Mixed Langmuir and Langmuir−Blodgett Films of a Proton Sponge

J. Phys. Chem. B 2007, 111, 2845-2855

2845

Mixed Langmuir and Langmuir-Blodgett Films of a Proton Sponge and a Fatty Acid: Influence of the Subphase Nature on the Interactions between the Two Components Marta Haro, Pilar Cea, Ignacio Gasco´ n, Fe´ lix M. Royo, and M. Carmen Lo´ pez* Departamento de Quı´mica Orga´ nica-Quı´mica Fı´sica, Facultad de Ciencias, Plaza de San Francisco, Ciudad UniVersitaria, 50009 Zaragoza, Spain ReceiVed: September 12, 2006; In Final Form: December 4, 2006

The H+ acceptor activity of a proton sponge, namely, diphenyl bis(octadecylamino)phosphonium bromide, has been studied at the air-liquid interface using several subphases. Mixed Langmuir and Langmuir-Blodgett (LB) films containing the proton sponge and a fatty acid (behenic acid) in the whole composition range have been prepared. Surface pressure versus area per molecule isotherms were recorded and excess Gibbs energies of mixing calculated. The existence of strong interactions between the proton sponge and the fatty acid is observed when the subphase is either pure water or a NaOH aqueous solution. A stoichiometric 1:1 reaction between both molecules takes place at the air-water interface. This reaction has an efficiency close to 100% at high surface pressures, provided the majority anion present in the subphase is OH-. However, when the majority anion is another one, this complex is hardly formed. From the experimental results, we conclude that the acid-base reaction is highly dependent on the protonation state of the proton sponge at the airliquid interface that is a function of the present counterion in the subphase. The floating films were also transferred onto solid substrates and characterized by means of IR spectroscopy, atomic force microscopy (AFM), and X-ray diffraction to investigate in more detail the complex formation. The interactions between the complex (when formed) and the excess component have been studied in terms of the subphase nature. It was found that the complex is immiscible with the proton sponge, yielding films made of different domains. Nevertheless, the complex is miscible with the fatty acid when the subphase used is an alkaline solution, presumably due to electrostatic interactions between the carboxylate group of the acid and the complex.

1. Introduction Proton sponges are organic diamines with unusually high basicity due to the presence of two basic nitrogen centers in the molecule, which have an orientation that allows the uptake of one proton to yield a stabilized intramolecular hydrogen bond [N‚‚‚H‚‚‚N]+.1,2 They have attracted considerable interest because of their very high proton affinity, low nucleophilicity, and slow protonation/deprotonation processes.3,4 Due to these characteristics, proton sponges have several potential applications which are the focus of current research. We can mention the role that proton sponges play in enzymatic catalysis,5 as well as the presence of [N‚‚‚H‚‚‚N]+ hydrogen bonds in important biological systems. They have also been studied as models of proton substraction,6 in the field of gene therapy,7 as organic base catalysts in green chemistry,8 and as effective H+ scavengers in nanocluster formation.9 Langmuir monolayers are molecular organized systems at the air-water interface which can be considered as two-dimensional models to study the effect of the subphase nature on chemical or biochemical reactions as well as intermolecular forces between the floating molecules and the ions present in the subphase. These monolayers can be transferred onto solid substrates using the Langmuir-Blodgett (LB) technique to obtain films with a controlled molecular density and thickness. In the context of this paper, studying fatty acid-strong base mixed layers, we could remark its interest in the fabrication of pyroelectric materials.10,11 * To whom correspondence should be addressed. E-mail: mcarmen@ unizar.es. Phone: + 34 976 76 11 96. Fax: + 34 976 76 12 02.

Figure 1. Molecular structure of diphenyl bis(octadecylamino)phosphonium bromide (DPOPBr).

