Effect of Functional Group on the Monolayer Structures of

Oct 24, 2013 - The flat surface of a hemicylindrical CaF2 prism (UV grade, d = 25 mm, l = 25 mm) was used as the substrate for the SFG measurements...
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Effect of Functional Group on the Monolayer Structures of Biodegradable Quaternary Ammonium Surfactants Aimin Ge,† Qiling Peng,† HengLiang Wu,† Huijin Liu,† Yujin Tong,† Takuma Nishida,† Naoya Yoshida,‡ Keigo Suzuki,§ Takaya Sakai,∥ Masatoshi Osawa,† and Shen Ye*,† †

Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, Tokyo 192-0015, Japan § R&D - Fabric & Home Care Research - Household Products Research, Kao Corporation, 1334, Minato Wakayama-shi, Wakayama 640-8580, Japan ∥ R&D - Eco-Innovation Research, Kao Corporation, 1334, Minato Wakayama-shi, Wakayama 640-8580, Japan ‡

S Supporting Information *

ABSTRACT: The monolayer structures and conformational ordering of cationic surfactants including the biodegradable quaternary ammonium molecules have been systematically characterized by π−A isotherm, surface potential, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and sum frequency generation (SFG) vibrational spectroscopy. It was found that the monolayer of the typical dialkyl dimethylammonium on the water surface was less densely packed along with many conformational gauche defects. The packing density and ordering of these monolayers were improved as halide ions were added to the subphase. A similar condensation effect was also observed when amide or ester groups are present in the alkyl tails of the surfactant. These results are discussed on the basis of the repulsive electrostatic interactions between the terminal ammonium moieties, the hydrogen bonding between the functional groups in the alkyl chains, as well as the flexibility of the alkyl chains in these surfactants. The present study is crucial to understanding the relationship between the interfacial structures and the functionalities of the biodegradable quaternary ammonium surfactants.

1. INTRODUCTION Cationic surfactants are widely used as surface active materials in different fields such as germicides, fungicides, colloidal stabilization, fabric softeners in home laundry processes, as well as hair conditioners.1,2 These surfactants can adsorb onto solid surfaces which are usually negatively charged and, thus, functionalize the surfaces with special characteristics. The long-chain dialkyl dimethylammonium halide, such as dioctadecyldimethylammonium chloride (DOAC) or bromide (DOAB), is one of the well-known synthetic dialkyl quaternary ammonium cationic surfactants used as textile softeners.1,2 However, these synthetic cationic surfactants are hard to completely decompose in the natural environment.2 To obtain a better biodegradation performance, the quaternary ammonium surfactants containing ester, amide, and other functional groups, which show higher decomposition ability in nature, have been developed.2−6 The thermodynamic and morphological properties of the cationic surfactant monolayers at the interfaces have been widely investigated. It has been reported that the presence of a counterion in the subphase,7−13 surface coverage,13,14 chain length,15 and mixing with other molecules15−17 can efficiently reduce the repulsive electrostatic interaction between the terminal ammonium moieties and increase the chain−chain © 2013 American Chemical Society

interactions and thus improve the packing densities of the monolayers.7−12 Although much information has been obtained about these cationic surfactants, some points are still controversial or unknown. For example, some studies emphasize that the chain−chain interaction between the surfactant molecules in the monolayer plays the most important role in the monolayer’s structure and conformation16 while other studies mainly focus on the electrostatic repulsion between the charged ammonium terminal moieties.7−12 Furthermore, most of the previous studies focused on the typical cationic surfactants of dialkyl dimethylammonium which are hardly decomposed in the nature environment. No systematic study has yet been reported on the biodegradable cationic surfactants containing ester, amide, and other functional groups, which have recently been developed for much wider markets.2,3 Such information will be useful for designing novel environmentally friendly surfactants and understanding their functional mechanism. In the present study, the effect of functional groups attached to the cationic surfactants on the structure and conformation of Received: September 11, 2013 Revised: October 22, 2013 Published: October 24, 2013 14411

