Reversible Phase Transitions in the Phospholipid Monolayer

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Reversible Phase Transitions in the Phospholipid Monolayer Lu Xu† and Yi Y. Zuo*,†,‡ †

Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States Department of Pediatrics, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii 96826, United States

‡

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S Supporting Information *

ABSTRACT: The polymorphism of phospholipid monolayers has been extensively studied because of its importance in surface thermodynamics, soft matter physics, and biomembranes. To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance. However, because of experimental artifacts caused by film leakage, the Langmuir balance fails to study the reversibility of two-dimensional surface phase transitions. We have developed a novel experimental methodology called the constrained drop surfactometry capable of providing a leakage-proof environment, thus allowing reversibility studies of two-dimensional surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC) as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structures of the DPPC monolayer also show reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.

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temperature (Tc) of 44.1 °C.13 Second, the CDS enables the possibility of maintaining a constant surface pressure while ramping up temperature, thus allowing the study of isobaric phase transitions upon various surface pressures. Using the CDS, we have shown that phase transitions in the DPPC monolayer can be induced not only by isothermal compression but also by isobaric heating at a surface pressure below a critical value of (πc) of 57 mN/m.14 Together, our recent studies on the DPPC monolayer using the CDS significantly extended current understanding of phospholipid phase transitions and surface thermodynamics. Despite the extensive studies of phase transitions in the phospholipid monolayer, reversibility of the phase transitions is still largely unknown. Reversibility of the phase transitions is a critical property of any phase change materials.15,16 It is of particular importance to understand the reversibility of phase transitions in phospholipid monolayers because many monolayers and biomembranes undergo cyclic phase transitions, such as the pulmonary surfactant film11 and the tear film.17,18 Nevertheless, the traditional Langmuir balance is intrinsically lack of the capacity of studying reversibility of the phase transitions because of film leakage.11 Because of losing monolayer materials from the air−water interface at the end

INTRODUCTION The polymorphism of phospholipid monolayers has been extensively studied in the past decades because of its importance in surface thermodynamics,1 soft matter physics,2 interfacial rheology,3,4 microbubble stability,5 and biomembranes.6 To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance.2 Using the Langmuir balance, numerous studies have shown phase transitions in the phospholipid monolayer upon lateral compression at room temperature.7−9 A first-order phase transition from a disordered liquid-expanded (LE) phase to an ordered tiltedcondensed (TC) phase is demonstrated by a plateau region in the compression isotherm obtained with the Langmuir balance.2,10 Nucleation and growth of the TC domains from the LE phase upon lateral compression have been visualized with various microscopy techniques.10−12 Recently, owing to the development of a novel experimental methodology called the constrained drop surfactometry (CDS), the study of phospholipid phase behavior has been generalized in two perspectives. First, the CDS ensures rigorous control of the environmental temperature, thus allowing the study of compression isotherms at various temperatures. Using the CDS, we have shown that isothermal phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer exist in a large temperature range between a triple-point temperature (T0) of 17.5 °C and a critical © XXXX American Chemical Society

Received: May 10, 2018 Revised: June 25, 2018 Published: July 3, 2018 A

DOI: 10.1021/acs.langmuir.8b01544 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Reversibility of the isothermal phase transitions in the DPPC monolayer. (a) Five consecutive quasi-static compression−expansion cycles obtained with the CDS at room temperature. The compression and expansion isotherms superimpose with each other, indicating reversible isothermal phase transitions. (b) AFM topographic images of the DPPC monolayer at three representative surface pressures of 2, 6, and 20 mN/m during both compression and expansion processes. At these representative surface pressures, as indicated by I−VI in the compression−expansion isotherms, the DPPC monolayer is in a single disordered LE phase, two-phase coexistence, and a single ordered TC phase, respectively. AFM images in the first row have the same scanning area of 20 × 20 μm and a z range of 5 nm. Images in the second row show the zoom in and zoom out of the monolayer topography as indicated by the white boxes.

