Thermodynamic Characterization of Mixed Monolayers of Two Similar

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J. Phys. Chem. B 2008, 112, 10514–10519

Thermodynamic Characterization of Mixed Monolayers of Two Similar Amide Amphiphiles Different Only by Exchange of Substituents Position D. Vollhardt*,† and V. B. Fainerman‡ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, Medical Physicochemical Centre, Donetsk Medical UniVersity, 16 Ilych AVenue, Donetsk 83003, Ukraine ReceiVed: April 18, 2008; ReVised Manuscript ReceiVed: June 6, 2008

The monolayer features of two very similar amphiphiles, N-tridecyl-β-hydroxypropionic acid amide (THPA) and N-(β-hydroxyethyl) tridecanoic acid amide (HETA) and their 1:1 mixture are studied. The behavior of the mixed monolayer is of special interest as already the pure components reveal large differences of the surface pressure-area (π-A) isotherms, in particular, because of the second phase transition between two condensed phases at low temperatures in the THPA monolayer. This second phase transition occurs in the mixed monolayer, as well. The thermodynamic characteristics of the mixed HETA/THPA monolayers are compared with those of the pure components. The thermodynamic analysis is performed on the basis of the equations of state for the monolayer in the fluid (G, LE)/condensed (LC) transition region (A < Ac) with bimodal distribution (large clusters and monomers) and in the fluid (G, LE) state (A g Ac) considering the contribution of the entropy nonideality, caused by the mixing of monomers and clusters. Good agreement between the experimental π-A isotherms and the theoretical results are obtained. The results of the thermodynamic analysis allow conclusions on the specific phase properties of the mixed HETA/THPA monolayers. Finally the additive model applied for the theoretical description at higher temperatures provides good agreement with the experimental results at temperatures of g20 °C. Introduction The characteristic features of Langmuir monolayers are sensitively determined by the chemical structure of the amphiphiles.1,2 In general, specific modification of the chemical headgroup structure has provided new knowledge about the influence of the most important functional groups, such as -OH, -COOH, -COOR, -O-, -NH-CO-, and -NH2-, on phase behavior and packing properties of the condensed-phase monolayers. In recent years, interesting information has been obtained on the effect of small changes in the molecular structure of the amphiphile on the main characteristics of the monolayer, in particular the surface pressure-area per molecule (π-A) isotherms, the shape and texture of the condensed phase domains, and the two-dimensional lattice structure. For example, this is demonstrated by a systematic alteration of the headgroup structure in four monoglycerol amphiphiles by an amide, ether, ester, and amine group,3 by phospholipids different only with respect to the number of methyl groups at the nitrogen of the headgroup,4 or by fatty acids with the slightly changed OHsubstitution in the midpositions of the alkyl chain.5,6 Furthermore striking chiral discrimination effects observed in the main characteristics of amphiphilic monolayers, made evident their high sensitivity to smallest changes in the molecular structure.7,8 Mimetic monolayer studies of model amphiphiles containing amide and amine groups attracted attention9 because these groups are integral part of the general structure of sphingolipid.10,11 The enigmatic properties of sphingolipid seem to play an important role in those membrane parts that allow the transport into the cells. The main monolayer characteristics of various tailored amphiphiles have been studied whose headgroups * Corresponding author. † Max Planck Institute of Colloids and Interfaces. ‡ Medical Physicochemical Centre, Donetsk Medical University.

consist of an acid amide group and one or two hydroxyl groups separated by one or more (two or three) methylene groups.12–22 The two-dimensional miscibility of two insoluble amphiphiles in the monolayer at the air/water interface has attracted attention over several decades. Usually conclusions about the mixing process of two monolayer components can be drawn by comparing surface pressure-area per molecule (π-A) isotherms of the mixed and the pure monolayers.23–28 In the early history of monolayer research, phase diagrams of Langmuir monolayers of fatty acids and their esters were obtained on the basis of thermodynamic considerations and the comparison with the three-dimensional polymorphism. Later, the distinction between phase separation and ideal mixing has been substantiated using Brewster angle microscopy (BAM)29–32 and grazing incidence X-ray diffraction (GIXD).33,34 The mixing ability of two components is mainly affected by the steric compatibility of the amphiphilic molecules36–40 and their lateral interaction.41 Recently, the monolayer characteristics of two very similar amphiphiles, N-tridecyl-β-hydroxypropionic acid amide (THPA) and N-(β-hydroxyethyl) tridecanoic acid amide (HETA) have been studies.42 The only difference in the chemical structure between them is that the position of two substituents at the acid amide group is interchanged. These small changes in the chemical structure give rise to large differences in the monolayer features, as demonstrated by surface pressure-area per molecule (π-A) isotherms, BAM imaging and GIXD measurements. The characteristics of the π-A isotherms show large differences in the phase behavior of the two amide amphiphiles and reveal an additional phase transition between two condensed phases at T < 10 °C for the THPA monolayers. The objective of the present work is to study the thermodynamic characteristics of the 1:1 mixed HETA/THPA monolayers and to compare them with those of the pure components. The thermodynamic analysis is performed on the basis of the

