Relaxation phenomena in phospholipid monolayers at the air–water interface

In this work we are concerned with the study of long-term relaxation phenomena in dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers spread at the air–water interface as a function of the surface pressure and the aqueous phase pH (pH 5, 7, and 9). Long-term relaxation phenomena were determined in an automated Langmuir-type film balance at constant temperature (20 °C). Two kinds of experiments were performed to analyze relaxation mechanisms. In one, the surface pressure (π) was kept constant, and the area (A) was measured as a function of time (θ). In the second, the area was kept constant at monolayer collapse and the surface pressure was decreased. This decrease was measured as a function of time. Various relaxation mechanisms, including monolayer molecular loss by dissolution, collapse, and/or organization/reorganization changes, can be fitted to the results derived from these experiments. These relaxation mechanisms are pH and phospholipid dependent. In the discussion, special attention will be given to the effect of the relaxation phenomena on the hysteresis in π–A isotherms before and after the relaxation experiment. At π lower than the equilibrium spreading pressure (π_e) the relaxation phenomena are mainly due to the loss of DPPC or DOPC molecules by desorption into the bulk aqueous phase. The formation of interfacial macroscopic vesicles, which are dissolved into the bulk phase, makes the phospholipid monolayer molecular loss irreversible. At the collapse point (at π > π_e), the relaxation phenomena may be due either to collapse for DPPC and/or to a complex mechanism including competition between desorption and monolayer collapse for DOPC.     

Persistence of metastability after expansion of a supercompressed fluid monolayer

Fluid monolayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine collapse from an air/water interface to form a three-dimensional bulk phase at the equilibrium spreading pressure (pie) of approximately 47 mN/m. This phase transition limits access to higher surface pressures under equilibrium conditions or during slow continuous compressions. We have shown previously that these films avoid collapse and become metastable when compressed on a captive bubble to surface pressures above 60 mN/m and that the metastability persists during expansion at least to pie. Here, we first documented the extent of this persistent metastability. Rates of isobaric collapse during expansion of the metastable films were up to 3 orders of magnitude slower than those during the initial compression to high surface pressures. Recovery of the ability to collapse depended on the surface pressure to which the films were expanded and how long they were held there. Films reverted after brief exposure to 20 mN/m and after 1 h at 35 mN/m. At pie, films remained capable of reaching high surface pressures during slow compressions after 65 h, although an increase in compressibility above 55 mN/m suggested somewhat increased rates of collapse. We also determined if the films remained metastable when they acquired sufficient free area to allow reinsertion of collapsed material. Faster isobaric expansion in the presence of more collapsed material and with further deviation below pie supported the existence of reinsertion. The persistence of metastability to pie shows that films with sufficient free area to allow reinsertion remain resistant to collapse. Observations that suggest heterogeneous reinsertion, however, argue that free area may be distributed heterogeneously and leave open the possibility that metastability persists because significant regions retain a restricted free area.     

Effect of metal ions on monolayer collapses

A Langmuir monolayer of stearic acid on pure water and in the presence of certain divalent metal ions such as Cd and Pb at pH approximately 6.5 of the subphase water collapses at constant area, while for other divalent ions such as Mg, Co, Zn, and Mn at the same subphase pH the monolayer collapses nearly at constant pressure. Films of stearic acid with Cd, Pb, Mn, and Co in the subphase (at pH approximately 6.5) have been transferred onto hydrophilic Si(001) using a horizontal deposition technique, just after and long after collapse. Electron density profiles obtained from X-ray reflectivity analysis show that a three-molecular-layer structure starts to form just after constant area collapse, where in the lowest molecular layer, in contact with the substrate, molecules are in asymmetric configuration, i.e., both hydrocarbon tails are on the same side of the metal-bearing headgroup that touches the substrate, while the molecules above the first layer are in symmetric conformation of the tails with respect to the headgroups. Further along collapse, when the surface pressure starts to rise again with a decrease in area, more layers with molecules in the symmetric configuration are added, but the coverage is poor. On the other hand, only bimolecular layers form after constant pressure collapse, with the lower and upper layers having molecules in asymmetric and symmetric configurations, respectively, and the upper molecular layer density increases with compression of the monolayer after collapse. A "Ries mechanism" for constant area collapse and a "folding and sliding mechanism" for constant pressure collapse have been proposed.     