of film compression, the subsequent expansion of the monolayer can hardly be reversible. Consequently, an evident hysteresis loop between the compression and expansion isotherms is usually observed with the Langmuir balance.19−26 This artifact of the Langmuir balance prevents the reversibility study of surface phase transitions. Here, we used the CDS to study the reversibility of both isothermal and isobaric phase transitions in the DPPC monolayer. We showed that the CDS ensures a leakageproof environment that makes it capable of studying the reversibility of phospholipid phase transitions. In addition to surface thermodynamics, we have provided direct evidence of reversible phase transitions by visualizing the lateral structure of the DPPC monolayer, under both isothermal and isobaric conditions, using atomic force microscopy (AFM). With these technological advances in removing experimental artifacts, we concluded that both isothermal and isobaric phase transitions in the DPPC monolayer are reversible. Our study has implications in understating two-dimensional phase change materials, biomembranes, pulmonary surfactants, and tear films.

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two-dimensional thermodynamic system, that is, the surface pressure (π), molecular area (A), and temperature (T). The spread monolayer can be compressed and expanded quasi-statically or dynamically by regulating liquid flow into and out of the droplet with a motorized syringe. Both surface pressure and surface area of the monolayer are simultaneously determined by analyzing the shape of the droplet in real-time using axisymmetric drop shape analysis (ADSA).28 The system temperature is maintained by an electrothermostatic environmental chamber in which the temperature can be either controlled at a constant with an accuracy of ±0.1 °C or controlled to ramp up or down at a quasi-static rate of 1 °C/min.14 During the temperature ramp, the phospholipid monolayer can be maintained at a constant surface pressure with an accuracy of ±0.5 mN/m using the newly developed closed-loop ADSA (CL-ADSA).29 Isothermal and Isobaric Phase Transitions. A DPPC (Avanti Polar Lipids, Alabaster, AL) monolayer was spread at the surface of a water (Millipore) droplet and was left undisturbed for 1 min to allow evaporation of solvent (chloroform). For the study of isothermal phase transitions, the DPPC monolayer was compressed and expanded at a quasi-static rate of 0.05 cm2/min, that is, 0.25 Å2/ molecule/s, at room temperature (20 ± 1 °C). This quasi-static rate of monolayer compression and expansion is close to what used in previous studies (i.e., 0.2 Å2/molecule/s).30 The resultant surface pressure−molecular area (π−A) isotherms were recorded with ADSA. For the study of isobaric phase transitions, the DPPC monolayer, maintained at a constant surface pressure between 10 and 50 mN/m, was heated and cooled at a quasi-static rate of 1 °C/min. The resultant temperature−molecular area (T−A) isobars were recorded with CL-ADSA. All experiments were repeated for at least three times. Results were shown as mean ± standard deviation. AFM Imaging. To directly visualize the two-dimensional phase transitions, the DPPC monolayer was first Langmuir−Blodgett

EXPERIMENTAL SECTION

Constrained Drop Surfactometry. CDS is a novel droplet-based miniaturized Langmuir-type film balance recently developed in our laboratory.13,14,27 The Langmuir monolayer is spread at the air−water interface of a 3 mm sessile drop constrained on a carefully machined pedestal, which uses its knife-sharp edge to prevent film leakage and to keep the droplet integrity even at high surface pressures (∼72 mN/ m). CDS allows rigorous control of the intensive properties of the B


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Langmuir transferred from the droplet surface by lifting a small piece of freshly peeled mica sheet across the air−water interface of the droplet at a rate of 1 mm/min. The immobilized DPPC monolayers were scanned using an Innova AFM (Bruker, Santa Barbara, CA) in air in tapping mode using an antimony-doped silicon cantilever with a spring constant of 42 N/m and a tip radius of 8 nm. Image analysis was conducted with NanoScope Analysis (version 1.5). The size of phospholipid domains was determined with ImageJ.

