About this Journal Submit a Manuscript Table of Contents
ISRN Physical Chemistry
Volume 2012 (2012), Article ID 835397, 6 pages
doi:10.5402/2012/835397
Research Article

Hysteresis of Isotherms of Mixed Monolayers of N-Octadecyl-N′-phenylthiourea and Stearic Acid at Air/Water Interface

Department of Chemistry, University of Botswana, PB 00704, Gaborone, Botswana

Received 11 October 2012; Accepted 2 December 2012

Academic Editors: J. J. Lopez Cascales, G. Pellicane, and P. O. Westlund

Copyright © 2012 Siji Sudheesh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

1. Introduction

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 [1], they serve as precursors to Langmuir-Blodgett (LB) films [24], and they are used to incorporate nanoparticles [514] into LB films. They can also serve as templates for two-dimensional crystal growth [1519] and they can be used to control crystal nucleation [2027]. 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 [2834]. 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 [35], of 1-phenyl-1-hexadecanol over chromic acid [36], mixed monolayers of octadecylamine and 1-octadecanethiol [37], and several others [3842].

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 [4345]. 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 [46].

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 [47]. 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 [47], activity on the central nervous system [48], antimycobacterial activity [49], and antimicrobial activity [50].

2. Experimental

2.1. Materials

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 [51]. 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)].

835397.fig.001
Figure 1: Structure of N-octadecyl-N′-phenylthiourea (OPT).

For monolayer studies, a 4.043 × 10−3 M solution of OPT was prepared in chloroform.

2.2. Method

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.

835397.fig.002
Figure 2: Surface pressure area hysteresis isotherms of premixed OPT/SA monolayers of approximately equimolar composition at 25°C. Recompression of the monolayer after the first compression and expansion does not retrace the first cycle. The second and subsequent cycles, however, coincide.

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.

835397.fig.003
Figure 3: Surface pressure area hysteresis isotherms of premixed OPT/SA monolayers with mole fraction ratio about 0.6 : 0.4.

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.

835397.fig.004
Figure 4: Surface pressure area hysteresis isotherms of monolayer formed by spreading a premixed OPT/SA solution of the molar ratio 0.9 : 0.1, over water at 25°C.

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.

tab1
Table 1: Limiting area/molecule of OPT/SA premixed monolayer for coinciding curves.

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.

835397.fig.005
Figure 5: Area per molecule assuming the monolayer comprises 1 : 1 complex and excess SA versus mole fraction of SA in the spreading mixture. Any excess OPT is assumed removed from the monolayer. The solid line indicates constant area/molecule. At low mole fractions of SA deviation sets in from the constant value obtained at mole fractions of SA 0.4 or higher.
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.

835397.fig.006
Figure 6: Surface pressure area hysteresis isotherm of stearic acid spread over air/water interface at 25°C.

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.

4. Conclusion

-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.