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30 min, a PTFE barrier was compressed at the rate of 0.15 mm/s to obtain the π−A isotherm. A surface potential sensor SPOT from KSV NIMA was employed to monitor the surface potential of the monolayers by the vibrating plate capacitor method using the same LB trough. For the surface potential measurements, the surfactant monolayers were prepared in a manner similar to that for the surface pressure measurements. Normally, changes in the π−A isotherm only appear when a closely packed monolayer begins to form while changes in the surface potential occur as soon as the molecules with dipoles appear at the surface.18,19 This method has been employed to detect the small changes in the effective dipole moments during compressing the monolayer and is sensitive to changes in the molecular orientation and electronic structure of the molecules in a compressed film. To avoid confusion in the experimental results, only the surface potentials observed on the pure water and 1 mM NaBr subphases are provided in the paper. Film Preparation. In addition to the characterization by the π−A isotherm and surface potential, the monolayers were further characterized by other surface science techniques after the samples were transferred onto a solid substrate under ex situ conditions. Since the surface structure of the monolayers with very low coverage may be significantly changed after transfer to the substrate surface, only the monolayer at higher coverage (typically at a constant surface pressure of 25 mN/m) was prepared. The monolayer was first compressed to the desired surface pressure (25 mN/m in the study) and kept for 30 min in order to reach equilibrium, and then transferred onto the substrate surface by the vertical method at the same pressure. A freshly cleaved mica surface and an oxidized Si(100) wafer surface were used as the substrates for the AFM and XPS measurements, respectively. The flat surface of a hemicylindrical CaF2 prism (UV grade, d = 25 mm, l = 25 mm) was used as the substrate for the SFG measurements. The bottom surface of the CaF2 surface was coated with a hydrophilic SiO2 thin-film by the sol−gel method.20,21 The surface was cleaned in an ozone cleaner for 45 min before the monolayer deposition. AFM Measurement. AFM observations were carried out in air using an Agilent 5500 atomic force microscope (Agilent Technologies, USA) at room temperature (ca. 25 °C).17,22 Tapping mode measurements were performed at the scan rate of 1 Hz using SiN cantilevers (Olympus, AC160TS-C2) with a spring constant of 42 N/ m, a resonance frequency of 300 kHz and a tip radius less than 7 nm. Typically, it required 4 min to record each AFM image (256 lines per image). All the AFM images were analyzed by the Scanning Probe Image Processor SPIP 5.1.2 (Image Metrology A/S, Denmark). SFG Measurement. As a second-order nonlinear spectroscopic technique, SFG is known to have intrinsic surface selectivity and submonolayer sensitivity, and is especially useful to reveal the chain orientation and conformational ordering/disordering in organic thin films.23−27 Details about our broad-band SFG system have been previously described.17,20,21 All SFG spectra were collected at the CaF2/air interface. The SFG experiments were conducted in the internal total reflection mode in air at room temperature (23 °C). The angles of incidence for the broad-band IR beam (3.5 μJ, 120 fs) and narrow-band visible beam (30 μJ, 10 ps) were 50° and 70°, respectively. The SFG spectra in the C−H stretch region were recorded by the ssp (s-polarized SFG, s-visible and p-IR) and sps polarization combinations with an acquisition time of 1 min. The SFG spectra in the N−H stretch, amide I and II regions were recorded by the ssp polarization combination with an acquisition time of 20 min. All the SFG spectra were normalized by an SFG spectrum from a gold film evaporated on the prism with the same optical alignment. The spectral intensities, I, were fitted based on the following eq 1:28,29

their monolayers on the air/water interface has been investigated by π−A isotherm, surface potential, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and sum frequency generation (SFG) vibrational spectroscopy. The monolayers of the quaternary ammonium surfactants with amide ((C17H35CONHC3H6)2N+(CH3)2Cl¯, QDA, Figure 1)

Figure 1. Chemical structures of cationic surfactants used in this study.

and ester groups ((C17H35COOC2H4)2N+(CH3)2Cl¯, QDE) have been characterized in the subphase containing different counterions, and compared to a typical cationic surfactant, DOAC ((C18H37)2N+(CH3)2Cl¯). The present results revealed that the hydrogen bonding interaction between the amide groups in QDA and the chain flexibility in QDE significantly contribute to the condensation of the monolayer structure. Furthermore, the presence of a halide counterion in the subphase can also condense the structure of the monolayer of DOAC as well as QDA, but expand that of the QDE monolayer.

2. EXPERIMENTAL SECTION Materials. The chemical structures of the quaternary ammonium surfactants used in the study are shown in Figure 1. DOAC was obtained from Tokyo Kasei Ltd. QDA and QDE were obtained from the Kao Corporation. The ammonium moieties in all the surfactants are bound with two long alkyl chain and two methyl groups. The main structural differences between these surfactants are found in their long alkyl chains (Figure 1). Two long alkyl chains in DOAC are simple C18 chains. Each long alkyl chain in QDA contains an amide group, locating at a position three-carbons away from the terminal ammonium moiety. In addition to the amide group, QDA has two more methylenes (CH2) in the chain than DOAC. The ester group in each alkyl chain of QDE is located two carbons away from the terminal ammonium moiety (Figure 1). QDE has one more CH2 than DOAC in addition to the ester group. All surfactants were further purified by multiple recrystallizations before use. Chloroform (HPLC grade) and ethanol (guaranteed regent) were obtained from Nakalai Tesque, Inc. and were used without further purification. Halide salts (NaCl, NaBr, and NaI) were purchased from Wako Pure Chemicals Industries, Ltd. Milli-Q water (resistivity >18.2 MΩcm) was used to prepare the 1 mM salt solutions as the subphase. All the salt solutions were freshly prepared before the experiments. Surface Pressure and Surface Potential Measurements. The surfactant monolayers were prepared by spreading a 1 mg/mL chloroform solution of DOAC and QDE, or a 1 mg/mL chloroform− ethanol (4:1, v/v) solution of QDA on the water surface in an LB trough (FSD-500, USI) at 22 °C. After the solvent had evaporated in

