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Langmuir and Langmuir–Blodgett films of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of amino acids
Journal of Colloid and Interface Science 310 (2007) 337–347 www.elsevier.com/locate/jcis
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Langmuir and Langmuir–Blodgett films of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of amino acids M. Kanthimathi, Aruna Dhathathreyan ∗ Chemical Laboratory, CLRI, Adyar, Chennai 600020, India Received 4 September 2006; accepted 6 January 2007 Available online 16 February 2007
Abstract Langmuir and Langmuir–Blodgett monolayers of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of glycine, tyrosine, and phenylalanine were studied using π –A isotherms and photoelastic modulated FTIR (PEM-FTIR). Based on compression modulus and interaction parameters, mixed monolayers of these compounds with stearylamine (SAM) showed well-organized monolayers compared to mixed systems with stearic acid (SA) and stearyl alcohol (SAL). The pure amphiphiles exhibited fairly well-ordered packing in the films, and in the mixtures, the ordering increased and showed a triclinic packing arrangement. For the phenylalanine amphiphile the packing showed slight disorder compared to the other two compounds. Surface properties of the LB films of these compounds on solid substrates were analyzed using static and dynamic contact angles of a series of liquids. The surface tension of coated substrates reflected clearly the highly acidic character. Fluidlike monolayers having a molecularly rough surface indicated high wettability for n-alkanes. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettability and small hysteresis. © 2007 Elsevier Inc. All rights reserved. Keywords: Lipoamino acids; LB films; Wettability; Mixed monolayers
1. Introduction In recent years, investigations of amino acid amphiphiles have gained much importance, since amino acids are known to play an important role in model membrane studies [1–9]. Interesting work on aggregation properties of amino acid amphiphiles has been reported by Huang and co-workers [10]. Acidic amphiphiles based on L-alanine surfactant with specific structure-directing properties have been explored by Gin and co-workers [11]. New materials such as gel emulsions from these amphiphiles for specific applications have been achieved [12]. An interesting application of amino acid amphiphiles in discriminating between N- and C-termini of peptides has been reported in the literature [13]. Roy and Dey’s paper on self-organization of sodium N -(11-acrylamidoundecanoyl)-L-valinate using urea dealt with changes in local clusters of water near air/water interfaces [14]. * Corresponding author. Fax: +91 44 24911589.
E-mail address: [email protected] (A. Dhathathreyan). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.01.016
Langmuir–Blodgett (LB) films of N -octadecanoyl-L-alanine amphiphile, the simplest chiral amino acid derivative, have been studied by FTIR spectroscopy [1–3], where the enantiomeric molecules assemble regularly through an extended intermolecular hydrogen-bonding network and twist from neighbor to neighbor to develop chirality of the aggregate in the twodimensional crystalline array [4,5]. Intermolecular hydrogenbonding interactions and chiral effects between hydrophilic head groups enhancing the interactions between hydrophobic chains in these long-chain amphiphiles of amino acids have been demonstrated [1–6]. Assemblies of such amphiphilic derivatives of amino acids and their mixed films with lipid monolayers seem to be good model systems for studying cell– protein interactions. In order to design ordered structures using LB films of any amphiphile, one needs to first assemble a monolayer at the air/ water interface. It is now well known that the polar groups of the amphiphile have a pronounced effect on the structure of the condensed phase domains in monolayers [15,16]. It is expected that the problem of understanding molecular interactions in monolayers (two-dimensional (2d) systems) is relatively easy
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compared with that in three-dimensional systems, due to reduction of one degree of freedom. Also, control over an external variable (pressure) enables one to vary the intermolecular separation, and therefore the molecular interaction is relatively easier to tune in monolayers than in bulk solids or liquids, which are less compressible. The competition between the line tension and the electrostatic repulsion [15,17–22] in different polar groups has been shown to be an important factor in the shape transitions of nonchiral amphiphiles. A recent study on N -palmitoyl aspartic acid, N -stearoylserine methyl ester, N -palmitoylallothreonine methyl ester, and N -stearoylallothreonine methyl ester showed the importance of orientation and minimal energy conformations in determining the packing arrangements of the thin monofilament [23], which have potential as models in fibril formations in proteins. Thus, amino acid amphiphiles and short peptides constitute an important class of surfactants. Several amino acids have been investigated for this purpose, with the majority of papers dealing with basic amino acids such as arginine and lysine, from which cationic surfactants can easily be prepared. Most synthetic procedures are based on organic synthesis, but enzymatic processes have also been explored [24–26]. An exhaustive and excellent review on amino acid-based surfactants has been recently published by Infante et al. [27]. In this study we have prepared N -(4-octadecyloxy-2-hydroxybenzylidene derivatives of glycine (compound 1), tyrosine (compound 2), and phenylalanine (compound 3) using a procedure similar to that of Huang and co-workers [10] and have explored the properties of monolayers as well as their ability to form stable LB films on solid surfaces. The Langmuir films at air/water interfaces have been characterized using Brewster angle micrographs (BAM) and the LB films of these amphiphiles transferred to solid substrates have been analyzed using static contact-angle measurements. Further changes in the conformation of the packed films and their mixtures with stearic acid (SA), stearyl alcohol (SAL), and stearylamine (SAM) have been studied using PEM-FTIR. 2. Materials and methods All solvents used in this investigation, chloroform, methanol, DMSO, and hexadecane, were HPLC-grade solvents of high purity and used as received. 2,4-Dihydroxybenzaldehyde, glycine, phenylalanine, tyrosine, and 1-bromooctadecane were obtained from Aldrich Chemicals, USA and were more than 99% pure. The synthesis of the amphiphiles followed a proce-
Scheme 1. Energy-minimized structures of C18 Gly (compound 1), C18 Phe (compound 2), and C18 Tyr (compound 3).
