2018 Volume 59 Issue 7 Pages 1087-1090
Interaction of carboxylic acid headgroups of arachidic acid (AA) monolayers with trivalent ions Fe3+ and La3+ is studied by using vibrational sum-frequency generation (VSFG) spectroscopy. Comparing SFG spectra of AA Langmuir monolayers on 10−3 M LaCl3 solutions and 10−3 M FeCl3 solutions with various pH values, we show that, the cation La3+ binds to carboxylic groups by Coulombic interaction as the AA monolayers deprotonate at high pH (pH ≥ 7). On the other hand, insoluble Fe(OH)3 complexes in the FeCl3 solution bind to the carboxylic headgroups of AA molecules enhances the ordering structure of AA monolayers as well as that of the OH network at the interfacial layers.
Ions in intracellular/extracellular fluids are essential for living organisms, they can diffuse across the cell membrane and can govern important cell functions. For example, sodium and potassium ions can regulate muscle contraction and determine the transmission of nerve impulses in the brain as well as from the brain to other parts of the body.1) Langmuir monolayers are widely used as a two-dimensional model to study the interaction mechanism of ions on biological membranes.2,3)
Since being developed in the early 1980s, sum-frequency generation (SFG) spectroscopy has been extensively used as a highly surface-specific technique for studies of surfaces and interfaces.4,5) As a second-order nonlinear process, SFG is forbidden in centrosymmetric media under the dipole approximation, so that the SFG spectroscopy techniques are selectively sensitive to structures at surfaces and interfaces where the inversion symmetry is broken.6,7)
To make predictions about surface phenomena related to biological and atmospheric aerosol interfaces, an in-depth understanding of the binding of divalent and trivalent cations to the carboxylic headgroups of Langmuir monolayers is necessary. Recently, Cheng Y. Tang et al. have investigated effects of alkali ions (Na+ and K+) and divalent cations (Mg2+ and Ca2+) binding to the COOH headgroup of a palmitic acid monolayer by using SFG spectroscopy.8,9) Binding type determination of trivalent cations such as Fe3+ and La3+ is more complicated than monovalent and divalent cations because those trivalent cations can exist in water as ions or complexes. Wenjie Wang et al.10) have used a surface-sensitive X-ray scattering technique to investigate ionic specificity of Fe3+ and La3+ bound to carboxylic groups of AA monolayers on FeCl3 and LaCl3 solutions. Their results showed that La3+ cations bound to AA monolayers by Coulombic interaction, whereas the Fe3+ cations which have the same charge with La3+ formed covalent bonds by iron complexes of Fe(OH)3 and/or Fe(OH)2+. However, the iron complexes binding to AA monolayers have not been studied in detail because X-ray scattering is not able to probe the subphase water structure. Besides, iron complexes in magnetic bacteria have also drawn attention and been investigated using FMR (Ferromagnetic Resonance) measurements.11)
In this study, we use vibrational sum-frequency spectroscopy to investigate the interaction of trivalent ions Fe3+ and La3+ with AA Langmuir monolayers on the surfaces of 10−3 M FeCl3 and 10−3 M LaCl3 solutions. The interfacial surface charge and the composition of Fe in the solutions are controlled by varying pH levels. We investigate VSFG spectra in the CH stretch range (2800–3000 cm−1) and the OH stretch range (3000–3600 cm−1). The result shows different interactions of La3+, Fe3+, and iron complexes with the carboxylic groups of AA monolayers at the interface.
Previous studies10,12,13) on AA monolayers on water often used a molecular area value of ∼20 Å2. In this paper, we will discuss why using a molecular area of 22.6 Å2 leads to more sensitive effects on the AA monolayer, which help distinguish the roles of iron complexes Fe(OH)2+ and Fe(OH)3 that the previous studies did not point out.
Arachidic acid (CH3(CH2)18COOH) in the solid state (purity > 99%, Sigma–Aldrich) was dissolved in chloroform solvent (purity > 99%, Sigma–Aldrich) to make 10−3 M AA solution. Iron(III) chloride and lanthanum(III) chloride salt solutions with the same concentration of 10−3 M were prepared by dissolving appropriate amounts of FeCl3 (purity > 99%, Merck) and LaCl3 (purity > 99%, Merck) in ultrapure water (Millipore, Milli-Q, resistivity ∼ 18 MΩ cm). The pH values of these salt solutions, just after preparation, were 2.86 for the FeCl3 solution and 7.0 for the LaCl3 solution. We then changed those pH values by adding solutions of HCl or NaOH (purity > 99%, Merck). A Langmuir monolayer was formed within 10 minutes after spreading the AA solution on the water surface of the salt solution.