10.1021/jp803388t CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

Thermodynamic Characterization of Mixed Monolayers

J. Phys. Chem. B, Vol. 112, No. 34, 2008 10515 in an experimental setup consisting of a self-made computerinterfaced film balance.42 Using the Wilhelmy method the surface pressure was measured with a roughened glass plate with an accuracy of (0.1 mN m-1 and the area per molecule with (5 × 10-3 nm2. Theory The equation of state for the monolayer in the fluid (G, LE)/ condensed (LC) transition region (A < Ac) with the bimodal distribution (large clusters and monomers) was derived in43–46

π)

Figure 1. π-A isotherms of HETA monolayers spread on water measured in the temperature range between 15 and 30 °C.

kTRβ 1 - πcoh m A - ω[1 + ε(Rβ - 1)]

(1)

where π is the surface pressure, k is the Boltzmann constant, T is temperature, ω is the partial molecular area for monomers (or the limiting area of molecule in the gaseous state), A is the area per molecule, πcoh is the cohesion pressure, m is the association (m > 1) or dissociation (m < 1) degree of the amphiphilic molecules in the fluid monolayer state, ε ) 1s ωcl/ω, ωcl is the area per monomer in a cluster, and Ac the molecular area which corresponds to the onset of the phase transition (i.e., at π ) πc). In this equation the parameter R expresses the dependence of the aggregation constant on the surface pressure:

R)

[

]

π - πc A exp -ε ω Ac kT

(2)

the parameter β defines the fraction of the monolayer free from aggregates:

β ) 1 + ω(1 - ε)(R - 1)/A

Figure 2. π-A isotherms of THPA monolayers spread on water measured in the temperature range between 3 and 20 °C.

π) equations of state for the monolayer in the fluid (G, LE)/ condensed (LC) transition region (A < Ac) with the bimodal distribution (large clusters and monomers) and in the fluid (G, LE) state (A g Ac). The experimental π-A isotherms of the mixed monolayers are compared with the theoretical results obtained on the basis of an equation of state that considers the contribution of the entropy nonideality, caused by the mixing of monomers and clusters. Finally the applicability of the additive model is tested for the theoretical description of the experimental results at higher temperatures. Experimental Section The two tailored amphiphiles HETA (C13H27-CO-NHCH2CH2OH) and THPA (C13H27-NH-CO-CH2-CH2OH), different only in the position of the two substituents at the acid amide group, were synthesized and purified, as described elsewhere.42 The chemical purity of both amphiphiles (g99%) was checked by elemental analysis and HPLC. The used spreading solvent was chloroform (p.a. grade, Baker, Holland). Ultrapure water with a specific resistance of 18.2 MΩ used for the monolayer experiments was obtained from a Millipore desktop system. Equilibrium surface pressure (π-A) isotherms recorded at a compression rate of e0.1 nm2/(molecule min) were measured

(3)

and ε ) ε0 + ηπ, where ε0 is the relative jump of the area per molecule during phase transition, and η is a relative twodimensional compressibility of the condensed monolayer.46 For the case of R ) 1 (the phase transition is absent), the equation of state (eq 1) transforms in the Volmer type equation for monolayers in the fluid (G, LE) state (A g Ac):43–45

kT - πcoh m(A - ω)

(4)

In the equation of state (eq 1), the contribution of the entropy nonideality, caused by the mixing of monomers and clusters, to the surface pressure can be taken into account on the basis of the Flory-Huggins theory.47 The equation of state for the surface layer in the (G, LE)/LC transition region which takes into account the entropy nonideality of mixing of monomers and clusters takes the form48