References

  1. V. M. Kaganer, H. Möhwald, and P. Dutta, “Structure and phase transitions in Langmuir monolayers,” Reviews of Modern Physics, vol. 71, pp. 779–819, 1999.
  2. T. H. Chou and C. H. Chang, “Thermodynamic behavior and relaxation processes of mixed DPPC/cholesterol monolayers at the air/water interface,” Colloids and Surfaces B, vol. 17, no. 2, pp. 71–79, 2000. View at Publisher · View at Google Scholar · View at Scopus
  3. T. H. Chou and C. H. Chang, “Thermodynamic characteristics of mixed DPPC/DHDP monolayers on water and phosphate buffer subphases,” Langmuir, vol. 16, no. 7, pp. 3385–3390, 2000. View at Publisher · View at Google Scholar · View at Scopus
  4. J. M. Chovelon, K. Wan, and N. Jaffrezic-Renault, “Influence of the surface pressure on the organization of mixed Langmuir-Blodgett films of octadecylamine and butyrylcholinesterase. 1. Film preparation at the air-water interface,” Langmuir, vol. 16, no. 15, pp. 6223–6227, 2000. View at Publisher · View at Google Scholar · View at Scopus
  5. C. Malitesta, A. Tepore, L. Valli, A. Genga, and T. Siciliano, “X-Ray photoelectron spectroscopy characterisation of Langmuir-Blodgett films containing TiO2 nanoparticles grown by room-temperature hydrolysis of TiO(C2O4)22-,” Thin Solid Films, vol. 422, no. 1-2, pp. 112–119, 2002. View at Publisher · View at Google Scholar · View at Scopus
  6. B. I. Denis, E. A. Alexander, T. S. Dmitry, I. K. Vladimir, A. V. Vladimir, and K. A. Maria, “2D “soap”-assembly of nanoparticles via colloid-induced condensation of mixed Langmuir monolayers of fatty surfactants,” Langmuir, vol. 28, pp. 125–133, 2012.
  7. M. Weis, K. Gmucová, V. Nádaždy et al., “Control of single-electron charging of metallic nanoparticles onto amorphous silicon surface,” Journal of Nanoscience and Nanotechnology, vol. 8, no. 11, pp. 5684–5689, 2008. View at Publisher · View at Google Scholar · View at Scopus
  8. G. Clavel, C. Marichy, M. G. Willinger, S. Ravaine, D. Zitoun, and N. Pinna, “CoFe2O4-TiO2 and CoFe2O4-ZnO thin film nanostructures elaborated from colloidal chemistry and atomic layer deposition,” Langmuir, vol. 26, no. 23, pp. 18400–18407, 2010. View at Publisher · View at Google Scholar · View at Scopus
  9. J. H. Kim, H. S. Kim, J. H. Lee, S. W. Choi, Y. J. Cho, and J. H. Kim, “Hexagonally close packed langmuir-blodgett films from monodispersed silica nanoparticles,” Journal of Nanoscience and Nanotechnology, vol. 9, no. 12, pp. 7007–7011, 2009. View at Publisher · View at Google Scholar · View at Scopus
  10. E. Fulop, N. Nagy, A. Deak, and I. Barsony, “Langmuir-Blodgett films of gold/silica core/shell nanorods,” Thin Solid Films, vol. 520, pp. 7002–7005, 2012.
  11. N. Nagy, Z. Zolnai, A. Deák, M. Fried, and I. Bársony, “Various nanostructures on macroscopically large areas prepared by tunable ion-swelling,” Journal of Nanoscience and Nanotechnology, vol. 12, no. 8, pp. 6712–6717, 2012. View at Publisher · View at Google Scholar
  12. W. Wang, L. Liang, A. Johs, and B. Gu, “Thin films of uniform hematite nanoparticles: control of surface hydrophobicity and self-assembly,” Journal of Materials Chemistry, vol. 18, no. 47, pp. 5770–5775, 2008. View at Publisher · View at Google Scholar · View at Scopus
  13. P. Pienpinijtham, X. X. Han, S. Ekgasit, and Y. Ozaki, “An ionic surfactant-mediated Langmuir-Blodgett method to construct gold nanoparticle films for surface-enhanced Raman scattering,” Physical Chemistry Chemical Physics, vol. 14, no. 29, pp. 10132–10139, 2012. View at Publisher · View at Google Scholar
  14. K. Vegso, P. Siffalovic, M. Jergel et al., “Silver nanoparticle monolayer-to-bilayer transition at the air/water interface as studied by the GISAXS technique: application of a new paracrystal model,” Langmuir, vol. 28, pp. 9395–9404, 2012.
  15. M. Negar, D. Oksana, S. Kinga, and S. Patrick, “Monolayers of an amphiphilic para-carboxy-calix[4]arene act as templates for the crystallization of acetaminophen,” Journal of Colloid and Interface Science, vol. 377, pp. 450–455, 2012.
  16. E. Pechkova, D. Scudieri, L. Belmonte, and C. Nicolini, “Oxygen-bound hell's gate globin I by classical versus LB nanotemplate method,” Journal of Cellular Biochemistry, vol. 113, pp. 2543–2548, 2012.