dure similar to that of Huang and co-workers [10]. All three octadecyloxy derivatives were prepared using the same procedure from the corresponding dihydroxybenzilidine derivatives. Scheme 1 shows the energy-minimized structure of the three compounds. The proton NMR spectra of the samples dissolved in DMSO-D6 were recorded and the results are presented in Table 1. 2.1. Langmuir and Langmuir–Blodgett films (LB films) of compounds 1, 2, and 3 A NIMA single-barrier Model 601-S with a Wilhelmy balance and a thermostat was used to study the characteristics of the Langmuir films and the surface pressure–molecular area
Table 1 Characteristic chemical shifts for compounds C18 Gly, C18 Phe, and C18 Tyr Groups
C18 Gly
C18 Phe
C18 Tyr
Signals
δ-value
Signals
δ-value
Signals
δ-value
CH group in amino acid
(s, 2H, CH2 )
0.87
Long-chain hydrocarbon Azo group CH=N Aromatic ring attached to the long chain Aromatic ring present in the amino acid chain
(s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) –
1.25 2.05 6.66 –
(t, 1H, CH) (m, 2H, CH2 ) (s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) (m, 5H, phenyl H)
0.87 1.74 1.25 2.53 6.5 7.40
(t, 1H, CH) (m, 2H, CH2 ) (s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) (m, 4H, phenyl H)
0.82 1.75 1.20 2.45 6.67 7.61
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(π–A) isotherms. Surface pressure was measured with an accuracy of 0.1 mN/m. 2.2. Brewster angle microscopy In situ measurements on the spread monolayers were carried out using a BAM setup described elsewhere [28]. The amphiphiles were spread using chloroform (CHCl3 ) (HPLC grade) as a spreading solvent. A minimum waiting time of about 10 min elapsed before the films were compressed at the air/water interface. A compression speed of 5 Å2 /molecule/min was used for all the compounds, and the experiments were carried out at a temperature of T = 22 ◦ C. The subphase used was at pH 7, prepared using phosphate buffer (concentration 5 mM). The films of the pure compounds were transferred onto cleaned glass cover slips by the LB film technique at π = 20 mN/m. At this pressure all the monolayers were in the liquid condensed state (LC) and away from collapse. The films showed uniform X-type transfer and the transfer ratio was nearly 0.9. The contact angles were estimated for the three films using a NIMA 9005 tensiometer. The mixed films of the three amphiphiles with stearic acid (SA), stearyl alcohol (SAL), and stearylamine (SAM) were prepared for mol ratios between 0 and 0.9 (of the amphiphiles). The results reported here are for mole ratio 0.5. Using Young’s equation, the solid/liquid interfacial tension may be evaluated as γlv cos θ = γsv − γsl ,
(1)
where γlv , γsl , and γsv are the surface tension of the liquid at the liquid/vapor interface, the interfacial tension at the solid/liquid interface, and the surface tension of the solid substrate at the solid/vapor interface. θ is the angle made by the liquid in contact with the surface coated with the amphiphile. By estimating the change in contact angles for pure water, octadecane, and dimethyl sulfoxide (DMSO), the total surface p d energy γsv can be found from γsv (polar component) and γsv (dispersive component) using the equation γ = γ p + γ d,
(2)
where the superscripts d and p show dispersive and polar components. The polar component may further be divided into the acidic and basic parts given by γ + , γ − , respectively, using the equation + − 1/2 p γsv = 2 γsv (3) . γsv 2.3. PEM-FTIR measurements on Langmuir films The measurements of PEM-FTIR was carried out with a Brucker IF S48 spectrometer using a gold grid polarizer to obtain s- or p-polarized radiation modulated by a ZnSe photoelastic modulator (Hinds type II) at Brewster incidence. The frequencies of the methylene stretching vibration mode have been previously used as an indication of the ordered states of
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the packing of the monolayer in the condensed state. The frequencies, shapes, and intensities of the infrared C–H stretching bands provide information on the orientation, conformation, and packing of the hydrocarbon chain in the monolayer. The CH2 (asymmetric) and CH2 (symmetric) stretching bands appearing in the spectrum correspond to a preferential in-plane orientation of transition moments, which is consistent with the alkyl chains being quasi perpendicular to the interface. The signals from the hydrophilic groups immersed in water, being weak, cannot be observed directly in the spectrum. Hence only the bands corresponding to the long alkyl chains are indicated here. For different molecular areas during compression of the monolayer, the asymmetric and symmetric CH2 bands were recorded. 3. Results and discussion π–A isotherms of the three amphiphiles spread at the air/water interface are shown in Fig. 1. The ability of the lipoamino acids to form H-bonding networks was tested using mixed layers of these compounds with stearic acid, stearyl alcohol, and stearylamine at the air/water interface and the π –A isotherms studied. Repeated compression and expansion of the films did not show any appreciable hysteresis. In order to assess the stability of these films at air/water interface, all the films were maintained at a constant pressure of π = 20 mN/m. This surface pressure was chosen because almost all the monolayers showed a liquid condensed state at this pressure, and this pressure is below the collapse pressure. Further, the change in area at a constant surface pressure π = 20 mN/m as a function of time (carried out over 20 min) was within 3 to 5% of the initial area, indicating that the films were quite stable over the period and LB film transfer at this pressure can be effected. Thus all three compounds form fairly stable Langmuir films and show a liquid expanded (LE) to liquid condensed (LC) phase with average area varying between 0.192 nm2 (C18 Gly) (compound 1) and 0.254 nm2 (C18 Tyr) (compound 3) Å2 /molecule at π = 20 mN/m. In the case of the glycinederived lipoamino acid, the area is quite close to that of the stearyl chain, and the compactness in the layer could arise from the fact that glycine can form tighter H-bonds with water at the interface than the other compounds. Tyrosine is more nonpolar than glycine and has a larger polar group. Therefore the packing of the molecules at the interface is loose, leading to a larger area. In the case of the compound derived from phenylalanine (compound 2), the –OH group present on the aromatic ring in tyrosine is absent. Moreover, the aromatic rings seem to pack in such a way that they are almost tilted at an angle to the layer plane, thus leading to an area of about 0.2 nm2 /molecule. The compression modulus Cs−1 is a parameter used to define the different states of the monolayer [29,30] and the collapse pressure and is obtained from Cs−1 = −A(∂π/∂A). Generally its values lower than 12.5 mN/m are attributed to the gaseous state, values between 12.5 and 100 mN/m to the liquid
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Fig. 1. Surface pressure–molecular area (π –A) isotherms of C18 Gly, C18 Phe, and C18 Tyr at air/water interface, pH 7.0, T = 22 ◦ C.
expanded state, and 100 to 250 mN/m to the liquid condensed state, while Cs values greater than 250 mN/m are assigned to the solid state of the film. At the collapse pressure the value approaches zero. Using this criterion, the maximum collapse pressures for the different systems used in the present study have been estimated. The maximum collapse pressure varies for each compound. The mixed films of the two components were studied for mole fractions ranging from 0.1 to 0.9 of the amino acid amphiphile. It was found that beyond 0.5 there was not much change in the characteristics of the films, suggesting that the components are possibly phase-separating beyond a mole fraction of 0.5. Table 2 shows the area/molecule at π = 20 mN/m and the maximum collapse pressure πc for the pure compounds and in the mixed films. Fig. 2 presents the plots of πc versus matrix mole fraction, where the matrix means SA, SAL, or SAM, respectively. It is seen that for mixtures containing a wide range of mole fractions there is no dramatic change in collapse pressure after a mole fraction of 0.5. For values less than 0.5, in most cases it increases with increasing mole fraction of SA, SAL, or SAM. This could be interpreted as a mixing of SA, SAL, or SAM in Langmuir monolayers of compounds 1, 2, or 3 till a mol fraction of 0.5 is reached. Fig. 3a shows the isotherms of the compounds with stearic acid. The isotherms of the mixed films show that all the
Fig. 2. Plot of πc against mole fraction of matrix (SA, SAL, or SAM) for compounds C1, C2, and C3.
