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Hysteresis of Isotherms of Mixed Monolayers of N-Octadecyl-N′-phenylthiourea and Stearic Acid at Air/Water Interface
Surface pressure area isotherms of Langmuir monolayers formed by spreading mixed solutions of varying concentrations of N-octadecyl-N′-phenylthiourea (OPT) and octadecanoic acid or stearic acid (SA) over air-water interface are described. Examination of the hysteresis behavior and an analysis of the limiting area per molecule of the isotherms show that when the spread solution has an excess of OPT, the limiting surface area is consistent with a monolayer composed of equimolar amounts of the two components. This indicates that any excess thiourea, which on its own does not form a stable monolayer, is squeezed out and is not part of the monolayer. On the other hand, when the spreading mixture has an excess of SA over OPT, the isotherm indicates that the entire originally spread material is incorporated into the surface film. In this case, the values of area/molecule indicate that the monolayer is composed of SA : OPT complex with a ratio of 1 : 1 together with the excess SA remaining in the monolayer.
Single component and mixed Langmuir monolayers at air-water interface continue to be of interest because they provide a model for studying ordering in two dimensions , they serve as precursors to Langmuir-Blodgett (LB) films [2–4], and they are used to incorporate nanoparticles [5–14] into LB films. They can also serve as templates for two-dimensional crystal growth [15–19] and they can be used to control crystal nucleation [20–27]. They also serve as model systems for simulation of physicochemical properties at interfaces. In research on mixed monolayers, the study of interactions between the individual components, miscibility among the components, mutual interaction, and phase behavior are areas of interest [28–34]. Our group has been investigating interactions between components of mixed monolayers and between monolayers and the subphase with an objective to examine and control the reactivity in various systems such as the monolayer of monoterpenoid alcohol, nerol, over acidic subphase , of 1-phenyl-1-hexadecanol over chromic acid , mixed monolayers of octadecylamine and 1-octadecanethiol , and several others [38–42].
Some substances that do not form stable monolayers by themselves are able to form mixed Langmuir films with certain amphiphiles. Thus long-chain -alkanes and haloalkanes, which do not form monolayers on their own because of their strong hydrophobic character, can remain at the interface when mixed with suitable film-forming material [43–45]. Similarly some dicarboxylic acids have high solubility and pass into the aqueous subphase when added alone to air-water interface, but form mixed monolayers with film-forming substances because of the interaction between them .
In continuation of these studies, investigation of the interaction between a long-chain thiourea and a fatty acid was considered pertinent. The mechanism of interaction between thiourea and other substances within and across membranes would be of interest because thioureas constitute a well-known class of molecules with biological importance . Disubstituted thioureas are known to interact with several bioorganic molecules resulting in diverse types of biological activity such as anticancer activity against various types of leukemia and solid tumors , activity on the central nervous system , antimycobacterial activity , and antimicrobial activity .
Commercial stearic acid (SA) (90%, UnivAR, South Africa) was recrystallized from n-hexane, and a 3.585 × 10−3 M solution of it was prepared in n-hexane. OPT (Figure 1) was synthesized by the reaction of phenylisothiocyanate (1) (98%, Acros Organics, USA) and octadecan-1-amine (2) (98%, Aldrich chemicals) following the reported method . A solution of 5.0 mmol each of (1) and (2) in 25 mL of distilled ethanol was refluxed for 30 min in a 100 mL round-bottom flask. The white solid product (OPT) obtained after allowing the solution to attain the room temperature was filtered under suction and recrystallized from distilled ethanol three times. The melting point of the crystalline product was 87.3–88.1°C. The product was characterized by satisfactory spectral data [IR (Perkin-Elmer Spectrum 100 FTIR) cm−1: 3233 (N–H), 3054 (Ar C–H), 1346 (C=S); 1H NMR (CDCl3, ppm): 7.77–7.65 (br, 1H, NH), 7.48 (t, 2H, arom.), 7.35 (m, 1H, arom.), 7.25 (d, 2H), 6.09 (br, 1H, NH), 1.86 (t, 2H, N–CH2), 1.60 (m, 2H, CH2), 1.30 (s, 30H, fifteen methylene protons), 0.92 (t, 3H, CH3); 13C NMR (CDCl3, ppm): 180.7 (C=S), 136.1, 130.3, 127.4, 125.3 (Four arom. carbons), 45.7 (N–CH2), 31.9, 29.7, 29.6, 29.55, 29.5, 29.4, 29.2, 29.0, 26.9, 22.7, 14 (CH3)].