  17. W. Ren, Y. Li, M. Chen, B. Liu, X. Li, and M. Dong, “Oriented growth of single NaCl (100) crystal induced by Langmuir-Blodgett film,” Journal of Materials Research, vol. 26, no. 2, pp. 230–235, 2011. View at Publisher · View at Google Scholar · View at Scopus
  18. R. Gaynutdinov, V. Fridkin, and H. Kliem, “Growth and switching of ferroelectric nanocrystals from ultrathin film of copolymer of vinylidene fluoride and trifluoroethylene,” Journal of Nanotechnology, vol. 2011, Article ID 180104, 5 pages, 2011. View at Publisher · View at Google Scholar
  19. N. Bagkar, S. Choudhury, K. H. Kim, P. Chowdhury, S. I. Lee, and J. V. Yakhmi, “Crystalline thin films of transition metal hexacyanochromates grown under Langmuir monolayer,” Thin Solid Films, vol. 513, no. 1-2, pp. 325–330, 2006. View at Publisher · View at Google Scholar · View at Scopus
  20. L. H. Wee, Z. Wang, L. Tosheva, L. Itani, V. Valtchev, and A. M. Doyle, “Silicalite-1 films with preferred orientation,” Microporous and Mesoporous Materials, vol. 116, no. 1–3, pp. 22–27, 2008. View at Publisher · View at Google Scholar · View at Scopus
  21. F. Lu, X. Zhao, G. Zhou, H. S. Wang, and Y. Ozaki, “Control for oriented growth of large size KCl crystals by the competition between spontaneous and induced nucleation/growth on a Langmuir-Blodgett film,” Chemical Physics Letters, vol. 458, no. 1–3, pp. 67–70, 2008. View at Publisher · View at Google Scholar · View at Scopus
  22. F. Lu, G. Zhou, H. J. Zhai, Y. B. Wang, and H. S. Wang, “Nucleation and growth of glycine crystals with controllable sizes and polymorphs on langmuir-blodgett films,” Crystal Growth and Design, vol. 7, no. 12, pp. 2654–2657, 2007. View at Publisher · View at Google Scholar · View at Scopus
  23. K. Sato, Y. Kumagai, K. Watari, and J. Tanaka, “Hierarchical texture of calcium carbonate crystals grown on a polymerized Langmuir-Blodgett film,” Langmuir, vol. 20, no. 7, pp. 2979–2981, 2004. View at Publisher · View at Google Scholar · View at Scopus
  24. A. P. Girard-Egrot, R. M. Morelis, and P. R. Coulet, “Dependence of Langmuir-Blodgett film quality on fatty acid monolayer integrity. 2. Crucial effect of the removal rate of monolayer during Langmuir-Blodgett film deposition,” Langmuir, vol. 9, no. 11, pp. 3107–3110, 1993. View at Scopus
  25. N. P. Hughes, D. Heard, C. C. Perry, and R. J. P. Williams, “Controlled deposition of strontium sulphate on behenic acid Langmuir-Blodgett multilayers,” Journal of Physics D, vol. 24, no. 2, pp. 146–153, 1991. View at Publisher · View at Google Scholar · View at Scopus
  26. E. M. Landau, S. G. Wolf, J. Sagiv et al., “Design and surface synchrotron x-ray structure analysis of Langmuir films for crystal nucleation,” Pure and Applied Chemistry, vol. 61, pp. 673–684, 1989.
  27. C. Lendrum and K. M. McGrath, “Toward controlled nucleation: balancing monolayer chemistry with monolayer fluidity,” Crystal Growth and Design, vol. 10, no. 10, pp. 4463–4470, 2010. View at Publisher · View at Google Scholar · View at Scopus
  28. S. Pathirana, W. C. Neely, L. J. Myers, and V. Vodyanoy, “Interaction of valinomycin and stearic acid in monolayers,” Langmuir, vol. 8, no. 8, pp. 1984–1987, 1992. View at Scopus
  29. M. Puggelli, G. Gabrielli, and G. Caminati, “Langmuir-Blodgett monolayers and multilayers of stearic acid and stearyl amine,” Thin Solid Films, vol. 244, no. 1-2, pp. 1050–1054, 1994. View at Scopus
  30. R. Stosch and H. K. Cammenga, “Molecular interactions in mixed monolayers of octadecanoic acid and three related amphiphiles,” Journal of Colloid and Interface Science, vol. 230, no. 2, pp. 291–297, 2000. View at Publisher · View at Google Scholar · View at Scopus
  31. W. Cordroch and D. Möbius, “Incorporation of non-amphiphilic compounds into host monolayers,” Thin Solid Films, vol. 210-211, no. 1, pp. 135–137, 1992. View at Scopus
  32. A. M. P. S. Gonçalves da Silva, D. A. Armitage, and R. G. Linford, “Mixed langmuir films of 1-heptadecanoic acid and 1-bromohexadecane,” Journal of Colloid And Interface Science, vol. 156, no. 2, pp. 433–437, 1993. View at Publisher · View at Google Scholar · View at Scopus
  33. A. M. Gonçalves da Silva, J. C. Guerreiro, N. G. Rodrigues, and T. O. Rodrigues, “Mixed monolayers of heptadecanoic acid with chlorohexadecane and bromohexadecane. Effects of temperature and of metal ions in the subphase,” Langmuir, vol. 12, no. 18, pp. 4442–4448, 1996. View at Scopus
