Two-Dimensional Miscibility Behavior of Two Chemically Similar

Article pubs.acs.org/JPCC

Two-Dimensional Miscibility Behavior of Two Chemically Similar Amide Amphiphiles C. Stefaniu, G. Brezesinski, and D. Vollhardt* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany ABSTRACT: The monolayer characteristics of two chemically similar amphiphiles 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) and tetradecanoic acid-(2-hydroxyethyl)amide (TDAHA) and their selected mixtures are studied. Despite the slight structural difference (the position of the two substituents at the acid amide group), the pure components reveal large differences and peculiarities in the surface pressure−area (π-A) isotherms, the monolayer morphologies, and structures. Therefore, their miscibility behavior in monolayers is of special interest. At low temperatures (T ≤ 10 °C), the π-A isotherms of the pure components show a striking second critical point accompanied by an abrupt change of important 2D lattice parameters, indicating the existence of a second phase transition between two condensed phases. This second phase transition is strongly temperature-dependent for TDAHA, nearly temperature-independent for HTPA, and not occurring in the investigated mixtures. The results of Brewster angle microscopy and grazing incidence X-ray diffraction support ideal miscibility of HTPA-TDAHA monolayers already suggested by the linear relationship between the main phase transition pressure and the mole fraction. The lattice parameters of the mixed TDAHA-HTPA monolayers measured for three mole fractions (0.25, 0.5, and 0.75) at 5 °C are compared and discussed. The tilt angle in the mixed TDAHA-HTPA monolayers passes through a minimum, which is connected to the largest cross-sectional area and the smallest entropy change during the main LE/LC (liquid expanded/ liquid condensed) transition. An HTPA-TDAHA phase diagram that illustrates the transitions between the different phases is proposed.

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acylation,29,30 giving the possibility for the synthesis of a variety of derivatives. To understand the effect of substituents at the amide group on the monolayer features of N-acylethanolamines, two similar amphiphiles N-myristoyl-ethanolamine (C 13H 27 -CO-NHC2H4OH; TDAHA)31 and 3-hydroxy-N-tridecyl propanoic acid amide (C13H27-NH-CO-C2H4OH; HTPA)32 have been prepared and compared.33,34 These small changes in the chemical structure (position of the substituents) created pronounced differences in the monolayer characteristics, as demonstrated by surface pressure−area per molecule (π-A) isotherms, Brewster angle microscopy (BAM) imaging, and grazing incidence X-ray diffraction (GIXD) measurements. The miscibility of two insoluble amphiphiles in the monolayer at the air/water interface has attracted attention over several decades. In the early history of monolayer research, phase diagrams of Langmuir monolayers of fatty acids and their esters were obtained on the basis of π-A isotherms and the comparison with the 3-D polymorphism.35−39 Later on, BAM40,41 and GIXD42,43 have been additionally used to distinguish between complete phase separation and ideal miscibility. The mixing capability of two components can be affected by the steric compatibility of the amphiphilic

INTRODUCTION The chemical structure of amphiphiles affects sensitively the characteristic features of Langmuir monolayers.1,2 Previous work proved the influence of biologically important functional groups, such as −OH, −COOH, −COOR, −O−, −NH−CO−, −NH2, on the phase behavior and packing properties in condensed-phase monolayers.5,6 Systematic alterations of the headgroup structure have a strong influence on the monolayer properties, as found in monoglycerol amphiphiles using amide, ether, ester or amine groups,3 in phospholipids with a different number of methyl groups at the nitrogen of the headgroup4 or in striking chiral discrimination effects.7,8 In biological systems, the role of the amide group as integral part of sphingolipids is of special interest. Therefore, biomimetic monolayer studies have focused on adequate model systems containing amides and amines.9−11 Systematic information was obtained about the main monolayer characteristics of various tailored amphiphiles, whose headgroup consists of an acid amide group and one or two hydroxyl groups separated by one or more methylene groups.12−22 Special interest found amphiphilic derivatives of ethanolamine not only because of their occurrence in a wide variety of animals, plants, and microbes23,24 but also due to their interesting biological, pharmaceutical, and medicinal properties.25−28 The existence of an amino group and a hydroxy group in ethanolamines offers the possibility for N- or O© 2012 American Chemical Society

Received: December 2, 2011 Revised: January 11, 2012 Published: January 13, 2012 6268

dx.doi.org/10.1021/jp211610k | J. Phys. Chem. C 2012, 116, 6268−6274

The Journal of Physical Chemistry C

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molecules44−48 and their lateral interaction.49 Therefore, it was of general interest to understand the miscibility behavior of the two chemically similar acid amide amphiphiles TDAHA and HTPA. The objective of the present work is to study the major interfacial characteristics of the two amphiphiles in mixed TDAHA/HTPA monolayer systems and to compare them with those of the pure components.