(2) 2 I ∝ |χR(2) + χNR |

χR(2) = N ∑ n

(1a)

An ωIR − ωυ , n + i Γn

(1b)

where I is the SFG intensity and N is the molecular density on the (2) surface. χ(2) R and χNR are the second-order resonant and nonresonant 14412

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Figure 2. (a) π−A and surface potential isotherms of DOAC monolayers at the air/water interface. The concentration of salt in the subphase is 1 mM. Schematic illustrations of DOAC monolayer on (b) pure water surface and (c) halide containing subphase surface are also given. susceptibilities, respectively. χ(2) R is related to the resonant contribution for the species on the surface. An, ωυ,n, and Γn are the amplitude, resonant frequency, and line width of the nth vibrational mode, respectively.

on the monolayer surface. For example, it was found that the Br/N ratio of the DOAB monolayer on the pure water surface was 0.26, much lower than that on the 1 mM NaBr subphase (0.90) (see the Supporting Information). This suggests that most of the ammonium moiety (ca. 90%) is bound with Br− on the 1 mM NaBr subphase, higher than that on the pure water (ca. 26%). Thus, one can expect a weaker repulsive interaction between surfactant molecules on the halide subphase surface due to the higher concentration of Br− coadsorbed on the monolayer surface. Figure 2a also shows the surface potential of DOAC on the subphase of pure water and 1 mM NaBr (right axis). In the present study, a small amount of the sample solution was used to detect the surface potential at a low molecular density, which is more interesting to us. The surface potentials in a full compression process, especially for that close to the LC phase for each monolayer, were not measured due to the relatively large dimension of the potential sensor on the LB trough. On the pure water subphase, a surface potential of +120 mV is observed at the high surface area (≥2.0 nm2), indicative of a positive charge from the terminal ammonium moiety of the DOAC monolayer. The surface potential decreases to +25 mV on the 1 mM NaBr subphase at the same molecular density (2.0 nm2), implying that part of the surface charges is neutralized by adding the Br− counterion to the subphase. This is in a good agreement with the experimental results of the π−A isotherms and XPS measurements mentioned above showing that the halide ion in the subphase will strongly bind to the ammonium moieties. Similar decreases by the halide counterion have been previously reported by other groups and attributed to the decrease in the effective charge on the monolayer surface.8−10,12 During the compression process, the DOAC monolayer on the pure water surface shows a fast increase in the surface potential (ca. 80 mV) as the area per molecule becomes less than 1.6 nm2 although the π−A isotherm is still flat in the LE region (Figure 2). A transition of the surface potential (ca. 100 mV) is also observed on the 1 mM NaBr subphase at a higher surface density (1.3 nm2 /molecule). With further compression of the monolayer, the surface potentials of the DOAC on both subphases slowly increase while no steep change is observed corresponding to a further phase transition in the π−A isotherm.8−10,12 Generally, the surface potential is known to arise from the dipole moments associated with the water molecules, hydrophilic

3. RESULTS AND DISCUSSION π−A Isotherms and Surface Potentials. Figure 2a shows the π−A isotherms of DOAC on different subphases (left axis). The surface pressure of DOAC on the pure water surface appears when the area per molecule becomes less than 1.2 nm2. The surface pressure monotonically increases with the compression and finally collapses around 42 mN/m and no clear transition from liquid expanded (LE) phase to liquid condensed (LC) phase is observed. As 1 mM halides are added to the subphase, the π−A isotherms move to the left side in the sequence of Cl−, Br−, and I−. The π−A isotherms on the halide subphase show a similar limiting area of 0.68 nm2 /molecule, lower than that on the pure water surface. A narrow plateau was observed on the NaCl or NaBr subphase before reaching the limiting area region. The surface pressure on the NaI subphase appears around 0.78 nm2 and collapses at a pressure less than 30 mN/m. The π−A isotherms indicated that the DOAC monolayer is condensed by the halide counterions in the subphase as schematically shown in Figure 2b and c. By adding the halides to the subphase, the repulsive interaction between the positively charged ammonium moieties of the DOAC molecules on the subphase surface are shielded by the coadsorbed halide ions.8−11 The shielding ability decreases with the nucleophilicity and polarizability of the halide ions in the sequence of I−, Br−, and Cl−. The present dependence is in good agreement with the Hofmeister series dealing with the interaction between the anions and proteins or surfactants.30−32 The Hofmeister series predicts that affinity of halides toward cationic surfactant decreases in the sequence of I− > Br− > Cl−. The fully hydrated Cl− is unable to efficiently shield repulsive interaction between the headgroup of the cationic surfactants while partially hydrated Br− and weakly hydrated I− strongly interact the headgroup as ion-pairs. Furthermore, the XPS measurement was employed to study the coverage of the counterions of the surfactant monolayers on the pure water and halide subphases. Since one nitrogen atom is present in a surfactant, the elemental ratio between the halide (X) and nitrogen (X/N) estimated from the XPS spectra is used as an indicator for the relative coverage of the counterion 14413