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Table 2 Area/molecule and maximum collapse pressure for the different amphiphiles and the mixed films with SA, SAL, and SAM Compound
Area/molecule (nm2 )
Maximum collapse pressure (mN/m)
C18 Gly—compound 1 C18 Phe—compound 2 C18 Tyr—compound 3 Compound 1 + SA Compound 2 + SA Compound 3 + SA Compound 1 + SAL Compound 2 + SAL Compound 3 + SAL Compound 1 + SAM Compound 2 + SAM Compound 3 + SAM
0.19 0.20 0.25 0.52 0.57 0.63 0.53 0.63 0.48 0.25 0.31 0.32
28.1 26.5 40.1 26.1 31.1 39.9 56.9 43.1 35.8 37.0 44.5 43.5
Scheme 2. Scheme for mixed films of the compounds with SA or SAL (mole ratio 1:1).
isotherms show an intermediate phase between the LE and LC states. The appearance of a plateau is seen for compounds 2 and 3, while for 1 there is a slight change in the slope. The area/molecule now lies between 0.52 (compound 1) and 0.63 nm2 /molecule (compound 3). Doubling of the area in all the cases is possible due to a dimer-like structure formed between stearic acid and stearyl alcohol and the compounds as shown in Scheme 2. Fig. 3b shows the isotherms for the mixed films of the amino acid derivatives with stearyl alcohol. Here again there is an expansion in the average area/molecule and the new phase between the LE and LC phases is seen clearly as a change in slope in compound 1. The areas range from 0.48 nm2 (compound 3) to 0.53 and 0.63 nm2 per molecule (compounds 1 and 2). The maximum collapse pressure is also now higher for all three films, compared to that for the pure compounds. It was found that as the mole fraction of the additive increases from 0 to 0.5, the collapse pressure also increases, indicating that the two components are fairly miscible. Stearyl alcohol is expected to have an annealing effect in monolayers [27] and aids in more homogeneous packing of the molecules. This could, in turn, lead to higher collapse pressures. Fig. 3c shows the isotherms of the three lipoamino acids with stearylamine. In the mixed films with C18 Phe and C18 Tyr, there is a steep change in the slope at a π value of about 12.5 mN/m, and the area also ranges between 0.25 and 0.32 nm2 /molecule for all three compounds. It is well established that the amino group in SAM can form compact H-bonds, and therefore the
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monolayers show lower areas than SA and SAL mixed films. Here again the maximum collapse pressure is higher than the pure films. An excess free energy of mixing (Gexc ) at a surface pressure [31,32] given by exc (G ) = A12 − (A1 X1 + A2 X2 ) dπ for limits 0 to π and an interaction parameter α defined as α = (Gexc )/RT X1 X22 + X2 X12 are generally used for evaluating miscibility and phase separations in mixed films. In the present study these values were evaluated for surface pressures of 5, 10, and 20 mN/m. It was found that mixed films of both SA and SAL showed a positive value of α ranging between 2.7 and 2.9, while with SAM the values were negative. The positive sign of α suggests less interaction between the amphiphilic amino acids and SA and SAL, compared to the interaction between the molecules of the amphiphiles themselves. On the other hand, mixed films of all three compounds with SAM showed negative values, suggesting that the interactions between the amino acid amphiphiles and SAM are attractive. Figs. 4a–4c show the plots of the interaction parameter α as a function of mole fraction of SA, SAL, or SAM in mixed films of compounds 1, 2, and 3 for surface pressures of 5, 10, and 20 mN/m, respectively. For mixed films of the compounds with both SA and SAL, the α values are positive, which suggests that perhaps the interactions between the molecules are not attractive. Regarding mixtures with SAM, values of α are lower than for the mixtures with SA or SAL. These results suggest the possibility of a single-phase system for particular compositions of compounds 1, 2, and 3, especially with respect to SAM. Fig. 5a shows the IR spectra of compounds 1, 2, and 3 in the close-packed structure, while Fig. 5b shows the wavenumber dependence of the alkyl chains in compounds 1, 2, and 3 during compression as a function of the area/molecule. The frequencies of the methylene stretching vibration modes have been previously used as an indication of the ordered states of the packing of the monolayer in the condensed state [33]. The frequencies, shapes, and intensities of the infrared C–H stretching bands provide information on the orientation, conformation, and packing of the hydrocarbon chain in the monolayer. The ν as CH2 (asymmetric) and νs CH2 (symmetric) bands appearing in the spectrum correspond to a preferential in-plane orientation of transition moments, which is consistent with the alkyl chains being quasi-perpendicular to the interface. It has been reported that the ν as CH2 vibrations are sensitive to conformation and can be correlated with the trans/gauche ratio of the hydrocarbon chain [33]. Normally lowering of wavenumbers is characteristic of highly ordered conformations with preferential all-trans characteristics, while the number of gauche conformers increases with increasing wavenumber and width of the band. The increased wavenumber of the methylene stretching vibration for a gauche rotamer is caused by a coupling between the carbon
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(a)
(b)
(c) Fig. 3. Mixed films of C18 Gly, C18 Phe, and C18 Tyr with (a) SA, (b) SAL, and (c) SAM at air/water interface; mole ratio 1:1, pH 7.0, T = 22 ◦ C.
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(a)
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(b)
Fig. 4. Plots of interaction parameter α against mole fraction of SA, SAL, or SAM for (a) compound 1, (b) compound 2, and (c) compound 3. Curves 1, 2, and 3 correspond to π = 5, 10, and 20 mN/m.
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(a)
(b) Fig. 5. (a) PEM-FTIR spectra of compounds C18 Gly, C18 Phe, and C18 Tyr. (b) Area/molecule against change in wavenumber corresponding to asymmetric methylene stretch for the three amphiphiles.