This paper completes a previous study12 concerning the surface behavior at the air-water interface of mixed films containing diphenyl bis(octadecylamino)phosphonium bromide (DPOPBr), depicted in Figure 1), and behenic acid (BAH). Strong interactions between the two compounds occur in Langmuir monolayers spread onto pure water, yielding a stoichiometric 1:1 acid-base reaction involving a complex formation (DPOPH BA)+.12 However, such a reaction hardly takes place in conventional organic solvents. In this paper, we wish to provide deeper insight into the surface chemistry of these materials, investigating how the subphase nature determines the efficiency of this reaction, enhancing or inhibiting the complex formation. With this aim, we have prepared pure and mixed Langmuir and Langmuir-Blodgett films of DPOPBr and BAH on several aqueous solutions (NaOH, HCl, KCl, HNO3) in order to elucidate the influence of the pH and the nature of the ions present in the subphase on the (DPOPH BA)+ complex formation.

10.1021/jp065954b CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007

2846 J. Phys. Chem. B, Vol. 111, No. 11, 2007

Figure 2. Surface pressure vs area per molecule isotherms of DPOP spread onto water (pH 5.5), NaOH (pH 9), HCl (pH 3), HNO3 (pH 3), and KCl (pH 6) aqueous solutions.

2. Experimental Section The synthesis of diphenyl bis(octadecylamino)phophonium bromide was carried out by Prof. Palacios and Prof. Aparicio from the Organic Chemistry Department of the Basque Country University (Spain), with the synthesis protocol described elsewhere.13,14 Its purity (better than 99%) was checked by spectroscopic techniques. The behenic acid was obtained from Fluka (purity >99%) and was used without further purification. Solutions of DPOPBr, BAH, and their mixtures were prepared in chloroform (HPLC 99.9%) that was provided by Aldrich. The NaOH and HCl subphase solutions were made adding these compounds to Millipore Milli-Q water (resistivity 18.2 MΩ· cm) until the desired pH was obtained. The pH value was determined using a Crison pH meter. The reagents used were of the best grade commercially available. The films were prepared on a 210 × 460 mm2 homemade Teflon trough. The Langmuir films were fabricated spreading

Haro et al. 1.2 mL of the pure or mixed solution (10-4 M in all cases) by means of a Hamilton syringe onto the maximum available area of the trough. After waiting 15 min to allow the solvent to evaporate, the compression slowly began at a constant sweeping speed of 0.02 nm2/(molecule‚min). Each compression isotherm was recorded at least three times to ensure the reproducibility of the obtained results. All of the experiments were performed in a clean room whose temperature was kept constant at 20 ( 1 °C. The monolayer formation was also studied by Brewster angle microscopy (BAM) using a mini-BAM instrument from Nanofilm Technologie GmH. The LB films were fabricated by the vertical dipping method with a lifting speed of 6 mm/min. The cleaning procedure of the substrates (CaF2, mica, and glass used for the IR, atomic force microscopy (AFM), and X-ray diffraction studies, respectively) has been published elsewhere.15,16 The IR transmission spectra were recorded with a Jasco 410 Fourier transform infrared (FTIR) spectrometer using a normal incident angle with respect to the film plane. The AFM investigations were performed by means of a multimode extended microscope with Nanoscope IIIA electronics from Digital Instruments, using the tapping mode. The data were collected with a silicon cantilever provided by nanoworld, with a force constant of 42 mN/m, and operating at a resonant frequency of 285 kHz. The images were collected with a scan rate of 1 Hz and under ambient air conditions. The interlayer spacing was obtained by a Philips PW 1730 X-ray diffractometer. 3. Results and Discussion 3.1. Langmuir Films. Pure Compounds. Surface pressurearea (π-A) isotherms of pure Langmuir films containing BAH

Figure 3. mini-BAM photographs of a pure DPOP monolayer spread onto NaOH (pH 9) and HCl (pH 3) aqueous subphases as well as onto a pure water subphase at several surface pressures.

