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On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study
Colloids and Surfaces A: Physicochemical and Engineering Aspects 170 (2000) 199 – 208 www.elsevier.nl/locate/colsurfa
On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study Kai-Bin Chen, Chien-Hsiang Chang *, Yu-Min Yang, Jer-Ru Maa Department of Chemical Engineering, National Cheng Kung Uni6ersity, Tainan, 70101, Taiwan, ROC Received 25 November 1999; accepted 16 March 2000
Abstract This study investigated the mixed monolayer behavior of dipalmitoyl phosphatidylcholine (DPPC) with normal long-chain alcohols at the air/water interface. Surface pressure – area isotherms of mixed DPPC/C18OH and DPPC/ C20OH monolayers at 37°C were obtained and compared with previous results for the mixed DPPC/C16OH system. The negative deviations from additivity of the areas and the variation of the collapse pressure with composition imply that DPPC and long-chain alcohols were miscible and formed non-ideal monolayers at the interface. At lower surface pressures, it seems that the attractive intermolecular force was dominant in molecular packing in the mixed monolayers. At higher surface pressures, the data suggest that the molecular packing in mixed DPPC/C16OH monolayers may be favored by the packing efficiency or geometric accommodation. Furthermore, negative values of excess free energy of mixing were obtained and became significant as the hydrocarbon chain length of alcohols increased, which indicates there were attractive interactions between DPPC and long-chain alcohols. In each free energy of mixing–composition curve, there was only one minimum and thus a phase separation did not exist for mixed DPPC/long-chain alcohol monolayers. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Air/water interface; Mixed monolayer; Molecular interaction; Monolayer miscibility; Surface pressure – area isotherm
1. Introduction Many studies have been conducted on the mixed monolayer behavior at the air/water interface. Investigating the surface properties of mixed monolayers is very important since it allows one * Corresponding author. Tel.: + 886-6-2757575, ext. 62671; fax: +886-6-2344496. E-mail address: [email protected] (C.-H. Chang)
to obtain information on the molecular interactions between the monolayer components [1,2]. An insoluble monolayer at the air/water interface is usually considered as a two-dimensional solution, and the surface properties of mixed monolayers are generally studied based on the measurement of the surface pressure–area per molecule (p–A) isotherms of the monolayers. By comparing the p–A isotherms of individual monolayers with those of mixed monolayers, one
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is able to gain knowledge about the free energies of mixing and interactions between monolayerforming molecules [3 – 15]. In previous studies, mixed monolayer behavior of dipalmitoyl phosphatidylcholine (DPPC) with n-hexadecanol (C16OH) at the air/water interface as a function of temperature has been investigated, and it has been found that the two monolayers appeared to be miscible [16,17]. The analysis of p – A isotherms indicated that the areas of mixed monolayers exhibited negative deviations from the ideal values at all surface pressures and compositions. Moreover, a negative value of the free energy of mixing, that is, a mixture characterized by a higher thermodynamic stability was found at all compositions and one can conclude that the mixed monolayer with XDPPC = 0.5 was the most stable. The results are of special interest for potential applications in the lung surfactant therapy field, because DPPC and C16OH constitute the major part of the artificial lung surfactant formulation, Exosurf [18 – 20]. However, in the previous studies, the system was restricted to mixtures of DPPC and C16OH. With regard to the therapeutic use of DPPC/C16OH mixtures, it appears important to obtain more generalized information concerning the two-dimensional molecular interactions between DPPC and various long-chain alcohols as mixed monolayers at the air/water interface. A study to elucidate the effects of hydrocarbon chain length of long-chain alcohols on the miscibility and stability of mixed DPPC/long-chain alcohol monolayers may provide an insight on how to improve the surface properties of available surfactant therapy formulations. A miscibility analysis of binary phosphatidylethanolamine monolayers has been carried out to investigate how the differences in the chain lengths of mixing components with the same head group affect their miscibility properties [21]. The results demonstrated that the miscibility behavior in the binary monolayers was dependent on the monolayer state, and a successive demixing trend was found with increasing chain length differences. The effects of carbon chain length on mixed monolayer properties have been studied for the mixed systems of tetradecanoic acid with n-pe-
rfluorocarboxylic acid with n equal to 10–18 by Shibata et al. [22]. Their results suggested that for n equal to 10 or 12, the two monolayers seemed miscible with each other at the interface. However, when n=14, the two acids formed a completely immiscible monolayer. As for n= 16 or 18, the monolayers of tetradecanoic acid with n-perfluorocarboxylic acid were miscible in the expanded state but immiscible in the condensed state. In addition, Baker et al. have investigated the mixed monolayer behavior of tetrakis (cumylphenoxy) phthalocyanine and long-chain alcohols with chain lengths of 13–24 carbons long [23]. By changing the hydrocarbon chain length of alcohols, they have deduced a model of interaction based on the surface pressure–area isotherms. In the model, the hydrocarbon chains of the alcohols act as a stabilizing hydrophobic support around phthalocyanine mounds of a preferred stack height at the air/water interface. As the surface pressure increases, the mounds are pushed above the alcohol layer, and a single or double collapse is observed, depending on the chain length. The aim of this work was to study the miscibility and stability of mixed monolayers formed by DPPC and long-chain alcohols with the consideration of hydrocarbon chain length effects. In order to establish the mutual miscibility and the molecular interaction between DPPC and longchain alcohols at the interface, the p–A isotherms of the pure components and their mixtures were measured and analyzed. Although non-negligible desorption of the monolayers might occur and then complicate the mixed monolayer behavior [24,25], the desorption of DPPC or long-chain alcohols, if there is any, was ignored in the analysis.