(c) Fig. 4. (continued)
atoms and the methylene hydrogen, which due to interconversion around the C–C bond is positioned in the plane defined by the carbon atoms, resulting in an increased force constant for that C–H bond. In contrast, for an all-trans conformation all methylene hydrogens are out of the plane. From Fig. 5a it is seen that the anti-symmetric methylene stretching vibrations for the three compounds appear between 2922 and 2916 cm−1 , while the symmetric stretching occurs between 2850 and 2844 cm−1 . For compounds, the carbonyl stretching appears at around 1703 and 1692 cm−1 , while those with SA, SAM, and SAL appear between 1729 and 1716 cm−1 . Such shifts to higher wavenumbers have been assigned to unprotonated and protonated carbonyls by Gericke and Hühnerfuss [1]. In the present study it is possible that with the Hbonding taking place in the presence of SA, SAM, or SAL, the carbonyl stretching shifts to higher wave numbers. The amide I and II bands appear between 1620 and 1608 cm−1 , while the
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(a)
(b)
(c) Fig. 6. BAM micrographs of (a) C18 Gly, (b) C18 Phe, and (c) C18 Tyr and their mixed films at air/water interface, pH 7.0, T = 22 ◦ C. Top: LE phase (surface pressure π = 5 mN/m); bottom: LC phase (surface pressure π = 20 mN/m) (scale bar denotes 20 µm). The micrographs from left to right correspond to amino acid amphiphiles and their mixtures with SA, SAL, and SAM, respectively.
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Table 3 Alkyl stretch and scissoring vibrational frequencies corresponding to the different amphiphiles in close packed monolayers Sample Compound 1 Compound 1 + SA Compound 1 + SAM Compound 1 + SAL Compound 2 Compound 2 + SA Compound 2 + SAM Compound 2 + SAL Compound 3 Compound 3 + SA Compound 3 + SAM Compound 3 + SAL
Vibrational frequencies (cm−1 ) νas (CH2 )
νs (CH2 )
δ (CH2 )
2921.7 2917.8 2916.6 2917.5 2925.0 2922.4 2919.2 2920.3 2928.5 2923.4 2922.1 2923.8
2890.2 2855.4 2852.7 2858.7 2850.9 2838.5 2835.4 2835.6 2852.5 2850.9 2849.4 2850.2
1464.2 1471.1 1472.0 1470.1 1467.1 1467.6 1468.9 1464.1 1457.0 1468.2 1474.1 1474.1
bands around 1459–1447 cm−1 are attributed to methylene scissoring vibrations. For mixtures of compounds 1, 2, and 3 with SA, SAM, and SAL, the changes in the molecular order can be seen from the shifts in the νas and νs stretching vibrations corresponding to the CH2 . For the sake of brevity only the CH2 stretching vibrations, asymmetric and symmetric and scissoring band δ(CH2 ), are presented here in Table 3. Fig. 5b shows that there is an overall decrease in the wavenumbers as the films are compressed, giving rise to an overall increase in order of the alkyl chains. From the table of values it is seen that the asymmetric νas wavenumbers generally decrease in the mixed monolayers, suggesting that the order in the chains improve in these layers. Further, among the different mixed systems, the monofilms with SAM show the maximum decrease, suggesting that these mixed films have the best order. Generally, from the scissoring CH2 vibrations, the overall molecular structure may be determined. Further, the δ(CH2 ) did not show any splitting, suggesting that the packing of subcell structure may not be orthorhombic. Moreover, if the functional groups in the polar region of the compounds do indeed take part in H-bonding, the CH2 groups nearer the head group may show partial disorder, whereas the rest of the alkyl chain may still be highly ordered. From the position of the δ(CH2 ) bands, which are generally considered to be indicators for intrachain interactions, some general predictions can be made. In the present case, in compounds 1 and 3 the band at 1464.2 and 1457 cm−1 shifts to higher wavenumbers between 1470 and 1474 cm−1 . The 1472 cm−1 band is usually attributed due to the methylene chains packing in a parallel arrangement corresponding to a triclinic subcell. For compound 2, the shift is not very dramatic between the pure compound and the mixed systems, indicating that the intrachain interactions may not be significant enough to cause large changes in the packing. Figs. 6a–6c show the Brewster angle micrographs (BAM) of the three pure amphiphiles in the LE and LC phase (left extreme) and their mixed films with SA, SAL, and SAM (right extreme), respectively. From the micrographs it is seen that all the compounds show some anisotropy in the LE phase, which
Table 4 Surface tension components for the pure amphiphiles Sample C18 Gly C18 Phe C18 Tyr
Surface energy (mN/m) components γs
γs
γsd
γ−
γ+
45.1 35.7 29.0
35.7 22.8 13.1
9.39 12.9 16.2
26.2 32.3 11.5
12.19 4.02 3.75
p
disappears in the LC phase. In the case of compounds 1 and 3 there seem to be local assemblies that are highly directional, while for compound 2, the domains formed seem to have similar orientations. The LC phase shows fairly uniform contrast, indicating fairly oriented close-packed structures. It was found that for most mixed films, the domain formation did not change appreciably. However, the sizes of the domains decreased considerably with larger zero-contrast areas, indicating that mixtures are more homogeneous than pure films. The surface tension and the different components of the surface tension are estimated using the contact angles of water, hexadecane, and DMSO (see Table 4). From the values of total surface energy γs it is seen that C18 Tyr shows a nonpolar p character, and this is borne out also by the low polar γs and d high dispersive γs components of the surface energy. Further, the acidic component γ + also shows a low value. Tyrosine, an aromatic amino acid, is derived from phenylalanine by hydroxylation in the para position. While tyrosine is hydrophobic, it is significantly more soluble than phenylalanine. The phenolic hydroxyl group of tyrosine is significantly more acidic and thus should show a stronger acidic character. In the case of mixed films, the average surface properties did not change drastically and hence have not been presented here. 4. Conclusion The values of average surface excess (maximum surface pressure π ) and the average area/molecule of the lipoamino acids compare well with the similar amino acid derivatives reported earlier [1–8]. The amino acid-based surfactants are possible candidates for pharmaceutical applications. The lipoamino acids used in the present study could be used as model nanoadsorbants, whose structure is simple and robust, perfectly defined, and known in detail to some extent. They could be used to study local polarities and changes in overall surface properties. Acknowledgment The authors thank the Department of Science and Technology, Government of India, for a project grant under which part of this work was carried out. References [1] [2] [3] [4]
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[23] N. Nandi, D. Vollhardt, Colloids Surf. A Physicochem. Eng. Aspects 198– 200 (2000) 207. [24] E. Vulfson, in: K. Holmberg (Ed.), Novel Surfactants, Dekker, New York, 1998, p. 279. [25] R. Valivety, P. Jauregi, J. Am. Oil Chem. Soc. 74 (1997) 879. [26] M.R. Infante, L. Pérez, A. Pinazo, in: K. Holmberg (Ed.), Novel Surfactants, Dekker, New York, 1998, p. 87. [27] M.R. Infante, L. Pérez, A. Pinazo, P. Clapés, M.C. Morán, M. Angelet, M.T. García, M.P. Vinardell, C. R. Chim. 7 (2004) 583. [28] D. Honig, D. Mobius, J. Phys. Chem. 95 (1991) 4590. [29] W.D. Harkins, The Physical Chemistry of Surface Films, Reinhold, New York, 1954, p. 107. [30] J.T. Davies, E.K. Rideal, Interfacial Phenomena, second ed., Academic Press, New York, 1963, p. 265. [31] P. Dnarowicz-Latka, K. Kita, Adv. Colloid Interface Sci. 79 (1999) 1. [32] J.N. Israelachvili, Intermolecular and Surface Forces, second ed., Academic Press, San Diego, 1992. [33] A. Gericke, R. Mendelsohn, Langmuir 12 (1996) 758.
