磷酯酰乙醇胺与硬脂酸和十八醇二元混合体系单层膜的热力学特性和AFM观测

利用表面压力-平均分子面积(π-A)曲线的关系, 分别研究了在水/空气界面上形成的磷酯酰乙醇胺(PE)与硬脂酸(SA)和十八醇(OD)二元混合体系的热力学特性. 根据对弹性模量(C_S^-1)、过量分子面积(A_ex)以及表面过量吉布斯自由能(ΔG_ex)等热力学参数的数据计算, 定量分析了混合单层膜分子之间的相互作用. 实验结果表明, PE/SA和PE/OD单层膜两种组分之间摩尔比对其热力学参数有影响. 热力学分析表明, 在X_SA=0.2, 0.8和X_OD=0.8处, PE/SA和PE/OD二元混合体系的热力学参数[过量分子面积(A_ex)和表面过量吉布斯自由能(ΔGex)]相对理想状态均表现为负偏差作用, 说明分子之间的作用为引力作用. 相反, PE/OD二元体系在X_OD=0.2, 0.4, 0.6处, 表现为正偏差作用, 这说明分子之间的作用为斥力作用. AFM观测为PE/SA和PE/OD单层膜热力学特性的理论分析提供了有力的支持.     

胆固醇与磷脂酰乙醇胺/磷脂酰胆碱混合膜的热力学特性及其单层膜形态的AFM观察

研究了不同比例下胆固醇(Chol)对磷脂酰乙醇胺/磷脂酰胆碱(PE/PC,1∶1物质的量比)混合膜的影响,并在表面压力-平均分子面积(π-A曲线)等温线基础上,通过对过量平均分子面积(ΔAex)和过量吉布斯自由能(ΔGex)的计算分析,研究了不同比例Chol与PE/PC(1∶1物质的量比)三元混合膜的热力学特性.实验结果表明:Chol在一定程度上加速了混合膜的相变,增强了膜的凝聚性;当XChol=0.2,0.6,0.8时,过量平均分子面积和过量吉布斯自由能在所研究的表面压力下都为负值,分子之间相互作用力表现为引力,促使混合膜的凝聚,而在XChol=0.4时,过量平均分子面积和过量吉布斯自由能在15,20,25,30 mN/m下为正值,分子之间相互作用力表现为斥力,促使熵的增加,并且在20 mN/m压力下出现极值.实验中利用LB膜技术制备了不同比例Chol与PE/PC(1∶1物质的量比)混合膜,并在原子力显微镜下对其结构形态进行了观测.     

Effects of Calcium Ions on Thermodynamic Properties of Mixed Bilirubin/Cholesterol Monolayers

The mixed monolayer behavior of bilirubin/cholesterol was studied through surface pressure-area (?-A) isotherms on aqueous solutions containing various concentrations of calcium ions. Based on the data of ?-A isotherms, the mean area per molecule, collapse pressure, surface compressibility modulus, excess molecular areas, free energy of mixing, and excess free energy of mixing of the monolayers on different subphases were calculated. The results show an expansion in the structure of the mixed monolayer with Ca2+ in subphase, and non-ideal mixing of the components at the air/water interface is observed with positive deviation from the additivity rule in the excess molecular areas. The miscibility between the components is weakened with the increase of concentration of Ca2+ in subphase. The facts indicate the presence of coordination between Ca2+ and the two components. The mixed monolayer, in which the molar ratio of bilirubin to cholesterol is 3:2, is more stable from a thermodynamic point of view on pure water. But the stable 3:2 stoichiometry complex is destroyed with the increase of the concentration of Ca2+ in subphase. Otherwise, the mixed monolayers have more thermodynamic stability at lower surface pressure on Ca2+ subphase.     

The controlling effect of pH on oxidation of Cr(III) by manganese oxide minerals

Effects of the subphase temperature on the surface pressure (pi)-area (A) isotherms of mixed monolayers of miltefosine (hexadecylphosphocholine), a potential anticancer drug, and cholesterol were investigated at the air/water interface, which were supplemented with Brewster angle microscopy (BAM) observations. Comparison of the collapse pressure values, mean molecular areas, excess areas and excess free energy of mixing between the mixed monolayer at various molar ratios and the pure component monolayers showed that, regardless of the subphase temperature, the investigated miltefosine-cholesterol system is much more stable than that the pure component monolayers, suggesting strong attractive interactions between miltefosine and cholesterol in mixed monolayers. As a consequence, it was postulated that stable "complexes" of the two components could form at the interface, for which stoichiometry may vary with the subphase temperature. Such "surface complexes" should be responsible for the contraction of the mean molecular area and thus the high stability of the mixed monolayer.     