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headgroup and the hydrophobic tail of the surfactant molecules in the monolayer18,19 The sharp increase in the surface potentials has been related to a rapid decrease in the local relative permittivity in the vicinity of the polar head groups in the monolayer from 80 (pure water) to 7.6 upon monolayer compression.18 Based on the in situ SFG observations, Roke et al.33 reported a sharp structural transition for the lipid monolayer in the similar LE region and attributed it to the uncurling of the hydrophobic alkane chains upon compression. One expects that the uncurling process of the DOAC alkyl chain may also occur in the region by compression on the water surface. As Br− is present in the subphase, the domains with the curled alkyl chain conformation may be stabilized with a decrease in the positive charge from the terminal group. In the following sections, we will focus our attention on the profile difference in the surface potentials for the different surfactant monolayers in the low density region. Figure 3 shows π−A isotherms of the QDA monolayers on different subphases (left axis). A π−A isotherm of DOAC on

chains of QDA. The hydrogen bonding interactions between the amide groups of QDA molecule are expected to play significant roles in the molecular packing, resulting in a more condensed structure than that of the DOAC. We will further discuss this issue later based on our SFG observations. The surface potential of the QDA monolayer on the pure water surface shows a value of +100 mV (at 2.0 nm2/molecule) and decreases to +20 mV when Br− is added to the subphase (right axis, Figure 3). Similar results are also observed on the DOAC monolayer (Figure 2), indicating that the amide groups in the alkyl chain of QDA do not affect its initial effective charge of the monolayer in the LE phase on the different subphase. On the other hand, the transition of the surface potential for the QDA monolayer occurs at 1.3 nm2 on the pure water and 1.2 nm2 on the 1 mM NaBr subphase, both occurring at the higher molecular density than those of DOAC, implying a stronger interaction between the QDA. Figure 4 shows the π−A isotherms of the QDE monolayers on the different subphase surfaces. The π−A isotherms of the

Figure 3. π−A and surface potential isotherms of QDA monolayers at the air/water interface. The concentration of salt in the subphase is 1 mM. The isotherm of DOAC on the pure water subphase is shown for comparison.

Figure 4. π−A and surface potential isotherms of QDE monolayers at the air/water interface. The concentration of salt in the subphase is 1 mM. The isotherms of DOAC and QDA on pure water subphase are shown for comparison.

the pure water surface (dotted trace) is also shown for comparison. The surface pressure of QDA appears from ca. 1.1 nm2/molecule on the pure water surface. A plateau (10−20 mN/m) is found between 0.8 and 0.6 nm2, corresponding to the coexistence of the LE and LC phase. With further compression, the surface pressure quickly increases to form a LC phase with a limited area of 0.6 nm2. The π−A isotherm of the QDA is also significantly influenced by the halide counterion in the subphase (Figure 3). The LE−LC coexistence phase on the NaCl subphase appears as a plateau at a lower pressure (∼5 mN/m), but disappears on both the NaBr and NaI subphases which shows a direct formation of the LC phase from ca. 0.5 nm2. The QDA monolayers on the subphase containing different halides give a similar limiting area per molecule (0.5 nm2), smaller than that of DOAC (0.68 nm2). The condensation effect of the halide ions on the QDA increases in the sequence of Cl− < Br− ∼ I−, similar to that observed in the DOAC monolayer. The molecular densities of the QDA monolayer are always higher than that of the DOAC monolayers under the same conditions (Figures 2 and 3). This can be attributed to the presence of amide groups in the alkyl

DOAC monolayer (black dotted trace) and QDA monolayer (blue dotted trace) on the pure water surface are also shown in the same figure for comparison. It is a notable feature that QDE monolayer on the pure water surface shows the most condensed π−A isotherm of the three surfactants. The surface pressure appears at 0.6 nm2 and rapidly increases with compression and collapses around 60 mN/m. The limiting area is estimated to be 0.44 nm2 /molecule (or 0.22 nm2 /chain), comparable to that of the long-chain fatty acid monolayer at room temperature (0.22 nm2/molecule),21 which is known as one of most densely packed monolayer systems. As a halide counterion is present in the subphase, the QDE monolayer becomes more expanded than that on the pure water subphase (Figure 4). No condensation by the halide is observed for the QDE monolayer, totally different from DOAC and QDA. The π−A isotherms of QDE on the halide subphase are almost identical, showing a larger limiting area of 0.50 nm2/ molecule than that on the pure water subphase (Figure 4). The packing density of QDE on the halide subphase is almost identical to that of the QDA on the NaI or NaBr subphase. 14414

