Crystalline Phase Transition Temperature Research Articles

Hydrophobic bio-composites of stearic acid starch esters and micro fibrillated cellulose processed by extrusion

Fatty acid starch esters are potential candidates for obtaining hydrophobic starch-based materials. The objective was to evaluate thermoplastic bio-composites with stearic acid cassava starch esters (SAE) as matrices reinforced with micro fibrillated cellulose (MFC). SAE with 0.11 and 0.14 of DS and native cassava starch were processed with 5 % and 10 % MFC by extrusion and then thermo-compression to obtain bio-composite films. Performance of bio-composites as a function of the degree of substitution (DS) of starch esters and the content of MFC, was evaluated. Starch esterification reduced the crystalline phase and glass transition temperature of bio-composites, as confirmed by structural and thermal analysis. The increase of DS reduced the degradation temperature, which was compensated by adding MFC, favoring thermal stability. The hydrophobicity of films increased with the degree of starch esterification. Treatments with 0.14 DS formed stiffer and less tensile strength films. There was an increase in the modulus of elasticity and a reduction in elongation by increasing MFC. Fracture microscopy of films showed defects by incomplete starch melting and lack of interfacial adhesion with MFC caused by starch esterification. Starch esterification and MFC dispersion were determinants in the performance of bio-composites. In consequence, starch esterification with stearic acid reduces the compatibility with the MFC and, consequently, the performance of composites.

Photoinduced bidirectional mesophase transition in vesicles containing azobenzene amphiphiles.

The functionality and efficiency of proteins within a biological membrane are highly dependent on both the membrane lipid composition and the physiochemical properties of the solution. Lipid mesophases are directly influenced by changes in temperature, pH, water content or due to individual properties of single lipids such as photoswitchability. In this work, we were able to induce light- and temperature-driven mesophase transitions in a model membrane system containing a mixture of 1,2-dipalmitoyl-phosphatidylcholine phospholipids and azobenzene amphiphiles. We observed reversible and reproducible transitions between the lamellar and Pn3m cubic phase after illuminating the sample for 5 min with light of 365 and 455 nm wavelengths, respectively, to switch between the cis and trans states of the azobenzene N=N double bond. These light-controlled mesophase transitions were found for mixed complexes with up to 20% content of the photosensitive molecule and at temperatures below the gel-to-liquid crystalline phase transition temperature of 33°C. Our results demonstrate the potential to design bespoke model systems to study the response of membrane lipids and proteins upon changes in mesophase without altering the environment and thus provide a possible basis for drug delivery systems.

Controlling the formation of ionic complex vesicles through double-tailed surfactants.

Liposomes are often used in cosmetics since they are naturally derived and have excellent texture enhancing capabilities. However, when preparing them by using phospholipids with unsaturated acyl groups, they easily suffer from oxidative degradation. Accordingly, hydrogenated phospholipids are preferred, however, it is difficult to prepare stable liposomes due to its high gel-liquid crystalline phase transition temperature. On the other hand, although dialkyl dimethyl ammonium type cationic surfactants are widely known to form vesicles, they have rarely been used for skincare products except for water-in-oil type emulsion creams stabilized by organically modified clay minerals. We decided to overcome all of the problems above through ionic complex vesicles formed by double-tailed cationic and anionic surfactants. Distearyl dimethyl ammonium chloride (DSAC) and sodium dilauramidoglutamide lysine (DLGL) were selected as cationic and anionic surfactants, respectively. Differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SWAXS) measurements were performed to confirm the DSAC/DLGL/water ternary phase diagram. Newly developed ionic complex vesicle formation was confirmed by cryogenic transmission electron microscopy (cryo-TEM). The adsorbed cosmetic film structure on the skin invivo was evaluated through the polarized infrared external reflection (PIR-ER). Finally, a cosmetic lotion formula was developed and the vesicle size was determined by dynamic light scattering (DLS). DSC and SWAXS data indicated that stable vesicles could be obtained at a molar ratio of DLGL to DSAC = 6:4. At this molar ratio, multi lamellar vesicles with diameters less than 100 nm were observed through cryo-TEM. PIR-ER data revealed that the developed vesicles formed a highly perpendicular orientation to the human skin surface. We have succeeded in formulating a cosmetic lotion containing developed vesicles with a mean diameter of 63.2 nm, which was stable over 1 month at 0, 37, and 50°C. Our newly developed vesicles can be easily obtained through a coagulation process. Also, the adsorbed film structure supported by PIR-ER experiments implies that the developed lotion has an excellent texture that is the same as cosmetic lotions containing liposomes. Therefore, it's possible that this ionic complex vesicle could take the place of liposomes.