Reversibility of the Isobaric Phase Transitions. Figure 2a shows the reversibility of the isobaric heating−cooling cycles of the DPPC monolayer at five representative surface pressures 10, 20, 30, 40, and 50 mN/m, respectively. Also shown in Figure 2a is the surface pressures maintained at constant while ramping up and down the temperature. The complete experimental data sets of surface area, surface pressure, and experimental temperature can be found in Figure S4. Reproducibility of these isobars is demonstrated in Figure S5. It is found that for all studied surface pressures, the isobaric heating and cooling curves completely superimpose with each other, thus indicating reversible isobaric phase transitions in the DPPC monolayer. For example, at 20 mN/m, the isobaric heating and cooling curves both show an overlapping plateau region between the molecular areas of 54 and 65 Å2/molecule in which the temperature varies by only 1 °C around the phase transition temperature of 32 °C. Either below or above this phase transition temperature, the molecular area of the DPPC monolayer changes much more significantly with respect to temperature variations. From the isobars shown in Figure 2a, we have determined

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RESULTS AND DISCUSSION Reversibility of the Isothermal Phase Transitions. Figure 1a shows the reversibility of five consecutive isothermal compression−expansion cycles of the DPPC monolayer at room temperature. Reproducibility of these isothermal compression−expansion cycles can be found in Figure S1 of the Supporting Information. As shown in Figure 1a, the compression and expansion isotherms, ranging from zero to the collapse surface pressure of 72 mN/m, in each cycle closely superimpose with each other. In addition, all compression and expansion isotherms in five consecutive cycles closely overlap with each other (see Figure S1). These thermodynamic measurements indicate that the two-dimensional isothermal phase transitions are reversible upon lateral compression and expansion of the DPPC monolayer. The quasi-static compression and expansion isotherms obtained with the CDS (Figure 1a) are essentially different from those produced by the classical Langmuir film balance under comparable experimental conditions. As summarized in Figure S2, the compression and expansion isotherms of the DPPC monolayer obtained with the classical Langmuir film balance showed significant hysteresis in each compression− expansion cycle and between cycles. In contrast, the compression and expansion isotherms of the DPPC monolayer obtained with the CDS showed negligible intercycle hysteresis and only minimum intracycle hysteresis, mainly around the phase transition surface pressure. The source of this slight intracycle hysteresis is unknown, but the potential kinetic effect may be ruled out as the hysteresis remains even when the rate of compression and expansion was reduced by 10-fold, that is, at 0.025 Å2/molecule/s (Figure S3). These results strongly indicate the reversible nature of phase transitions at the DPPC monolayer, revealed by the leakage-proof environment of the CDS but masked by the classical Langmuir film balance. We have studied the reversibility of the surface phase transitions by directly visualizing the lateral structure of the DPPC monolayer using AFM. Figure 1b shows the lateral structures of the DPPC monolayer at 2, 6, and 20 mN/m during both compression and expansion processes. The DPPC monolayer at 2 mN/m during both compression and expansion processes is in a homogeneous disordered LE phase with a low lipid packing density, indicated by grooves/ stripes found in the monolayer. At 20 mN/m, during both compression and expansion processes, the DPPC monolayer is in a homogeneous ordered TC phase with a high lipid packing density. At the phase transition surface pressure of 6 mN/m, during both compression and expansion processes, the DPPC monolayer shows two-phase coexistence as demonstrated by the signature kidney-shaped TC domains.12 The average domain size in the compressed monolayer is 11.9 ± 0.8 μm, whereas the domain size in the expanded monolayer is 12.1 ± 1.0 μm. The similarities in the domain size and morphology provide direct evidence of reversible phase transitions during the compression and expansion processes.