π)

kTRβ 1 kT - (1 - Rβ) - πcoh (5) m A - ω[1 + ε(Rβ - 1)] A

In what follows, the numerical estimates and results of fitting of the experimental data for monolayers of HETA and THPA as well as their 1:1 mixtures according to eqs 1–5 are presented. Results and Discussion As demonstrated in a previous paper,42 the main characteristics of the HETA and THPA monolayers are completely different by exchanging the position of the two substituents at the acid amide group. Accordingly, the π-A isotherms (Figures 1 and 2) indicate large differences in the phase behavior of the two amide amphiphiles and reveal an additional phase transition between two condensed phases at T < 10 °C for the THPA monolayers.

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TABLE 1: Parameters of THPA Monolayers at Different Temperatures temperature, °C ω, nm2 Ac, nm2 πcoh, mN/m ε0 η, m/mN m

20 0.16 0.253 8.0 0.05 0.006 1.05

15 0.17 0.284 8.3 0.04 0.004 1.05

10 0.175 0.314 8.9 0.0 0.004 1.02

6 0.19 0.364/0.336 9.7 0.0/0.10 0.002/0.004 1.0

3 0.195 0.39/0.35 9.9 0.0/0.10 0.003/0.006 1.0

Figure 1 shows five π-A isotherms of HETA monolayers, spread on pure water, in the temperature range between 15 and 30 °C. As most of the usual monolayers of amphiphiles,35 the isotherms show a sharp inflection at the first-order phase transition point (Ac) from a fluid (LE, G) phase to a condensed (LC) phase. The fluid phase broadens to lower Ac values with the temperature. The Ac values corresponding to the onset of this main phase transition exist in the accessible surface pressure range first above 17 °C and become smaller with the increase of temperature. Figure 2 shows five π-A isotherms of the THPA monolayers in the temperature range between 3 and 20 °C. Even at the lowest temperature, the main phase transition starts already in the positive pressure range. It is interesting to note that, at low temperatures (T < 10 °C), the π-A isotherms of the THPA monolayers display a noticeable second critical point at A < Ac indicating the existence of a second phase transition between two condensed phases. Recent GIXD studies have shown that at the second phase transition an abrupt transition between two different oblique lattice structures takes place demonstrated convincingly by the abrupt change of the polar tilt angle of the molecules and the cross-section area per molecule. Thus the second phase transition observed at low temperatures (T < 10 °C) between the two condensed phases is due to an abrupt transition between two different oblique lattice structures. According to the objective of this work, the thermodynamic characteristics of the 1:1 mixed HETA/THPA monolayers are studied and compared with those of the pure components. It is of special interest whether a second phase transition between two condensed phases at A < Ac exists in the π-A isotherms of the 1:1 mixed monolayers, as well. To improve the clarity of presentation, four π-A isotherms at T g 15 °C are presented in Figure 3a and three isotherms at T e 10 °C are shown in Figure 3b. The four π-A isotherms at T g 15 °C (Figure 3a) resemble those of usual amphiphiles. The isotherms show a sharp inflection at the first-order phase transition point (Ac) from the fluid phase to the condensed phase. For a selected temperature, the first order phase transition of the mixed monolayer is at a surface pressure between that of the two pure components. However, the two-phase coexistence region (A < Ac) is more inclined than those of the pure components. The three isotherms at T e 10 °C (Figure 3b) reveal a small cusp at a similar surface pressure as the pure HTPA monolayer for the same temperature. This also indicates a phase transition between two condensed phases similar to that observed for the pure HTPA monolayer. At these low temperatures, the main phase transition of the pure HETA monolayer occurs already at zero pressure, and thus, the plateau region of the isotherm is shifted to zero pressure, as well. Correspondingly, and under consideration of the temperature dependent shift of the first order phase transition point (see Figure 3a) it can be concluded that at T e 10 °C the first order phase transition of the 1:1 mixed HETA/HTPA monolayer is also at zero pressure and cannot be observed.

Figure 3. π-A isotherms of the 1:1 mixed HETA/THPA monolayers spread on water (a) at T g 15 °C, and (b) at T e 10 °C.

