Phase Transitions In Monolayers Research Articles

Phase Transition of n-C36H74 Monolayer on Pt(111) Covered with Monolayer Graphene Studied by C K-NEXAFS

The temperature-induced phase transition of the n-C36H74 monolayer on the monolayer graphene grown on a Pt(111) surface is studied by X-ray absorption fine structure spectroscopy in the carbon near the K-edge (C K-NEXAFS). The C K-NEXAFS spectra of the n-C36H74 monolayer is separated from the spectra of the monolayer graphene by subtraction. The σCC* resonance predominantly observed in the normal incidence (NI) spectra is consistent with a chain axis lying flat on the graphene sheet at 120 K (crystalline phase) and 300 K (smectic phase). The σCH*/R resonance at 287.2 eV is characteristic of the monolayer, which is attributed to the σCH* orbitals distributed in an open space above the monolayer. The intensity ratio of the resonance for grazing incidence (GI) spectra with respect to that for normal incidence (NI) spectra decreases as a function of the temperature, which reflects the orientation change of the carbon plane with respect to the graphene sheet, from perpendicular (edge-on) to parallel (flat-on) ...

Interactions of Pluronic block copolymers with lipid monolayers studied by epi-fluorescence microscopy and by adsorption experiments

The interactions of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers, i.e. Pluronics F87, F88 and F127, with monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were investigated with different monolayer techniques. Surface pressure–area isotherms were recorded of co-spread Pluronic/lipid mixtures with different Pluronic content to determine the influence of the polymers on the monolayer phase transitions. The squeeze-out pressure of the polymers upon film compression was dependent on the PPO block length. The monolayer compression experiments were coupled with fluorescence microscopy to visualize the phase separation into polymer-rich and lipid-rich domains and to monitor morphological changes of the lipid domains in the monolayer. Extensive phase separation was observed in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) lipid phases, where pure polymer domains coexisting with round LE-domains containing polymer, and polymer-free LC-domains were seen. We also investigated the adsorption of Pluronics to a lipid monolayer after injecting a polymer solution underneath a pre-formed lipid monolayer by following the change in pressure at constant area. The results show that polymer adsorption is a superposition of two individual processes with different kinetics. Pluronics with a higher hydrophobicity and with a smaller molecular weight adsorb faster and the type and phase state of the lipid determines the surface pressure where no further Pluronic molecules adsorb to the interface. This critical surface pressure depends on the PPO block length, whereas the strength of the interaction with the lipids is determined by the relative PEO content. This indicates that also interactions between the PEO blocks and the lipid headgroup region are occurring. The interactions with the unsaturated lipid POPC in the liquid-expanded phase turn out to be stronger than for lipids in the liquid-condensed phase, where the polymers are excluded.

Monolayers of novel Gemini amphiphiles with phthalimide headgroups at the air/water interface: pH and alkyl chain length effects

A series of novel pH sensitive Gemini amphiphiles of N,N′-dialkyl-N,N′-di(ethyl-phthalimide) ethylenediamines (Di-CnPh, n=6, 8, 10, 12) was synthesized and characterized. It was found that relatively stable spread monolayers of Di-CnPh can be formed at the air/water interface once n⩾6, and their stability was improved by increasing the hydrophobic chain length or adding electrolyte. The surface pressure–area (π–A) isotherms of Di-CnPh have been studied in detail by employing the axisymmetric drop shape analysis as penetration Langmuir balance. Furthermore, Brewster angle microscopy (BAM) was employed to confirm the monolayer phase transition from gaseous phase to liquid condensed phase via a liquid expanded phase. Moreover, dilatational rheological measurements of spread Di-CnPh monolayers at the air/water interface were performed, and the influence of the hydrophobic chain length and pH on the dilatational elasticity (ε) and viscosity (η) was also investigated. The results have suggested that the spread Di-CnPh monolayers are spontaneously elastic.