the isobaric thermal expandability απ =

1 dA A dT π

( )

of the DPPC

monolayer during the heating and cooling processes. As shown in Figure 3, the change of thermal expandability is found to be completely symmetric about the heating and cooling processes. The peak location of the thermal expandability indicates the phase transition temperature (Ttr) at the studied surface pressure. As summarized in Table 1, the phase transition temperatures, along with the location of the phase transition plateaus at various surface pressures, are identical for the heating and cooling processes, indicating excellent reversibility. Figure 2b shows the lateral structures of the DPPC monolayer at 20, 32, and 40 °C during both heating and cooling processes at the surface pressure 20 mN/m. At both 20 and 40 °C, the DPPC monolayer is in a single phase, with more rigid lipid molecules and higher molecular packing in the TC phase at 20 °C than the fluid LE phase at 40 °C. At 32 °C, the DPPC monolayer is in TC−LE coexistence. The domain morphology, however, is different from that shown in Figure 1b. This difference is because the phase coexistence shown in Figures 1b and 2b was maintained at two different conditions, one at 20 °C and 6 mN/m (Figure 1b) while another one at 32 °C and 20 mN/m (Figure 2b). We therefore scrutinized the coexisting domain morphology during both heating and cooling processes, for a range of surface pressures between 10 and 50 mN/m. For each of these experiments, we maintained a constant surface pressure while quasi-statically ramping up and down the temperature. The constant surface pressure was maintained by naturally relaxing the DPPC monolayer to compensate for the temperature variations, using CL-ADSA.14,29 Lateral structures of the DPPC monolayers were examined at the corresponding phase transition temperature (Table 1) for both heating and cooling processes. As shown in Figure 4, morphology and size of the domains obtained during the heating and cooling processes are very alike at each studied surface pressure and temperature, thus providing direct evidence of reversible phase transitions in the DPPC monolayer. It is worth noting that the domain size reduces with the increasing phase transition temperature and surface pressure, in excellent agreement with our previous findings.14 The phase coexistence disappears at a critical point of (Tc, πc) = (44 °C, 57 mN/m).13,14 C


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Figure 3. Isobaric thermal expandability of the DPPC monolayer during (a) heating and (b) cooling processes. For both heating and cooling processes, five representative surface pressures 10, 20, 30, 40, and 50 mN/m, were studied with the CDS in a temperature range between 10 and 50 °C. The peak location of the thermal expandability indicates the phase transition temperature at the corresponding surface pressure. The phase transition temperatures determined from the thermal expandability are consistent for both heating and cooling processes (Table 1), indicating reversible phase transitions.

Implications in Surface Thermodynamics and Biophysics. Most previous studies of phospholipid phase transitions relied on the classical Langmuir film balance. It was found that there exists a significant hysteresis loop between the compression and expansion isotherms of phospholipid monolayers obtained with the Langmuir balance.19−26 The area enclosed in a π−A hysteresis loop represents the net loss of energy per compression−expansion cycle, which is a crucial biophysical property in evaluating the surface activity of pulmonary surfactant films.11 A good pulmonary surfactant film should enable reversible compression−expansion cycles that minimize the hysteresis area, thus consuming little to no energy in normal tidal breathing.31 Using captive bubble surfactometry (CBS), Schürch and coworkers have proven that the hysteresis produced by the Langmuir balance is largely due to film leakage.31 Here, taking advantage of the leakage-proof environment of the CDS, we showed that the isothermal phase transitions in the DPPC monolayer are reversible (Figure 1). Previous studies using the Langmuir balance suggested that addition of hydrophobic surfactant proteins (such as SP-B) is necessary to reduce the hysteresis area of the DPPC monolayer.26 Our results showed that the isothermal phase transitions in the DPPC monolayer

Figure 2. Reversibility of the isobaric phase transitions in the DPPC monolayer. (a) Quasi-static heating (open symbols) and cooling (solid symbols) cycles at five representative surface pressures of 10, 20, 30, 40, and 50 mN/m, obtained with the CDS in a temperature range between 10 and 50 °C. The heating and cooling isobars superimpose with each other, indicating reversible isobaric phase transitions. Also shown (coordinates in orange color) is the surface pressures maintained at constant while ramping up (open symbols) and down (solid symbols) the temperature. (b) AFM topographic images of the DPPC monolayer at three representative temperatures of 20, 32, and 40 °C, during both heating and cooling processes at the surface pressure 20 mN/m. At these representative temperatures, as indicated by I−VI in the heating and cooling isobars, the DPPC monolayer is in a single ordered TC phase, two-phase coexistence, and a single disordered LE phase. AFM images in the first row have the same scanning area of 20 × 20 μm and a z range of 5 nm. Images in the second row show the zoom in and zoom out of the monolayer topography as indicated by the white boxes. D