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Table 1. Root-Mean-Square Roughness (Rq) of the Surfactant Monolayers Deposited from the Different Subphases Evaluated by AFM Observations sample Rq (nm)

DOAC-water 0.22 ± 0.02

DOAC-Cl‑ 0.11 ± 0.02

DOAC-Br‑ 0.10 ± 0.02

DOAC-I‑ 0.05 ± 0.02

QDA-water 0.10 ± 0.03

QDE-water 0.07 ± 0.01

subphase to a freshly cleaved mica surface at 25 mN/m. Table 1 summarizes the root-mean-square roughness (Rq) for each monolayer obtained from the AFM measurement. Each value was obtained by at least averaging five different samples. As reported in our previous paper,17 the DOAC monolayer on the mica surface shows a high number of small islands and holes as irregular domains with a Rq of 0.22 ± 0.02 nm (Table 1, also see the Supporting Information for the AFM images), much rougher than that of the mica substrate observed in the present work (0.05 ± 0.01 nm). This very rough morphology has been attributed to the large repulsive interactions between the DOAC molecules which make the DOAC molecules poorly packed. The morphology of the DOAC monolayers deposited from the halide subphase obviously becomes smooth with the wider domain, in comparison to that on the pure water subphase (see the Supporting Information). As shown in Table 1, Rq clearly drops and its distribution also becomes narrower on the halide subphase. One can also see that Rq decreases in the sequence of Cl¯ (0.11 ±0.02 nm), Br¯ (0.10 ± 0.01 nm), and I¯ (0.05 nm ±0.02 nm), showing that the surface morphology of the DOAC monolayer is improved by adding the halide counterion into the subphase. This change is in agreement with the change in the packing density of the DOAC monolayer on these subphases expected from the π−A isotherm (Figure 2). The shielding effect by the halide ions coadsorbed on the monolayer surface improves both the packing quality and surface morphology by reducing the repulsive interaction between the DOAC molecules. The roughness of the surfactant monolayers noticeably decreases from the DOAC monolayer (0.22 ± 0.02) to the QDA monolayer (0.10 ± 0.03) and QDE monolayer (0.07 ± 0.01). These results agree with their packing densities estimated from the π−A isotherms (Figures 2−4). In addition to the influence of the halide counterion, the presence of a functional group, especially the ester group, can significantly improve the packing density and surface morphology of the cationic surfactant molecules. SFG Characterizations of Surfactant Monolayers. In this section, SFG vibrational spectroscopy was employed to study the conformation and structures of the cationic surfactant monolayers prepared from the different subphases. This information is useful to fully understand the other physical properties of the cationic surfactant monolayers obtained in the previous sections. a. C−H Stretching Region for the Alkyl Chains. Figures 5 and 6 show the ssp- and sps-polarized SFG spectra of (a) DOAC, (b) QDA, and (c) QDE monolayers in the C−H stretching region (2800−3000 cm−1), respectively, for their alkyl chains. In addition to the SFG spectra of these monolayers prepared on the water subphase (25 mN/m, bottom traces, Figures 5 and 6), the SFG spectra of the surfactant monolayers prepared on the 1 mM halide subphase are also given in the figure (25 mN/m, top three traces, Figures 5 and 6). The open symbols are the experimentally observed results and the solid lines are the fitting results based on eq 1. The assignment of the SFG peaks of DOAC on the water subphase were determined in our previous paper.17 The peaks at 2850 and 2920 cm−1 are