Light-induced phase transitions of colloidal monolayers with crystalline order

We experimentally study the phase behavior of a charge-stabilized two-dimensional colloidal crystal which is subjected to a one-dimensional periodic light field. Such light fields are created by a scanned optical line tweezer which allows the variation of the periodicity without optical realignments. In order to realize a wide range of line spacings relative to the lattice constant, we use a suspension of silica particles in bromobenzene. This colloidal system has a Debye screening length of about 4.6 μm which results in the formation of crystals with lattice constants up to 20 μm. Because the refractive index of bromobenzene is larger than that of the colloids, optical gradient forces lead to the attraction of particles at regions where the intensity is smallest. Depending on the depth and periodicity of the optical potential, we observe the light-induced assembly of colloids into triangular, rhombic and square phases.

Spontaneous desorption and phase transitions of self-assembled alkanethiol and alicyclic thiol monolayers chemisorbed on Au(1 1 1) in ultrahigh vacuum at room temperature

Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10−7Pa at room temperature (RT). STM imaging revealed that after storage for 7days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV.

Phase transitions in monolayer formed by hyperbranched polyester with alkyl-terminated branches at the air/water interface

Investigations of Langmuir and Langmuir–Blodgett molecular films of hyperbranched polyester with alkyl-terminated branches over a wide temperature range revealed an unusual phase transitions. The measured surface pressure–surface area isotherms clearly show that the hyperbranched polyester formed stable and well-defined monolayers at the air/water interface. At temperatures below 313 K ice floe-like structures of a condensed phase were formed already from very low surface pressures. On the increase of the surface pressure the floes of the condensed phase merged forming a uniform monolayer. Above 313 K a surface liquid phase was formed at the interface. It was shown that the phase transition from the surface liquid phase to the condensed phase occurred on temperature decrease. At lower temperatures the compression process was not reversible. The increase of temperature up to about 323 K made the compression process reversible. The monolayers were transferred from the air/water interface onto silicon and mica substrates using the Langmuir–Blodgett technique at different temperatures. The structure of the polyester monolayer formed at the substrates' surfaces was investigated.

Commensurate Solid–Solid Phase Transitions in Self-Assembled Monolayers of Alkylthiolates Lying on Metal Surfaces

A temperature-induced commensurate solid-solid phase transition in self-assembled monolayers (SAMs) of alkylthiolates lying on Pt(111) is predicted from molecular dynamics simulations based on ab initio potential energy surfaces. As the system cools down from room temperature to low enough temperature, SAMs of alkylthiolates with more than ~12 carbon atoms undergo an abrupt change of orientation from a nearly upright to a tilted configuration. As the initial hexagonal arrangement of the sulfur headgroups is kept fixed during the simulations, the phase transition is entirely governed by chain-chain interactions. Similar commensurate phase transitions are predicted for hexagonally arranged SAMs with lattice spacings of the order of 4.7-4.9 Å, which, among others, excludes the well-known cases of densely packed SAMs of alkylthiolates on Au(111) and Ag(111). These findings could be relevant for the design of novel electronic or optical devices controllable by temperature.

Dual functional performance of fiber Bragg gratings coated with metals using flash evaporation technique

Metallic and other type of coatings on fiber Bragg grating (FBG) sensors alter their sensitivity with thermal and mechanical stress while protecting the fragile optical fiber in harsh sensing surroundings. The behavior of the coated materials is unique in their response to thermal and mechanical stress depending on the thickness and the mode of coating. The thermal stress during the coating affects the temperature sensitivity of FBG sensors. We have explored the thermal response of FBGs coated with Al and Pb to an average thickness of 80nm using flash evaporation technique where the FBG sensor is mounted in a region at room temperature in an evacuated chamber having a pressure of 10−6Torr which will minimize any thermal stress during the coating process. The coating thickness is chosen in the nanometer region with the aim to study thermal behavior of nanocoatings and their effect on FBG sensitivity. The sensitivity of FBGs is evaluated from the wavelengths recorded using an optical sensing interrogator sm 130 (Micron Optics) from room temperature to 300°C both during heating and cooling. It is observed that the sensitivity of the metal coated fibers is better than the reference FBG with no coating for the entire range of temperature. For a coating thickness of 80nm, Al coated FBG is more sensitive than the one coated with Pb up to 170°C and it reverses at higher temperatures. This point is identified as a reversible phase transition in Pb monolayers as the 2-dimensional aspects of the metal layers are dominant in the nanocoatings of Pb. On cooling, the phase transition reverses and the FBGs return to the original state and for repeated cycles of heating and cooling the same pattern is observed. Thus the FBG functions as a sensor of the phase transitions of the coatings also.