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Table 1. Phase Transition Temperature (Ttr) and the Saturation Molecular Areas (ATC and ALE) of the LE−TC Phase Coexistence during Isobaric Heating and Cooling of the DPPC Monolayer isobaric heating π (mN/m) 10 20 30 40 50

Ttr (°C) 25.9 32.3 35.4 41.1 44.6

± ± ± ± ±

0.1 0.3 0.1 0.2 0.2

2

ATC (Å /mol) 56.1 53.7 52.1 50.2 47.2

± ± ± ± ±

0.3 0.6 0.9 0.6 0.8

isobaric cooling 2

ALE (Å /mol) 73.2 65.1 59.8 56.4 49.2

± ± ± ± ±

0.5 0.3 0.4 0.7 1.1

Ttr (°C) 25.9 31.9 34.9 40.9 44.7

± ± ± ± ±

0.1 0.3 0.1 0.2 0.2

ATC (Å2/mol) 56.2 53.9 51.8 50.4 47.3

± ± ± ± ±

0.4 0.4 0.5 0.3 0.5

ALE (Å2/mol) 73.1 65.2 59.9 56.6 49.0

± ± ± ± ±

0.2 0.4 0.6 0.5 0.7

Figure 4. Phase coexistence in the DPPC monolayer during the isobaric (a) heating and (b) cooling processes at surface pressures 10, 20, 30, 40, and 50 mN/m. Lateral structures of the DPPC monolayer were examined at the phase transition temperatures (summarized in Table 1 and also indicated in the topographic images) corresponding to the studied surface pressures. AFM topographic images in the first row have the same scanning area of 20 × 20 μm and a z range of 5 nm. Images in the second row show the zoom in and zoom out of the monolayer topography as indicated by the white boxes. (c) Quantification results of the domain size during the isobaric heating (open bar) and cooling (solid bar) processes. Both domain size and morphology are found to be similar at each studied surface pressure, indicating reversible phase transitions.

are intrinsically reversible. Hence the main benefit of adding surfactant proteins and/or fluid lipids into the DPPC is probably to compensate for the artifact of the Langmuir balance. In addition to isothermal phase transitions induced by physical compression and expansion of the monolayer, the CDS allows the study of isobaric phase transitions caused by increasing and decreasing temperature. Such isobaric phase transitions in the two-dimensional DPPC monolayer are directly analogous to melting and freezing in bulk pure substances, such as pure water. In general, the Langmuir balance is incapable of such studies because of its incapability of controlling the environmental temperature. Hall and co-

workers have conducted isobaric heating experiments using the CBS.32−34 However, cyclic heating and cooling processes may become difficult to operate in the CBS especially when the DPPC monolayer is heated close to its critical point. Using the CDS, here we showed that the isobaric phase transitions in the DPPC monolayer are reversible from the point of view of both surface thermodynamics and direct microstructural observations (Figures 2−4). The phase transition temperature under a certain surface pressure is fixed but independent of history and the specific thermodynamic process, that is, heating or cooling (Table 1). This conclusion is also consistent with the Gibbs phase rule for a pure substance, which predicts that the system has only one degree of freedom during phase transitions. E


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Langmuir Together with our previous findings,13,14 we conclude that the two-dimensional surface phase transitions in the phospholipid monolayer are in direct analogy to phase transitions in bulk pure substances.