2. Materials and methods L-a-Dipalmitoyl phosphatidylcholine (DPPC) (\99% pure), n-octadecanol (C18OH) (99% pure), and n-eicosanol (C20OH) ( 99% pure) were purchased from Sigma Chemical Company, USA and were used as received. Ethanol ( 99.5% pure) was supplied by Seoul Chemical In-
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dustry Co., Ltd., Korea and HPLC grade hexane (\ 99% pure) was obtained from Ferak Laborat GmbH, Germany. All the mixtures with desired proportions were prepared by dissolving appropriate weights of materials in an ethanol/hexane (1:9 v/v) solution and were stored at 4°C. The water used in all experiments was prepared by using a Milli-Q plus water purification system, and its resistivity was 18.2 MV-cm. The surface tension of the purified water was about 72 mN m − 1 as measured by a Wilhelmy plate tensiometer (model CBVP-A3, Kyowa Interface Science, Japan) at 25°C. Before used, all glassware in contact with the samples was exhaustively rinsed with purified water. An automatic controlled KSV minitrough (KSV Instruments Ltd., Finland), equipped with a platinum Wilhelmy plate, was used to obtain the surface pressure – area per molecule (p–A) isotherms of monolayers at the air/water interface. The measurements were performed with an accuracy of 90.004 mN m − 1 in surface pressure and 9 1% in surface area, according to the instrument specification. Before each run, the Teflon trough (trough size, 320×7.5 mm2) was washed with ethanol and rinsed with purified water. The platinum plate was cleaned between each experiment by rinsing with purified water and heating to red heat. For all the experiments, the trough was filled with purified water as the subphase, and the temperature was maintained at 379 1°C by an external circulator. The air/subphase interface could be compressed and expanded symmetrically with two Teflon barriers at a desired rate. The cleanliness of the trough and subphase was ensured before each experiment by cycling through the full range of subphase surface and aspirating the surface while at minimum surface area. When the surface pressure fluctuation was found to be less than 0.2 mN m − 1 during the compression cycle, a sample containing monolayer-forming materials was then evenly spread on the subphase surface by the use of a microsyringe (Hamilton Co., USA). Ten minutes were allowed for solvent evaporation and monolayer equilibration before an experiment was started. After allowing for the solvent to evaporate, the monolayer at the air/water interface was continu-
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ously compressed at a rate of 1 A, 2 molecule − 1 min − 1 to obtain the p–A isotherms. There is no significant difference in the p–A isotherms when the isotherms were obtained at a higher compression rate of 2 A, 2 molecule − 1 min − 1. As a monolayer was compressed to be in a condensed phase, the isotherm generally exhibited a sharp break followed by an abnormal change of surface pressure upon further compression, which was referred to as the collapse point of the monolayer under the given experimental condition.
3. Evaluation of miscibility and stability of mixed monolayers According to the phase rule, if the monolayer components are immiscible in the condensed and collapse states, the isotherm will show two distinct collapse pressures corresponding to pure components [26,27]. However, collapse pressures of mixed monolayers composed of two miscible components will varied with composition. Moreover, the excess area, Aex, for a mixed monolayer can also be an indication of miscibility. The excess area per molecule of a mixed monolayer consisting of components 1 and 2 at a given surface pressure can be expressed by the relation [1,2]: Aex = A12 − (X1A1 + X2A2)
(1)
where A12 is the mean area per molecule of the mixed monolayer, A1 and A2 are the areas per molecule of pure monolayers of components 1 and 2, respectively, and X1 and X2 are their corresponding mole fractions in the mixed monolayer. If an ideal mixed monolayer is formed or the two components are completely immiscible in the two-dimensional state, the excess area will be zero and a plot of A12 as a function of X1 or X2 at a given surface pressure would be a straight line. Deviations from the straight line indicate various types of interactions which occur in the mixed monolayers [28,29]. Thus, the additive rule of area per molecule at constant surface pressures allows one to decide on the ideality and miscibility of the mixed monolayers.
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The interaction between the components in a mixed monolayer and the thermodynamic stability of a mixed monolayer compared with pure components can be investigated from the calculation of excess free energy of mixing, DGex, or free energy of mixing, DGmix [30]. For a process of two pure monolayers to form a mixed monolayer at a constant surface pressure and temperature, the expression for DGex is given as,
DGex =
&
Thus, the DGex can be calculated from the p–A isotherm. The DGmix is then given by the relation: DGmix = DGid + DGex
(3)
where the ideal free energy of mixing, DGid, can be calculated from the equation: DGid = kT(X1 ln X1 + X2 ln X2)
(4)
where k is the Boltzmann’s constant and T is the temperature.
p
[A12 −(X1A1 +X2A2)] dp
(2)
0
4. Results and discussion
4.1. DPPC/C18OH mixed monolayers
Fig. 1. Surface pressure–area per molecule isotherms of (a) DPPC/C18OH and (b) DPPC/C20OH mixed monolayers of various compositions at 37°C.