Mixed Langmuir and Langmuir-Blodgett Films or DPOPBr have been obtained on NaOH (pH 9), HCl (pH 3), HNO3 (pH 3), and KCl (10-3 M, pH 6) aqueous subphases. The π-A isotherms of BAH monolayers on the studied subphases are consistent with those previously reported in the literature.17-19 Figure 2 shows the π-A isotherms of DPOP monolayers prepared on several subphases (DPOPBr molecules at the air-aqueous solution interface give a positively charged DPOP monolayer; meanwhile, the Br- ions go into the subphase).20 It is remarkable that when the majority subphase counterion is OH- (H2O and NaOH aqueous subphases), the π-A isotherms show an overshoot followed by a plateau, attributed to bilayer formation.20 Nevertheless, neither the overshoot nor the plateau are observed when the counterions are others such as Cl- (HCl and KCl aqueous subphases) or NO3- (HNO3 aqueous subphase). When the subphase is a HNO3 aqueous solution, the isotherm takes off at a lower area per molecule, and it is more expanded at high surface pressures compared to those recorded when the subphase contains Cl-. These results reveal that not only the pH of the subphase but also the nature of the counterion has a remarkable influence on the behavior of the DPOP monolayer. We have also recorded BAM images of DPOP monolayers at different stages of the compression process to complement the information provided at the macroscopic level by π-A isotherms. Figure 3 shows some representative images of the monolayers prepared on NaOH and HCl aqueous subphases at 0 and 20 mN/m and just before the collapse surface pressure. The images of the monolayer fabricated onto a pure water subphase are also shown for comparison purposes. The most relevant result is the different growth of the monolayers. In water, the monolayer grows in the characteristic fatty amine islands,21 whereas, in a NaOH aqueous subphase, no domains are observable within the mini-BAM resolution. When the subphase is a HCl aqueous solution, parallel stripes of different brightness are observed which might be due to (i) the monolayer growth in a certain orientation where the stripes are domains of different molecular tilts22 or (ii) the formation of a liquid crystalline phase.23 Mixed Monolayers. Figure 4 shows the π-A isotherms of pure and some representative mixed monolayers of BAH and DPOP of different molar fractions on several subphases: NaOH, HCl, HNO3, and KCl aqueous solutions. The π-A isotherms of the mixed monolayers at the airNaOH aqueous solution interface (Figure 4a) show an overshoot followed by a plateau region when the composition is xBAH < 0.5. The length of the plateau decreases gradually until it disappears as the proportion of BAH in the mixture increases. A similar behavior was observed when the subphase was pure water.12 On the other hand, the profiles of the π-A isotherms of the mixed films fabricated onto HCl (Figure 4b), HNO3 (Figure 4c), and KCl (Figure 4d) aqueous subphases are quite similar to each other, and very different to those recorded on subphases where the majority counterion is OH-. The “additivity rule” gives the expected area per molecule for ideal mixing.24 Thus, deviations from the additivity rule are indicative of attractive or repulsive interactions between both components, and can be related to partial miscibility processes.25,26 In Figure 5, the curves of area per molecule versus molar fraction of BAH at surface pressures of 5, 15, and 30 mN/m (25 mN/m in the NaOH aqueous subphase) are shown. At this point, it is worth noting the following: (i) There is a large negative deviation of the area per molecule with respect to the ideal behavior when the counterion present

J. Phys. Chem. B, Vol. 111, No. 11, 2007 2847

Figure 4. Surface pressure vs area per molecule isotherms of BAH and DPOP as well as their mixed film isotherms at the indicated molar fractions referred to BAH, xBAH, spread onto (a) NaOH (pH 9), (b) HCl (pH 3), (c) HNO3 (pH 3), and (d) KCl (pH 6) aqueous solution subphases.