(5) Kwan, J. J.; Borden, M. A. Lipid monolayer collapse and microbubble stability. Adv. Colloid Interface Sci. 2012, 183−184, 82− 99. (6) Brezesinski, G.; Möhwald, H. Langmuir monolayers to study interactions at model membrane surfaces. Adv. Colloid Interface Sci. 2003, 100, 563−584. (7) Yu, Z.-W.; Jin, J.; Cao, Y. Characterization of the liquidexpanded to liquid-condensed phase transition of monolayers by means of compressibility. Langmuir 2002, 18, 4530−4531. (8) Gradella Villalva, D.; Diociaiuti, M.; Giansanti, L.; Petaccia, M.; Bešker, N.; Mancini, G. Molecular packing in langmuir monolayers composed of a phosphatidylcholine and a pyrene lipid. J. Phys. Chem. B 2016, 120, 1126−1133. (9) Lee, K. Y. C. Collapse mechanisms of Langmuir monolayers. Annu. Rev. Phys. Chem. 2008, 59, 771−791. (10) Wüstneck, R.; Perez-Gil, J.; Wüstneck, N.; Cruz, A.; Fainerman, V. B.; Pison, U. Interfacial properties of pulmonary surfactant layers. Adv. Colloid Interface Sci. 2005, 117, 33−58. (11) Zuo, Y. Y.; Veldhuizen, R. A. W.; Neumann, A. W.; Petersen, N. O.; Possmayer, F. Current perspectives in pulmonary surfactant inhibition, enhancement and evaluation. Biochim. Biophys. Acta, Biomembr. 2008, 1778, 1947−1977. (12) McConlogue, C. W.; Vanderlick, T. K. A close look at domain formation in DPPC monolayers. Langmuir 1997, 13, 7158−7164. (13) Zuo, Y. Y.; Chen, R.; Wang, X.; Yang, J.; Policova, Z.; Neumann, A. W. Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers. Langmuir 2016, 32, 8501−8506. (14) Xu, L.; Bosiljevac, G.; Yu, K.; Zuo, Y. Y. Melting of the Dipalmitoylphosphatidylcholine Monolayer. Langmuir 2018, 34, 4688−4694. (15) Wang, J.; Liu, K.; Xing, R.; Yan, X. Peptide self-assembly: thermodynamics and kinetics. Chem. Soc. Rev. 2016, 45, 5589−5604. (16) Li, X.; Fei, J.; Xu, Y.; Li, D.; Yuan, T.; Li, G.; Wang, C.; Li, J. A Photoinduced reversible phase transition in a dipeptide supramolecular assembly. Angew. Chem., Int. Ed. 2018, 57, 1903−1907. (17) Rantamäki, A. H.; Telenius, J.; Koivuniemi, A.; Vattulainen, I.; Holopainen, J. M. Lessons from the biophysics of interfaces: lung surfactant and tear fluid. Prog. Retinal Eye Res. 2011, 30, 204−215. (18) Patterson, M.; Vogel, H. J.; Prenner, E. J. The effect of repeated lateral compression and expansions mimicking blinking on selected tear film polar lipid monofilms. Biochim. Biophys. Acta, Biomembr. 2017, 1859, 319−330. (19) Rana, F. R.; Mautone, A. J.; Dluhy, R. A. Surface chemistry of binary mixtures of phospholipids in monolayers. Infrared studies of surface composition at varying surface pressures in a pulmonary surfactant model system. Biochemistry 1993, 32, 3169−3177. (20) Taneva, S. G.; Keough, K. M. W. Dynamic surface properties of pulmonary surfactant proteins SP-B and SP-C and their mixtures with dipalmitoylphosphatidylcholine. Biochemistry 1994, 33, 14660− 14670. (21) Notter, R. H.; Taubold, R.; Mavis, R. D. Hysteresis in saturated phospholipid films and its potential relevance for lung surfactant function in vivo. Exp. Lung Res. 1982, 3, 109−127. (22) Baekmark, T. R.; Elender, G.; Lasic, D. D.; Sackmann, E. Conformational transitions of mixed monolayers of phospholipids and polyethylene oxide lipopolymers and interaction forces with solid surfaces. Langmuir 1995, 11, 3975−3987. (23) McConlogue, C. W.; Malamud, D.; Vanderlick, T. K. Interaction of DPPC monolayers with soluble surfactants: electrostatic effects of membrane perturbants. Biochim. Biophys. Acta, Biomembr. 1998, 1372, 124−134. (24) Georgiev, G. A.; Gurov, R.; Jordanova, A.; Vassilieff, C. S.; Lalchev, Z. Properties of alkyl-phosphatidylcholine monolayers in the presence of surface-active three-block copolymers. Colloids Surf., B 2010, 80, 40−44. (25) Zhai, X.; Brezesinski, G.; Möhwald, H.; Li, J. Thermodynamics and structures of amide phospholipid monolayers. J. Phys. Chem. B 2004, 108, 13475−13480.