Commensurate-incommensurate phase transition in chlorine monolayer chemisorbed on Ag(111): Direct observation of crowdion condensation into a domain-wall fluid

The commensurate-incommensurate phase transition in chlorine monolayer on Ag(111) has been studied with low-temperature scanning tunneling microscopy and low-energy electron diffraction. We report the observation of self-interstitial defects: two-dimensional (2D) crowdions formed at the initial stage of the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R30${}^{\ensuremath{\circ}}$ lattice compression. After critical coverage of 0.34 monolayer (ML) the crowdions condense into a domain-wall fluid. As coverage further increases, the domain-wall crystal solidifies with average wall separation of 23 \AA{}.

Properties of compatible solutes in aqueous solution

We have performed Molecular Dynamics simulations of ectoine, hydroxyectoine and urea in explicit solvent. Special attention has been spent on the local surrounding structure of water molecules. Our results indicate that ectoine and hydroxyectoine are able to accumulate more water molecules than urea by a pronounced ordering due to hydrogen bonds. We have validated that the charging of the molecules is of main importance resulting in a well defined hydration sphere. The influence of a varying salt concentration is also investigated. Finally we present experimental results of a DPPC monolayer phase transition that validate our numerical findings.

Phase Transition of Octaneselenolate Self-assembled Monolayers on Au(111) Studied by Scanning Tunneling Microscopy

We investigated the surface structure and wetting behavior of octaneselenolate self-assembled monolayers (SAMs) on Au(111) formed in a 50 ethanol solution according to immersion time, using scanning tunneling microscopy (STM) and an automatic contact angle (CA) goniometer. Closely-packed, well-ordered alkanethiol SAMs would form as the immersion time increased; unexpectedly, however, we observed the structural transition of octaneselenolate SAMs from a molecular row phase with a long-range order to a disordered phase with a high density of vacancy islands (VIs). Molecularly resolved STM imaging revealed that the missing-row ordered phase of the SAMs could be assigned as a superlattice containing three molecules in the rectangular unit cell. In addition, CA measurements showed that the structural order and defect density of VIs are closely related to the wetting behaviors of octaneselenolate SAMs on gold. In this study, we clearly demonstrate that interactions between the headgroups and gold surfaces play an important role in determining the physical properties and surface structure of SAMs.

Odd–Even Effects on the Structure, Stability, and Phase Transition of Alkanethiol Self-Assembled Monolayers

Molecular dynamics simulations were conducted to predict the structural properties and phase transition temperatures of n-alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 4 ≤ n ≤ 22) self-assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including tilt and orientation angles, and on phase transition temperature. We found clear dependence of the structural properties, on both the number of carbon atoms, n; and on n being odd or even. Alkanethiols with n ≤ 7 show liquid-like behavior and large rotational mobility, whereas those with n ≥ 12 are well-ordered and stable. For 12 ≤ n ≤ 15, odd-even effects are observed, where for n = odd, larger tilt angles, oriented in the direction of their next next nearest neighbor (NNNN), and for n = even, lower tilt angles, mostly tilted toward next nearest neighbor (NNN), were observed. For 15 ≤ n ≤ 19, we find tilt angle and orientation to be independent of n. For all alkanethiols, a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. Order-disorder phase transitions occurred at a certain temperature. This was signified by abrupt instabilities in the tilt orientation angle. This transition temperature showed an enhancement of ∼67-100 °C over the melting point of the corresponding n-alkane bulk system. This enhancement depended on n, and was larger for n = odd. Overall, we found that odd alkanethiols show better structural and thermal stability, and smaller gauche defects.