Exploring the Physical Properties of Lipid Membranes with Polyhydroxy Oxanorbornane Head Group Using NBD-Conjugated and DPH Fluorescent Probes.

The present study focuses on exploring the physical properties of lipid membranes based on the polyhydroxy oxanorbornane (PH-ONB) headgroup, designed as synthetic analogues of naturally occurring archaeal lipid membranes. Specifically, we study two variants of PH-ONB headgroup-based lipids differing in the number of hydroxy groups present in the headgroup, with one having two hydroxy groups (ONB-2OH) and the other having three (ONB-3OH). These lipids form stable bilayer membranes. The study begins with a comprehensive analysis of the fluorescence characteristics of nitrobenzoxadiazole (NBD)-tagged ONB-based lipids in different solvent environments and within a model lipid membrane 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Subsequently, the physical properties of the ONB-based membranes were examined by using an NBD-tagged ONB-based probe and a commonly used extrinsic 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probe. The steady-state and time-resolved fluorescence properties of the NBD-tagged ONB-based probe and DPH were used to compare the physical properties of the ONB-based membranes, including polarity, fluidity, phase transition, order, hydration, location, heterogeneity, and rotational diffusion. The solid gel to liquid crystalline phase transition temperatures of ONB-2OH and ONB-3OH lipid membranes are found to be (68 ± 1) °C and (74 ± 1) °C, respectively. The variation in organization (size), fluidity, and phase transition temperature of ONB-based lipid membranes is explained by the extent of hydrogen bonding interactions between lipid head groups. ONB-based membranes exhibit characteristics similar to those of phospholipid membranes and possess a notably high phase transition temperature. These properties make them a promising and cost-effective synthetic alternative to archaeal lipid membranes with a wide range of potential applications.

Synthesize and Characterization of Crystalline Structure and Phase Transition Temperature of Ce Substituted Ba- TiO3 Ceramics

Characterization and synthesize of Ba1-xCexTiO3 (x=0.01mol.) annealed at three different temperatures (1100oC, 1200 oC, 1300 oC) have been studied. For preparing powder of (x = 0.01 moles), Sol- Gel synthesis was carried out according to the stoichiometric conditions. The powder obtained palletized at 1.5 ton /mm2 pressure into a 10 mm dia. Pellet using PVA (polyvinyl alcohol) as binder. The pellets were then annealed at 1100oC for 1 hour. Then silver paste was applied on pellet by fully coating the one side and applying a single dot on other side, for making the pellet as capacitor. The XRD pattern reveals that the lattice parameter increases with the Ce doping and c/a ratio decreased which shows that tetragonality decreases. In all investigated samples it is assumed that the phase transition analysis, after doping Ce in BaTiO3 the Curie temperature decreases.

Controllable synthesis of crystal-amorphous heterostructures in transition metal phosphide and enhancement mechanism for overall water splitting

The crystal-amorphous (c-a) heterostructure is a promising structure for electrocatalysts in water electrolysis. In this study, stable, efficient, and abundant c-a interfaces were formed by synchronously synthesizing crystalline FeP and amorphous MoP on foam ferrum by controlling the crystalline phase transition temperature. In 1 M KOH, the a-MoP/FeP@FF catalyst showed overpotentials of 75 mV and 253 mV for HER and OER processes, respectively, at a current density of j10. The Tafel slopes were small at 60 mV∙dec−1 and 77 mV∙dec−1 for HER and OER, respectively. Theoretical calculations indicate that the c-a interface optimizes the overall electronic structure, enabling Mo and Fe to have excellent HER and OER activity, respectively. The two types of active sites collaborate fully to promote overall water splitting. This study confirms that the c-a heterostructure can significantly enhance the catalytic performance of transition metal phosphides and explains the mechanism of enhanced catalytic activity. This synthesis method and reaction mechanism not only apply to other transition metal phosphides but also provide reference for other types of catalysts.