is OH-. This result is indicative of significant attractive interactions between BAH and DPOP. Conversely, in all of the studied cases where OH- is not the majority counterion in the subphase, the deviations from the additivity rule are small or even negligible, indicating that somehow the interaction between acid and base moieties is inhibited. (ii) Both the majority counterion and the pH of the subphase determine the behavior of the mixed monolayers (i.e., the mixed monolayers do not behave exactly in the same way on water as on the NaOH aqueous subphase, neither do the mixed monolayers fabricated onto HCl and HNO3 aqueous subphases). In Figure 6, the area per group (DPOPBr + nBAH) versus n (molecules of BAH per DPOPBr molecule) is shown (calculated at 25 and 30 mN/m in NaOH and HCl aqueous solutions, respectively). In the mixed monolayers with n e 1 fabricated


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Figure 5. Average area per molecule vs behenic acid molar fraction at several surface pressures for the monolayers spread onto (a) NaOH (pH 9), (b) HCl (pH 3), (c) HNO3 (pH 3), and (d) KCl (pH 6) aqueous solution subphases. The area for the ideal mixtures mixing is indicated by dashed lines.

onto the NaOH aqueous subphase, the area per group is practically equal to the area of one molecule of DPOPBr. This means that if we take a pure DPOP monolayer and we gradually add BAH, the insertion of BAH has no net effect on the area per group until n ) 1. When n > 1, there is an almost linear increase of the area per group when n increases, with the slope of the line being approximately equal to the value of the area per molecule of a BAH molecule at the indicated surface pressure (0.21 nm2). An identical result was previously reported for mixed monolayers spread onto pure water,12 where the fact that the area per group does not increase until n > 1 was interpreted in terms of an acid-base reaction between the BAH and the DPOP at the air-water interface, with this reaction being more efficient the higher the surface pressure. Such a reaction yields a complex formation where the acid moiety is situated on top of the DPOP one. This explains why the progressive addition of BAH hardly induces any change on the area per group until the 1:1 stoichiometry in the film mixture is reached. We believe that the same explanation could be applied to mixed films spread onto a basic subphase. More data to support this hypothesis will be presented throughout this paper. On the other hand, when the subphase is the HCl aqueous solution, the area per group hardly increases when BAH is gradually added from n ) 0 to n ) 0.25, and when n > 0.25, there is an almost linear

increase of the area per group, with a slope approximately equal to 0.21. According to the above explanation, the acid-base reaction is rather limited on the HCl aqueous subphase. The excess Gibbs energy of mixing, ∆GEm, provides information about intermolecular interactions and can be calculated by means of the equation developed by Goodrich27 and Pagano and Gershfeld:28

∆GEm )

∫0πA dπ - x1 ∫0π A1 dπ - x2 ∫0πA2 dπ

where A is the experimental area per molecule in the mixed film at a given surface pressure; A1 and A2 are the area per molecule at that surface pressure for the pure films, and x1 and x2 are the mole fractions. The excess Gibbs energies of mixing versus BAH molar fraction at different surface pressures are represented in Figure 7. The large negative values of ∆GEm for mixed films spread onto a NaOH aqueous subphase are indicative of strong attractive interactions between the two materials. However, ∆GEm values are considerably less negative when the subphase is a HCl aqueous solution, again pointing out a weaker interaction between BAH and DPOP on an acid subphase, and even a certain repulsion at xBAH ) 0.75, where the ∆GEm values are slightly positive.

Mixed Langmuir and Langmuir-Blodgett Films

Figure 6. Area per group (DPOPBr + nBAH) vs n (number of BAH molecules per DPOPBr molecule) at 25 and 30 mN/m in NaOH (pH 9) and HCl (pH 3) aqueous solutions, respectively.

Figure 7. Excess Gibbs energy of mixing vs behenic acid molar fraction, xBAH, at several surface pressures of the monolayers spread onto (a) NaOH (pH 9) and (b) HCl (pH 3) aqueous solution subphases.