Molecular dynamics simulation of phase transitions in model lung surfactant monolayers

To explore the role of individual lung surfactant components in liquid-condensed (LC)/liquid-expanded (LE) phase transitions the MARTINI coarse-grained (CG) model is used to simulate monolayers containing DPPC and additional lipid or peptide components. Our analysis suggests that the LC phase forms from the LE phase via a nucleation and growth mechanism, while the LC–LE transition occurs by melting that originates from defects in the monolayer. On the time scale of our simulations, DPPC monolayers display a substantial hysteresis between the ordering and disordering transitions, which is decreased by the addition of a second component. In binary mixtures of DPPC with lung surfactant peptide fragment SP-B1–25, the ordered side of the hysteresis loop is abolished altogether, suggesting that SP-B1–25 effectively nucleates disorder in the monolayer on heating. SP-B1–25 is observed to perturb the packing of the surrounding lipids leading to local fluidization of the monolayer and to aggregate within the LE phase. In 1:1 DPPC:POPC monolayers, a high concentration of unsaturated phospholipid leads to a substantial decrease in the LC–LE and LE–LC transition temperatures. Adding cholesterol to pure DPPC increases the LC–LE and LE–LC transition temperatures and increases the order on the disordered side of the hysteresis loop leading to a phase of intermediate order, which could be the liquid-disordered (Ld) phase. Cholesterol is also observed to show a preference for LC–LE domain boundaries. The results of our molecular dynamics simulations coincide with many experimental observations and can help provide insight into the physiological roles of individual surfactant components.

Effect of Saline on Transitions in Poly(ethylene glycol)-Grafted Succinyl-Phosphoethanolamine Monolayers Bearing C16Aliphatic Chains

To investigate the effect of saline on miscibility, phase, and conformational transitions in binary mixtures of a succinyl-phosphoethanolamine bearing C(16) aliphatic chains, DPPE-succinyl, and a poly(ethylene glycol) (PEG)-phospholipid conjugate with a PEG molecular weight of 2000, DPPE-PEG2000, we have compared the properties of monolayers spread on water and on phosphate buffered saline (PBS). A comparative analysis of monolayer surface pressure, surface potential, compressibility, and epifluorescence microscopy data has revealed that spreading on PBS induces unfavorable interactions between the two phospholipids, which stabilizes immiscible phases in mixed monolayers. Strikingly, the conformational transition in grafted PEG2000 chains on PBS could not be easily described by the existing interpretive schemes. Plausibly, this transition becomes partially impaired due to interactions with PBS. Thus, saline has a significant effect on miscibility, phase, and conformational transitions in these PEG-grafted monolayers bearing C(16) aliphatic chains, which may have implications for understanding the behavior of PEG-grafted phospholipid surfaces in aqueous media of biological relevance.

In honour of the 65th birthday of Valentin B. Fainerman

This Honorary Note is dedicated to the 65th birthday of Valentin Fainerman and summarizes some of his contributions to the field of interfacial dynamics. First of all, he made the maximum bubble pressure tensiometry the most frequently used methodology in the short time range of surfactant adsorption at liquid surfaces. This work allows us now to use experimental data down to the time range of sub-milliseconds for analyzing adsorption mechanisms of surfactants and polymers and their mixtures. The contributions of V.B. Fainerman to the quantitative understanding of the thermodynamics of adsorption represent a significant step ahead and describe adsorption layers even of rather complex nature, such as mixed protein–surfactant layers. These models consider molecular interfacial reorientation and aggregation. His thermodynamic approach is able to explain various interfacial systems which includes for example also phase transitions in insoluble monolayers. Based on diffusional transport and the proposed thermodynamic models, the adsorption kinetics and dilational rheology of liquid interfacial layers have reached a new level of understanding.

Structural Manipulation of Hydrogen-Bonding Networks in Amide-Containing Alkanethiolate Monolayers via Electrochemical Processing

We report the electrochemically driven phase transformation of amide-containing alkanethiol, 3-mercapto-N-nonylpropionamide (1ATC9) self-assembled monolayers (SAMs) into a linear nanostructure. Hydrogen-bonding interactions between buried amide groups cause multistep electrochemical desorption and enable an unusual phase change, affording a less dense, textured structure. Single-component 1ATC9 SAMs prepared in solution at room temperature for 24 h consist of two phases with different apparent heights in scanning tunneling microscope images; these phases are readily manipulated by controlling solution temperature and deposition time. Intermolecular hydrogen-bonding interactions give high thermal stability to the films. The presence of two independent cathodic peaks in 1ATC9 monolayer voltammograms indicates two-step reductive desorption. A monolayer phase transition occurs after the first cathodic peak, transforming a close-packed monolayer into a striped phase that is energetically favored at low surface...