The surface pressure–area per molecule (p –A) isotherms obtained right before collapse points for mixed DPPC/C18OH monolayers at various molar ratios at 37°C are shown in Fig. 1(a). One can observe that mixed monolayers of DPPC/ C18OH showed a somewhat intermediate isotherm behavior compared with those shown by pure monolayers, and the monolayer state was dependent on composition. The shape of the p–A isotherm for a C18OH monolayer demonstrated that the C18OH monolayer may be considered as a condensed monolayer. However, a DPPC monolayer showed a phase transition between liquid-expanded and liquid-condensed states at surface pressures around 35 mN m − 1. At compositions corresponding to XC18OH \ 0.4, the phase transition to the liquid-condensed state disappeared in the isotherms, which is similar to the condensing effect of cholesterol on lecithin [7,31– 33]. Furthermore, it is clearly seen from Fig. 1(a) that the collapse pressures of mixed monolayers varied with composition. If the two components are completely immiscible in the monolayers, a lower collapse surface pressure corresponding with the collapse pressure of one of the components will be observed in all the mixed monolayer isotherms [34,35], as predicted by the phase rule [26,27]. Thus, it can be concluded based on the observed composition-dependent collapse surface pressures that DPPC and C18OH were miscible in the condensed and collapsed states at the air/water interface.
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Fig. 2. (a) Area per molecule and (b) Aex/Aid as a function of composition for mixed DPPC/C18OH monolayers at various surface pressures.
The mean areas per molecule and the condensing effects for the mixed DPPC/C18OH monolayers were determined from the isotherms at various surface pressures and plotted against monolayer composition, XC18OH (Fig. 2). Since a linear relationship between molecular area and composition was not satisfied, DPPC and C18OH were considered to be miscible and form non-ideal monolayers at the interface [1,2]. The excess area as a function of composition indicates strong negative deviations, and a significant condensation effect can be observed for all compositions at various surface pressures. The condensation effects were
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relatively large at XC18OH = 0.6, and the effects of molecular interactions between DPPC and C18OH on molecular packing at the interface depended on the surface pressure, as one can see in Fig. 2(b). For a mixed monolayer, it always results in a negative Aex if attractive intermolecular force or geometric accommodation (efficient packing) occurs, which in most cases will result in an enhanced dispersive interaction [36]. The deviations from additivity of the areas observed in Fig. 2 and the variations of the collapse pressure with composition observed in Fig. 1(a) imply the mutual compatibility of DPPC and C18OH at the air/water interface. Normally, one would expect the negative deviation in Aex to be larger at lower surface pressures because the monolayers were more extended and the effects of intermolecular interaction on molecular packing will be more significant. When the mixed monolayer was rich in C18OH molecules, the condensing effect of C18OH on a DPPC monolayer indeed became significant as the surface pressure decreased. However, for monolayers with XC18OH 5 0.4, the condensing effect of C18OH on a DPPC monolayer increased with decreasing surface pressure at higher surface pressures, but was similar at lower surface pressures, p5 30 mN m − 1. It is interesting to note that for XC18OH 5 0.4 and p5 30 mN m − 1, the mixed monolayer existed in a liquid-expanded state. By means of Eqs. (2)–(4), the values of DGex and DGmix for various compositions of mixed DPPC/C18OH monolayers at different surface pressures were calculated (Fig. 3). From the dependence of DGex on composition, one can see that all the values of DGex were negative, which implies attractive interactions and compatibility between DPPC and C18OH. The interaction became more significant as the surface pressure increased, since the distance between molecules became smaller as the mixed monolayer was compressed. Moreover, since there was only one minimum in each DGmix –composition curve, which corresponded to XC18OH = 0.6, one can conclude that a phase separation in the mixed DPPC/ C18OH monolayers did not exist. The existence of a minimum indicates that the mixed monolayers were thermodynamically more stable than the
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monolayers with separation between individual components, especially around XC18OH = 0.6. Besides, the values of DGmix became more negative with increasing surface pressure, which suggests that the stability of a mixed monolayer compared with pure components was higher at higher surface pressures. This can be explained by stronger interactions between DPPC and C18OH as the mixed monolayer was more condensed.
Fig. 4. (a) Area per molecule and (b) Aex/Aid as a function of composition for mixed DPPC/C20OH monolayers at various surface pressures.
4.2. DPPC/C20OH mixed monolayers
Fig. 3. (a) Excess free energy of mixing and (b) free energy of mixing as a function of composition for mixed DPPC/C18OH monolayers at various surface pressures.
The p–A curves corresponding to pure and mixed monolayers formed by DPPC and C20OH in different proportions at 37°C are presented in Fig. 1(b). The curves displayed features similar to those of DPPC/C18OH monolayers of comparable compositions. The phase transition at p $35 mN m − 1, which was characteristic of a DPPC monolayer, disappeared as XC20OH \ 0.5, and the mixed monolayers existed only as condensed monolayers. Again, it seems that DPPC and C20OH were miscible at the air/water interface since the col-
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lapse pressures of the mixed monolayers changed with composition. The miscibility of DPPC/ C20OH mixed monolayers can be seen more clearly in Fig. 4, which shows the mean areas per molecule and the condensing effects at various surface pressures as a function of C20OH mole fraction. Apparently, the mean areas per molecule had lower values than those calculated for ideal mixed monolayers within the surface pressure range 5–40 mN m − 1, especially at higher proportions of C20OH. This indicates that DPPC and C20OH were miscible and formed non-ideal monolayers at the air/water interface. Moreover,
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the deviations were negative for all compositions at different surface pressures, and were more pronounced at lower surface pressures, except at XC20OH = 0.2. The contraction in the molecular packing was mainly ascribed to the molecular interactions in the hydrocarbon region of the monolayers, as for the DPPC/C16OH and DPPC/ C18OH systems. However, the condensation effect of C20OH on a DPPC monolayer showed slightly different dependence on composition and surface pressure compared with what observed in mixed DPPC/C18OH monolayers. Fig. 5 represents the values of DGex and DGmix as a function of composition for mixed DPPC/ C20OH monolayers at different surface pressures. One can find that the values of DGex were all negative, especially for the compositions around XC20OH = 0.6, which was the same as what observed in mixed DPPC/C18OH monolayers. Again, the data of DGex and DGmix suggest that there were strong interactions between DPPC and C20OH molecules at the interface, and the magnitude of interactions became stronger as the surface pressure was higher or the mixed monolayer was in a more condensed state. In addition, no phase separation was observed from the DGmix – composition curves for this mixed monolayer system.