The foregoing data made us propose several models for the pure and mixed monolayers depending on the subphase nature. These models focus on interactions related to the hydrophilic groups. Although not indicated in the models, alkyl tail organization probably changes when BA-DPOP molecules

J. Phys. Chem. B, Vol. 111, No. 11, 2007 2849 associate. Figure 8 summarizes these models that we tried to bear out by means of further experiments (IR, AFM, and X-ray diffraction) that, as we will see shortly, have effectively corroborated these models. Thus, we propose a complex formation with efficiency close to 100% in H2O and NaOH aqueous subphases; meanwhile, such a complex is not formed on the other studied subphases (only the HCl subphase has been represented in Figure 8 for shortening purposes). As indicated in Figure 8, the proton sponge here studied is protonated at the air-liquid interface in acid or neutral subphases provided the majority counterion is not the OH-. If OH- is the majority anion, OH- anions prevent the protons from reaching the interface where the DPOP cations are. The characteristic overshoot and plateau in the π-A isotherms recorded in the water and NaOH aqueous subphase, assigned to bilayer formation,20 also support this hypothesis. However, such a plateau does not exist when the counterions are Cl- or NO3-. It has been shown that ionization prevents the formation of bilayers due to Coulombic forces between the ionized monolayer and subphase counterions.29 If the proton sponge is protonated, a DPOPH2+ monolayer is formed at the air-water interface, and then, strong Coulombic forces with the subphase anions hinder the formation of a bilayer; consequently, neither the overshoot nor the plateau is observed. However, the subphase nature not only affects the DPOP moieties but also affects the BAH ones. Thus, for subphases with pH e 6 (i.e., pure water, HCl, KCl, and HNO3), the BAH is expected to be almost totally protonated,30 but when the BAH is spread onto a NaOH aqueous solution, it is unprotonated (i.e., in the carboxylic form, BA-) according to previous studies.31 BAM experiments were also performed to gain some insight into the homogeneity of the monolayers. In Figure 9, some representative images are presented. On NaOH aqueous solution, the images obtained for the mixed film of xBAH ) 0.25 are similar to those of the pure DPOP monolayer, whereas at higher BAH ratio the films are more homogeneous. On the other hand, the images obtained for the mixed films at the three molar ratios studied on the HCl aqueous solution subphase are similar to each other. 3.2. Langmuir-Blodgett Films. Pure and mixed LB films containing BAH and DPOP moieties have been transferred onto CaF2, mica, and glass and studied by IR spectroscopy, AFM, and X-ray diffraction techniques, respectively. The films prepared on the HCl aqueous solution were transferred at 30 mN/m. Nevertheless, the pure DPOP monolayer at the airNaOH aqueous solution interface reaches the overshoot before this surface pressure, and this is the reason why the surface pressure of transference in the alkaline subphase was 25 mN/ m. The deposition modes and the transfer ratios for the studied subphases are gathered in Table 1. IR Spectroscopy. Figure 10 shows the IR spectra of the mixed LB films (of 29 layers) at several molar ratios, and Table 2 shows the assignment of the main bands. The spectra of pure DPOP LB films are also plotted in the same figure to remark differences in the vibrations assigned to amine groups. The IR spectra of pure BAH films are in good agreement with those previously reported in the literature.32-34 Differences in the spectra of pure DPOP LB films obtained on HCl and NaOH aqueous subphases are relevant: LB film obtained on NaOH solution shows two bands at 3337 and 3205 cm-1, attributed to the N-H stretching mode in the “free” (nonhydrogen-bonded) and hydrogen-bonded secondary amine group.35,36 Besides, a band centered at 1614 cm-1 corresponding to the free N-H scissoring mode is observed.37 These bands


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Figure 8. Schematic representation of the pure DPOP and mixed Langmuir films on water (pH 5.5), NaOH (pH 9), and HCl (pH 3) aqueous subphases.