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CONCLUSIONS The leakage-proof environment of the CDS eliminates the common experimental artifact of the classical Langmuir-type film balances, thus allowing reversibility studies of twodimensional surface phase transitions. Using DPPC as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structure of the DPPC monolayer also shows reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01544.

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Reproducibility of the compression−expansion isotherms of the DPPC monolayer; surface area−surface pressure−temperature as a function of time; and reproducibility of the heating and cooling isobars of the DPPC monolayer (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 808-956-9650. Fax: 808956-2373. ORCID

Yi Y. Zuo: 0000-0002-3992-3238 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF grant no. CBET-1254795 (Y.Y.Z.). REFERENCES

(1) Neumann, A. W.; David, R.; Zuo, Y. Applied Surface Thermodynamics, 2nd ed.; Surfactant Science Series; Taylor & Francis, 2012; Vol. 151. (2) Kaganer, V. M.; Möhwald, H.; Dutta, P. Structure and phase transitions in Langmuir monolayers. Rev. Mod. Phys. 1999, 71, 779− 819. (3) Miller, R.; Ferri, J. K.; Javadi, A.; Krägel, J.; Mucic, N.; Wüstneck, R. Rheology of interfacial layers. Colloid Polym. Sci. 2010, 288, 937− 950. (4) Yu, K.; Yang, J.; Zuo, Y. Y. Droplet Oscillation as an Arbitrary Waveform Generator. Langmuir 2018, 34, 7042−7047. F


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Langmuir (26) Wang, Z.; Baatz, J. E.; Holm, B. A.; Notter, R. H. Contentdependent activity of lung surfactant protein B in mixtures with lipids. Am. J. Physiol.: Lung Cell. Mol. Physiol. 2002, 283, L897−L906. (27) Valle, R. P.; Wu, T.; Zuo, Y. Y. Biophysical influence of airborne carbon nanomaterials on natural pulmonary surfactant. ACS Nano 2015, 9, 5413−5421. (28) Yang, J.; Yu, K.; Zuo, Y. Y. Accuracy of Axisymmetric Drop Shape Analysis in Determining Surface and Interfacial Tensions. Langmuir 2017, 33, 8914−8923. (29) Yu, K.; Yang, J.; Zuo, Y. Y. Automated droplet manipulation using closed-loop axisymmetric drop shape analysis. Langmuir 2016, 32, 4820−4826. (30) Ou-Yang, W.; Weis, M.; Manaka, T.; Iwamoto, M. Study of relaxation process of dipalmitoyl phosphatidylcholine monolayers at air−water interface: Effect of electrostatic energy. J. Chem. Phys. 2011, 134, 154709. (31) Schürch, S.; Bachofen, H.; Goerke, J.; Green, F. Surface properties of rat pulmonary surfactant studied with the captive bubble method: adsorption, hysteresis, stability. Biochim. Biophys. Acta, Biomembr. 1992, 1103, 127−136. (32) Crane, J. M.; Putz, G.; Hall, S. B. Persistence of phase coexistence in disaturated phosphatidylcholine monolayers at high surface pressures. Biophys. J. 1999, 77, 3134−3143. (33) Yan, W.; Biswas, S. C.; Laderas, T. G.; Hall, S. B. The melting of pulmonary surfactant monolayers. J. Appl. Physiol. 2007, 102, 1739−1745. (34) Rugonyi, S.; Biswas, S. C.; Hall, S. B. The biophysical function of pulmonary surfactant. Respir. Physiol. Neurobiol. 2008, 163, 244− 255.

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Reversible Phase Transitions in the Phospholipid Monolayer

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