Transformation diagrams for the collapse of a phospholipid monolayer

The kinetics of phase transitions in three-dimensional bulk materials are commonly presented in transformation diagrams. Time-temperature transformation (TTT) and continuous-cooling-transformation (CCT) diagrams plot the time required to transform specific fractions of the material to the new phase by cooling below a transition temperature. Transformation occurs isothermally for the TTT diagrams and during continuous cooling through a range of temperatures for CCT curves. Here we present analogous transformation diagrams for two-dimensional monolayers, which collapse at the equilibrium spreading pressure (pi e) to form a three-dimensional bulk phase. Time-surface pressure-transformation (TpiT) diagrams give the time required for specific fractions of the film to collapse when surface pressure is constant, and continuous-compression-transformation diagrams give the same information when surface pressure varies continuously. The diagrams, constructed here from previously reported data for 1-palmitoyl-2-oleoyl phosphatidylcholine, provide insights into the behavior of the films. The TpiT diagrams successfully predict the existence and approximate magnitude of a threshold rate for compressing the films to high surface pressures above pi e and the approximate shape of isotherms obtained with different rates of interfacial compression. The diagrams also caution that the behavior of mixed monolayers, explained previously in terms of compositional changes, can instead result from collapse that varies with surface pressure. Finally, the similarity between the shapes of the TTT and TpiT diagrams, with the time for transformation passing through a minimum and then increasing as the systems deviate further from equilibrium, suggests that analogous mechanisms determine the behavior of both systems.     

Relationships between equilibrium spreading pressure and phase equilibria of phospholipid bilayers and monolayers at the air-water interface

The intricate interplay between the bilayer and monolayer properties of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE) phospholipids, in relation to their polar headgroup properties, and the effects of chain permutations on those polar headgroup properties have been demonstrated for the first time with a set of time-independent bilayer-monolayer equilibria studies. Bilayer and monolayer phase behavior for PE is quite different than that observed for PC and PG. This difference is attributed to the characteristic biophysical PE polar headgroup property of favorable intermolecular hydrogen-bonding and electrostatic interactions in both the bilayer and monolayer states. This characteristic hydrogen-bonding ability of the PE polar headgroup is reflected in the condensed nature of PE monolayers and a decrease in equilibrium monolayer collapse pressure at temperatures below the monolayer critical temperature, T(c) (whether above or below the monolayer triple point temperature, T(t)). This interesting phenomena is compared to equilibrated PC and PG monolayers which collapse to form bilayers at 45 mN/m at temperatures both above and below monolayer T(c). Additionally, it has been demonstrated by measurements of the equilibrium spreading pressure, pie, that at temperatures above the bilayer main gel-to-liquid-crystalline phase-transition temperature, T(m), all liquid-crystalline phospholipid bilayers spread to form monolayers with pie around 45 mN/m, and spread liquid-expanded equilibrated monolayers collapse at 45 mN/m to form their respective thermodynamically stable liquid-crystalline bilayers. At temperatures below bilayer T(m), PC and PG gel bilayers exhibit a drop in bilayer pi(e) values < or =0.2 mN/m forming gaseous monolayers, whereas the value of pic of spread monolayers remains around 45 mN/m. This suggests that spread equilibrated PC and PG monolayers collapse to a metastable liquid-crystalline bilayer structure at temperatures below bilayer T(m) (where the thermodynamically stable bilayer liquid-crystalline phase does not exist) and with a surface pressure of 45 mN/m, a surface chemical property characteristically observed at temperatures above bilayer T(m) (monolayer T(c)). In contrast, PE gel bilayers, which exist at temperatures below bilayer T(m) but above bilayer T(s) (bilayer crystal-to-gel phase-transition temperature), exhibit gel bilayer spreading to form equilibrated monolayers with intermediate pie values in the range of 30-40 mN/m; however, bilayer pie and monolayer pic values remain equal in value to one another. Contrastingly, at temperatures below bilayer T(s), PE crystalline bilayers exhibit bilayer pie values < or =0.2 mN/m forming equilibrated gaseous monolayers, whereas spread monolayers collapse at a value of pic remaining around 30 mN/m, indicative of metastable gel bilayer formation.     