HASYLAB, DESY (Hamburg, Germany). At BW1, a Langmuir film balance equipped with a Wilhelmy plate was positioned in a hermetically closed container flushed with helium. A monochromatic synchrotron X-ray beam (λ = 1.304 Å) was adjusted to strike the helium/water interface at a grazing incidence angle αi = 0.85αc (αc = 0.13°) lighting up roughly 2 × 50 mm2 of the surface. The trough was laterally moved during the measurements to prevent any sample damage by the strong X-ray beam. For the measurement of the diffracted signal a MYTHEN detector system (PSI, Villigen, Switzerland) was rotated to scan the in-plane Qxy component values of the scattering vector. The vertical strips of the MYTHEN measured the out-of-plane Qz component of the scattering vector between 0.0 and 0.75 Å−1. The diffraction data consisted of Bragg peaks at diagnostic Qxy values. The in-plane lattice repeat distances d of the ordered monolayer structure were calculated from the Bragg peak positions: d = 2π/Qxy. The in-plane coherence length Lxy was approximated from the full width at half-maximum (fwhm) of the Bragg peak using Lxy ≈ 0.9(2π)/ fwhm (Qxy). The diffracted intensity normal to the interface was integrated over the Qxy window of the diffraction peak to calculate the corresponding Bragg rod. Experimental details have been described in the literature.54

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EXPERIMENTAL SECTION Materials. The amphiphilic amides TDAHA (tetradecanoic acid-(2-hydroxyethyl)amide, C13H27-CO-NH-C2H4OH) and HTPA (3-hydroxy-N-tridecyl propanoic acid amide, C13H27NH-CO-C2H4OH) were synthesized, as previously described in detail.31,32 The chemical purity of the amphiphiles was checked by elemental analysis, 1H NMR, and HPLC. The chemical structures of the single-chain amides with exchanged position of the two substituents at the acid amide group TDAHA and HTPA are shown below.

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RESULTS AND DISCUSSION The experimental π-A isotherms obtained at two different temperatures (5 and 15 °C) for the pure TDAHA and HTPA are presented together with those of selected mixtures in Figure 1. The exchange of the position of the two substituents at the acid amide group (TDAHA compared with HTPA) triggers clear differences in the π-A isotherms. At 5 °C, TDAHA exhibits a fully condensed isotherm, whereas HTPA shows a first-order phase transition at 11.6 mN/m. As expected, increasing temperature increases the transition pressure from the fluid to the condensed phase. Therefore, at 15 °C, TDAHA has a transition pressure of 5.5 mN/m and HTPA of 25 mN/m. Below 11 °C, both pure components show an interesting second transition between two condensed phases (Figure 1A). The area change at the second inflection point is very small compared with the main phase transition; for example, ΔS of TDAHA (−23 J/(mol·K)) at 10 °C is only one-tenth of the ΔS value for the transition from the fluid (LE (liquid-expanded), G (gas-analogous)) to the condensed phase. It is interesting to note that the pressure of this second transition is nearly

The used spreading solvent was chloroform (p.a. grade, Baker, Holland). Ultrapure water produced by “Purelab Plus” was used as subphase. Surface Pressure Measurements and Brewster Angle Microscopy. An experimental setup consisting of a self-made computer interfaced film balance coupled to a Brewster angle microscope (BAM1+, NFT, Göttingen, Germany) was used to measure the equilibrium surface pressure (π-A) isotherms at a compression rate of ≤10 Å2/(molecule·min).50 Using the Wilhelmy method, the surface pressure was measured with a filter paper plate with an accuracy of ±0.1 mN·m−1 and the area per molecule with ±0.5 Å2. The lateral resolution of the BAM1+ was ∼4 μm. Simple imaging processing software was used to improve the contrast. Detailed information about the BAM method is given elsewhere.51−53 X-ray Diffraction Measurements. The lattice structure in the condensed monolayers was investigated using grazing incidence X-ray diffraction measurements at the BW1 beamline,

Figure 1. π-A isotherms of TDAHA (xTDAHA = 1), the mixtures with xTDAHA = 0.75, 0.5, 0.25, and HTPA (xTDAHA = 0) (from bottom to top) monolayers spread on water and measured at 5 (A) and 15 °C (B). 6269