  34. Y. L. Lee, Y. C. Yang, and Y. J. Shen, “Monolayer characteristics of mixed octadecylamine and stearic acid at the air/water interface,” Journal of Physical Chemistry B, vol. 109, no. 10, pp. 4662–4667, 2005. View at Publisher · View at Google Scholar · View at Scopus
  35. J. Ahmad and K. B. Astin, “Conformer selection by monolayer compression,” Journal of the American Chemical Society, vol. 108, no. 23, pp. 7434–7435, 1986. View at Scopus
  36. J. Ahmad and K. B. Astin, “Oxidation of alcohol monolayers by chromic acid,” Langmuir, vol. 4, no. 3, pp. 780–781, 1988. View at Scopus
  37. J. Ahmad, “Reactions in monolayers: oxidation of 1-octadecanethiol catalyzed by octadecylamine,” Langmuir, vol. 12, no. 4, pp. 963–965, 1996. View at Scopus
  38. J. Ahmad and K. B. Astin, “Influencing reactivity by monolayer compression: an alcohol dehydration,” Journal of the American Chemical Society, vol. 110, no. 24, pp. 8175–8178, 1988. View at Scopus
  39. J. Ahmad and K. B. Astin, “Reaction of a monolayer with subphase: dehydration of a secondary alcohol over sulfuric acid,” Langmuir, vol. 6, no. 6, pp. 1098–1101, 1990. View at Scopus
  40. J. Ahmad and K. B. Astin, “Reactions in monolayers: base-catalyzed ester hydrolysis revisited,” Langmuir, vol. 6, no. 12, pp. 1797–1799, 1990. View at Scopus
  41. J. Ahmad and K. B. Astin, “Switching reaction mechanism by monolayer compression: an ester hydrolysis,” Angewandte Chemie, vol. 29, no. 3, pp. 306–308, 1990. View at Publisher · View at Google Scholar · View at Scopus
  42. J. Ahmad and K. B. Astin, “Reactions in monolayers: the oxidation of thiols to disulfides,” Colloids and Surfaces, vol. 49, pp. 281–287, 1990. View at Scopus
  43. J. M. Berg and L. G. T. Eriksson, “Mixed monolayers and Langmuir-Blodgett films consisting of a fatty amine and a bipolar substance,” Langmuir, vol. 10, no. 4, pp. 1213–1224, 1994. View at Scopus
  44. S. S. Gayathri and A. Patnaik, “Interfacial behaviour of brominated fullerene (C60Br24) and stearic acid mixed Langmuir films at air-water interface,” Chemical Physics Letters, vol. 433, no. 4–6, pp. 317–322, 2007. View at Publisher · View at Google Scholar · View at Scopus
  45. J. M. Corkill, J. F. Goodman, S. P. Harrold, and J. R. Tate, “Monolayers formed by mixtures of anionic and cationic surface-active agents,” Transactions of the Faraday Society, vol. 63, pp. 247–256, 1967. View at Scopus
  46. A. Asnacios, D. Langevin, and J. F. Argillier, “Complexation of cationic surfactant and anionic polymer at the air-water interface,” Macromolecules, vol. 29, no. 23, pp. 7412–7417, 1996. View at Scopus
  47. H. Q. Li, P. C. Lv, T. Yan, and H. L. Zhu, “Urea derivatives as anticancer agents,” Anti-Cancer Agents in Medicinal Chemistry, vol. 9, no. 4, pp. 471–480, 2009. View at Scopus
  48. J. Stefanska, D. Szulczyk, A. E. Koziol et al., “Di-substituted thiourea derivatives and their activity on CNS: synthesis and biological evaluation,” European Journal of Medicinal Chemistry, vol. 55, pp. 205–213, 2012.
  49. S. van Poecke, H. Munier-Lehmann, O. Helynck, M. Froeyen, and S. V. Calenbergh, “Calenbergh, Synthesis and inhibitory activity of thymidine analogues targeting Mycobacterium tuberculosis thymidine monophosphate kinase,” Bioorganic & Medicinal Chemistry, vol. 19, pp. 7603–7611, 2011.
  50. A. Saeed, U. Shaheen, A. Hameed, and S. Z. H. Naqvi, “Synthesis, characterization and antimicrobial activity of some new 1-(fluorobenzoyl)-3-(fluorophenyl)thioureas,” Journal of Fluorine Chemistry, vol. 130, no. 11, pp. 1028–1034, 2009. View at Publisher · View at Google Scholar · View at Scopus
  51. L. J. Wilson, S. R. Klopfenstein, and M. Li, “A traceless linker approach to the solid phase synthesis of substituted guanidines utilizing a novel acyl isothiocyanate resin,” Tetrahedron Letters, vol. 40, no. 21, pp. 3999–4002, 1999. View at Publisher · View at Google Scholar · View at Scopus
  52. Y. S. Kang, D. K. Lee, and P. Stroeve, “FTIR and UV-vis spectroscopy studies of Langmuir-Blodgett films of stearic acid/γ-Fe2O3 nanoparticles,” Thin Solid Films, vol. 327-329, no. 1-2, pp. 541–544, 1998. View at Scopus
  53. A. Sakai, S. H. Wang, L. O. Peres, and L. Caseli, “Controlling the luminescence properties of poly(p-phenylene vinylene) entrapped in Langmuir and Langmuir-Blodgett films of stearic acid,” Synthetic Metals, vol. 161, pp. 1753–1759, 2011.