4.3. Effects of alcohol chain length
Fig. 5. (a) Excess free energy of mixing and (b) free energy of mixing as a function of composition for mixed DPPC/C20OH monolayers at various surface pressures.
In Fig. 6, the condensing effects for mixed DPPC/Cn OH (n= 16, 18 and 20) monolayers at different surface pressures are demonstrated as a function of composition. For all three mixed monolayer systems, the molecular structures only differ in the hydrocarbon chain length of alcohols. At lower surface pressures (p= 10 and 20 mN m − 1), the condensing effect of alcohol on a DPPC monolayer became more significant as the hydrocarbon chain length of alcohols increased (Fig. 6(a) and (b)). Since all these mixed monolayers had same hydrophilic interactions, the difference in condensing effect was attributed to the hydrophobic interaction and the packing efficiency by the hydrocarbon moiety of the monolayers. At lower surface pressures, since each molecule occupies a larger area, one might expect
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ing in mixed DPPC/C16OH monolayers at higher surface pressures. It is not clear that this is due to the shorter hydrocarbon chain of C16OH compared with C18OH or due to the same hydrocarbon chain length of C16OH as DPPC. Comparisons of DGex and DGmix values for mixed monolayers of DPPC/C16OH, DPPC/ C18OH and DPPC/C20OH are shown in Figs. 7 and 8. All three mixed monolayer systems showed negative DGex values, and the negative DGex became significant as the hydrocarbon chain length of alcohols increased. Apparently, the attractive
Fig. 6. Aex/Aid as a function of composition for mixed DPPC/ C16OH ( ), DPPC/C18OH (+ ), and DPPC/C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
that the influence of packing efficiency in the condensing effect was not as important as that of intermolecular attraction. Thus, the longer the alcohol chain length, the stronger the intermolecular attraction and the more significant the condensing effect. However, at higher surface pressures, the effect of packing efficiency became important since the molecules were in a crowded state. Fig. 6(c) shows that at p= 40 mN m − 1, the larger condensing effect was observed for C16OH than for C18OH. These data suggest that the packing efficiency may favor the molecular pack-
Fig. 7. Excess free energy of mixing as a function of composition for mixed DPPC/C16OH ( ), DPPC/C18OH ( + ), and DPPC/C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
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curves for a monolayer system composed of DPPC and C16OH showed symmetric behavior around the composition XDPPC = 0.5 or XC16OH = 0.5, at which the minimum occurred [17]. However, for both DPPC/C18OH and DPPC/C20OH mixed systems, the DGmix –composition curves showed similar skewed behavior with the minimum occurred at XDPPC = 0.4 or Xalcohol = 0.6.
5. Conclusions
Fig. 8. Free energy of mixing as a function of composition for mixed DPPC/C16OH ( ), DPPC/C18OH ( + ), and DPPC/ C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
intermolecular force resulting from the hydrophobic interactions between hydrocarbon chains played a more important role than the packing efficiency in the mixing processes of monolayers of DPPC and long-chain alcohols with alcohol chain lengths from 16 to 20. Moreover, there was only one minimum in each DGmix –composition curve for the three mixed monolayer systems at given surface pressures, indicating a single phase existing across the entire composition range. The DGmix – composition
From a detailed analysis of surface pressure– area isotherms, one can conclude that DPPC and normal long-chain alcohols with chain lengths of 16, 18, and 20 were miscible and formed non-ideal monolayers at the air/water interface. The nonideality of the mixed monolayers was evident in the area–composition figures, in which the mixed monolayers exhibited significantly negative deviations from the ideal ones. It seems that the hydrophobic interaction was dominant in the molecular packing of the mixed monolayers at lower surface pressures. However, the packing efficiency or geometric accommodation may favor the molecular packing of mixed DPPC/C16OH monolayers at higher surface pressures, resulting in unexpected stronger condensing effects of C16OH than C18OH on a DPPC monolayer. Based on the results of excess free energies of mixing, the hydrophobic interaction appeared to play a more important role than the packing efficiency in the mixing processes of DPPC with long-chain alcohols at the interface. Moreover, the free energy of mixing–composition curves indicate that there was no phase separation existing across the entire composition range, and the mixed monolayers appeared to be more stable than the monolayers with separation between individual components.
Acknowledgements This research was supported by the National Science Council of the Republic of China through Grants c NSC85-2214-E-006-006 and NSC862214-E006-025.