Long chain ceramides raise the main phase transition of monounsaturated phospholipids to physiological temperature

Little is known about the molecular mechanisms of ceramide-mediated cellular signaling. We examined the effects of palmitoyl ceramide (C16-ceramide) and stearoyl ceramide (C18-ceramide) on the phase behavior of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) using differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS, WAXS). As previously published, the presence of ceramides increased the lamellar gel-to-lamellar liquid crystalline (Lβ–Lα) phase transition temperature of POPC and POPE and decreased the Lα-to-inverted hexagonal (Lα–HII) phase transition temperature of POPE. Interestingly, despite an ~ 30° difference in the main phase transition temperatures of POPC and POPE, the Lβ–Lα phase transition temperatures were very close between POPC/C18-ceramide and POPE/C18-ceramide and were near physiological temperature. A comparison of the results of C16-ceramide in published and our own results with those of C18-ceramide indicates that increase of the carbon chain length of ceramide from 16 to 18 and/or the small difference of ceramide content in the membrane dramatically change the phase transition temperature of POPC and POPE to near physiological temperature. Our results support the idea that ceramide signaling is mediated by the alteration of lipid phase-dependent partitioning of signaling proteins.

Anticarcinogenic trimethoxybenzoate of catechin stabilizes the liquid crystalline bilayer phase in phosphatidylethanolamine membranes

The anticarcinogenic properties of catechins stand out among the great variety of biological actions attributed to these compounds. The capacity of catechins to interact with lipids and their participation in membrane related processes points out to the membrane as their potential site of action. Phosphatidylethanolamine is an abundant phospholipid in mammalian membranes that has tendency to form non lamellar phases, it is associated with important cellular processes, and it has been related to cancer. In order to shed light into the molecular effect of the anticarcinogenic 3,4,5-trimethoxybenzoate of catechin (TMBC) on lipid polymorphism and membrane structure and dynamics, we present a combined experimental and computational study of the interaction between this semisynthetic catechin and biomimetic membranes composed of unsaturated phosphatidylethanolamine. Our experimental evidence reveals that TMBC is readily incorporated into unsaturated phosphatidylethanolamine system where it is able to shift the gel to liquid crystalline phase transition temperature to lower values, decreasing the cooperativity and the enthalpy change of the transition. The presence of TMBC is able to promote the formation of gel phase immiscibility and to block the formation of the inverted hexagonal phase. In the bilayer liquid crystalline phase, the catechin decreases the interlamellar repeat distance, it increases the fluidity of the membrane, and it alters the hydrogen bond pattern of the interfacial region of the bilayer. Our molecular dynamics results concur with the experimental data and locate TMBC forming different domains near the interfacial region of the bilayer where it modifies the lateral pressure profile of the membrane leading to a stabilization of the bilayer in the liquid crystalline phase and to a potential alteration of the function of the membrane.

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

Polyoxyethylene Sorbitan Monolaurate induced vesicle-to-micelle transition of aqueous dimethyldioctadecylammonium bromide dispersion

The interactions of micelle forming nonionic surfactant, Polyoxyethylene Sorbitan Monolaurate (Tween 20), with spontaneously formed vesicle dispersions of Dimethyldioctadecylammonium bromide (DDOAB) have been investigated. DDOAB vesicles are solubilized in the Tween 20 micelles and undergo vesicle-to-micelle transition. Differential scanning calorimetry (DSC) measurements show that the addition of Tween 20 lowers the gel-to-liquid crystalline phase transition temperature (Tm) of DDOAB vesicles. The thermogram peak shifts to a lower temperature, broadens and ultimately disappears when the vesicle-to-micelle transition is complete. Fluorescence anisotropy and Turbidity data indicate that the vesicle-to-micelle transition takes place in three stages: the vesicle bilayer is first saturated with Tween 20, the bilayer then disintegrates, and finally a complete solubilization takes place, leading to the formation of mixed micelles. Dynamic light scattering study shows an initial increase of the average vesicle size upon the addition of Tween 20 up to the saturation point and then a decrease upon complete solubilization of vesicles into mixed micelles. The results have been confirmed by observing Transmission Electron Microscopy (TEM). TEM images show evidence of direct transformation of the vesicles to mixed micelles without the formation of intermediate aggregates.