Note
Langmuir and Langmuir–Blodgett films of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of amino acids M. Kanthimathi, Aruna Dhathathreyan ∗ Chemical Laboratory, CLRI, Adyar, Chennai 600020, India Received 4 September 2006; accepted 6 January 2007 Available online 16 February 2007
Abstract Langmuir and Langmuir–Blodgett monolayers of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of glycine, tyrosine, and phenylalanine were studied using π –A isotherms and photoelastic modulated FTIR (PEM-FTIR). Based on compression modulus and interaction parameters, mixed monolayers of these compounds with stearylamine (SAM) showed well-organized monolayers compared to mixed systems with stearic acid (SA) and stearyl alcohol (SAL). The pure amphiphiles exhibited fairly well-ordered packing in the films, and in the mixtures, the ordering increased and showed a triclinic packing arrangement. For the phenylalanine amphiphile the packing showed slight disorder compared to the other two compounds. Surface properties of the LB films of these compounds on solid substrates were analyzed using static and dynamic contact angles of a series of liquids. The surface tension of coated substrates reflected clearly the highly acidic character. Fluidlike monolayers having a molecularly rough surface indicated high wettability for n-alkanes. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettability and small hysteresis. © 2007 Elsevier Inc. All rights reserved. Keywords: Lipoamino acids; LB films; Wettability; Mixed monolayers
1. Introduction In recent years, investigations of amino acid amphiphiles have gained much importance, since amino acids are known to play an important role in model membrane studies [1–9]. Interesting work on aggregation properties of amino acid amphiphiles has been reported by Huang and co-workers [10]. Acidic amphiphiles based on L-alanine surfactant with specific structure-directing properties have been explored by Gin and co-workers [11]. New materials such as gel emulsions from these amphiphiles for specific applications have been achieved [12]. An interesting application of amino acid amphiphiles in discriminating between N- and C-termini of peptides has been reported in the literature [13]. Roy and Dey’s paper on self-organization of sodium N -(11-acrylamidoundecanoyl)-L-valinate using urea dealt with changes in local clusters of water near air/water interfaces [14]. * Corresponding author. Fax: +91 44 24911589.
E-mail address: [email protected] (A. Dhathathreyan). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.01.016
Langmuir–Blodgett (LB) films of N -octadecanoyl-L-alanine amphiphile, the simplest chiral amino acid derivative, have been studied by FTIR spectroscopy [1–3], where the enantiomeric molecules assemble regularly through an extended intermolecular hydrogen-bonding network and twist from neighbor to neighbor to develop chirality of the aggregate in the twodimensional crystalline array [4,5]. Intermolecular hydrogenbonding interactions and chiral effects between hydrophilic head groups enhancing the interactions between hydrophobic chains in these long-chain amphiphiles of amino acids have been demonstrated [1–6]. Assemblies of such amphiphilic derivatives of amino acids and their mixed films with lipid monolayers seem to be good model systems for studying cell– protein interactions. In order to design ordered structures using LB films of any amphiphile, one needs to first assemble a monolayer at the air/ water interface. It is now well known that the polar groups of the amphiphile have a pronounced effect on the structure of the condensed phase domains in monolayers [15,16]. It is expected that the problem of understanding molecular interactions in monolayers (two-dimensional (2d) systems) is relatively easy
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compared with that in three-dimensional systems, due to reduction of one degree of freedom. Also, control over an external variable (pressure) enables one to vary the intermolecular separation, and therefore the molecular interaction is relatively easier to tune in monolayers than in bulk solids or liquids, which are less compressible. The competition between the line tension and the electrostatic repulsion [15,17–22] in different polar groups has been shown to be an important factor in the shape transitions of nonchiral amphiphiles. A recent study on N -palmitoyl aspartic acid, N -stearoylserine methyl ester, N -palmitoylallothreonine methyl ester, and N -stearoylallothreonine methyl ester showed the importance of orientation and minimal energy conformations in determining the packing arrangements of the thin monofilament [23], which have potential as models in fibril formations in proteins. Thus, amino acid amphiphiles and short peptides constitute an important class of surfactants. Several amino acids have been investigated for this purpose, with the majority of papers dealing with basic amino acids such as arginine and lysine, from which cationic surfactants can easily be prepared. Most synthetic procedures are based on organic synthesis, but enzymatic processes have also been explored [24–26]. An exhaustive and excellent review on amino acid-based surfactants has been recently published by Infante et al. [27]. In this study we have prepared N -(4-octadecyloxy-2-hydroxybenzylidene derivatives of glycine (compound 1), tyrosine (compound 2), and phenylalanine (compound 3) using a procedure similar to that of Huang and co-workers [10] and have explored the properties of monolayers as well as their ability to form stable LB films on solid surfaces. The Langmuir films at air/water interfaces have been characterized using Brewster angle micrographs (BAM) and the LB films of these amphiphiles transferred to solid substrates have been analyzed using static contact-angle measurements. Further changes in the conformation of the packed films and their mixtures with stearic acid (SA), stearyl alcohol (SAL), and stearylamine (SAM) have been studied using PEM-FTIR. 2. Materials and methods All solvents used in this investigation, chloroform, methanol, DMSO, and hexadecane, were HPLC-grade solvents of high purity and used as received. 2,4-Dihydroxybenzaldehyde, glycine, phenylalanine, tyrosine, and 1-bromooctadecane were obtained from Aldrich Chemicals, USA and were more than 99% pure. The synthesis of the amphiphiles followed a proce-
Scheme 1. Energy-minimized structures of C18 Gly (compound 1), C18 Phe (compound 2), and C18 Tyr (compound 3).