For monolayer studies, a 4.043 × 10−3 M solution of OPT was prepared in chloroform.
The film balance used for the measurements was manufactured by Nima Technology, Coventry, England. The trough was made from a single slab of PTFE with two PTFE barriers. Near one barrier was a measuring balance with a Wilhelmy plate cut from a filter paper, which dipped into the aqueous phase. The force on the plate was measured by a microbalance, which displayed the reading on a computer screen. The monolayer was compressed at a speed of 10 cm2/min. The instrument displayed a graph between the area available to the monolayer and the surface pressure.
The inside of the trough was cleaned with soapy water, followed by thorough rinsing with distilled water. It was then cleaned with n-hexane using tissue paper. Finally it was thoroughly rinsed with triply distilled water, which was prepared by taking distilled water and redistilling it in a two-stage all-quartz still.
The measurements were made at a constant temperature of 25°C, by allowing the substrate water to come to this temperature by storing it in a water bath. Blank runs on pure water and with spread pure solvent were made to ensure that there was no surface impurity.
The sample containing the monolayer-forming material was spread on the surface using a micro syringe. Appropriate quantities of SA and OPT solutions were premixed and spread on the subphase. The solvent was allowed to evaporate and the monolayer was compressed to get the surface pressure area per molecule (-) isotherm.
3. Results and Discussion
OPT alone does not form a stable monolayer. The isotherm of stearic acid has been reported earlier by other workers [52, 53]. The curve shows that when the surface pressure is around 49 mN/m, it decreases sharply, indicating a collapse. With further compression, the surface pressure essentially remains constant.
The behavior of a monolayer formed by spreading an equimolar mixture of the two components and subjected to repeated compression-expansion cycles is shown in Figure 2. The initial compression is the right-most trace in the diagram. As the monolayer is compressed initially the behavior is that of a liquid expanded film. Around a surface pressure of 15 mN/m the curve starts to flatten out indicating a phase transition. As the area available to the film is decreased further, the surface pressure starts to rise steeply again.
When the monolayer is reexpanded from a value of surface pressure of around 38 mN/m, there is a steep drop in the surface pressure to around 8 mN/m, followed by a gentler drop to about zero. On recompression the surface pressure increases but the second cycle does not retrace the curve of the first one. During the second cycle, the area per molecule is much smaller and the phase change that was pronounced during the first compression has largely disappeared. The expansion curve, however, follows the path of the first expansion.
Subsequent cycles largely coincide with the second cycle indicating that it was during the first compression that changes took place in the monolayer, and the state of the monolayer is largely preserved through the subsequent compression-expansion cycles. The indication is that the film has reached a stable state and the composition of the monolayer remains unchanged during subsequent compressions and expansions.
The behavior of the monolayer formed by spreading a 0.6 : 0.4 OPT : SA mixture is similar and is shown in Figure 3.
The monolayers formed by spreading mixtures containing a mole fraction of SA 0.4 and above showed a plateau region around 14–17 mN/m. This plateau region may be due to an intermolecular proton transfer from the carboxylic acid to thiocarboxamide functional groups of adjacently located molecules. The attraction between the resulting ions draws them closer reducing the area, which shows as the flat region.
This proton transfer can lead to a new monolayer phase and the behavior of which does not correspond to either a pure OPT or to a pure SA monolayer.
When OPT in the spreading mixture is in excess; however, the isotherms do not fully coalesce even after several cycles. In such monolayers, successive compression-expansion cycles progressively shift towards a lower area, indicating that the monolayer is progressively losing film material. By noting that this loss does not take place when SA is in excess, we can conclude that it is OPT which is being squeezed out of the interface. The difference between the positions of the isotherms for successive cycles, however, becomes smaller with each new cycle till the curves nearly coincide as shown in Figure 4.
The limiting area of the composite film is obtained by extrapolating the straight line portion of the coinciding isotherm to zero pressure. Three quantities were calculated: total limiting area per molecule deposited, total limiting area per stearic acid molecule deposited, and total limiting area per OPT molecule deposited. A comparison of these three quantities allows the estimation of the relative amount of each of the components in the monolayer, assuming that all the stearic acid deposited remains in the monolayer.