The surface potentials of the QDE monolayer on the pure water and NaBr subphase show values of +100 and +20 mV, respectively, at 2.0 nm2/molecule, comparable to that of DOAC and QDA under the same conditions. This demonstrates that the initial effective charges of the surfactant monolayers are not affected by the functional group when the molecular density of the monolayer is low, while the presence of a halide counterion in the subphase can significantly lower the surface potential due to the decrease in the surface effective charge by the shielding effect of the halide ions. The surface potentials of the QDE monolayer on both subphases are almost constant during compression until ca. 1.0 nm2/molecule and then show steep increase of approximately 100 mV. The higher onset for the transition of the surface potential implies that the interaction between the QDE molecules is higher than the other two monolayers, i.e., DOAC and QDA. The high density of the QDE monolayer on the water surface is expected to associate with the high flexibility of its alkyl chain due to the presence of the ester group, which allows molecules to adjust to the best conformation for dense molecular packing for the ammonium surfactant which gives rise to a strong van der Waals interaction between the QDE molecules. The different effects of the amide and ester groups on the molecular packing density and structures have been discussed for different monolayer systems.5,6,34,35 The packing density of a cationic surfactant with an ester group was found to be higher than one with an amide group.5,6 Although hydrogen bonding can create a lateral interaction between the molecules or even in the molecule, it requires a certain spatial distance and conformation. This can restrict the rotation around the C−C bonds near the amide moieties and thus can generate a destructive effect on the packing density when such a conformation is not available. As mentioned above, halide counterions in the subphase lower the packing density of the QDE monolayer more than that in pure water, opposite to that of DOAC or QDA. As mentioned above, the halide anions are expected to be adsorbed between the ammonium moieties of these surfactant molecules to shield their positive charges. As the molecular density of the monolayer is low, there is enough space for the adsorption of the halide counterion between the surfactants. However, when the packing density is higher than a critical value, the halide will be unable to coadsorb into the monolayer due to its ionic radius (Cl− 0.188 nm, Br¯ 0.195 nm and I¯ 0.216 nm). As shown in Figure 4, the QDE monolayer expands to 0.5 nm2/molecule on the halide subphase, similar to the limited value for QDA on the NaI or NaBr subphase. The molecular density of 0.5 nm2/molecule is considered as a critical value for the coadsorption of halide ions in the cationic surfactant monolayer. The spatial limitation of the adsorbed halide can hinder the further condensation of the QDE molecules when halide counterions are added to the subphase. As will be shown by the SFG measurement, it was found that the ordering of the QDE monolayer on the halide subphase surface is lower than that of the QDE monolayer on the pure water. Surface Morphology of the Surfactant Monolayers. The AFM was used to characterize the surface morphology of these surfactant monolayers after transferring them from each 14415

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Figure 5. ssp-Polarized SFG spectra of (a) DOAC, (b) QDA, and (c) QDE monolayers (25 mN/m) at the CaF2/air interface. Open circles are SFG data. Solid curves are spectral fits. All the SFG spectra are offset for clarity.

Figure 6. sps-Polarized SFG spectra of (a) DOAC, (b) QDA, and (c) QDE monolayers (25 mN/m) at the CaF2/air interface. Open circles are SFG data. Solid curves are spectral fits. All the SFG spectra are offset for clarity.

As a specific feature of the SFG vibrational spectroscopy, it is extremely sensitive to the local symmetry of the hydrocarbon chains. In a densely packed organic monolayer in which all hydrocarbon chains should take the all-trans conformation, the vibrational modes for the CH2 groups are SFG-inactive and only those for the terminal CH3 group is SFG-active. The SFG peaks for the CH2 groups only appear when the local symmetry is broken, such as the appearance of gauche defects, which modify the all-trans conformation of the hydrocarbon chain. The amplitude ratio of ACH2ss/ACH3ss has been employed as an indicator for disordering of the monolayer.17 The lower the ratio, the fewer gauche defects are expected in the monolayer. Figure 7 summarizes the values of ACH2ss/ACH3ss from the ssppolarized SFG spectra shown in Figures 5 and 6 (ACH2as/ACH3as from sps-polarized SFG spectra are given in the Supporting Information). The ratio significantly depends on the type of cationic surfactants and composition of the subphase. First, the DOAC (QDE) monolayers always show the highest (lowest) value for the ratio ACH2ss/ACH3ss on all subphases. For example, the ACH2ss/ACH3ss values of the DOAC, QDA, and QDE monolayers on the water subphase were found to be 0.64, 0.34, and 0.09, respectively. This indicates that the alkyl chains of

attributed to the C−H symmetric stretching (CH2ss) and asymmetric stretching (CH2as) of the methylene groups on the alkyl chains. The peaks at 2880, 2940, and 2960 cm−1 can be assigned to the C−H symmetric stretching (CH3ss), Fermi resonance of the symmetric stretching mode (CH3FR) and asymmetric stretch (CH3as) of the terminal methyl groups on the alkyl chains of the DOAC, respectively. The symmetric stretching modes mainly appear in the ssp-polarized spectra while the asymmetric stretching modes appear in the spspolarized spectra. This indicates that the alkyl chains are not significantly tilted from the surface normal based on the resonance interaction between the IR field and dipole moment of each C−H vibrational mode. The general features of the DOAC monolayer on the halide subphase are similar to that on water, but the relative peak intensities of CH2 decrease less in the sequence of Cl¯, Br¯, and I¯ (Figures 5a and 6a). The relative peak intensities of the CH2 group of the QDA monolayer on water surface is clearly weaker than that of DOAC under the same condition and becomes even weaker on the halide subphase (Figures 5b and 6b). The QDE monolayers show much stronger peak intensities for the CH3 groups, but very weak relative peak intensities for the CH2 groups in comparison with the DOAC and QDA monolayers (Figures 5c and 6c). 14416