2.2 Optical setupSFG spectra were conducted on a SFG spectrometer (EKSPLA-SF41) established by EKSPLA Lithuania company. The optical layout of this spectrometer is depicted in Fig. 1. In this system, a mode-locked Nd:YAG pico-second laser (PL2251A) was used as a pump source which has energy of 50 mJ/pulse, pulse width of 30 ps, and repetition rate of 50 Hz at the fundamental wavelength of 1064.2 nm. This fundamental beam is directed into a second-harmonic unit (H500). The second harmonic at 532.1 nm and fundamental beam at 1064.2 nm from H500 were used to pump an OPG/OPA/DFG system (EKSPLA–PG501) and obtained tunable waves covering a mid-infrared range from 2,3–10 µm. This IR beam and the VIS beam at 532.1 nm were guided to the samples with angles at φVIS = 60° and φIR = 55° respectively. Once the two incident waves satisfied the phase-matching condition, the sum-frequency (SF) wave was generated in the reflection direction with an angle of 59,7° ± 0,35°. The SF wave was selected by a monochromator (MS3504) and then detected by photomultiplier tubes. All the spectra were taken in SSP polarization combination (referring to SF output, VIS input, and IR input, respectively).
Optical setup for vibrational sum-frequency generation spectroscopy.
SFG spectra from AA monolayers on neat water with surface areas per molecule of 20.4 and 22.6 (Å2/molecule) are shown in Fig. 2. There are two prominent peaks at 2880 cm−1 and 2945 cm−1 corresponding to the symmetric stretch mode (CH3SS) and the CH3 Fermi resonance (CH3Fr) of the methyl group.14) A weakly observed peak at ∼2850 cm−1 from the symmetric stretch mode of the methylene group (CH2SS) indicate that alkyl chains in the monolayer organize in the all-trans conformation.8,14) These observations indicate that AA monolayers have been well formed in a liquid-condensed (LC) phase on the water surface at surface areas of 20.4 and 22.6 (Å2/molecule). This result is consistent with pressure-area (π–A) isotherm measurements for AA monolayers on water.15)
SFG spectra from AA monolayers on neat water with different molecular areas (a) in the CH range, and (b) in the OH stretch range.
Figure 2(b) shows the hydrogen-bonding OH stretch range of water molecules at the interface broadening from 3000 to 3600 cm−1. In this range, the peak at ∼3450 cm−1 is assigned to OH groups hydrogen-bonded in a disorder “liquid-like” structure. The peak at ∼3200 cm−1 is assigned to OH groups hydrogen-bonded in a well-ordering “ice-like” structure.16)
Phase transition studies of Langmuir monolayers showed that if compressing the surface area below ∼20 Å2/molecule, the monolayers will be collapsed.15,17,18) In this situation, we use a surface area value of 22.6 Å2/molecule that is slightly larger than that collapse value to clarify the effect of the trivalent ions on the monolayer structure. In Fig. 2, intensities of the CH3SS, CH3Fr peaks, and the “ice-like” range for A = 22.6 Å2/molecule are significantly below than those for A = 20.4 Å2/molecule. This indicates that the molecules in the AA monolayer with the molecular area of 22.6 Å2 are loosely ordered than those of 20.4 Å2. Therefore, we are going to use the molecular area of 22.6 Å2 to observe a more sensitive effect of the trivalent ions on the interfacial structure in SFG spectra.
