Małgorzata Jurak

Surface free energy and topography of mixed lipid layers on mica

The Langmuir-Blodgett/Schaefer (LB/LS) method, AFM technique and contact angle measurements were used to investigate the formation, structure and energetic properties of lipid films, which were constituted of single-component or binary systems formed by phospholipids: saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and triglyceride: glyceryl trioleate (GTO) or their mixture DPPC/DOPC and DPPC/GTO with a different DOPC or GTO molar ratio. The surface free energy and its components (apolar and polar interaction) for single and binary lipid mono- and bilayer systems were evaluated from the advancing and receding contact angles of water, formamide and diiodomethane, using two different models of interfacial interactions, namely, van Oss et al. (LWAB) and the contact angle hysteresis (CAH). The molecular structure of these layers was studied by means of atomic force microscopy (AFM). The thermodynamic analysis of the excess area and excess free energy of DPPC/DOPC mixture indicated that lateral phase separation took place for these two components. The AFM images also confirmed the phase separation, which was visualized as condensed DPPC-rich domains and a fluid DOPC matrix. Saturated DPPC mono- and bilayers exhibited much lower surface free energy than unsaturated DOPC ones, probably due to the differences in the packing of molecules. In the DPPC/DOPC mixed layers the energy increased with increasing mole fraction of DOPC in the layer. For DPPC/GTO layers a similar trend in the energy changes as a function of GTO amount was observed. These studies provided a fundamental framework for better understanding of the interactions occurring in the used lipid systems.

Stability of binary model membranes--prediction of the liposome stability by the Langmuir monolayer study.

In this paper, usefulness of the Langmuir monolayer study is demonstrated for predictions of the stability of liposomes composed of dipalmitoyl phosphatidylcholine (DPPC) and cholesterol (Chol). Thermodynamic analysis of the surface pressure (π)-area (A) isotherms of the DPPC/Chol systems was performed, which allowed for concluding on miscibility of the components, their molecular packing, and the interactions between molecules. It was found that the most stable system, in which the strongest interactions between molecules occured, was DPPC/Chol at x(Chol)=0.25. The stability of liposomes of the same composition as that in the Langmuir monolayers was analyzed by determining the size distribution of vesicles and the polydispersity as a function of time. The changes of these parameters confirmed that the system of the greatest stability is that with low Chol content.

Comparison of contact angle hysteresis of different probe liquids on the same solid surface

Advancing and receding contact angles of water, formamide and diiodomethane were measured on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) layers deposited on three different solid supports—glass, mica and poly(methyl methacrylate). Up to five statistical monolayers were deposited on the surfaces by spreading DPPC solution. It was found that even on five statistical DPPC monolayers, the hysteresis of a given liquid depends on the kind of solid support. Also on the same solid support the contact angle hysteresis is different for each probe liquid used. The AFM images show that the heights of roughness of the DPPC films cannot be the primary cause of the observed hysteresis because the heights are too small to cause the observed hystereses. It is believed that the hysteresis is due to the liquid film present right behind the three-phase solid surface/liquid drop/gas (vapour) contact line and the presence of Derjaguin pressure. The value of contact angle hysteresis depends on both the solid surface and liquid properties as well as on intermolecular interactions between them.

Changes in stability of the DPPC monolayer during its contact with the liquid phase.

The Langmuir-Blodgett (LB) method was applied and a few series of advancing and receding contact angles measurements as a function of time were performed to examine stability of model phospholipid monolayers during their contact with water, formamide and diiodomethane droplets. The studied monolayer was single-component saturated phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) transferred onto mica surface. When the time of the contact angle measurements is prolonged in these systems, some changes in the DPPC layer structure occur due to the contact with probing liquid, especially water, which is reflected in the changes of measured contact angle. Generally, with increasing time of the droplet contact with DPPC monolayer the contact angle decreases. Some correlation between the contact angle decrease and molecular rearrangements of initially hydrophobic DPPC monolayer is observed if it comes into contact with water. On the other hand, the contact angle completed within the first few seconds can faithfully reflect the original structure of the layer, and thus its energetic state, because during this time the structure changes are insignificant. Basing on the measured contact angles the monolayers apparent surface free energy and its components, corresponding to different contact times of the droplets, were calculated. These results are helpful for better characterization of the processes taking place in the phospholipid layers being in contact with polar (water and formamide) and nonpolar (diiodomethane) liquids.

Properties of Artificial Phospholipid Membranes Containing Lauryl Gallate or Cholesterol

Lauryl gallate (LG) is an antioxidant agent. However, it exhibits poor solubility in water. Its interactions with the membrane result in structure evolution thus affecting the membrane functionality. In this paper the Brewster angle microscope coupled with the Langmuir trough was applied to determine the morphology, phase behaviour, thickness and miscibility of ternary Langmuir monolayers with equal mole fractions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and an increasing mole fraction of LG. The results were discussed as regards analogous systems where cholesterol (Chol) was the third component. Moreover, the phosphatidylcholine–lauryl gallate (PC–LG) interactions were monitored by the attenuated total reflectance Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectrometry. Besides lipid composition, the addition of LG was found to be a significant factor to modulate the model membrane properties. The LG molecules adjust themselves to the PC monolayer structure. The hydrophobic fragment is dipped into the membrane interior while the hydroxyl groups of phenolic gallate moiety associate with the polar groups of PC mainly through hydrogen bonding inducing the compacting effect. LG is found to be deeply submerged within DOPC, closer to the double bonds, and its insertion practically does not affect the DPPC/DOPC membrane fluidity. This is crucial for getting more profound insight into the role of LG in stabilizing the non-raft domains, mostly exposed to oxidation in which LG can co-localize and serve its antioxidant function.

WETTABILITY OF HYBRID CHITOSAN/PHOSPHOLIPID COATINGS

Solution spreading and Langmuir-Blodgett/Schaefer techniques were used for preparation of chitosan/phospholipid (DPPC) films on air plasma treated polyethylene terephthalate (PET) plates. The surface wetting properties were determined based on the measurements of the advancing and receding contact angles of water, formamide and diiodomethane. The contact angle hysteresis model of Chibowski was applied to estimate the total surface free energy values. The coatings properties were found to be affected by molecular organisation and packing depending on the preparation technique. Phospholipid molecules modified the chitosan film surface by changing the kind and magnitude of interactions, which is revealed in the values of surface free energy. These findings may be helpful for the development of new generation polymersupported biocompatible coatings with anti-thrombogenic and antibactericidal properties, thus expanding the spectrum of chitosan applications.