The monolayer formed by depositing equimolar mixture of the two components gives coinciding curves after the first compression, indicating that there is negligible loss of film material through repeated compression-expansion cycles. This shows that all the material deposited on the interface remains incorporated in the monolayer in this case and the equilibrium state of the monolayer is achieved just after the first compression. Since the area per molecule is known for stearic acid in a monolayer of pure stearic acid, the total area occupied by stearic acid in the mixed monolayer can be calculated. The area per molecule of OPT can then be calculated by subtracting this value from the total area occupied by the mixture of stearic acid and OPT. The value of the area per molecule allows us to calculate the amount of OPT remaining on the surface for those mixed films where some of OPT is squeezed out of the monolayer before a stable monolayer is formed.
In the equimolar mixed film, the limiting area per molecule is found to be close to the area/molecule of a pure stearic acid monolayer. Moreover the limiting area of the mixed equimolar film divided by the number of molecules of stearic acid alone is found to be twice the area/molecule in the pure stearic acid film. This indicates that half the area in the mixed film is occupied by the molecules of each component. This is as to be expected since both the molecules have similar hydrophobic chain. Thus, total area/molecule OPT = total area/molecule stearic acid = 2(area/(molecule stearic acid + OPT)).
This leads us to conclude that the composition of the film is 1 : 1 and all OPT molecules are incorporated in the monolayer in the equimolar film.
This 1 : 1 composition is confirmed by the behavior of the monolayer spread from solutions of other compositions. Table 1 shows the observed limiting areas per molecule for other compositions.
The values of area/molecule shown in the last column are about constant for all monolayers that achieve a stable reproducible form when subjected to several compression-expansion cycles. Deviation from the constant value sets in for those mixtures where OPT is in large excess. It is noteworthy that for these monolayers, hysteresis curves do not coincide. Figure 5 depicts this deviation graphically; the solid line in the figure represents constant value of the area/molecule obtained on the basis of those monolayers where SA was in excess.
3.1. OPT Excess
For a monolayer formed by spreading OPT and stearic acid in the ratio 0.8 : 0.2 mole fractions only as many molecules of OPT will be incorporated into the 1 : 1 complex as are the molecules of stearic acid. The rest will be removed from the film. With repeated cycles of the hysteresis curves, excess OPT is progressively removed. A stage is reached when successive hysteresis curves coincide and the remaining monolayer remains stable. The observed area/molecule at this equilibrium stage should be about one half of the available area divided by the number of stearic acid molecules. As Table 1 shows, this is what is observed experimentally for the spread mixtures with an excess of OPT. For the mixtures in which OPT is vastly in excess (0.8 : 0.2 and 0.9 : 0.1), successive curves do not fully coincide even after several cycles, though they get closer. This explains why the values of area per molecule start deviating from the values for other compositions as the excess of OPT becomes large, as shown in Figure 5. Keeping in view the fact that for 0.9 : 0.1 mixture the calculation assumes that ninth of the amount of OPT deposited remains in the film as part of the 1 : 1 complex and ignores the rest, the closeness of the area per molecule is remarkable. This is further confirmation of 1 : 1 complex.
3.2. SA Excess
The result of the film formed by spreading mixtures where stearic acid is in excess is in accordance with the conclusions outlined above. When stearic acid is in excess, the hysteresis curves show a reversible behavior after the first compression as shown in Figure 2. The value of the limiting area per molecule can only be explained by assuming that all OPT remains in the monolayer. This value is also consistent with all OPT molecules being incorporated in a 1 : 1 complex with excess stearic acid molecules remaining at the surface.
Figure 6 shows hysteresis curves for pure SA, which were obtained for the purpose of comparison. Except for the expected loss of some film material over time, all the cycles show similar behavior.
The pure SA isotherm is a smoothly rising curve with a lift off area 20.2 Å2 and collapsed at about 49 mN/m. A transition to a steeper portion of the curve occurred at about 25 mN/m indicating the attainment of solid phase. The area/molecule of pure SA was 20.0 Å2 at 25 mN/m.
-Octadecyl--phenylthiourea does not form a stable monolayer on air-water interface but when spread together with stearic acid, it can get incorporated into the monolayer. Hysteresis measurements on such mixed monolayers show that the area/molecule is consistent with the monolayer having a 1 : 1 composition of the two components in addition to any excess stearic acid deposited. For monolayers which are formed by spreading mixtures containing excess of the thiourea, the values correspond to a simple 1 : 1 mixture with excess thiourea not remaining in the monolayer.
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