Temperature tunable flexible photo absorbers based on near-infrared 1D photonic crystal hybridized W-doped VO2 nanostructures

Vanadium dioxide is a potential candidate for energy efficient smart windows and have crystalline phase transition temperature (T c) at 68 °C. So far, literatures mainly emphasis on different synthetic strategies of tungsten doped VO2 which is a most effective dopant to reduce T c of VO2 to near room temperatures. Until now, there is no report shows the incorporation of flexible 1D photonic crystals as spectrally selective, temperature tunable device to control the changes in optical transmission modulations of W-VO2 nanostrtcures, especially in the near IR region for smart window application. W-doped VO2 with various tungsten contents were synthesized with a facile hydrothermal route. We found that, with 1.1 at% of tungsten doping in intrinsic VO2, the metal to insulator transition temperature is brought down to 37 °C from 68 °C. IR transmission of VO2 thin film can be reduced from 70% to 40% around room temperature, after doping. Significant absorption enhancement has been observed for both VO2 and W-doped VO2 films, deposited over tunable SiO2/Ta2O5 based distributed Bragg reflector (DBR) fabricated over flexible PET (poly-ethylene terephthalate) substrates. On depositing VO2 over ∼70% reflecting DBR, optical transmission is reduced to ∼15% from 35% while the temperature varies to 380 K from 300 K in IR regime. Number of stacks plays a crucial role for effective IR extinctions. A high quality DBR is fabricated by increasing no. of stacks from 4 to 7, with optical transmission of DBR reduced to nearly 5% in stop band. However, with 1.1 at% of W-VO2 over such 95% reflecting flexible DBR, optical transmission vanishes nearly, around room temperature itself in the stop bands of that DBR, which clearly indicates the significant absorption enhancement. W-VO2/DBR hybrid can substantially modulate the solar heat flux and also imbuing DBR over flexible PET substrates offers retrofitting of the existing windows for energy economy. Thus these structures have promising potential applications for optical devices and practical design for smart windows.

A Facile Route to Synthesis of Ferromagnetic and Antiferromagnetic Phases of Iron Oxide Nanoparticles by Controlled Heat Treatment of Ferritin

Magnetic nanoparticles of ferromagnetic and antiferromagnetic phases were synthesized from controlled heat treatment of dry ferritin powder in a bottom-up approach. Heat treatment paves the way for synthesis of Fe2O3 solid phase from iron molecular complexes stored in the cavity of ferritin; the strong increase (~ 150 times) in saturation magnetization is a sign of the presence of ferromagnetic exchange coupling which exists only in solid phase. In this experimental study, vibrating sample magnetometer (VSM) was used to study magnetic induction. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed for analysis of structure and morphology of the samples. Differential scanning calorimetry (DSC) was used to search for indicators of structural phase transformation through scanning of temperature. The nanoparticle size with highly narrow distribution (mean size ~ 6 nm) was observed by SEM for sample annealed at 430 °C. As indicated by XRD measurements, the majority phase of Fe2O3 nanoparticles was amorphous up to 500 °C, whereas DSC demarcates the crystalline phase transition temperature at 545 °C. The annealing temperature range from 400 to 500 °C was found to be suitable for growing ferromagnetic nanoparticles endowed with high saturation magnetization and low coercivity. At higher range of annealing temperature (500–700 °C), XRD confirms the presence of α-Fe2O3 (haematite) phase which is an antiferromagnetic crystalline system with weak magnetization. A systematic decline of magnetization on increasing the annealing temperature beyond 500 °C was attributed to finite size effects and increased purity of antiferromagnetic phase.

Urea-Assisted Reconstitution of Discoidal High-Density Lipoprotein.

High-density lipoprotein (HDL) is a naturally occurring composite of lipids and lipid-binding proteins. The cholate dialysis method, first reported by Jonas in 1969, is the most widely used approach for reconstituting discoidal HDL (dHDL) in test tubes with phospholipids and the most dominant protein, apolipoprotein A-1 (apoA-I). Here, we show that a dHDL-relevant complex can also be prepared by gently mixing 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and apoA-I or its mutants in ethanol/H2O solutions containing urea at a concentration of a few molar and then incubating the mixture at the gel-liquid crystalline phase transition temperature in test tubes. Subsequent purification steps involve quick dialysis following size exclusion chromatography. The yields (73 ± 3% and 70 ± 1% protein and DMPC, respectively) of the resulting HDL-like nanoparticles, designated as uHDL, were comparable to the values of 68 ± 9% and 71 ± 12% obtained in the cholate dialysis method. Using apoA-I and two mutants, the key factor in this method was found to be urea at the folded and unfolded transition midpoint concentration. By using this urea-assisted method in the presence of a hydrophobic drug, all-trans-retinoic acid (ATRA), one-step preparation of ATRA-loaded uHDL was also possible. The loading efficiency was comparable to that in the mixing of ATRA and uHDL or dHDL reconstituted by the cholate dialysis method. Atomic force microscopy analysis revealed that uHDL and ATRA-loaded uHDL were discoidal. Our urea-assisted method is an easy and efficient method for reconstituting dHDL and can be utilized to prepare various drug-dHDL complexes.