Figure 9. mini-BAM photographs of the mixed monolayers fabricated onto NaOH (pH 9) and HCl (pH 3) aqueous subphases at 25 mN/m.

are also present in pure DPOP LB films obtained on pure water, while they do not appear in pure LB films obtained on HCl aqueous solution, where a weak broad band in the 3588-3180 cm-1 region is observed. It has been reported before38 that IR bands of intramolecular hydrogen bonds are sometimes extremely broad, and the stronger the intramolecular hydrogen bond, the lower the intensity. This seems to confirm our previous hypothesis: DPOP should be unprotonated on aqueous solutions

where the majority counterion is OH-, while it is protonated on the other subphases (the proton sponge sequesters a proton from the subphase and forms an intramolecular [N‚‚‚H‚‚‚N] hydrogen bond). The CH2 scissoring band at 1468 cm-1, sensitive to intermolecular interactions,39 is observable in all LB films fabricated on both NaOH and HCl aqueous subphases. The single peak observed provides an indication of a triclinic package with one


J. Phys. Chem. B, Vol. 111, No. 11, 2007 2851 TABLE 2: Infrared Bands in Wavenumber (cm-1) and Assignments

TABLE 1: Transference Type, Transference Ratio, and Interlayer Distance for Pure and Mixed LB Films Containing BAH and DPOP xBAH ) xBAH ) xBAH ) 0.25 0.5 0.75 BAH DPOP NaOH Aqueous Solution transference type Y Y Y transference ratio 1 1 1 interlayer distancea (nm) 3.1 6.0 5.9

Y 0.8 5.8

Y 1 5.6

HCl Aqueous Solution transference type Z Y Y transference ratio 0.6 0.7 0.9 interlayer distancea (nm) 6.4 6.3

Y 0.6 6.2

Y 1 5.6

a

The error estimation in the interlayer distance is (0.1 nm.

Figure 10. FTIR spectra of mixed LB films (xBAH ) 0.25, 0.5, and 0.75) and pure DPOP films transferred from (a) NaOH (pH 9) and (b) HCl (pH 3) aqueous subphases.

molecule per unit cell, or a hexagonal one.40 The band that appears at 1437 cm-1 can be attributed to the P-C stretching mode.41 With respect to mixed films spectra, the most relevant fact is that the CdO stretching vibration of the behenic acid appears at 1560 cm-1 when the subphase used was NaOH aqueous solution; meanwhile, in the acid subphase, the dominant band is around 1720 cm-1. The CdO stretching vibration indicates the grade of ionization of the fatty acid: the peak is situated around 1720 cm-1 in a carboxylic group (COOH), whereas it

wavenumber (cm-1) NaOH aqueous subphase 3337 3205 2956 2917 2849 1614 1560 1468 1438

HCl aqueous subphase

2956 2917 2849 1720 1468 1438

assignment ν(NsH) in free NsH group ν(NsH) hydrogen-bonded NsH group ν(CsH) in CH3 group νA(CsH) in CH2 group νS(CsH) in CH2 group ν(CdO) in COOH group δ(N-H) in free NsH group νA(CdO) in COO- group δ(CsH) in CH2 group ν(PsC)