dure similar to that of Huang and co-workers [10]. All three octadecyloxy derivatives were prepared using the same procedure from the corresponding dihydroxybenzilidine derivatives. Scheme 1 shows the energy-minimized structure of the three compounds. The proton NMR spectra of the samples dissolved in DMSO-D6 were recorded and the results are presented in Table 1. 2.1. Langmuir and Langmuir–Blodgett films (LB films) of compounds 1, 2, and 3 A NIMA single-barrier Model 601-S with a Wilhelmy balance and a thermostat was used to study the characteristics of the Langmuir films and the surface pressure–molecular area
Table 1 Characteristic chemical shifts for compounds C18 Gly, C18 Phe, and C18 Tyr Groups
C18 Gly
C18 Phe
C18 Tyr
Signals
δ-value
Signals
δ-value
Signals
δ-value
CH group in amino acid
(s, 2H, CH2 )
0.87
Long-chain hydrocarbon Azo group CH=N Aromatic ring attached to the long chain Aromatic ring present in the amino acid chain
(s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) –
1.25 2.05 6.66 –
(t, 1H, CH) (m, 2H, CH2 ) (s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) (m, 5H, phenyl H)
0.87 1.74 1.25 2.53 6.5 7.40
(t, 1H, CH) (m, 2H, CH2 ) (s, 37H, C18 H37 ) (s, 1H, CH) (m, 3H, phenyl H) (m, 4H, phenyl H)
0.82 1.75 1.20 2.45 6.67 7.61
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(π–A) isotherms. Surface pressure was measured with an accuracy of 0.1 mN/m. 2.2. Brewster angle microscopy In situ measurements on the spread monolayers were carried out using a BAM setup described elsewhere [28]. The amphiphiles were spread using chloroform (CHCl3 ) (HPLC grade) as a spreading solvent. A minimum waiting time of about 10 min elapsed before the films were compressed at the air/water interface. A compression speed of 5 Å2 /molecule/min was used for all the compounds, and the experiments were carried out at a temperature of T = 22 ◦ C. The subphase used was at pH 7, prepared using phosphate buffer (concentration 5 mM). The films of the pure compounds were transferred onto cleaned glass cover slips by the LB film technique at π = 20 mN/m. At this pressure all the monolayers were in the liquid condensed state (LC) and away from collapse. The films showed uniform X-type transfer and the transfer ratio was nearly 0.9. The contact angles were estimated for the three films using a NIMA 9005 tensiometer. The mixed films of the three amphiphiles with stearic acid (SA), stearyl alcohol (SAL), and stearylamine (SAM) were prepared for mol ratios between 0 and 0.9 (of the amphiphiles). The results reported here are for mole ratio 0.5. Using Young’s equation, the solid/liquid interfacial tension may be evaluated as γlv cos θ = γsv − γsl ,
(1)
where γlv , γsl , and γsv are the surface tension of the liquid at the liquid/vapor interface, the interfacial tension at the solid/liquid interface, and the surface tension of the solid substrate at the solid/vapor interface. θ is the angle made by the liquid in contact with the surface coated with the amphiphile. By estimating the change in contact angles for pure water, octadecane, and dimethyl sulfoxide (DMSO), the total surface p d energy γsv can be found from γsv (polar component) and γsv (dispersive component) using the equation γ = γ p + γ d,
(2)
where the superscripts d and p show dispersive and polar components. The polar component may further be divided into the acidic and basic parts given by γ + , γ − , respectively, using the equation + − 1/2 p γsv = 2 γsv (3) . γsv 2.3. PEM-FTIR measurements on Langmuir films The measurements of PEM-FTIR was carried out with a Brucker IF S48 spectrometer using a gold grid polarizer to obtain s- or p-polarized radiation modulated by a ZnSe photoelastic modulator (Hinds type II) at Brewster incidence. The frequencies of the methylene stretching vibration mode have been previously used as an indication of the ordered states of
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the packing of the monolayer in the condensed state. The frequencies, shapes, and intensities of the infrared C–H stretching bands provide information on the orientation, conformation, and packing of the hydrocarbon chain in the monolayer. The CH2 (asymmetric) and CH2 (symmetric) stretching bands appearing in the spectrum correspond to a preferential in-plane orientation of transition moments, which is consistent with the alkyl chains being quasi perpendicular to the interface. The signals from the hydrophilic groups immersed in water, being weak, cannot be observed directly in the spectrum. Hence only the bands corresponding to the long alkyl chains are indicated here. For different molecular areas during compression of the monolayer, the asymmetric and symmetric CH2 bands were recorded. 3. Results and discussion π–A isotherms of the three amphiphiles spread at the air/water interface are shown in Fig. 1. The ability of the lipoamino acids to form H-bonding networks was tested using mixed layers of these compounds with stearic acid, stearyl alcohol, and stearylamine at the air/water interface and the π –A isotherms studied. Repeated compression and expansion of the films did not show any appreciable hysteresis. In order to assess the stability of these films at air/water interface, all the films were maintained at a constant pressure of π = 20 mN/m. This surface pressure was chosen because almost all the monolayers showed a liquid condensed state at this pressure, and this pressure is below the collapse pressure. Further, the change in area at a constant surface pressure π = 20 mN/m as a function of time (carried out over 20 min) was within 3 to 5% of the initial area, indicating that the films were quite stable over the period and LB film transfer at this pressure can be effected. Thus all three compounds form fairly stable Langmuir films and show a liquid expanded (LE) to liquid condensed (LC) phase with average area varying between 0.192 nm2 (C18 Gly) (compound 1) and 0.254 nm2 (C18 Tyr) (compound 3) Å2 /molecule at π = 20 mN/m. In the case of the glycinederived lipoamino acid, the area is quite close to that of the stearyl chain, and the compactness in the layer could arise from the fact that glycine can form tighter H-bonds with water at the interface than the other compounds. Tyrosine is more nonpolar than glycine and has a larger polar group. Therefore the packing of the molecules at the interface is loose, leading to a larger area. In the case of the compound derived from phenylalanine (compound 2), the –OH group present on the aromatic ring in tyrosine is absent. Moreover, the aromatic rings seem to pack in such a way that they are almost tilted at an angle to the layer plane, thus leading to an area of about 0.2 nm2 /molecule. The compression modulus Cs−1 is a parameter used to define the different states of the monolayer [29,30] and the collapse pressure and is obtained from Cs−1 = −A(∂π/∂A). Generally its values lower than 12.5 mN/m are attributed to the gaseous state, values between 12.5 and 100 mN/m to the liquid
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Fig. 1. Surface pressure–molecular area (π –A) isotherms of C18 Gly, C18 Phe, and C18 Tyr at air/water interface, pH 7.0, T = 22 ◦ C.