Reversible Phase Transitions in Self-Assembled Monolayers at the Liquid−Solid Interface: Temperature-Controlled Opening and Closing of Nanopores

We present a variable-temperature study of monolayer self-assembly at the liquid-solid interface. By means of in situ scanning tunneling microscopy (STM), reversible phase transitions from a nanoporous low-temperature phase to a more densely packed high-temperature phase are observed. The occurrence of the phase transition and the respective transition temperature were found to depend on the type of solvent and solute concentration. Estimates of the entropic cost and enthalpic gain upon monolayer self-assembly suggest that coadsorption of solvent molecules within the cavities of the nanoporous structure renders this polymorph thermodynamically stable at low temperatures. At elevated temperatures, however, desorption of these relatively weakly bound solvent molecules destabilizes the nanoporous polymorph, and the densely packed polymorph becomes thermodynamically favored. Interestingly, the structural phase transition provides external control over the monolayer morphology and, for the system under discussion, results in an effective opening and closing of supramolecular nanopores in a two-dimensional molecular monolayer.

Dipolar electrostatic energy effect on relaxation process of monolayers at air-water interface: Analysis of thermodynamics and kinetics

In order to understand the effect of electrostatic energy on phase transition from monolayer to multilayer, isobaric relaxation process of Langmuir monolayers composed of stearic acid or ferroelectric polyvinylidene fluoride and trifluoroethylene copolymer with various vinylidene fluoride (VDF) ratios is investigated in terms of thermodynamic and kinetic analysis. A monotonous decreasing tendency of material loss with respect to temperature is observed for stearic acid monolayer, which is due to thermal activation effect on phase transition from monolayer to multilayer. In contrast, for the ferroelectric monolayer it presents a nonmonotonous behavior of losing materials with a peak position near the Curie temperature, which is not only owing to thermal activation but also dipole moment change. This observation is confirmed for the copolymer monolayers with other VDF content ratios. Amazingly, for the ferroelectric monolayers a good correspondence is found for critical temperatures evaluated from several independent methods including the analysis on slow collapse. This finding again tells the fact that the relaxation process, namely phase transition from monolayer to multilayer, is greatly influenced by dipolar electrostatic energy. Moreover, the study of time dependent relaxation process reveals a diffusionlike behavior of multilayer structure formation, which cannot be interpreted by classical models. Hence a new model based on diffusion-driven material transfer is proposed and diffusivity of the copolymer molecules is estimated with a value of 0.4x10(-5) cm(2)/s. As a whole, this research reflects the importance of dipolar electrostatic energy for phase transition of monolayers at air-water interface.

Spectroscopic manifestations of orientational phase transition in adsorbate monolayer

In the framework of the virtual crystal approximation, the temperature dependences of frequencies and integral intensities of infrared absorption spectral lines are established for an adsorbate monolayer near the point of the order-to-disorder orientational phase transition. The results obtained are consistent with the experimentally observed temperature dependences of the vibrational spectra for the 2 × 1 monolayer CO/NaCl (100). For this system, the Davydov splitting does not vanish on the phase transition due to a nonzero value of the average dipole moment (unlike molecular crystals and orientationally planar monolayers in which the average dipole moment becomes zero at the phase transition temperature).

Study of phase transition of two-dimensional ferroelectric copolymer P(VDF-TrFE) Langmuir monolayer by Maxwell displacement current and Brewster angle microscopy

Phase transition of ferroelectric Langmuir monolayers composed of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer with various VDF ratios, was investigated. The thermodynamic, dielectric, and structural properties of these two-dimensional ferroelectric systems were analyzed by coupling surface pressure-area isotherms, Maxwell displacement currents (MDCs), and Brewster angle microscopy (BAM) images. A low-temperature phase transition, specific for the ferroelectric monolayer, was found out with an order-disorder phase transition. The phase transition temperature was dependent on the composition ratio of the copolymer, and the Gibbs free energy reached maximum at this temperature. A negative MDC peak representing dipole moment oriented to the subphase was observed during monolayer compression and disappeared above the order-disorder transition. Interestingly, the BAM images showed decrease of domain size with increase of temperature. These results were well explained based on the order-disorder transition.