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independent of temperature for HTPA32,33 (∼17.5 mN/m), whereas it is strongly dependent on temperature for TDAHA.31 The transition pressure of TDAHA increases (∼1.9 mN/ (m·K)) with increasing temperature. This second transition cannot be observed at temperatures above 10 °C because either the main transition pressure is already higher than the expected pressure of this second phase transition (in the case of HTPA) or the transition pressure is above 40 mN/m and not accessible because of the reduced stability of the monolayer (in the case of TDAHA). Therefore, the diffraction experiments with the HTPA/TDAHA mixtures have been performed at 5 °C. It is interesting to note that the second condensed/condensed phase transition observed at low temperatures for both pure components does not occur in any mixture investigated. The main phase transition pressure is between the values of the pure components. Plotting this main phase transition pressure versus the mole fraction shows that the dependence can be described by a linear behavior. (See Figure 2.) This is valid for all

comparison with the domain shapes of the single components shows a clear resemblance to the HTPA domains. On further compression, additional straight branches on both sides of the straight arms are developed in the growth process. The effect of TDAHA on the mesoscopic domain shape is hardly observed. The domains indicate high crystallinity in the equimolar mixture. To provide unequivocal evidence of the mixing characteristics of HTPA and TDAHA on the angstrom scale, we have performed GIXD experiments. Therefore, the condensed phases formed upon compression have been characterized. It is reasonable to assume that the two components are miscible in the disordered fluid (LE) phase. If demixing occurs during the main phase transition, then GIXD should be able to show the coexistence of two condensed structures. However, in no case have coexisting condensed phases been observed. (See Figure 4.) The lattice parameters of the mixed TDAHA-HTPA monolayers at mole fractions xTDAHA = 0.25, 0.5, and 0.75 measured at 5 °C are listed in Tables 1−6. The two single components exhibit different structures of the condensed phases. At low pressure, HTPA forms an oblique phase (obl 1), which is very similar to the NN (nearest neighbors) tilted orthorhombic structure.32 The alkyl chains are strongly tilted. The diffraction pattern changes (obl 2) at the condensed-condensed phase transition pressure (∼17.5 mN/ m). The most amazing observation was that the cross-sectional area A0 increases on monolayer compression (from 19.5 Å2 at low pressure to 20.0 Å2 at pressures above the phase transition).32 Differences in the dichroic ratio in the IRRA spectra indicated changes in the hydrogen bonding system. Computational studies showed that a shortening of the hydrogen bond separation at higher pressure in the headgroup region drives obviously the increase in separation between the alkyl chains as observed by GIXD. In the case of TDAHA, the three Bragg peaks observed at pressures below the condensedcondensed phase transition show that the structure of the condensed monolayer phase is also oblique (obl 3).31 The alkyl chains are strongly tilted in a nonsymmetry direction. However, the deviation from the NNN (next-nearest neighbor) tilted orthorhombic phase is not very pronounced. Above the condensed-condensed transition pressure, the monolayer phase changes to the orthorhombic structure with NNN tilted chains. The distortion is in NN direction; therefore, this orthorhombic phase can be designated as L2′. In contrast with HTPA, the condensed-condensed phase transition in the TDAHA monolayer is connected with a decrease in the cross-sectional area (from 19.3 Å2 to 18.8 Å2). In the L2′ phase, the dimensions of the in-plane rectangular unit cell containing two molecules are 4.89 × 7.68 Å2. Such a dense molecular packing is a fingerprint of herringbone (HB) arrangement. Addition of TDAHA to HTPA (xTDAHA = 0.25) leads to the further distortion of the lattice. The diffraction pattern with three Bragg peaks is typical for an oblique lattice and does not remarkably change upon compression. The tilt decreases from 26° at 15 mN/m to 18° at 40 mN/m keeping the crosssectional area constant at 19.4 Å2, which is the same as in the obl 1 phase of HTPA. The The following use of keep-together tags is outside the context of a table.1:1 mixture shows the diffraction pattern of an orthorhombic structure with NNN tilt and NNN distortion. Compression changes the tilt angle from 25° at 10 mN/m to 14° at 40 mN/m without changing the lattice structure. The cross-sectional area is slightly larger (19.6 Å2) compared with the other mixtures. In the 3:1 mixture

Figure 2. Main phase transition pressure πc as a function of the mole fraction of TDAHA xTDAHA of the corresponding monolayers spread on water and measured at 5, 10, and 15 °C.