expanded state, and 100 to 250 mN/m to the liquid condensed state, while Cs values greater than 250 mN/m are assigned to the solid state of the film. At the collapse pressure the value approaches zero. Using this criterion, the maximum collapse pressures for the different systems used in the present study have been estimated. The maximum collapse pressure varies for each compound. The mixed films of the two components were studied for mole fractions ranging from 0.1 to 0.9 of the amino acid amphiphile. It was found that beyond 0.5 there was not much change in the characteristics of the films, suggesting that the components are possibly phase-separating beyond a mole fraction of 0.5. Table 2 shows the area/molecule at π = 20 mN/m and the maximum collapse pressure πc for the pure compounds and in the mixed films. Fig. 2 presents the plots of πc versus matrix mole fraction, where the matrix means SA, SAL, or SAM, respectively. It is seen that for mixtures containing a wide range of mole fractions there is no dramatic change in collapse pressure after a mole fraction of 0.5. For values less than 0.5, in most cases it increases with increasing mole fraction of SA, SAL, or SAM. This could be interpreted as a mixing of SA, SAL, or SAM in Langmuir monolayers of compounds 1, 2, or 3 till a mol fraction of 0.5 is reached. Fig. 3a shows the isotherms of the compounds with stearic acid. The isotherms of the mixed films show that all the
Fig. 2. Plot of πc against mole fraction of matrix (SA, SAL, or SAM) for compounds C1, C2, and C3.
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Table 2 Area/molecule and maximum collapse pressure for the different amphiphiles and the mixed films with SA, SAL, and SAM Compound
Area/molecule (nm2 )
Maximum collapse pressure (mN/m)
C18 Gly—compound 1 C18 Phe—compound 2 C18 Tyr—compound 3 Compound 1 + SA Compound 2 + SA Compound 3 + SA Compound 1 + SAL Compound 2 + SAL Compound 3 + SAL Compound 1 + SAM Compound 2 + SAM Compound 3 + SAM
0.19 0.20 0.25 0.52 0.57 0.63 0.53 0.63 0.48 0.25 0.31 0.32
28.1 26.5 40.1 26.1 31.1 39.9 56.9 43.1 35.8 37.0 44.5 43.5
Scheme 2. Scheme for mixed films of the compounds with SA or SAL (mole ratio 1:1).
isotherms show an intermediate phase between the LE and LC states. The appearance of a plateau is seen for compounds 2 and 3, while for 1 there is a slight change in the slope. The area/molecule now lies between 0.52 (compound 1) and 0.63 nm2 /molecule (compound 3). Doubling of the area in all the cases is possible due to a dimer-like structure formed between stearic acid and stearyl alcohol and the compounds as shown in Scheme 2. Fig. 3b shows the isotherms for the mixed films of the amino acid derivatives with stearyl alcohol. Here again there is an expansion in the average area/molecule and the new phase between the LE and LC phases is seen clearly as a change in slope in compound 1. The areas range from 0.48 nm2 (compound 3) to 0.53 and 0.63 nm2 per molecule (compounds 1 and 2). The maximum collapse pressure is also now higher for all three films, compared to that for the pure compounds. It was found that as the mole fraction of the additive increases from 0 to 0.5, the collapse pressure also increases, indicating that the two components are fairly miscible. Stearyl alcohol is expected to have an annealing effect in monolayers [27] and aids in more homogeneous packing of the molecules. This could, in turn, lead to higher collapse pressures. Fig. 3c shows the isotherms of the three lipoamino acids with stearylamine. In the mixed films with C18 Phe and C18 Tyr, there is a steep change in the slope at a π value of about 12.5 mN/m, and the area also ranges between 0.25 and 0.32 nm2 /molecule for all three compounds. It is well established that the amino group in SAM can form compact H-bonds, and therefore the
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monolayers show lower areas than SA and SAL mixed films. Here again the maximum collapse pressure is higher than the pure films. An excess free energy of mixing (Gexc ) at a surface pressure [31,32] given by exc (G ) = A12 − (A1 X1 + A2 X2 ) dπ for limits 0 to π and an interaction parameter α defined as α = (Gexc )/RT X1 X22 + X2 X12 are generally used for evaluating miscibility and phase separations in mixed films. In the present study these values were evaluated for surface pressures of 5, 10, and 20 mN/m. It was found that mixed films of both SA and SAL showed a positive value of α ranging between 2.7 and 2.9, while with SAM the values were negative. The positive sign of α suggests less interaction between the amphiphilic amino acids and SA and SAL, compared to the interaction between the molecules of the amphiphiles themselves. On the other hand, mixed films of all three compounds with SAM showed negative values, suggesting that the interactions between the amino acid amphiphiles and SAM are attractive. Figs. 4a–4c show the plots of the interaction parameter α as a function of mole fraction of SA, SAL, or SAM in mixed films of compounds 1, 2, and 3 for surface pressures of 5, 10, and 20 mN/m, respectively. For mixed films of the compounds with both SA and SAL, the α values are positive, which suggests that perhaps the interactions between the molecules are not attractive. Regarding mixtures with SAM, values of α are lower than for the mixtures with SA or SAL. These results suggest the possibility of a single-phase system for particular compositions of compounds 1, 2, and 3, especially with respect to SAM. Fig. 5a shows the IR spectra of compounds 1, 2, and 3 in the close-packed structure, while Fig. 5b shows the wavenumber dependence of the alkyl chains in compounds 1, 2, and 3 during compression as a function of the area/molecule. The frequencies of the methylene stretching vibration modes have been previously used as an indication of the ordered states of the packing of the monolayer in the condensed state [33]. The frequencies, shapes, and intensities of the infrared C–H stretching bands provide information on the orientation, conformation, and packing of the hydrocarbon chain in the monolayer. The ν as CH2 (asymmetric) and νs CH2 (symmetric) bands appearing in the spectrum correspond to a preferential in-plane orientation of transition moments, which is consistent with the alkyl chains being quasi-perpendicular to the interface. It has been reported that the ν as CH2 vibrations are sensitive to conformation and can be correlated with the trans/gauche ratio of the hydrocarbon chain [33]. Normally lowering of wavenumbers is characteristic of highly ordered conformations with preferential all-trans characteristics, while the number of gauche conformers increases with increasing wavenumber and width of the band. The increased wavenumber of the methylene stretching vibration for a gauche rotamer is caused by a coupling between the carbon
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(a)
(b)
(c) Fig. 3. Mixed films of C18 Gly, C18 Phe, and C18 Tyr with (a) SA, (b) SAL, and (c) SAM at air/water interface; mole ratio 1:1, pH 7.0, T = 22 ◦ C.
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(a)
343
(b)
Fig. 4. Plots of interaction parameter α against mole fraction of SA, SAL, or SAM for (a) compound 1, (b) compound 2, and (c) compound 3. Curves 1, 2, and 3 correspond to π = 5, 10, and 20 mN/m.
