acid (CZO)show similar, though less well-defined, spikes at pH I4.5. Beyond Czo, compression isotherms at low pH are identical to those at high pH. Alkanoic acids with chain lengths shorter than c16 display quasi-liquid compression iso-
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Discussion The major question posed by the anomalous compression isotherms of stearic acid at low pH is the nature of the molecular process that is occurring. We explain the compression behavior using the model illustrated in Figure 7. A film that has been compressed to about 0.20 nm2/ molecule exhibits a surface pressure of 20 mN/m. This film is in a two-dimensional close-packed array (Figure 7a), as indicated by the fact that it sharply resists further compression. On further compression, the two-dimensional layer distorts until folds appear at a surface pressure around 40 mN/m (Figure 7b). These folds ride over the underlying monolayer (Figure 7c), leading to relief of surface pressure which falls to 25 mN/m or lower. The initiation of trilayer formation is a slow process that is accelerated by increased surface pressure. Once a “breakthrough” occurs, however, the growth of the trilayer is relatively facile. This model is consistent with all observed features of the isotherms. Surface pressure builds steeply until the first override takes place, but once this happens, pressure is relieved as layers slide over one another. The surface pressure then remains constant at the value required to promote the sliding transition until nearly all the monolayer has been converted into a trilayer. Then the pressure
Organized Collapse of Fatty Acid Monolayers a,) monolayer, 20 mN/m
b )distorted monolayer, 40 mN/m
c ) formation of trilayer, 25 mN/m
Figure 7. Model for the transformation of a monolayer film into a trilayer film. Buckling of the monolayer film is followed by head-to-headformation of a bilayer that slides over the top of the original monolayer.
rises steeply once again, increasing until it is sufficient to fracture the ordered trilayer into less ordered solid fragments. This conversion from trilayer to solid behaves just like the monolayer-to-solid conversion for typical monolayers. The mainevidence supportingthis plausible explanation for the anomalous isotherms is as follows. First, the “spike” at low pH occurs at the same surface area per molecule as the “wall” at high pH (see Figure 2), indicating that the “spike”represents a transition from a close-packed monolayer to some more compact array. Second,for both stearic acid (Figure 4) and heptacosa-10-12-diynoicacid (Figure 5),,the second barrier to compression occurs at an apparent area per molecule that is one-third the area of a monolayer. Third, irregular structural patterns can be observed on layers compressed beyond the point of conversion into three-dimensional solid, but no such patterns appear as stearic acid is compressed beyond a monolayer but short of atrilayer. Fourth, heptacosa-10-12-diynoicacidreadily polymerizes in the trilayer conformation to give smooth polymer platelets. Thispolymerization is known to require highly ordered molecular layer^.^ Fifth, electron micrographs of collapsed monolayers taken by Ries shown patterns indicative of layered structures rather than chaotic structures.2 The sensitivity of this phenomenon to pH is consistent with our model. At low pH, all carboxylicacid headgroups are neutral and can form extensive hydrogen-bonding networks with one another as well as with solvent molecules. These networks are not seriously disrupted in the foldingprocess that leads from monolayersto trilayers. However,when the pH rises above 5, cooperativeformation of trilayers is hindered, and above pH = 6, isotherms indicate that direct collapse occurs to give three-dimensional crystallites. Interestingly, the pH range over which we observe this transition in collapse is lower by 2 pH units than the pH range over which Joos observed an increase in the collapse pressure for stearic acid? Using Joos’ value of pK, = 8.2 for a surface layer of stearic acid,
Langmuir, Vol. 9, No. 4, 1993 1039 we calculatethat only 0.6 7% of acid groups are deprotonated at pH = 6. Apparently a small fraction of ionization is sufficient to prevent the cooperativeformation of trilayers, which suggests that the cooperative transition observed at low pH requires a specific alignment of polar headgroups that is disrupted as ionizationbegins. Even a small fraction of ionized groups may be sufficient to attract cations into the surface domain, generating Coulombic forces that prevent a molecular double layer from forming and sliding over the surface monolayer. The confinement of this phenomenon to alkanoic acids with carbon chain lengths between 16 and 20 indicates that trilayer formation requires unique conditions where layers slide over one another in preference to fragmenting. When the alkyl chains is shorter than 15 atoms, the monolayer fractures relatively easily and there is only a small barrier to formation of three-dimensional arrays. Beyond chain lengths of 20 atoms, the monolayers become too thick for overlapping collapse to take place, instead collapsing to three-dimensional structures regardless of the pH. Heptacosa-10,12-diynoic acid can collapse to trilayers,despite ita long hydrophobic chain. We speculate that r - interactions ~ among ita diyne linkages cause these molecules to be well-aligned in ita monolayer array and that highly aligned molecules are favorably oriented for cooperative transition to a trilayer. The semireversibility of formation of tbe trilayer structure can be attributed to ita ordered structure. The collapse of a two-dimensional film to three-dimensional crystallites is irreversiblebecause their molecular clusters cannot readily disperse over a surface. The process of sliding a bilayer over a monolayer, in contrast, can readily reverse itself. When the surface pressure drops below the transition pressure, the trilayer “unzips” to regenerate a monolayer. This unzipping process leaves nucleation sites that anneal over time. Thus, immediate recompression shows a lower pressure “spike” (Figure 3) while recompression after annealing reproduces the original compression isotherm. Our data establish upper and lower bounds on the equilibrium spreading pressure for interconversion between monolayers and trilayers. During compression, the surface pressure holds relatively constant at a value that depends on compression rate. At the slowest compression achievable with our film balance, 1X nm2/(molecule s), this pressure is 16 mN/m. During expansion, on the other hand, the surface pressure gradually falls from 12.5 to 10 mN/m. Thus, monolayers and trilayers are at equilibrium under a surface pressure of 14 f 2 mN/m.
Acknowledgment. The film balance used in this work was purchased under an NSF Scientific Equipment Grant. All of the results were obtained in the course of undergraduate research projects carried out by Craig McFate and Douglas Ward. Undergraduate students Chris Lee, Eric Kantarowski, and Mike Constant also assisted in obtaining some of the isotherms. (9)Joos, P.Bull. SOC.Chim. Belg. 1971, 80, 277.