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On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study Kai-Bin Chen, Chien-Hsiang Chang *, Yu-Min Yang, Jer-Ru Maa Department of Chemical Engineering, National Cheng Kung Uni6ersity, Tainan, 70101, Taiwan, ROC Received 25 November 1999; accepted 16 March 2000
Abstract This study investigated the mixed monolayer behavior of dipalmitoyl phosphatidylcholine (DPPC) with normal long-chain alcohols at the air/water interface. Surface pressure – area isotherms of mixed DPPC/C18OH and DPPC/ C20OH monolayers at 37°C were obtained and compared with previous results for the mixed DPPC/C16OH system. The negative deviations from additivity of the areas and the variation of the collapse pressure with composition imply that DPPC and long-chain alcohols were miscible and formed non-ideal monolayers at the interface. At lower surface pressures, it seems that the attractive intermolecular force was dominant in molecular packing in the mixed monolayers. At higher surface pressures, the data suggest that the molecular packing in mixed DPPC/C16OH monolayers may be favored by the packing efficiency or geometric accommodation. Furthermore, negative values of excess free energy of mixing were obtained and became significant as the hydrocarbon chain length of alcohols increased, which indicates there were attractive interactions between DPPC and long-chain alcohols. In each free energy of mixing–composition curve, there was only one minimum and thus a phase separation did not exist for mixed DPPC/long-chain alcohol monolayers. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Air/water interface; Mixed monolayer; Molecular interaction; Monolayer miscibility; Surface pressure – area isotherm
1. Introduction Many studies have been conducted on the mixed monolayer behavior at the air/water interface. Investigating the surface properties of mixed monolayers is very important since it allows one * Corresponding author. Tel.: + 886-6-2757575, ext. 62671; fax: +886-6-2344496. E-mail address: [email protected] (C.-H. Chang)
to obtain information on the molecular interactions between the monolayer components [1,2]. An insoluble monolayer at the air/water interface is usually considered as a two-dimensional solution, and the surface properties of mixed monolayers are generally studied based on the measurement of the surface pressure–area per molecule (p–A) isotherms of the monolayers. By comparing the p–A isotherms of individual monolayers with those of mixed monolayers, one
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is able to gain knowledge about the free energies of mixing and interactions between monolayerforming molecules [3 – 15]. In previous studies, mixed monolayer behavior of dipalmitoyl phosphatidylcholine (DPPC) with n-hexadecanol (C16OH) at the air/water interface as a function of temperature has been investigated, and it has been found that the two monolayers appeared to be miscible [16,17]. The analysis of p – A isotherms indicated that the areas of mixed monolayers exhibited negative deviations from the ideal values at all surface pressures and compositions. Moreover, a negative value of the free energy of mixing, that is, a mixture characterized by a higher thermodynamic stability was found at all compositions and one can conclude that the mixed monolayer with XDPPC = 0.5 was the most stable. The results are of special interest for potential applications in the lung surfactant therapy field, because DPPC and C16OH constitute the major part of the artificial lung surfactant formulation, Exosurf [18 – 20]. However, in the previous studies, the system was restricted to mixtures of DPPC and C16OH. With regard to the therapeutic use of DPPC/C16OH mixtures, it appears important to obtain more generalized information concerning the two-dimensional molecular interactions between DPPC and various long-chain alcohols as mixed monolayers at the air/water interface. A study to elucidate the effects of hydrocarbon chain length of long-chain alcohols on the miscibility and stability of mixed DPPC/long-chain alcohol monolayers may provide an insight on how to improve the surface properties of available surfactant therapy formulations. A miscibility analysis of binary phosphatidylethanolamine monolayers has been carried out to investigate how the differences in the chain lengths of mixing components with the same head group affect their miscibility properties [21]. The results demonstrated that the miscibility behavior in the binary monolayers was dependent on the monolayer state, and a successive demixing trend was found with increasing chain length differences. The effects of carbon chain length on mixed monolayer properties have been studied for the mixed systems of tetradecanoic acid with n-pe-
rfluorocarboxylic acid with n equal to 10–18 by Shibata et al. [22]. Their results suggested that for n equal to 10 or 12, the two monolayers seemed miscible with each other at the interface. However, when n=14, the two acids formed a completely immiscible monolayer. As for n= 16 or 18, the monolayers of tetradecanoic acid with n-perfluorocarboxylic acid were miscible in the expanded state but immiscible in the condensed state. In addition, Baker et al. have investigated the mixed monolayer behavior of tetrakis (cumylphenoxy) phthalocyanine and long-chain alcohols with chain lengths of 13–24 carbons long [23]. By changing the hydrocarbon chain length of alcohols, they have deduced a model of interaction based on the surface pressure–area isotherms. In the model, the hydrocarbon chains of the alcohols act as a stabilizing hydrophobic support around phthalocyanine mounds of a preferred stack height at the air/water interface. As the surface pressure increases, the mounds are pushed above the alcohol layer, and a single or double collapse is observed, depending on the chain length. The aim of this work was to study the miscibility and stability of mixed monolayers formed by DPPC and long-chain alcohols with the consideration of hydrocarbon chain length effects. In order to establish the mutual miscibility and the molecular interaction between DPPC and longchain alcohols at the interface, the p–A isotherms of the pure components and their mixtures were measured and analyzed. Although non-negligible desorption of the monolayers might occur and then complicate the mixed monolayer behavior [24,25], the desorption of DPPC or long-chain alcohols, if there is any, was ignored in the analysis.