Figure 4. π-A dependences for the pure monolayers of HETA, THPA, and their 1:1 mixture spread on water at 3 °C. The symbols represent the experimental results and the solid lines the theoretical results.

Figures 4–9 show the experimental π-A dependences for the individual monolayers of HETA, THPA and their 1:1 mixture at the different temperatures of 25, 20, 15, 10, 6, and 3 °C respectively. The theoretical π-A isotherms are plotted, as calculated from eq 4 for A g Ac and eqs 1 and 5 for A < Ac. The red lines represent the theory, and the black symbols result from the experiment (measured continuously but presented as

Thermodynamic Characterization of Mixed Monolayers


J. Phys. Chem. B, Vol. 112, No. 34, 2008 10517

Figure 7. π-A dependences for the pure monolayers of HETA, THPA, and their 1:1 mixture spread on water at 15 °C. The symbols represent the experimental results and the solid lines the theoretical results (cyan calculated by the additive model).


symbols because of clarity). The corresponding parameters of the π-A isotherms are listed in Tables 1–3. The parameter values 0 and η calculated using eqs 1 and 5 for the of fluid/ condensed phase transition region, differ from each other insignificantly. Using eq 5, these parameters are generally 10-30% smaller than those calculated with eq 1. As recently shown,48 the consideration of the mixing entropy nonideality provided by eq 5 results in more realistic molecular characteristics of the amphiphilic monolayers. Therefore the parameters 0 and η values calculated on the basis of eq 5 are presented in Tables 1–3. For THPA at the temperatures of 3 and 6 °C (see Figure 2) and for the HETA/THPA mixture at the temperatures of 3, 6 and 10 °C (see Figure 3b), the π-A isotherms show a second phase transition between two condensed phases. For these temperatures the parameter values in the numerator of Tables 1 and 3 correspond to the first fluid/condensed phase transition, and the values shown as the denominator were calculated for the second phase transition between two condensed phases. As at the temperatures of 3, 6, and 10 °C for the mixed HETA/ THPA monolayers, the critical point of the first fluid/condensed phase transition s is at π ≈ 0 (A > 0.5 nm2), and it cannot be seen in Figures 7–9. For the HETA/THPA mixture both phase transitions refer to the condensation of the various components

Figure 8. π-A dependences for the pure monolayers of HETA, THPA, and their 1:1 mixture spread on water at 20 °C. The symbols represent the experimental results and the solid lines the theoretical results (cyan calculated by the additive model).

TABLE 2: Parameters of HETA Monolayers at Different Temperatures temperature, °C ω, nm2 Ac, nm2 πcoh, mN/m ε0 η, m/mN m

25 0.23 0.34 8.0 0.11 0.002 1.90

20 0.24 0.41 8.8 0.07 0.002 2.00

15 0.24 0.47 9.1 0.04 0.002 2.00

of the monolayer. It should be noted that for the systems with two phase transitions only the lowest (thermodynamically stable)46,49 branch of theoretical π-A isotherms is shown in Figures 7–9. The agreement between the theory and the experiment is seen to be quite satisfactory. The dependence of the parameters Ac and πcoh on the temperature in the pure and mixed monolayers have the usual character.43–46 As follows from Tables 1–3, both parameters decrease as the temperature increases. As expected, 46,49 the presence of the second phase transition between the two

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TABLE 3: Parameters of HETA/THPA mixed monolayers at different temperatures temperature, °C ω, nm2 Ac, nm2 πcoh, mN/m ε0 η, m/mN m