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.     

On the stability of Langmuir films of pyramidic mesogens

Pyramidic mesogens forming thermotropic liquid crystal bulk phases were spread in an air-water interface. Pressure surface measurements and polarizing microscopy on the Langmuir films were used to characterize the various states of these pyramidic-like molecules. For two compounds bearing short lateral aliphatic chains, the surface pressure isotherms exhibit a large plateau region corresponding to a metastable monolayer in which the molecules may adopt an 'edge-on' arrangement. The coexistence of multilayered, anisotropic, slowly growing domains with the monolayer in the plateau region has been observed at long time scale. The film area relaxation kinetics at constant surface pressure show the existence of two nucleation mechanisms for the formation of these domains.     

Hydrolysis and Condensation of Alkyltrimethoxysilanes in Monomolecular Films

Hydrolysis and condensation of monomolecular alkyloxysilane films by the Langmuir technique is presented. Octadecyltrimethoxysilane formed monolayers on aqueous subphases with different properties depending on the bulk pH. At pH 1 a solid condensed film was directly formed with a molecular area of 23 Ų and a surface pressure/surface area variation similar to that on non-ionized stearic acid. At pH 5.6 and 11 several phase transitions were observed during the compression of the monolayer with a final collapse at a molecular area of 20 Ų. Relaxation measurements confirmed the stability of the films for longer than 12 hours at different surface pressures below a critical value.     

Interaction of DMPC liposomes with a DMPA monolayer: related study of Langmuir-Blodgett films

The formation of a phospholipidic layer was achieved in two steps: (1) a dimyristoyl-l-α-phosphatidic acid (DMPA) Langmuir monolayer was formed by spreading a chloroform/methanol DMPA solution onto an aqueous subphase; after a 10 min period, the monolayer was compressed at 5mNm⁻¹; and (2) keeping the area of the DMPA monolayer constant, a dimyristoyl-l-α-phosphatidylcholine (DMPC) liposomal suspension was added. The progressive incorporation of DMPC molecules into the DMPA monolayer was studied by monitoring the variation of surface pressure with time at constant film area. Three parameters involved in the formation of the interfacial layer DMPA/liposomal DMPC (DMPA/ lip-DMPC) were studied: liposome addition, aqueous subphase composition and initial surface pressure of the DMPA monolayer. The transfer of this mixed layer was controlled through a traceable fluorescent probe incorporated in the liposomes. The thickness and homogeneity of the Langmuir-Blodgett films thus obtained were assessed through Fourier transform infra-red spectroscopy and Nomarski microscopy, respectively. This study shows that the DMPA/lip-DMPC monolayer could be transferred without dragging of aggregates or mesophases.     

Formation of langmuir-blodgett films containing two-dimensional monoatomic arrays of rare-earth cations

The work is focused on the preparation and characterisation of rare-earth containing stearic acid Langmuir-Blodgett (LB) films. Stearic acid monolayer behavior on the rare-earth containing aqueous subphase have been analyzed by thermodynamic measurements. The compression isotherm shape changed substantially under varying the anion type at the constant rare-earth cation content in aqueous subphase. The structure of formed multilayer rare-earth containing LB films was studied using small angle X-rays diffraction. The considerable structural defects in multilayer films were found when rare-earth chloride solution was exploited as aqueous subphase, caused presumably by electrostatic interactions at the monolayer surface under high binding of rare-earth cations. The use of monodentate complexones like acetic acid was proposed to form bulk phase quasi-neutral rare-earth complexes. The electroneutral ligand exchange reactions of such complexes with stearic acid monolayer allowed to neutralise monolayer surface under rare-earth cations binding and to form condenced monolayer below the triple-point temperature. The compression of such monolayer to the state of minimal compressibility and subsequent monolayer deposition resulted in the formation of high ordered LB films with high content of adsorbed multivalent rare-earth cations arranged in two-dimensional monoatomic arrays.     