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(a)
(b) Fig. 5. (a) PEM-FTIR spectra of compounds C18 Gly, C18 Phe, and C18 Tyr. (b) Area/molecule against change in wavenumber corresponding to asymmetric methylene stretch for the three amphiphiles.
(c) Fig. 4. (continued)
atoms and the methylene hydrogen, which due to interconversion around the C–C bond is positioned in the plane defined by the carbon atoms, resulting in an increased force constant for that C–H bond. In contrast, for an all-trans conformation all methylene hydrogens are out of the plane. From Fig. 5a it is seen that the anti-symmetric methylene stretching vibrations for the three compounds appear between 2922 and 2916 cm−1 , while the symmetric stretching occurs between 2850 and 2844 cm−1 . For compounds, the carbonyl stretching appears at around 1703 and 1692 cm−1 , while those with SA, SAM, and SAL appear between 1729 and 1716 cm−1 . Such shifts to higher wavenumbers have been assigned to unprotonated and protonated carbonyls by Gericke and Hühnerfuss [1]. In the present study it is possible that with the Hbonding taking place in the presence of SA, SAM, or SAL, the carbonyl stretching shifts to higher wave numbers. The amide I and II bands appear between 1620 and 1608 cm−1 , while the
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345
(a)
(b)
(c) Fig. 6. BAM micrographs of (a) C18 Gly, (b) C18 Phe, and (c) C18 Tyr and their mixed films at air/water interface, pH 7.0, T = 22 ◦ C. Top: LE phase (surface pressure π = 5 mN/m); bottom: LC phase (surface pressure π = 20 mN/m) (scale bar denotes 20 µm). The micrographs from left to right correspond to amino acid amphiphiles and their mixtures with SA, SAL, and SAM, respectively.
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Table 3 Alkyl stretch and scissoring vibrational frequencies corresponding to the different amphiphiles in close packed monolayers Sample Compound 1 Compound 1 + SA Compound 1 + SAM Compound 1 + SAL Compound 2 Compound 2 + SA Compound 2 + SAM Compound 2 + SAL Compound 3 Compound 3 + SA Compound 3 + SAM Compound 3 + SAL
Vibrational frequencies (cm−1 ) νas (CH2 )
νs (CH2 )
δ (CH2 )
2921.7 2917.8 2916.6 2917.5 2925.0 2922.4 2919.2 2920.3 2928.5 2923.4 2922.1 2923.8
2890.2 2855.4 2852.7 2858.7 2850.9 2838.5 2835.4 2835.6 2852.5 2850.9 2849.4 2850.2
1464.2 1471.1 1472.0 1470.1 1467.1 1467.6 1468.9 1464.1 1457.0 1468.2 1474.1 1474.1
bands around 1459–1447 cm−1 are attributed to methylene scissoring vibrations. For mixtures of compounds 1, 2, and 3 with SA, SAM, and SAL, the changes in the molecular order can be seen from the shifts in the νas and νs stretching vibrations corresponding to the CH2 . For the sake of brevity only the CH2 stretching vibrations, asymmetric and symmetric and scissoring band δ(CH2 ), are presented here in Table 3. Fig. 5b shows that there is an overall decrease in the wavenumbers as the films are compressed, giving rise to an overall increase in order of the alkyl chains. From the table of values it is seen that the asymmetric νas wavenumbers generally decrease in the mixed monolayers, suggesting that the order in the chains improve in these layers. Further, among the different mixed systems, the monofilms with SAM show the maximum decrease, suggesting that these mixed films have the best order. Generally, from the scissoring CH2 vibrations, the overall molecular structure may be determined. Further, the δ(CH2 ) did not show any splitting, suggesting that the packing of subcell structure may not be orthorhombic. Moreover, if the functional groups in the polar region of the compounds do indeed take part in H-bonding, the CH2 groups nearer the head group may show partial disorder, whereas the rest of the alkyl chain may still be highly ordered. From the position of the δ(CH2 ) bands, which are generally considered to be indicators for intrachain interactions, some general predictions can be made. In the present case, in compounds 1 and 3 the band at 1464.2 and 1457 cm−1 shifts to higher wavenumbers between 1470 and 1474 cm−1 . The 1472 cm−1 band is usually attributed due to the methylene chains packing in a parallel arrangement corresponding to a triclinic subcell. For compound 2, the shift is not very dramatic between the pure compound and the mixed systems, indicating that the intrachain interactions may not be significant enough to cause large changes in the packing. Figs. 6a–6c show the Brewster angle micrographs (BAM) of the three pure amphiphiles in the LE and LC phase (left extreme) and their mixed films with SA, SAL, and SAM (right extreme), respectively. From the micrographs it is seen that all the compounds show some anisotropy in the LE phase, which
Table 4 Surface tension components for the pure amphiphiles Sample C18 Gly C18 Phe C18 Tyr
Surface energy (mN/m) components γs
γs
γsd
γ−
γ+
45.1 35.7 29.0
35.7 22.8 13.1
9.39 12.9 16.2
26.2 32.3 11.5
12.19 4.02 3.75
p
disappears in the LC phase. In the case of compounds 1 and 3 there seem to be local assemblies that are highly directional, while for compound 2, the domains formed seem to have similar orientations. The LC phase shows fairly uniform contrast, indicating fairly oriented close-packed structures. It was found that for most mixed films, the domain formation did not change appreciably. However, the sizes of the domains decreased considerably with larger zero-contrast areas, indicating that mixtures are more homogeneous than pure films. The surface tension and the different components of the surface tension are estimated using the contact angles of water, hexadecane, and DMSO (see Table 4). From the values of total surface energy γs it is seen that C18 Tyr shows a nonpolar p character, and this is borne out also by the low polar γs and d high dispersive γs components of the surface energy. Further, the acidic component γ + also shows a low value. Tyrosine, an aromatic amino acid, is derived from phenylalanine by hydroxylation in the para position. While tyrosine is hydrophobic, it is significantly more soluble than phenylalanine. The phenolic hydroxyl group of tyrosine is significantly more acidic and thus should show a stronger acidic character. In the case of mixed films, the average surface properties did not change drastically and hence have not been presented here. 4. Conclusion The values of average surface excess (maximum surface pressure π ) and the average area/molecule of the lipoamino acids compare well with the similar amino acid derivatives reported earlier [1–8]. The amino acid-based surfactants are possible candidates for pharmaceutical applications. The lipoamino acids used in the present study could be used as model nanoadsorbants, whose structure is simple and robust, perfectly defined, and known in detail to some extent. They could be used to study local polarities and changes in overall surface properties. Acknowledgment The authors thank the Department of Science and Technology, Government of India, for a project grant under which part of this work was carried out. References [1] [2] [3] [4]
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