Thermodynamic characterization of the prevailing molecular interactions in mixed floating monolayers of phospholipids and usnic acid

The investigation of the characteristics of mixed floating monolayers of phospholipids and usnic acid (UA), an active metabolite from lichens, can provide valuable information on how to prepare stable liposomes that could serve as carriers of UA for therapeutic proposes. The present paper is concerned with the thermodynamic analysis of the behavior of Langmuir monolayers formed by mixing different phospholipids (dibehenoylphosphatidylcholine, DBPC, dipalmitoylphosphatidylcholine, DPPC, and dioleoylphosphatidylcholine, DOPC) and UA at varied molar fractions. Relevant thermodynamic parameters such as excess areas, excess free energies and free energy of mixing were derived from the surface pressure data obtained from compression measurements performed in a Langmuir trough. For the largest lateral pressure examined (25 mN/m), negative values of the excess free energy were found only for the DOPC/UA monolayer, which should be the most stable of them. Based on the calculated values of the free energy of mixing, we note that the DBPC/UA and DPPC/UA systems present the best mixed character at low pressures and when the molar fraction of the UA is 0.5; at that relative concentration and at low values of the external pressure, the UA molecules can better mix and interact with the phospholipid molecules. The compression isotherms for mixed monolayers show no visible transitions, exhibiting a more organized phase that corresponds to a negative free energy of mixing. We have established that the most stable monolayers were those corresponding to DOPC/UA mixtures with a UA molar fraction of 0.75.     

Interaction of Egg-Sphingomyelin with DOPC in Langmuir Monolayers

Lipid rafts are a dynamic microdomain structure found in recent years, enriched in sphin-golipids, cholesterol and particular proteins. The change of structure and function of lipid rafts could result in many diseases. In this work, the monolayer miscibility behavior of mixed systems of Egg-Sphingomyelin (ESM) with 1, 2-dioleoyl-sn-glycero-3-phosphocholine was in-vestigated in terms of mean surface area per molecule and excess molecular area ΔA_ex at certain surface pressure, surface pressure and excess surface pressure Δπ_ex at certain mean molecular area. The stability and compressibility of the mixed monolayers was assessed by the parameters of surface excess Gibbs free energy ΔG_ex, excess Helmholtz energy ΔH_ex and elasticity. Thermodynamic analysis indicates ΔA_ex and Δπe_ex in the binary systems with positive deviations from the ideal behavior, suggesting repulsive interaction. The max-imum of ΔG_ex and ΔH_ex was at the molar fraction of ESM of 0.6, demonstrating the mixed monolayer was more unstable. The repulsive interaction induced phase separation in the monolayer     

Molecular organization of a water-insoluble iridium(III) complex in mixed monolayers

In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.     

Interactions between the ganglioside GM1 and hexadecylphosphocholine (miltefosine) in monolayers at the air/water interface

The ganglioside, GM1, was studied as Langmuir monolayers at the air/water interface with surface pressure-area measurements in addition to Brewster angle microscopy. A characteristic plateau transition, observed on aqueous subphases of pH 2 and 6, 20 degrees C, at the surface pressure of ca. 20 mN/m, was attributed to the reorientation of GM1 polar group upon film compression. This transition was found to disappear at alkaline subphases (pH 10) due to the hydration of fully ionized polar group, hindering its reorientation. The interactions between GM1 and hexadecylphosphocholine (miltefosine) were investigated in mixed monolayers and analyzed with the mean molecular areas, excess areas of mixing and the excess free energy of mixing versus film composition plots. The monolayers stability, quantified by the collapse pressure values, as well as the strength of interaction was found to diminish in the following order: pH 6>pH 2>pH 10. The strongest interaction occurs for mixed films of miltefosine molar fraction, XM=0.7-0.8, especially at low pressure region, and are explained as being due to the surface complex formation of 3:1 or 4:1 (miltefosine:ganglioside) stoichiometry (XM=0.75 or 0.8, respectively).     