temperatures investigated (5, 10, and 15 °C). A linear behavior is an indication for either ideal miscibility or complete immiscibility. In this respect, limited conclusions can be expected from the study of the mesoscopic morphology of the condensed domains appearing and growing in the fluid/condensed transition region. In general, the domains of the single-chain acid amide amphiphiles are dendritic. However, they show substancespecific features. This is demonstrated by the morphology of HTPA and TDAHA domains observed at 10 °C (Figure 3, top). The domain shape of HTPA is characterized by particularly high crystallinity. Six straight main axes grow from a center in a regular distance of ∼60° from each other, and the main axes develop numerous straight branches on both sides in the succeeding growth stages (Figure 3, top left).34 The TDAHA dendrites are less crystalline and tend to form large domains (Figure 3, top right), and the main axes are irregularly distributed around the center.32 The domain growth of the 1:1 HTPA-TDAHA mixture at 10 °C is presented in Figure 3, bottom. Only one domain type is formed, suggesting complete miscibility. This is in perfect agreement with the linear dependence of the main phase transition pressure on the mole fraction. (See Figure 2.) The 6270



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Figure 3. Top: BAM images taken in the fluid/condensed phase coexistence region during the compression of HTPA (left) and TDAHA (right) monolayers at 10 °C. Bottom: Growth process of typical domains formed above the main phase transition pressure within the fluid/condensed phase coexistence region of a 1:1 HTPA/TDAHA mixture at 10 °C. The images are taken at different compression stages (from left to right: 32, 30, and 25 Å/molecule).

Figure 4. GIXD contour plots of the corrected diffraction intensities as a function of the in-plane Qxy and out-of-plane Qz components of the scattering vector for TDAHA, HTPA, and selected mixtures (indicated by the mole fraction xTDAHA) at 5 °C and different pressures (bottom row: 20 mN/m (exception: HTPA at 17 mN/m because of the phase transition at 17.5 mN/m), top row: 30 mN/m).

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Table 4. Lattice Parameters a, b, c and α, β, γ of the Unit Cell; Lattice Distortion; Chain Tilt, t, from the Surface Normal; in-Plane Area, Axy, per Chain; and Chain CrossSectional Area, A0, of Mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.5 at 5 °C

Table 1. Bragg Peak (Qxy) and Bragg Rod (Qz) Positions and the Corresponding Full Widths at Half-Maximum Measured at Different Lateral Pressures π of Mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.25 at 5 °C π (mN/ m)

Qxy1 (Å−1)

Qz 1 (Å−1)

Qxy2 (Å−1)

Qz2 (Å−1)

Qxy3 (Å−1)

Qz3 (Å−1)

15

1.414 0.015 1.431 0.014 1.461 0.012 1.484 0.015

0.632 0.28 0.594 0.27 0.483 0.27 0.423 0.27

1.459 0.007 1.463 0.007 1.471 0.008 1.481 0.010

0.046 0.28 0.027 0.27 0.025 0.27 0.024 0.27

1.493 0.012 1.502 0.012 1.516 0.012 1.523 0.015

0.586 0.28 0.567 0.27 0.508 0.27 0.447 0.27

20 30 40

a, b, c (Å)

α, β, γ (deg)

15

4.851 5.005 5.122 4.841 4.949 5.081 4.826 4.859 5.007 4.803 4.813 4.940

122.8 119.8 117.4 122.3 120.2 117.5 121.5 120.8 117.7 121.0 120.8 118.2

20

30

40

distortion

t (deg)

Axy (Å2)

A0 (Å2)

0.06273597

26.0

21.6

19.4

0.05608534

24.6

21.3

19.3

0.04584840

20.1

20.8

19.5

0.03632508

17.5

20.4

19.4

Qxy1 (Å−1)

Qz1 (Å−1)

Qxy2 (Å−1)

Qz2 (Å−1)

10

1.401 0.022 1.433 0.026 1.464 0.029 1.500 0.032

0.664 0.27 0.570 0.28 0.498 0.27 0.372 0.28

1.475 0.012 1.485 0.013 1.496 0.013 1.506 0.010

0.332 0.27 0.285 0.28 0.249 0.27 0.186 0.28

40

10

4.840 5.096 5.096 4.831 5.006 5.006 4.816 4.921 4.921 4.811 4.830 4.830

123.3 118.4 118.4 122.3 118.8 118.8 121.4 119.3 119.3 120.3 119.9 119.9

40

Axy (Å2)