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clearly become higher on the halide subphase than that on the water surface, while those of CH3as are almost identical. The present features are in good agreement with the molecular density of the surfactant monolayers observed in the π−A isotherms (Figures 2−4). For example, the limiting area per molecule on all the subphases decreases in the sequence of DOAC, QDA and QDE, while the QDE monolayer shows the highest molecular density on the water subphase compared to those on the halide subphase. Since the gauche defects take more space in the monolayer than the all-trans conformation, the number of gauche defects in the monolayers significantly affects the packing density of the surfactant monolayers. The higher the number of gauche defects in the monolayer, the lower the molecular packing density. As mentioned above, the halide counterion can significantly reduce the repulsive interaction between the positively charged terminal groups and thus dominates the packing density, depending on the nucleophilicity and polarizability of the halide ions in the sequence of hydrated I¯, Br¯, and Cl¯. This halide dependence of the monolayer disordering works well on both the DOAC and QDA monolayers, but does not work on the QDE monolayer in which the monolayer disordering increases on the halide subphase (Figure 7). As discussed in the section about the π−A isotherm, the densely packed QDE monolayer provide no space for the additional halide coadsorption on the monolayer surface. In order to adsorb on the QDE surface, the QDE monolayer has to slightly expand, resulting in a lower molecular density (ca. 0.5 nm2/ molecule) and a higher disordering in comparison to that on pure water. The orientation angle (θ) of the terminal CH3 group (the angle between the symmetric axis of the methyl group and the surface normal) and the tilt angle (α) of the hydrocarbon chain, of the surfactant molecules can be estimated from the ssp- and sps-polarized SFG spectra (Supporting Information). However, the calculation can be only used for a system with an all-trans conformation. Rough estimations show that the tilt angle of QDA and QDE were approximately 5° from the surface normal assuming a δ-distribution. No significant difference in α was found for these monolayers under the estimation limitation. On the other hand, the calculation was not made for DOAC due to its high number of gauche defects in the monolayer.

Figure 7. ACH2ss/ACH3ss of DOAC, QDA, and QDE monolayers at the CaF2/air interface transferred from different subphases (25 mN/m). The values of ACH2ss and ACH3ss were obtained from the fitting results of the ssp-polarized SFG spectra.

DOAC (QDE) are highly disordered (ordered) on the water subphase, while the QDA shows a conformational ordering between them. Second, the ACH2ss/ACH3ss ratio depends on the subphase compositions. As schematically shown in Figures 2b and 2c, the packing ordering of DOAC monolayer is significantly improved by adding halide into the subphase. Furthermore, the conformational ordering for the DOAC monolayers increases in the sequence of water, Cl¯, Br¯, and I¯ expected from the relative values of ratio. Although all the ratios for QDA are lower than those for DOAC, the difference between the different halides for the QDA monolayer is smaller in the disordering order of water > Cl¯ ≈ Br¯ ≈ I¯. The disordering factor for the QDE monolayer on the water surface shows the lowest value but the disordering increases on the halide subphase in the sequence of water < Cl¯ ≈ Br¯ ≈ I¯. For example, as shown for the sps-polarized SFG for the QDE monolayer (Figure 6c), the intensities for the CH2as mode

Figure 8. (a) SFG spectra for N−H stretch mode of the QDA monolayers, overlapped with broad bands from O−H stretch of water molecules. (b) SFG spectra for amide I and II bands of the QDA monolayers on CaF2 surface deposited from the pure water and 1 mM NaBr subphases. 14417