2. Materials and methods L-a-Dipalmitoyl phosphatidylcholine (DPPC) (\99% pure), n-octadecanol (C18OH) (99% pure), and n-eicosanol (C20OH) ( 99% pure) were purchased from Sigma Chemical Company, USA and were used as received. Ethanol ( 99.5% pure) was supplied by Seoul Chemical In-
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dustry Co., Ltd., Korea and HPLC grade hexane (\ 99% pure) was obtained from Ferak Laborat GmbH, Germany. All the mixtures with desired proportions were prepared by dissolving appropriate weights of materials in an ethanol/hexane (1:9 v/v) solution and were stored at 4°C. The water used in all experiments was prepared by using a Milli-Q plus water purification system, and its resistivity was 18.2 MV-cm. The surface tension of the purified water was about 72 mN m − 1 as measured by a Wilhelmy plate tensiometer (model CBVP-A3, Kyowa Interface Science, Japan) at 25°C. Before used, all glassware in contact with the samples was exhaustively rinsed with purified water. An automatic controlled KSV minitrough (KSV Instruments Ltd., Finland), equipped with a platinum Wilhelmy plate, was used to obtain the surface pressure – area per molecule (p–A) isotherms of monolayers at the air/water interface. The measurements were performed with an accuracy of 90.004 mN m − 1 in surface pressure and 9 1% in surface area, according to the instrument specification. Before each run, the Teflon trough (trough size, 320×7.5 mm2) was washed with ethanol and rinsed with purified water. The platinum plate was cleaned between each experiment by rinsing with purified water and heating to red heat. For all the experiments, the trough was filled with purified water as the subphase, and the temperature was maintained at 379 1°C by an external circulator. The air/subphase interface could be compressed and expanded symmetrically with two Teflon barriers at a desired rate. The cleanliness of the trough and subphase was ensured before each experiment by cycling through the full range of subphase surface and aspirating the surface while at minimum surface area. When the surface pressure fluctuation was found to be less than 0.2 mN m − 1 during the compression cycle, a sample containing monolayer-forming materials was then evenly spread on the subphase surface by the use of a microsyringe (Hamilton Co., USA). Ten minutes were allowed for solvent evaporation and monolayer equilibration before an experiment was started. After allowing for the solvent to evaporate, the monolayer at the air/water interface was continu-
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ously compressed at a rate of 1 A, 2 molecule − 1 min − 1 to obtain the p–A isotherms. There is no significant difference in the p–A isotherms when the isotherms were obtained at a higher compression rate of 2 A, 2 molecule − 1 min − 1. As a monolayer was compressed to be in a condensed phase, the isotherm generally exhibited a sharp break followed by an abnormal change of surface pressure upon further compression, which was referred to as the collapse point of the monolayer under the given experimental condition.
3. Evaluation of miscibility and stability of mixed monolayers According to the phase rule, if the monolayer components are immiscible in the condensed and collapse states, the isotherm will show two distinct collapse pressures corresponding to pure components [26,27]. However, collapse pressures of mixed monolayers composed of two miscible components will varied with composition. Moreover, the excess area, Aex, for a mixed monolayer can also be an indication of miscibility. The excess area per molecule of a mixed monolayer consisting of components 1 and 2 at a given surface pressure can be expressed by the relation [1,2]: Aex = A12 − (X1A1 + X2A2)
(1)
where A12 is the mean area per molecule of the mixed monolayer, A1 and A2 are the areas per molecule of pure monolayers of components 1 and 2, respectively, and X1 and X2 are their corresponding mole fractions in the mixed monolayer. If an ideal mixed monolayer is formed or the two components are completely immiscible in the two-dimensional state, the excess area will be zero and a plot of A12 as a function of X1 or X2 at a given surface pressure would be a straight line. Deviations from the straight line indicate various types of interactions which occur in the mixed monolayers [28,29]. Thus, the additive rule of area per molecule at constant surface pressures allows one to decide on the ideality and miscibility of the mixed monolayers.
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The interaction between the components in a mixed monolayer and the thermodynamic stability of a mixed monolayer compared with pure components can be investigated from the calculation of excess free energy of mixing, DGex, or free energy of mixing, DGmix [30]. For a process of two pure monolayers to form a mixed monolayer at a constant surface pressure and temperature, the expression for DGex is given as,
DGex =
&
Thus, the DGex can be calculated from the p–A isotherm. The DGmix is then given by the relation: DGmix = DGid + DGex
(3)
where the ideal free energy of mixing, DGid, can be calculated from the equation: DGid = kT(X1 ln X1 + X2 ln X2)
(4)
where k is the Boltzmann’s constant and T is the temperature.
p
[A12 −(X1A1 +X2A2)] dp
(2)
0
4. Results and discussion
4.1. DPPC/C18OH mixed monolayers
Fig. 1. Surface pressure–area per molecule isotherms of (a) DPPC/C18OH and (b) DPPC/C20OH mixed monolayers of various compositions at 37°C.