appears at 1560 cm-1 (carbonyl (COO-) antisymmetric stretching vibration) when the carboxylic group is ionized.42,43 Therefore, the band in the region of 1560 cm-1 that increases with BAH/DPOP ratio, can be attributed to the carboxilate group (COO-) present in the complex (DPOPH BA)+ when the subphase used was a NaOH aqueous solution. Bands in the COOH region are always present in mixed LB films obtained on HCl aqueous solution, even at small mole fractions of the carboxylic acid. The IR spectra also give important information about conformational order and packing of the alkyl chains. The hydrocarbon tails in the LB films here reported present bands at 2917 and 2849 cm-1 indicative of well ordered alkyl chains with a trans zigzag conformation.44 AFM Microscopy. We have used the AFM technique to investigate in more detail the morphology of the films and the miscibility or phase separation of the components in the mixed films. Figure 11 shows the typical top view and line profiles for one layer of DPOP deposited on mica from different subphases. The DPOP film image when the subphase was a NaOH aqueous solution (Figure 11a) shows regions of bare mica and others with the transferred film. The height of the transferred film is 3.1 nm, and it is surrounded by thinner regions with a height of 1.7 nm. When the film is transferred from a HCl aqueous subphase (Figure 11b), a honeycomb is observed where each hexagon has a diameter of approximately 85 nm. Such a type of hexagonal structure has been observed before for a monolayer of gramicidin A (gA),45 and could be explained in terms of a DPOP hexametric aggregation that is arranged into another bigger hexameric base aggregation, and this operation is repeated until the observed hexagon ratio width, that is, 85 nm, is reached. The film transferred from a water subphase (Figure 11c) shows a hybrid aspect in between the film transferred from a NaOH aqueous subphase and the HCl one. Some domains 3.1 nm height with respect to the bare mica and some hexagons can be observed, but the honeycomb is not well-defined. To investigate the miscibility or possible phase separation of the components in the mixed LB films, we have applied bearing and section analysis (digital instruments software). Representative pictures of one-layer LB films are shown in Figure 12. The image of the mixed film with xBAH ) 0.33, transferred from NaOH aqueous solution (Figure 12a), shows two kinds of domains, whose height difference is around 1.7 nm. The area percentage of the higher domain is approximately 50%. The AFM image of the mixed film of xBAH ) 0.5 transferred from a NaOH aqueous solution (Figure 12b) shows two regions. However, there is not a significant height difference between them, as can be seen in the section analysis. Therefore, these domains are probably owing to regions with a different


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Figure 11. AFM of a one-layer LB film of DPOP transferred from (a) the NaOH aqueous subphase (pH 9), (b) the HCl aqueous subphase (pH 3), and (c) water (pH 5.5). The images on the left represent the top view, and those on the right, the line profiles.

tilt or orientation of the complex. The film of composition xBAH ) 0.66 (Figure 12c) is quite homogeneous without clear phase separation, and differs from those obtained on water and HCl aqueous subphases. The obtained results may be interpreted in terms of the miscibility of the complex and the excess component. Thus, for mixed films of composition xBAH e 0.5,

two phases are observed, whereas, for mixed monolayers with xBAH > 0.5, only one phase appears in the AFM images. If xBAH > 0.5, the excess compound is BAH that is mainly in carboxylate form on NaOH aqueous solution. Thus, it may interact with the positive charged complex, (DPOPH BA)+ BA-, and no separation of phases is observed.


J. Phys. Chem. B, Vol. 111, No. 11, 2007 2853

Figure 12. AFM of a one-layer mixed LB film transferred from a NaOH aqueous subphase (pH 9) with a composition of (a) xBAH ) 0.33, (b) xBAH ) 0.5, and (c) xBAH ) 0.66 and transferred from a HCl aqueous subphase (pH 3) with a composition of (d) xBAH ) 0.25. The images on the left represent the top view, and those on the right, the line profiles.


Haro et al. transferred to the LB films,46 so we suggest that this larger distance may be caused by the transference of Cl- ions from the subphase to the LB film. 4. Conclusions

Figure 13. Diffractograms of xBAH ) 0.5 mixed films transferred from (a) NaOH (pH 9) and (b) HCl (pH 3) aqueous subphases.

Figure 14. Schematic representation of the LB film architecture of DPOP transferred from a NaOH aqueous subphase (pH 9) according to the AFM and X-ray measurements.

In the films transferred from a HCl aqueous solution (Figure 12d), a grain structure where some mountains of 1.5 nm stand out is observed. The area of the mountains increases with the BAH proportion until a ratio of 1:1 BA-DPOP is reached, and it is drastically reduced with further addition of BAH. X-ray Diffraction. The interlayer distance has been determined by an X-ray study of 100-layer LB films (Table 1), with the exception of pure DPOP films fabricated onto a HCl aqueous subphase that yield diffractograms with nearly imperceptible Bragg’s peaks. Figure 13 shows two representative diffractograms of the mixed films (xBAH ) 0.5) fabricated onto NaOH and HCl aqueous subphases. The DPOP film distance between neighboring layers is 3.1 nm for films transferred from the NaOH aqueous subphase. Such an interlayer distance agrees with a Y-type film. From the AFM and X-ray results, we propose in Figure 14 a structural model for the DPOP LB film transferred from the alkaline subphase. In the mixed films, the interlayer distance agrees with the heights obtained by AFM. The interlayer distance is 0.3 and 0.4 nm higher when the subphase is a HCl aqueous solution than for with respect to those observed for water and NaOH. It is well-known that the present ions in the subphase can be