On the stability of Langmuir films of pyramidic mesogens

Abstract

Pyramidic mesogens forming thermotropic liquid crystal bulk phases were spread in an air-water interface. Pressure surface measurements and polarizing microscopy on the Langmuir films were used to characterize the various states of these pyramidic-like molecules. For two compounds bearing short lateral aliphatic chains, the surface pressure isotherms exhibit a large plateau region corresponding to a metastable monolayer in which the molecules may adopt an ‘edge-on’ arrangement. The coexistence of multilayered, anisotropic, slowly growing domains with the monolayer in the plateau region has been observed at long time scale. The film area relaxation kinetics at constant surface pressure show the existence of two nucleation mechanisms for the formation of these domains.     

Activation and inhibition of lipoprotein lipase in mixed monolayers of medium or long chain-triglycerides and phospholipids

 We evaluated the activation and inhibition effects of phosphatidylcholine (PC) and sphingomyelin (SM) on lipoprotein lipase (LPL) for medium or long chain-triglycerides (TG) in monolayers at the air/water interface. Monolayers of medium chain-TG, being in an expanded state at a surface pressure of 15 mN/m, showed low susceptibility to LPL in the subphase. Adding 50 mole% of PC or SM into these monolayers reduced the partial molecular area of the TG and enhanced the LPL activity. Monolayers of long chain-TG, being in a condensed state, showed high susceptibility of LPL either with or without PC. SM added to the long chain-TG monolayers, however, inhibited the LPL action. We investigated the interaction between TG and phospholipid on the basis of the collapse pressure-measurements of mixed monolayers. For medium chain-TG monolayers, PC and SM gave similar collapse pressure-composition profiles. Contrary to this, SM gave a markedly higher collapse pressure of long chain-TG than PC: SM stabilized the monolayer state of long chain-TG. These results suggested that I) orientation of the acyl chains of TG molecule in a monolayer is crucial for the LPL activity, and that II) strong interaction between SM and long chain-TG retards the substrate-transfer from the mixed monolayer to the catalytic pocket of LPL. Received: 6 March 1996 Accepted: 19 July 1996     

Phase behavior and viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface

A trisilanol polyhedral oligomeric silsesquioxane (POSS), trisilanolcyclohexyl-POSS (TCyP), has recently been reported to undergo a series of phase transitions from traditional Langmuir monolayers to unique rodlike hydrophobic aggregates in multilayer films that are different from "collapsed" morphologies seen in other systems at the air/water interface. This paper focuses on the phase transitions and morphology of films varying in average thickness from monolayers to trilayers and the corresponding viscoelastic properties of trisilanolcyclohexyl-POSS molecules at the air/water interface by means of surface pressure-area per molecule (Pi-A) isotherms, Brewster angle microscopy (BAM), and interfacial stress rheometry (ISR) measurements. The morphology studies by BAM reveal that the TCyP monolayer can collapse into different 3D structures by homogeneous or heterogeneous nucleation mechanisms. For homogeneous nucleation, analysis by Vollhardt et al.'s nucleation and growth model reveals that TCyP collapse is consistent with instantaneous nucleation with hemispherical edge growth at Pi = 3.7 mN.m(-1). Both surface storage (Gs') and loss (Gs") moduli obtained by ISR reveal three different non-Newtonian flow regimes that correlate with phase transitions in the Pi-A isotherms: (A) A viscous liquidlike "monolayer"; (B) a "biphasic regime"between a liquidlike viscous monolayer and a more rigid trilayer; and (C) an elastic solidlike "trilayer". These observations provide interesting insights into collapse mechanisms and structures in Langmuir films.     