Molecular interactions of cord factor with dipalmitoylphosphatidylcholine monolayers: implications for lung surfactant dysfunction in pulmonary tuberculosis

Molecular interactions between mycobacterial cell wall lipid, cord factor (CF) and the abundant surfactant lipid, dipalmitoylphosphatidylcholine (DPPC) were investigated using Langmuir monolayers at physiological temperatures (37 degrees C). Surface topography of the films was visualized by atomic force microscopy (AFM). Thermodynamic behavior of the mixed monolayers was evaluated by investigating the molecular area excess, excess Gibbs free energy of mixing and maximum compressibility modulus (SCM(max)). Cord factor formed immiscible and thermodynamically unstable monolayers with DPPC. Monolayer presence of cord factor altered the physical state of DPPC monolayers from liquid condensed to liquid expanded with the lowering of SCM(max) from 160 to 40 mN/m, respectively. AFM imaging exhibited smooth homogenous surface topography of DPPC films which in the presence of cord factor was markedly altered with the appearance of aggregates and increased surface roughness. The results highlight the capacity of cord factor to disturb DPPC monolayer organization and structure. Interfacial presence of cord factor results in DPPC monolayer fluidization. Lung surfactant function is attributed to its ability to form well packed low compressibility films. Such molecular interactions suggest a dysfunction of lung surfactant in pulmonary tuberculosis due to surfactant monolayer fluidization.     

The Equilibria of Phosphatidylethanolamine-Cholesterol and Phosphatidylcholine–Phosphatidylethanolamine in Monolayers at the Air/Water Interface

Monolayers of phosphatidylethanolamine (PE), cholesterol (Ch), phosphatidylcholine (PC), and binary mixtures (PC-PE or PE-Ch) were investigated at the air/water interface. The surface tension values of pure and mixed monolayers were used to calculate π -A isotherms. The surface tension measurements were carried out at 20°C using an improved Teflon trough and a Nima 9000 tensiometer. The Teflon trough was filled with a subphase of triple-distilled water. Known amounts of lipid dissolved in 1-chloropropane were placed at the surface using a syringe. The interactions between phosphatidylethanolamine and cholesterol as well as phosphatidylcholine and phosphatidylethanolamine result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC-PE and PE-Ch complexes. We considered the equilibrium between the individual components and the complex and established that phosphatidylethanolamine and cholesterol as well as phosphatidylcholine and phosphatidylethanolamine formed highly stable 1:1 complexes.     

Monolayer behavior of binary systems of betulinic acid and cardiolipin: thermodynamic analyses of Langmuir monolayers and AFM study of Langmuir-Blodgett monolayers

Betulinic acid (BA, a natural pentacyclic triterpene) can induce mitochondrial membrane damage and trigger the mitochondrial pathway of apoptosis in tumor cells. The monolayer behavior of binary systems of BA and cardiolipin (CL, a unique phospholipid found only in mitochondria membrane in animals) was studied by surface pressure-area (π-A) measurements and analyses and Atomic force microscopy (AFM) observation. The miscibility analysis presents that in mixed monolayers BA takes both tilted and nearly perpendicular orientations at surface pressure below 30 mN/m but only nearly perpendicular orientation at 30 mN/m. The thermodynamic stability analysis indicates that phase separation and repulsion occur in mixed BA/CL monolayers. The compressibility analysis shows that at 30 mN/m, 20% addition of BA does markedly translate the liquid-condensed CL monolayer to mixed BA/CL monolayer with the coexistence of liquid-condensed and liquid-expanded phases. The AFM images of supported monolayers give direct evidence of the conclusions obtained from the analyses of π-A isotherms. These results confirm that at high surface pressure near to real biologic situations, BA orients nearly perpendicularly with hydroxyl group toward water, causes phase separation and changes the permeability of CL film, which correlates with the mitochondrial membrane damage induced by BA.     