The surface pressure–area per molecule (p –A) isotherms obtained right before collapse points for mixed DPPC/C18OH monolayers at various molar ratios at 37°C are shown in Fig. 1(a). One can observe that mixed monolayers of DPPC/ C18OH showed a somewhat intermediate isotherm behavior compared with those shown by pure monolayers, and the monolayer state was dependent on composition. The shape of the p–A isotherm for a C18OH monolayer demonstrated that the C18OH monolayer may be considered as a condensed monolayer. However, a DPPC monolayer showed a phase transition between liquid-expanded and liquid-condensed states at surface pressures around 35 mN m − 1. At compositions corresponding to XC18OH \ 0.4, the phase transition to the liquid-condensed state disappeared in the isotherms, which is similar to the condensing effect of cholesterol on lecithin [7,31– 33]. Furthermore, it is clearly seen from Fig. 1(a) that the collapse pressures of mixed monolayers varied with composition. If the two components are completely immiscible in the monolayers, a lower collapse surface pressure corresponding with the collapse pressure of one of the components will be observed in all the mixed monolayer isotherms [34,35], as predicted by the phase rule [26,27]. Thus, it can be concluded based on the observed composition-dependent collapse surface pressures that DPPC and C18OH were miscible in the condensed and collapsed states at the air/water interface.
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Fig. 2. (a) Area per molecule and (b) Aex/Aid as a function of composition for mixed DPPC/C18OH monolayers at various surface pressures.
The mean areas per molecule and the condensing effects for the mixed DPPC/C18OH monolayers were determined from the isotherms at various surface pressures and plotted against monolayer composition, XC18OH (Fig. 2). Since a linear relationship between molecular area and composition was not satisfied, DPPC and C18OH were considered to be miscible and form non-ideal monolayers at the interface [1,2]. The excess area as a function of composition indicates strong negative deviations, and a significant condensation effect can be observed for all compositions at various surface pressures. The condensation effects were
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relatively large at XC18OH = 0.6, and the effects of molecular interactions between DPPC and C18OH on molecular packing at the interface depended on the surface pressure, as one can see in Fig. 2(b). For a mixed monolayer, it always results in a negative Aex if attractive intermolecular force or geometric accommodation (efficient packing) occurs, which in most cases will result in an enhanced dispersive interaction [36]. The deviations from additivity of the areas observed in Fig. 2 and the variations of the collapse pressure with composition observed in Fig. 1(a) imply the mutual compatibility of DPPC and C18OH at the air/water interface. Normally, one would expect the negative deviation in Aex to be larger at lower surface pressures because the monolayers were more extended and the effects of intermolecular interaction on molecular packing will be more significant. When the mixed monolayer was rich in C18OH molecules, the condensing effect of C18OH on a DPPC monolayer indeed became significant as the surface pressure decreased. However, for monolayers with XC18OH 5 0.4, the condensing effect of C18OH on a DPPC monolayer increased with decreasing surface pressure at higher surface pressures, but was similar at lower surface pressures, p5 30 mN m − 1. It is interesting to note that for XC18OH 5 0.4 and p5 30 mN m − 1, the mixed monolayer existed in a liquid-expanded state. By means of Eqs. (2)–(4), the values of DGex and DGmix for various compositions of mixed DPPC/C18OH monolayers at different surface pressures were calculated (Fig. 3). From the dependence of DGex on composition, one can see that all the values of DGex were negative, which implies attractive interactions and compatibility between DPPC and C18OH. The interaction became more significant as the surface pressure increased, since the distance between molecules became smaller as the mixed monolayer was compressed. Moreover, since there was only one minimum in each DGmix –composition curve, which corresponded to XC18OH = 0.6, one can conclude that a phase separation in the mixed DPPC/ C18OH monolayers did not exist. The existence of a minimum indicates that the mixed monolayers were thermodynamically more stable than the
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monolayers with separation between individual components, especially around XC18OH = 0.6. Besides, the values of DGmix became more negative with increasing surface pressure, which suggests that the stability of a mixed monolayer compared with pure components was higher at higher surface pressures. This can be explained by stronger interactions between DPPC and C18OH as the mixed monolayer was more condensed.
Fig. 4. (a) Area per molecule and (b) Aex/Aid as a function of composition for mixed DPPC/C20OH monolayers at various surface pressures.
4.2. DPPC/C20OH mixed monolayers
Fig. 3. (a) Excess free energy of mixing and (b) free energy of mixing as a function of composition for mixed DPPC/C18OH monolayers at various surface pressures.
The p–A curves corresponding to pure and mixed monolayers formed by DPPC and C20OH in different proportions at 37°C are presented in Fig. 1(b). The curves displayed features similar to those of DPPC/C18OH monolayers of comparable compositions. The phase transition at p $35 mN m − 1, which was characteristic of a DPPC monolayer, disappeared as XC20OH \ 0.5, and the mixed monolayers existed only as condensed monolayers. Again, it seems that DPPC and C20OH were miscible at the air/water interface since the col-
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lapse pressures of the mixed monolayers changed with composition. The miscibility of DPPC/ C20OH mixed monolayers can be seen more clearly in Fig. 4, which shows the mean areas per molecule and the condensing effects at various surface pressures as a function of C20OH mole fraction. Apparently, the mean areas per molecule had lower values than those calculated for ideal mixed monolayers within the surface pressure range 5–40 mN m − 1, especially at higher proportions of C20OH. This indicates that DPPC and C20OH were miscible and formed non-ideal monolayers at the air/water interface. Moreover,
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the deviations were negative for all compositions at different surface pressures, and were more pronounced at lower surface pressures, except at XC20OH = 0.2. The contraction in the molecular packing was mainly ascribed to the molecular interactions in the hydrocarbon region of the monolayers, as for the DPPC/C16OH and DPPC/ C18OH systems. However, the condensation effect of C20OH on a DPPC monolayer showed slightly different dependence on composition and surface pressure compared with what observed in mixed DPPC/C18OH monolayers. Fig. 5 represents the values of DGex and DGmix as a function of composition for mixed DPPC/ C20OH monolayers at different surface pressures. One can find that the values of DGex were all negative, especially for the compositions around XC20OH = 0.6, which was the same as what observed in mixed DPPC/C18OH monolayers. Again, the data of DGex and DGmix suggest that there were strong interactions between DPPC and C20OH molecules at the interface, and the magnitude of interactions became stronger as the surface pressure was higher or the mixed monolayer was in a more condensed state. In addition, no phase separation was observed from the DGmix – composition curves for this mixed monolayer system.