Interfacial characteristics of β-casein spread films at the air–water interface

Casein is well known to be a good protein emulsifier and β-casein is the major component of casein and commercial sodium caseinate. This work studies the behaviour of β-casein at the interface. The interfacial characteristics (structure and stability) of β-casein spread films have been examined at the air–water interface in a Langmuir-type film balance, as a function of temperature (5–40°C) and aqueous phase pH (pH 5 and 7). From surface pressure–area isotherms (π–A isotherms) as a function of temperature we can draw a phase diagram. β-Casein spread films present two structures and the collapse phase. That is, there is a critical surface pressure and a surface concentration at which the film properties change significantly. This transition depends on the temperature and the aqueous phase pH. The film structure was observed to be more condensed and β-casein interfacial density was higher at pH 5. β-Casein films were stable at surface pressures lower than equilibrium surface pressure. In fact, no hysteresis was observed in π–A isotherms after continuous compression-expansion cycles or over time. The relative area relaxation at constant surface pressure (10 or 20 mN m⁻¹) and the surface pressure relaxation at constant area near the monolayer collapse, can be fitted by two exponential equations. The characteristic relaxation times in β-casein films can be associated with conformation–organization changes, hydrophilic group hydration and/or surface rheology, as a function of pH.     

Fluorination of the hydrophilic head accelerates the collapse of the monolayer but stabilizes the bilayer of a long-chain trifluoroethyl ether on water

A comparison of the collapse of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water shows that in both films the 3D phase is formed layer-by-layer. The substitution of CH3 by a CF3 group in the hydrophilic head yields a more stable bilayer exhibiting lower equilibrium spreading pressure, pi(esp)(DFEE) < pi(esp)(DEE). Upon lateral compression, the DFEE bilayer fractures abruptly as a compact solid body whereas the DEE bilayer breaks down gradually as a polycrystalline material. A comparison of the collapse kinetics of the two films at the same constant supersaturation pi-pi(esp) = 7 mN/m shows that the fluorinated DFEE monolayer transforms more quickly, yielding a stable bilayer of closely packed upright molecules, whereas the DEE film undergoes a continuous monolayer-bilayer-multilayer transition. Brewster angle microscopy allows us to visualize different collapse mechanisms of the DFEE and DEE films; the domains of the fluorinated DFEE bilayer grow laterally at constant thickness and density, and the collapse of the nonfluorinated DEE monolayer occurs through a sequence of disordered stripelike and broken elongated textures. The characteristic molecular areas of the monolayer and bilayer collapse suggest that the 2D-3D transition in the DFEE and DEE films is accompanied by at least partial dehydration of their headgroups. The faster collapse of the fluorinated monolayer could result from a lower energy barrier due to the more hydrophobic CF3 group in the heads. The increased stability of the DFEE bilayer could be associated with the electrostatic attraction between the -C(F delta-)3 versus (H delta+)3C- terminals at the heads-to-tails contact plane of the top and the bottom layer, contrasting with the repulsion between the -C(H delta+)3 versus (H delta+)3C- terminals of the top-layer heads and the bottom-layer tails in the DEE bilayer.     

Penetration of beta-lactoglobulin into monoglyceride monolayers. dynamics, interactions, and topography of mixed films

In this work we have analyzed the penetration of betalactoglobulin into a monoglyceride monolayer (monopalmitin or monoolein) spread at the air-water interface and its effects on the structural, dilatational, and topographical characteristics of mixed films. Dynamic tensiometry, surface film balance, Brewster angle microscopy (BAM), and surface dilatational rheology have been used, maintaining the temperature constant at 20 degrees C and the pH and ionic strength at 7 and 0.05 M, respectively. The initial surface pressure (mN/m) of the spread monoglyceride monolayer (pii(MONOGLYCERIDE)) at 10, 20, and the collapse point is the variable studied. Beta-lactoglobulin can penetrate into a spread monoglyceride monolayer at every surface pressure. The penetration of beta-lactoglobulin into the monoglyceride monolayer with a more condensed structure, at the collapse point of the monoglyceride, requires monoglyceride molecular loss by collapse and/or desorption. However, the structural, topographical, and dilatational characteristics of monoglyceride penetrated by beta-lactoglobulin mixed monolayers are essentially dominated by the presence of monoglyceride (either monopalmitin or monoolein) in the mixed film. In fact, monoglyceride molecules have the capacity to re-enter the monolayer after expansion and recompression of the mixed monolayer. Thus, monoglyceride molecular loss by collapse and/or desorption is reversible. The topography of the monolayer under dynamic conditions corroborates these conclusions.     