A0 (Å2)

25.4 NNN

21.7

19.6

0.04694852 NNN

21.7 NNN

21.2

19.7

0.02861930 NNN

18.8 NNN

20.7

19.6

0.005315654 NNN

13.9 NNN

20.2

19.6

distortion

t (deg)

0.06741279 NNN

Table 5. Bragg Peak (Qxy) and Bragg Rod (Qz) Positions and the Corresponding Full Widths at Half-Maximum Measured at Different Lateral Pressures π of Mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.75 at 5 °C π (mN/ m)

Qxy1 (Å−1)

Qz 1 (Å−1)

Qxy2 (Å−1)

Qz 2 (Å−1)

Qxy3 (Å−1)

Qz3 (Å−1)

10

1.401 0.020 1.427 0.021 1.452 0.026

0.702 0.28 0.656 0.28 0.621 0.27

1.474 0.010 1.481 0.012 1.485 0.011

0.351 0.28 0.328 0.28 0.249 0.27

1.495 0.014

0.372 0.27

20 30

π (mN/m)

30

α, β, γ (deg)

30

Table 6. Lattice Parameters a, b, c and α, β, γ of the Unit Cell; Lattice Distortion; Chain Tilt, t, from the Surface Normal; In-Plane Area, Axy, per Chain; and Chain CrossSectional Area, A0, of Mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.75 at 5 °C

Table 3. Bragg Peak (Qxy) and Bragg Rod (Qz) Positions and the Corresponding Full Widths at Half-Maximum Measured at Different Lateral Pressures π of Mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.5 at 5 °C

20

a, b, c (Å)

20

Table 2. Lattice Parameters a, b, c and α, β, γ of the Unit Cell; Lattice Distortion; Chain Tilt, t, from the Surface Normal; in-Plane Area, Axy, per Chain; and Chain CrossSectional Area, A0, of mixed TDAHA-HTPA Monolayers at Mole Fraction xTDAHA = 0.25 at 5 °C π (mN/ m)

π (mN/ m)

π (mN/ m)

a, b, c (Å)

α, β, γ (deg)

10

4.845 5.097 5.097 4.841 5.025 5.025 4.829 4.939 4.972

123.3 118.4 118.4 122.4 118.8 118.8 121.7 119.5 118.8

20

30

(TDAHA/HTPA), the phase structure is again orthorhombic (NNN tilt and NNN distortion) and changes only at high pressure to oblique (which might be the result of too high compression of the layer and will be ignored in the phase diagram). The cross-sectional area (19.4 Å2) is again smaller than that in the 1:1 mixture. Figure 5 shows the tilt angle, observed at 20 mN/m, versus the mole fraction xTDAHA. The only exception is HTPA, for which the tilt angle measured at 17 mN/m was considered. The reason is the condensed-condensed phase transition (∼17.5 mN/m for HTPA). To be consistent, the comparison is made only for the condensed phase formed directly from the fluid

Axy (Å2)

A0 (Å2)

26.6 NNN

21.7

19.4

0.04889631 NNN

24.7 NNN

21.3

19.4

0.03504908

23.3

20.9

19.2

distortion

t (deg)

0.06654083 NNN

phase above the main transition pressure. One can clearly see that the tilt angle passes through a minimum around the equimolar mixture in accordance with the changes in the packing density. (The largest cross-sectional area has been observed for the 1:1 mixture.) This shows that the lower packing density in the 1:1 mixture allows the alkyl chains to be less tilted because of an increased rotational freedom. The 2D Clausius−Clapeyron equation can be used for calculating the entropy change ΔS of the first-order LE/LC phase transition by ΔS = (Ac − Ae)(dπt/dT), with Ae as the molecular area at the onset of the phase transition at the transition pressure πt and Ac as the area of the condensed phase 6272



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Figure 7. Pressure versus mole fraction of TDAHA phase diagram determined at 5 °C. LE denotes the fluid phase, obl 1 is the oblique phase with a cross-sectional area of ∼19.3 Å2, and obl 2 is the oblique phase with the larger cross-sectional area (∼20.0 Å 2 ); the orthorhombic phase always has the NNN tilt, but the distortion changes from NNN to NN depending on the mole fraction.