25 0.19 0.305 9.4 0.05 0.004 1.37

20 0.19 0.34 9.6 0.0 0.0033 1.35

15 0.20 0.385 9.8 0.0 0.004 1.35

condensed phases at higher surface pressure gives rise to a noticeable decrease of area per monomer in the cluster ωcl: values according to 0 ) 0.09-0.1 (see Tables 1 and 3). The comparison of the parameter values describing the pure HETA and THPA monolayers reveals essential differences of the parameters m expressing the association (m > 1) or dissociation (m < 1) degree of the amphiphilic molecules in the fluid monolayer state. Values of m ≈ 1 obtained for the THPA monolayer indicate that monomers are present in the fluid state whereas in the case of HETA monolayers, values of m ≈ 2 suggest dimerization in the fluid state. The parameter values m for mixed HETA/THPA monolayer (see Table 3) allow conclusions about structure and composition of the formed aggregates. The values of m ) 1.3-1.4 at T g 15 °C suggest the formation of mixed aggregates (or the primary aggregation of HETA), as both components are present in the fluid phase of the monolayer. On the contrary, at temperatures of 10 °C and lower (especially at 6 and 3 °C) the m values of the mixed HETA/THPA monolayer are close to 1, indicating that mainly individual THPA molecules are present in the fluid state at π > 0. Whereas in this low temperature range the fluid state of the mixed monolayer is formed by single THPA monomers, the condensed (LC) state is formed by HETA molecules, as at these temperatures the phase transition of HETA occurs at π ≈ 0 and A > 0.5 nm2. Therefore, at the low temperatures of 3-10 °C, the second phase transition in the mixed HETA/THPA monolayer presented in Figures 7–9 by the small cusp in the isotherm is mainly realized by the characteristics of the fluid/ condensed main phase transition of the pure THPA monolayer. The experimental π-A isotherms of the mixed HETA/THPA monolayer at temperatures of above 15 °C (see Figure 3a) to test their theoretical description on the basis of the additive model. The additive behavior of the mixed monolayer of HETA and THPA can be assumed similar to the model used in ref 50.

10 0.23 0.475/0.37 10.7 0.0/0.09 0.005/0.007 1.2

6 0.23 0.47/0.375 14.1 0.0/0.1 0.0065/0.008 1.1

3 0.22 0.47/0.38 16.4 0.0/0.1 0.0025/0.003 1.0

That means, if X1 is the molar fraction of HETA and X2 is the molar fraction of THPA in the mixed monolayer, all parameters in eqs 1–5, ω, Ac, 0, η, πcoh, can be written as follows:

x ) x1X1+x2X2

(6)

Here x is the parameter value for mixed monolayer, x1 is its value for the pure HETA monolayer, and x2 is its value for the pure THPA monolayer. Extrapolated model parameters for THPA at 25 °C were used to calculate the π-A isotherms of the mixed monolayer at 25 °C. The π-A isotherms calculated by the additive method are presented in Figures 4–6 by the blue solid lines. It is seen that the additive model provides good agreement with the experiment only at 20 and 25 °C, whereas at 15 °C, when the phase transition of the HETA monolayer occurs already at zero surface pressure, the agreement with the additive model becoming worse. Conclusions The large differences in the monolayer characteristics of the tailored amphiphiles HETA (C13H27-CO-NH-CH2CH2OH) and THPA (C13H27-NH-CO-CH2-CH2OH), different only in the position of the two substituents at the acid amide group, have drawn attention to their 1:1 mixture. The behavior of the mixed HETA/THPA monolayer is of special interest because of the indication of a second phase transition between two condensed phases at nearly the same low temperatures as well as in the pure THPA monolayer. The thermodynamic characteristics of the monolayers of the pure HETA and THPA components and the 1:1 HETA/THPA mixtures have been studied and compared with each other. The thermodynamic analysis performed on the basis of the equations of state for the monolayer in the fluid (G, LE)/condensed (LC) transition region (A < Ac) with bimodal distribution (large clusters and monomers) and in the fluid (G, LE) state (A g Ac) under consideration of the contribution of the entropy nonideality at mixing of monomers and clusters, provides good agreement with the experimental π-A isotherms. The results of the thermodynamic analysis allow conclusions on the specific phase properties of the mixed HETA/THPA monolayers in the accessible temperature range. In the low temperature range, mainly single THPA monomers are present in the fluid (LE) state of the mixed HETA/THPA monolayer whereas, the condensed (LC) state up to the second phase transition is formed by HETA molecules. The second phase transition in the mixed monolayer is related to the characteristics of the fluid/condensed main phase transition of the pure THPA monolayer. Finally the additive model applied for the theoretical description at higher temperatures provides good agreement with the experimental results at temperatures of g20 °C. References and Notes

Figure 9. π-A dependences for the pure monolayers of HETA, THPA and their 1:1 mixture spread on water at 25 °C. The symbols represent the experimental results and the solid lines the theoretical results (cyan calculated by the additive model).

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Thermodynamic Characterization of Mixed Monolayers of Two Similar

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