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b. Evidence for Hydrogen Bonding in QDA. In order to understand the chemical interaction in these biodegradable cationic surfactants, not only in the C−H stretching region for their alkyl chains, it is also important to investigate the structures of their specific functional groups in other frequency regions. As an example, the SFG observation is employed to investigate the amide group in the QDA monolayer. Figure 8a shows the ssp-polarized SFG spectra in the 3000− 3500 cm−1 region for the QDA monolayers prepared from the water subphase and 1 mM NaBr subphase. In the broad band region of the O−H stretching mode (3000−3500 cm−1), which may be from adsorbed water molecules in the QDA monolayer, a sharp peak is observed at 3320 cm−1. According to previous studies, the N−H stretching without hydrogen bonding produces a peak around 3450 cm−1.36,37 The red-shift of this band to 3320 cm−1 implies that the hydrogen bonding is formed between the amide groups in the QDA monolayer.36−39 The SFG measurements on the N−H stretching mode of the protein or peptide, which is strongly hydrogen bonded, also show a peak around 3300 cm−1.40−42 No significant difference in the peak position is observed for the N−H stretching mode of the QDA monolayer on the pure water subphase and 1 mM NaBr subphase (Figure 8a), indicating that the hydrogen bonding for the QDA monolayer prepared on the two subphases are not affected by the halide ions in the subphase. As further evidence for the hydrogen bonding interaction, the SFG measurements were also carried out in the IR frequency region between 1500−1700 cm−1 (Figure 8b), corresponding to the N−H bending mode (amide II) and CO stretching mode (amide I) of the amide group. As shown in Figure 8b, a broad peak is observed at 1570 cm−1 with a weak shoulder around 1665 cm−1. The peak at 1570 cm−1 can be assigned to the amide II mode. Normally, the bending N−H mode (amide II) for the amide group free hydrogen bonding has been reported to appear around 1510 cm−1 and this peak usually blue-shifts several tens of cm −1 by hydrogen bonding.43−47 The appearance of amide II mode at a higher frequency is also clear evidence that hydrogen bonding is present between the amide groups in the QDA monolayer.43−47 The peak at 1665 cm−1, which is assigned to the amide I mode, also demonstrates formation of the hydrogen bonding between the amide groups in the QDA monolayer.43−47 It is interesting to note that only amide II mode was previously observed in the amide-containing alkanethiol selfassembled monolayers (SAMs) on gold surfaces but N−H stretching mode and amide I mode were hardly observed there.36,44,45 Based on the previous discussions on the IR reflection absorption spectroscopy measurements, this implies that the direction of N−H stretching mode of QDA molecule is not parallel to the surface but may take slightly tilted conformation in the QDA monolayer, being IR active. With this conformation, in addition to the N−H stretching mode, amide I and II modes which are perpendicular to each other,45 will have IR active components in the direction of the surface normal, thus can be detected by the ssp-polarization used in the present SFG measurement. As mentioned above, both SFG measurements in the N−H stretching region and amide I/II region suggest that the amide groups of the QDA monolayers are strongly hydrogen bonded. A schematic illustration of the molecular structure of the QDA monolayer is shown in Figure 9. The halide ions on the QDA surface are believed to mainly reduce the electrostatic repulsive interaction between the terminal ammonium moieties but have

Figure 9. Schematic illustration of the hydrogen bonding on the QDA monolayer.

no apparent influence on the hydrogen bonding. It should be noted that in Figure 8a, the SFG signals from the O−H stretch demonstrated that there are some water molecules adsorbed on the substrate during the LB deposition. QDA-water and QDANaBr show different spectral features for the adsorbed water molecules, which should be due to the screening effect of the halide ions on the surface electric field; however, the water structures adsorbed on the surfactant monolayers are beyond our interest.

4. CONCLUSIONS In summary, SFG spectroscopy, surface pressure and potential measurements, and AFM have been used to study the effect of halide ions and effect of amide and ester groups on the dialkyl dimethylammonium surfactant monolayers. For DOAC and QDA, the adsorbed halide ions can significantly improve the monolayer packing by decreasing the repulsive interaction between the positively charged headgroups. However, for QDE, the adsorbed halide ions can slightly disorder the monolayer structure. It is believed that the halide ions, which penetrate into the QDE monolayer, have a spatial hindrance effect on the packing of the QDE molecules. The present results revealed that, in the loosely packed cationic surfactant monolayers, the electric screening effect of the halide ions is dominant. However, for the densely packed cationic surfactant monolayer, the spatial hindrance effect caused by adsorbed halide ions becomes more apparent. Furthermore, for the QDA monolayer, the hydrogen bonding of the amide groups, which is demonstrated by the SFG measurements in both the N−H stretching and amide I/II regions plays an important role in forming the densely packing monolayer. In the case of QDE, the flexibility of the ester groups on the QDE molecules is believed to induce the dense packing of the QDE monolayers. Due to this flexibility, the alkyl chains of QDE are anticipated to be close to each other, resulting in a strong van der Waals interaction. Such a strong van der Waals interaction should overcome the repulsive interaction between the positively 14418

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charged headgroups and keep the monolayer structure densely packed and stable. The present results are important to understand the intra/intermolecular interaction of cationic surfactants and specific ion effect on the monolayer structure, both of which are important in order to understand the behavior of biodegradable surfactants at interfaces.

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ASSOCIATED CONTENT

* Supporting Information S

XPS spectra and AFM images for the surfactant monolayers; analysis results for π−A isotherms and SFG spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination Program” 759 (24108701) and a Grant-in-Aid for Scientific Research (B) 23350058 from the Ministry of Education, Culture, Sports, Science & Technology (MEXT), Japan. M. O. acknowledges JSPS KAKENHI Grant 24550143. N. Y. and S. Y. also gratefully acknowledge financial support from the Cooperative Research Program of Catalysis Research Center, Hokkaido University (No. 12B1004).

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