The efficiency of a complex formation between a proton sponge and a fatty acid onto different subphases has been investigated. Pure BAH, DPOP, and mixed BAH-DPOP monolayers were prepared on aqueous solutions containing several ions and different pHs. The π-A isotherms and the area per molecule versus xBAH graphs are indicative of strong attractive interactions between both components when the majority counterion in the subphase is OH-, whereas when the majority counterion is another one, the interaction between BAH and DPOPBr molecules is weak. The area per group (DPOPBr + nBA) and the ∆GEm versus xBAH graphs suggest the formation of a stoichiometric 1:1 BAH-DPOPBr complex, previously observed on pure water. However, the complex is hardly formed in subphases where the majority counterion is different than OH-. The IR spectra corroborate the complex formation in the LB films fabricated onto a NaOH aqueous subphase. The BAM and AFM images show the different grades of homogeneity of the Langmuir and LB films, respectively. The AFM pictures are indicative of the immiscibility between the complex and the excess DPOP molecules as well as the miscibility with the excess unprotonated BA moieties, probably due to electrostatic interactions between the complex (positively charged) and the carboxylate group of the behenic acid. When the anion is OH-, DPOP is not protonated and can react with BAH at the air-water interface, giving the complex. However, when the counterion is another one, the proton sponge is protonated. The pH of the subphase also determines the interactions between the complex and the excess of BAH molecules. Thus, on an alkaline subphase, the behenic acid is unprotonated and electrostatic interactions could exist between the complex and the BAH excess. Besides, the pH subphase seems to determine the deposition type of the DPOP molecules as AFM and X-ray diffraction show. Acknowledgment. DPOPBr was kindly provided by Dr. Palacios and Dr. Aparicio from the Universidad del Paı´s Vasco. The authors appreciate very much the efforts of both Jordi Diaz and Ismael Diez in the AFM measurements. The authors wish to acknowledge the financial assistance from project BQU 200301765 of Ministerio de Educacioń y Ciencia. M.H. gratefully acknowledges her F.P.U. fellowships from Ministerio de Educacioń y Ciencia as well as her grant from CAI-Gobierno de Aragoń for a research stay at Manchester University. She also acknowledges both Prof. Richard Tredgold and Dr. Marıá Bardosova for their help at Manchester University. References and Notes (1) Alder, R. W. Chem. ReV. 1989, 89, 1215-1223. (2) Llamas-Saiz, A. L.; Foces-Foces, C.; Elguero, J. J. Mol. Struct. 1994, 328, 297. (3) Ash, J. M.; Sudmeier, J. L.; Fabo, E. C. D.; Bachovchin, W. W. Science 1997, 278, 1128. (4) Raab, V.; Kipke, J.; Gshwind, R. M.; Sundermeyer, J. Chem.s Eur. J. 2002, 8, 1682-1693. (5) Shan, S. O.; Loh, S.; Hershchlag, D. Science 1996, 272, 97. (6) Wille, A. E.; Su, K.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 1996, 118, 6407. (7) Kim, T. H.; Ihm, J. E.; Nah, J. W.; Cho, C. S. J. Controlled Release 2003, 93, 398-402. (8) Corma, A.; Iborra, S.; Rodrı´guez, I.; Sańchez, F. J. Catal. 2002, 211, 208-215. (9) O ¨ zkar, S.; Finke, R. G. Langmuir 2003, 19, 6247.

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Mixed Langmuir and Langmuir−Blodgett Films of a Proton Sponge

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