4.3. Effects of alcohol chain length
Fig. 5. (a) Excess free energy of mixing and (b) free energy of mixing as a function of composition for mixed DPPC/C20OH monolayers at various surface pressures.
In Fig. 6, the condensing effects for mixed DPPC/Cn OH (n= 16, 18 and 20) monolayers at different surface pressures are demonstrated as a function of composition. For all three mixed monolayer systems, the molecular structures only differ in the hydrocarbon chain length of alcohols. At lower surface pressures (p= 10 and 20 mN m − 1), the condensing effect of alcohol on a DPPC monolayer became more significant as the hydrocarbon chain length of alcohols increased (Fig. 6(a) and (b)). Since all these mixed monolayers had same hydrophilic interactions, the difference in condensing effect was attributed to the hydrophobic interaction and the packing efficiency by the hydrocarbon moiety of the monolayers. At lower surface pressures, since each molecule occupies a larger area, one might expect
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ing in mixed DPPC/C16OH monolayers at higher surface pressures. It is not clear that this is due to the shorter hydrocarbon chain of C16OH compared with C18OH or due to the same hydrocarbon chain length of C16OH as DPPC. Comparisons of DGex and DGmix values for mixed monolayers of DPPC/C16OH, DPPC/ C18OH and DPPC/C20OH are shown in Figs. 7 and 8. All three mixed monolayer systems showed negative DGex values, and the negative DGex became significant as the hydrocarbon chain length of alcohols increased. Apparently, the attractive
Fig. 6. Aex/Aid as a function of composition for mixed DPPC/ C16OH ( ), DPPC/C18OH (+ ), and DPPC/C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
that the influence of packing efficiency in the condensing effect was not as important as that of intermolecular attraction. Thus, the longer the alcohol chain length, the stronger the intermolecular attraction and the more significant the condensing effect. However, at higher surface pressures, the effect of packing efficiency became important since the molecules were in a crowded state. Fig. 6(c) shows that at p= 40 mN m − 1, the larger condensing effect was observed for C16OH than for C18OH. These data suggest that the packing efficiency may favor the molecular pack-
Fig. 7. Excess free energy of mixing as a function of composition for mixed DPPC/C16OH ( ), DPPC/C18OH ( + ), and DPPC/C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
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curves for a monolayer system composed of DPPC and C16OH showed symmetric behavior around the composition XDPPC = 0.5 or XC16OH = 0.5, at which the minimum occurred [17]. However, for both DPPC/C18OH and DPPC/C20OH mixed systems, the DGmix –composition curves showed similar skewed behavior with the minimum occurred at XDPPC = 0.4 or Xalcohol = 0.6.
5. Conclusions
Fig. 8. Free energy of mixing as a function of composition for mixed DPPC/C16OH ( ), DPPC/C18OH ( + ), and DPPC/ C20OH ( ) monolayers at (a) 10 mN m − 1, (b) 20 mN m − 1, and (c) 40 mN m − 1.
intermolecular force resulting from the hydrophobic interactions between hydrocarbon chains played a more important role than the packing efficiency in the mixing processes of monolayers of DPPC and long-chain alcohols with alcohol chain lengths from 16 to 20. Moreover, there was only one minimum in each DGmix –composition curve for the three mixed monolayer systems at given surface pressures, indicating a single phase existing across the entire composition range. The DGmix – composition
From a detailed analysis of surface pressure– area isotherms, one can conclude that DPPC and normal long-chain alcohols with chain lengths of 16, 18, and 20 were miscible and formed non-ideal monolayers at the air/water interface. The nonideality of the mixed monolayers was evident in the area–composition figures, in which the mixed monolayers exhibited significantly negative deviations from the ideal ones. It seems that the hydrophobic interaction was dominant in the molecular packing of the mixed monolayers at lower surface pressures. However, the packing efficiency or geometric accommodation may favor the molecular packing of mixed DPPC/C16OH monolayers at higher surface pressures, resulting in unexpected stronger condensing effects of C16OH than C18OH on a DPPC monolayer. Based on the results of excess free energies of mixing, the hydrophobic interaction appeared to play a more important role than the packing efficiency in the mixing processes of DPPC with long-chain alcohols at the interface. Moreover, the free energy of mixing–composition curves indicate that there was no phase separation existing across the entire composition range, and the mixed monolayers appeared to be more stable than the monolayers with separation between individual components.
Acknowledgements This research was supported by the National Science Council of the Republic of China through Grants c NSC85-2214-E-006-006 and NSC862214-E006-025.
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