Figure 5. Tilt angle t as a function of the mole fraction xTDAHA determined from the GIXD data (Figure 4) at 5 °C and 20 mN/m (exception: HTPA at 17 mN/m because of the phase transition at 17.5 mN/m).

at this pressure. Negative ΔS values are obtained according to the exothermic nature of the main phase transition at compression of amphiphilic monolayers and an increase in the ordering of the system. The ΔS values, which are determined from the isotherms measured at 5 °C, are wellcorrelated with the changes in the packing density. The |ΔS| values pass through a minimum around the equimolar mixture (Figure 6). Taking the ΔS values as an indication for the

monolayer exhibits at low pressures also an oblique phase (obl 3), but its structure is closer to an orthorhombic structure with NNN tilt and NNN distortion. The weak first-order transition leads to a clearly orthorhombic structure with NNN tilt and NN distortion. Mixing the two single components, which exhibit oblique phase structures just above the LE/LC transition, has different effects. Adding HTPA to TDAHA stabilizes the NNN tilted and NN distorted orthorhombic phase, which is now present over a large composition and pressure range. This structure changes between xTDAHA = 0.5 and 0.25 into an oblique one, or if we start on the other side of the phase diagram, adding TDAHA to HTPA leads to an increased distortion in the oblique phase before the whole structure changes to an orthorhombic one with NNN tilt and NN distortion. The observed behavior can be compared with a transition from NN to NNN tilted orthorhombic phases via a region of oblique tilt orientation. Usually, the NN-NNN transition can be triggered by increasing pressure, leading to a decrease in the tilt angle and occurs at a defined lateral pressure. However, such a transition can also occur in mixtures over a pressure range involving oblique lattice structures with tilt in an intermediate direction. This has been found for mixed monolayers of 1-monostearoyl-rac-glycerol (StGl) and 1-(12hydroxy)monostearoyl-rac-glycerol (12OHStGl).54 The incorporation of 12OH-StGl molecules into the lattice of StGl decreases the NN−NNN tilt transition pressure and leads to the appearance of an oblique phase. The pressure range of the intermediate oblique phase increases with increasing mole fraction of 12OH-StGl. As in the present case, the compounds were able to form hydrogen bonds, which seems to be important for the observed phase behavior.

Figure 6. Entropy change ΔS of the LE/condensed phase transition versus the mole fraction xTDAHA determined from the corresponding isotherms measured at 5 °C. The error bar of 4% is mainly due to uncertainties in the determination of Ac.

increase in order during the fluid (LE)/condensed (LC (liquidcondensed)) transition, the smallest entropy change is connected with the largest cross-sectional area observed in the condensed phase of the equimolar mixture. Figure 7 summarizes the results in a phase diagram. The two compounds, which are quite similar in their chemical structure and differ only by the orientation of the acid amide group with regard to the interface, exhibit different phase sequences. Just above the fluid/condensed (LE/LC) transition pressure, the HTPA monolayer has an oblique structure (obl 1), which is very close to an orthorhombic structure with NN tilt and NN distortion. The transition to the second oblique phase (obl 2) upon compression is a weak first-order transition. The TDAHA

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CONCLUSIONS The large differences in the monolayer characteristics of the tailored acid amide amphiphiles TDAHA and HTPA, different only in the position of the two substituents at the acid amide group, raised the question of their miscibility behavior. The pure components show two types of π-A isotherms: (i) at low temperatures (T < 11 °C) with two first-order phase transitions and (ii) at higher temperatures (T > 11 °C) with only one first6273



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order phase transition. The condensed-condensed transition is nearly independent of temperature for HTPA, whereas it is strongly dependent on temperature for TDAHA. Surprisingly, this second phase transition does not occur in any mixture of the two components. Ideal miscibility of the mixed HTPATDAHA monolayers is suggested by the linear relationship between the main phase transition pressure and the mole fraction and confirmed by BAM and GIXD experiments. The lattice parameters of the mixed TDAHA-HTPA monolayers show that the molecular tilt angle passes through a minimum at the equimolar mixture. This is connected to a maximum in the cross-sectional area and a minimum in the entropy change during the LE/LC phase transition. The observed phase diagram of the mixed TDAHA-HTPA monolayers supports the model of an oblique transition range between two orthorhombic phases with NNN and NN tilt, respectively. Such behavior seems to be connected to the ability of hydrogen bond formation in the headgroup region as previously proved for the pure components.

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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 We thank HASYLAB at DESY, Hamburg, Germany, for beamtime and excellent support and Irina Berndt for the BAM experiments. This work was supported by the Max Planck Society.

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REFERENCES

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Two-Dimensional Miscibility Behavior of Two Chemically Similar

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