SFG spectra from AA monolayers with the molecular area of 22.6 Å2 on various subphases are shown in Fig. 3. In these spectra, the peak intensities of CH3SS (in Fig. 3(a)) and the OH “ice-like” range (in Fig. 3(b)) are enhanced in the presence of the 10−3 M FeCl3 in the water subphase, which is in contrast to the case of the 10−3 M LaCl3 solution. These observations indicate that the AA monolayer and the interfacial water structure are well-ordered in the presence of FeCl3, whereas LaCl3 disturbs the monolayer and the interfacial water structures. For the 10−3 M LaCl3 solution having pH value of 7.0, taking the pKa value of 5.1 for AA,10) about 2% of the carboxylic headgroups deprotonate as calculated by using the Gouy–Chapman theory.14) This deprotonation produces a negatively charged surface at the interface that attracts La3+ ions leading to a charge-neutral surface. In this case, there is no resulting surface field to help the surface water molecules form an ordered hydrogen-bonding network at the interfacial water. Thus, the interaction between La3+ ions and the AA monolayer is a Coulombic interaction. For the pH = 2.86 of the 10−3 M FeCl3 solution, with the pKa = 5.1 of the AA, the headgroups of the AA monolayer are mostly neutral,10,12,14) so unlike for the La3+ ions, there is no Coulombic binding for the Fe3+ ions at the interfacial layers. Woongmo Sung et al.13) have performed a SFG study on these systems with the surface area of the AA monolayer maintained at ∼20 Å2/molecule. As this surface area value is nearly the collapsed limit of the liquid-condensed (LC) phase, they observed a relatively weak SF intensity from the monolayer in the presence of 1 mM LaCl3 in the subphase water. They attributed this observation to the spontaneously collapsed bilayer due to the presence of La3+ cations in water.
SFG spectra from AA monolayers on various subphases (a) in the CH range, and (b) in the OH stretch range.
SFG spectrum from the AA monolayer on the 3.10−3 M HCl solution (Fig. 4) shows that the intensities of CH3SS peak and the OH range are both lower than those from the AA monolayer on the 10−3 M FeCl3 solution. The Cl− ion concentration in both solutions are identical. With a pH of 2.52 that is well below the pKa value of 5.1 for AA, the carboxylic groups of the AA monolayer on the 3.10−3 M HCl solution are also almost neutral. This comparison indicates that the Cl− ions in the solutions did not enhance the ordering structure at the interface.
SFG spectra from AA monolayers on solutions with the same Cl− concentration (a) in the CH range, and (b) in the OH stretch range.
Calculations using the ion equilibrium equation and tabulated solubility constants10) give that at the pH value of 2.86, in the 10−3 M FeCl3 solution, there are > 90% insoluble Fe(OH)3 and < 10% molecular ions Fe(OH)2+. Under this condition, Fe(OH)3 and Fe(OH)2+ complexes can bind with the carboxylic groups of the AA monolayer to form an OH ordering network at the interface.
To clarify the role of each iron complex species, we reduced the pH of the 10−3 M FeCl3 solution to 1.71. At this value, there are about 75% of ions Fe3+, 25% of molecular ions Fe(OH)2+, and practically zero Fe(OH)3, as calculated by the equilibrium equation.10) The reducing of peak intensities of the CH3SS and the “ice-like” OH range in this case in comparison to the SFG spectrum from the monolayer on the pH = 2.86 solution (Fig. 5(a)) indicates a less ordering of the OH network at the interface. Additionally, in Fig. 5(b), the SFG spectrum from the AA monolayer on the 10−3 M FeCl3, pH = 1.71, is almost identical to that from the AA monolayer on the neat water with the same pH value. These observations suggest that the Fe3+ and Fe(OH)2+ ions do not bind to the carboxylic groups of the AA monolayers. Studies using X-ray scattering have suggested a possibility of iron complexes Fe(OH)2+ and Fe(OH)3 to form covalent bonds with the carboxylic groups.10,12) However, SFG spectra in this study show that only the Fe(OH)3 complex involves in binding with the carboxylic groups of the AA monolayer at the interface. We have also noticed that Woongmo Sung et al. have not addressed the effect of Fe(OH)2+ on the monolayer structure in their study.13)
SFG spectra from AA monolayers on (a) FeCl3 solutions with different pH values and (b) on neat water with the same pH value.
In summary, using sum-frequency spectroscopy, we investigate the interactions of trivalent ions Fe3+ and La3+ with AA monolayers on the surfaces of FeCl3 and LaCl3 solutions. These trivalent ions lead to different interfacial structures depending on the interaction. The cation La3+ binds to the negatively-charged interface of the AA monolayer by Coulombic interaction. Iron exists in the solution in three forms, soluble ions Fe3+, molecular ions Fe(OH)2+, and insoluble Fe(OH)3, but only Fe(OH)3 complexes bind to the carboxylic groups of the AA monolayer reinforcing the ordering structure at the interface. The vibrational SFG spectroscopy technique can be extended to the COO− vibrational range (∼1400 cm−1) to investigate more evidence of the interaction between irons and carboxylic groups at the interface.
We would like to thank Center for Materials Science of Faculty of Physics in VNU University of Science for supporting apparatuses and chemicals for this study.