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air-water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression-expansion cycle of the monolayer is not reversible and the second pi-A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.     

Stabilization of Langmuir monolayer of hydrophobic thiocholesterol molecules

The self-assembled monolayer of the thiocholesterol (TCh) exhibits interesting properties that can be used for various technological applications. TCh is predominantly a hydrophobic molecule, and it does not spread at the air–water interface to form a stable Langmuir monolayer. We have stabilized the TCh molecules in the cholesterol (Ch) monolayer. We find the mixed monolayer to be stable upto 0.75 mole fraction of TCh in Ch. The mixed monolayer shows an initial and a final collapse. On compressing the monolayer beyond the initial collapse, the TCh molecules squeeze out irreversibly from the mixed monolayer phase. The calculation of excess area per molecule for the TCh and Ch mixed monolayer system indicates an attractive interaction between the component molecules. Interestingly, the elasticity of the Ch monolayer reduces to less than half, and the monolayer becomes more fluidic due to the presence of even very minute quantity (5%) of TCh.     

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. I. Structure

The structure of a monolayer film of the branched alkane squalane (C30H62) adsorbed on graphite has been studied by neutron diffraction and molecular dynamics (MD) simulations and compared with a similar study of the n-alkane tetracosane (n-C24H52). Both molecules have 24 carbon atoms along their backbone and squalane has, in addition, six methyl side groups. Upon adsorption, there are significant differences as well as similarities in the behavior of these molecular films. Both molecules form ordered structures at low temperatures; however, while the melting point of the two-dimensional (2D) tetracosane film is roughly the same as the bulk melting point, the surface strongly stabilizes the 2D squalane film such that its melting point is 91 K above its value in bulk. Therefore, squalane, like tetracosane, will be a poor lubricant in those nanoscale devices that require a fluid lubricant at room temperature. The neutron diffraction data show that the translational order in the squalane monolayer is significantly less than in the tetracosane monolayer. The authors' MD simulations suggest that this is caused by a distortion of the squalane molecules upon adsorption on the graphite surface. When the molecules are allowed to relax on the surface, they distort such that all six methyl groups point away from the surface. This results in a reduction in the monolayer's translational order characterized by a decrease in its coherence length and hence a broadening of the diffraction peaks. The MD simulations also show that the melting mechanism in the squalane monolayer is the same footprint reduction mechanism found in the tetracosane monolayer, where a chain melting drives the lattice melting.     

Generation of nanostructures of mica supported lysozyme molecules in aqueous solution by atomic force microscopy

Nanostructures of lysozyme molecules adsorbed to mica were generated by the tip of an atomic force microscope in contact, tapping, and force-distance mode in aqueous solution. In contact mode at high ionic strength and adjusted lysozyme concentration a monolayer of defined pattern and orientation could be formed by the scan process of the tip. A lysozyme monolayer with minimal pattern size of about 60 nm was achieved by line scan. At larger loading forces besides a monolayer also 3D-aggregates of lysozyme molecules could be generated. In force-distance mode the volume of 3D-aggregates grows with increasing generation time, lysozyme concentration in the bulk phase, loading force, and frequency of up- and down-movement of the substrate toward the fixed cantilever. In tapping mode 3D-aggregates could be generated as well. It is postulated that reduction of electrostatic interaction between the oppositely charged lysozyme molecules and mica surface by sufficient high ionic strength is essential for monolayer formation. It is discussed that for the underlying mechanism of monolayer generation in contact mode lysozyme molecules of the bulk phase adsorb to the tip, become pulled off and attach to the mica surface by the scan process of the tip.