Surface properties of mixed monolayers of sulfobetaines and ionic surfactants

To study the influence of the head group in the properties of the mixed monolayers adsorbed at the air-water interface, the surface tension and surface potential of binary mixtures of surfactant have been determined as a function of the surfactant composition. Experiments were carried out with anionic-zwitterionic sodium dodecyl sulfate and dodecyl dimethyl ammoniopropane sulfonate (SDS/DDPS), and cationic-zwitterionic dodecyl trimethylammonium bromide and dodecyl dimethyl ammoniopropane sulfonate (DTAB/DDPS), and dodecyl trimethylammonium bromide and tetradecyl dimethyl ammoniopropane sulfonate (DTAB/TDPS). It was shown that mixed monolayers of cationic-zwitterionic surfactant exhibit small negative deviations of ideal behavior, whereas for SDS/DDPS monolayers show strong negative deviation from the ideality. Deviations of ideal behavior are interpreted by regular solution theory. The surface potential values agree very well with the concentration of the ionic component at the interface. The dynamic surface tension values show that the adsorption kinetics on the interface is a diffusion-controlled process. In monolayers with significant deviation of the ideal behavior, anionic-zwitterionic, there is some evidence of intermolecular attractions after diffusion of both surfactants at the interface.     

Study of gramicidin A--phospholipid interactions in Langmuir monolayers: analysis of their mechanical, thermodynamical, and electrical properties

The mechanisms of interactions between gramicidin A (gA) and dimyristoylphosphatidylcholine (DMPC) in monolayers formed at the air-water interface were studied by analyzing their mechanical, thermodynamical, and electrical properties evaluated from measurements of pressure-area isotherms and of Maxwell displacement currents (MDC). A contactless method of recording MDC enabled us to monitor changes in the charge state of the monolayer-constituting molecules and to find the relation between a phase state of the monolayer and structural transitions of gA. The peptide-lipid interactions were quantified in terms of the excess of Gibbs free energy, excess entropy, as well as the molecular dipole moments at various gA/DMPC molar ratios, at various temperatures (in the gel phase and also in the liquid-crystalline phase of DMPC molecule), and at various surface pressures. It was found that the strongest interactions between gA and DMPC took place at the gA/DMPC molar ratio at around 0.25. At this monolayer composition, the phospholipids, via their carbonyl moieties, dominantly interact with the single helical gA, which mostly stands upright on the surface and is anchored by its C-terminus to the water surface, and prevent the formation of the intertwined helical gA dimers. The optimum ratio was confirmed also by anomalous electrical behavior of electrical dipole moments derived from MDC measurements.     

Electrolyte effect on adsorption and the phase transition from microstructures to nanostructures in ionic/ionic surfactants mixture

The effect of salt concentration on intermicellar interactions and aggregate structures of anionic and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry, surface tension, zeta potential, cyclic voltammetry measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface excess (Γ(max)), and mean molecular surface area (A(min)) were determined from plots of the surface tension (γ) as a function of the log of total surfactant concentration. The electrochemical behavior of cationic-anionic (catanionic) mixed surfactant and self-assembled surfactant monomers at Pt wire electrode were studied by cyclic voltammetry (CV). A variation in the peak current versus the total concentration of surfactant allow us to evaluate the CAC and related parameters from regular solution theory along with the diffusion coefficient of the electroactive species. It was observed that, for both the planar air/aqueous interface and micellar systems, the nonideality decreased as the amount of electrolyte in the aqueous medium was increased. Finally, we investigated the variations of electrostatic, transfer and steric free energy in phase transition between mixed micelle and vesicle in the presence of electrolyte using the presented model by our groups.     

On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study

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.     

Nanoscale properties of mixed fengycin/ceramide monolayers explored using atomic force microscopy

To gain insight into the interactions between fengycin and skin membrane lipids, mixed fengycin/ceramide monolayers were investigated using atomic force microscopy (AFM) (monolayers supported on mica) and surface pressure-area isotherms (monolayers at the air-water interface). AFM topographic images revealed phase separation in mixed monolayers prepared at 20 degrees C/pH 2 and composed of 0.25 and 0.5 fengycin molar ratios, in the form of two-dimensional (2-D) hexagonal crystalline domains of ceramide surrounded by a fengycin-enriched fluid phase. Surface pressure-area isotherms as well as friction and adhesion AFM images confirmed that the two phases had different molecular orientations: while ceramide formed a highly ordered phase with crystalline chain packing, fengycin exhibited a disordered fluid phase with the peptide ring lying horizontally on the substrate. Increasing the temperature and pH to values corresponding to the skin parameters, i.e., 37 degrees C/pH 5, was found to dramatically affect the film organization. At low fengycin molar ratio (0.25), the hexagonal ceramide domains transformed into round domains, while at higher ratio (0.5) these were shown to melt into a continuous fengycin/ceramide fluid phase. These observations were directly supported by the thermodynamic analysis (deviation from the additivity rule, excess of free energy) of the monolayer properties at the air-water interface. Accordingly, this study demonstrates that both the environmental conditions (temperature, pH) and fengycin concentration influence the molecular organization of mixed fengycin/ceramide monolayers. We believe that the ability to modulate the formation of 2-D domains in the skin membrane may be an important biological function of fengycin, which should be increasingly investigated in future pharmacological research.     

Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

Mixed layers of DPPC and a linear poly(ethylene glycol)-b-hyperbranched poly(glycerol) block copolymer having a cholesteryl end group

Interactions of the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with the amphiphilic diblock copolymer Ch-lPEG₃₀-b-hbPG₂₄ (ChP) are studied at the air–water interface by surface pressure–mean molecular area (π–mmA) measurements of mixed Langmuir films and adsorption measurements of ChP to the air–water interface covered with DPPC monolayers at different initial surface pressure values π ₀. ChP is composed of a single hydrophobic cholesteryl (Ch) moiety covalently bound to a diblock copolymer consisting of a hydrophilic linear poly(ethylene glycol) (lPEG) block and a hydrophilic hyperbranched poly(glycerol) (hbPG) block. Langmuir isotherms and compression moduli of the mixed Langmuir films of different molar ratios reveal distinct interactions between DPPC and ChP during compression. It is demonstrated that the behavior of the DPPC/ChP mixtures is dominated by DPPC up to a molar ratio of 10:1, whereas the behavior is predominantly governed by ChP in mixtures with lower DPPC content (molar ratios of 5:1, 2:1, and 1:1). In adsorption measurements, a strong affinity of ChP to DPPC is observed after injection into the water subphase. The surface pressure value π _in up to which ChP is able to penetrate into DPPC monolayers is determined to the remarkably high value of 48.2 mN/m which attests the favorable interactions between DPPC and the Ch moiety of ChP. Atomic force microscopy on LB films of DPPC/ChP mixtures of different molar ratios transferred onto hydrophilic substrates confirms the presence of two different phases, a DPPC-rich phase and a ChP-rich phase.     

Influence of temperature on microdomain organization of mixed cationic-zwitterionic lipidic monolayers at the air-water interface

The thermodynamic behavior of mixed DOTAP-DPPC monolayers at the air-water interface has been investigated in the temperature range from 15 to 45 degrees C, covering the temperature interval where the thermotropic phase transition of DPPC, from solid-like to liquid-like, takes place. Based on the regular solution theory, the miscibility of the two lipids in the mixed monolayer was evaluated in terms of the excess Gibbs free energy of mixing DeltaG(ex), activity coefficients f(1) and f(2) and interaction parameter omega between the two lipids. The mixed DOTAP-DPPC film was found to have positive deviations from ideality at low DOTAP mole fractions, indicating a phase-separated binary mixture. This effect depends on the temperature and is largely conditioned by the structural chain conformation of the DPPC lipid monolayer. The thermodynamic parameters associated to the stability and the miscibility of these two lipids in a monolayer structure have been discussed in the light of the phase diagram of the DOTAP-DPPC aqueous mixtures obtained from differential scanning calorimetry measurements. The correlation between the temperature behavior of DOTAP-DPPC monolayers and their bulk aqueous mixtures has been briefly discussed.     

季铵盐型双子表面活性剂与十八醇的混合单分子膜

研究了双子表面活性剂12-2-16和12-2-12分别与十八醇(C18H37OH)在空气-水界面上混合单分子膜的π-A等温线. 在相分离表面压以下, 比较了不同表面压下和不同混合比单分子膜的混合表面过剩自由能ΔGMexo, 分析了双子表面活性剂与脂肪醇在空气-水界面上混合膜中的相容性. 结果表明, 12-2-16与C18H37OH在所有混合摩尔比下随着表面压增高, 自由能增大. 12-2-12与C18H37OH混合膜体系的相容性取决于两者的混合比, ΔGMexo随所加入C18H37OH摩尔分数的增加逐渐增大, 从异种分子间净的吸引作用转变到相互排斥作用体系, 转变点为C18H37OH加入量的摩尔分数0.65. 当混合为热力学自发过程时, 增大表面压将有利于混合; 而对相互排斥体系, 增加表面压将使体系内异种分子之间的相互排斥作用更大.