Investigation and Control of Charge Transport Anisotropy in Highly Oriented Friction-Transferred Polythiophene Thin Films.

Highly oriented thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) were prepared by friction-transfer technique followed by their characterization using polarized absorption spectroscopy, angle-dependent polarized Raman spectroscopy, and X-ray diffraction (XRD) techniques. Orientation in high-molecular-weight (MW) polymers is hampered by chain folding or entanglements, which limit their macromolecular orientation. Interestingly, utilizing high-molecular-weight PBTTT (MW > 50 kDa) and friction-transfer technique, successful fabrication of highly oriented thin films with very high dichroic ratio (∼30) was demonstrated. The role of the substrate's surface energy and its impact on the field-effect mobility (μ) of the oriented thin films were comprehensively investigated. The influence of annealing the thin films as prepared on the bare and self-assembled monolayer (SAM)-treated SiO2 surfaces exhibiting differential trends of μ was systematically investigated. This was explained by partial and complete conformational transformation of macromolecules on bare and SAM-treated SiO2 surfaces, respectively, after annealing them beyond liquid crystalline phase transition temperature, as revealed by in-plane and out-of-plane XRD results. On bare SiO2, optimum μ up to 0.03 cm2 V-1 s-1 along the backbone orientation was obtained for the thin films annealed to 120 °C; whereas, it reached up to 0.36 cm2 V-1 s-1 on SAM-treated SiO2 after annealing at 200 °C. Finally, a charge transport mechanism was proposed taking evidence from the anisotropic optical and electrical characteristics of the friction-transferred PBTTT films into consideration.

Fused Filament Fabrication of Biodegradable PLA/316L Composite Scaffolds: Effects of Metal Particle Content

Polylactic acid (PLA) is a biodegradable polymer suitable for fabricating porous scaffolds in bone tissue engineering. Fillers are often added into PLA matrix to fabricate composite scaffolds in order to improve the performance of pure PLA. In the present study, we fabricated PLA/316L composite scaffolds with stainless steel particle contents from 5 vol% to 15 vol% using fused filament fabrication (FFF) process. The effects on dimensions of pore and strut, surface roughness, as well as thermal, mechanical, and in-vitro degradation properties of the scaffolds were studied. The results showed that the dimensional accuracy was improved, and the surface roughness was tailored by the addition of steel powders that were dispersedly distributed on the strut surface. In addition, PLA/316L composite scaffolds exhibited a lower coefficient of thermal expansion than pure PLA, while glass transition temperature and crystalline phase transition temperature were not significantly affected by steel powder. Moreover, compressive strength and elastic modulus were enhanced when powder loading was set at 10 vol% or 15 vol%. In-vitro degradation tests showed no significant differences on degradation rates between PLA and PLA/316L composite scaffolds.

IMAGE ANALYSIS STUDIES IN RE-ENTRANT SMC PHASE ON HYDROGEN BONDED LIQUID CRYSTAL SA:10OBA

Ponte Academic JournalFeb 2020, Volume 76, Issue 2 IMAGE ANALYSIS STUDIES IN RE-ENTRANT SMC PHASE ON HYDROGEN BONDED LIQUID CRYSTAL SA:10OBAAuthor(s): K. Lakshmi Sarada ,A.V.N.Ashok Kumar, K. Mallika, S. Sreehari SastryJ. Ponte - Feb 2020 - Volume 76 - Issue 2 doi: 10.21506/j.ponte.2020.2.15 Abstract:Image analysis using MATLAB software for characterization of liquid crystalline (LC) phases and phase transition temperatures in hydrogen bonded liquid crystal dimer viz., Succinic acid (SA) (for n=10) with molecular formula nOBA:SA:nOBA is described. SmC, SmX, SmCRE and SmG phase variance is observed by optical textures through Polarized Optical Microscope and which are confirmed by Differential Scanning Calorimetry experimental technique. Influence of flexible end chain length and length of spacer moiety on the LC polymorphism is studied and the results are compared with reports on other HBLC dimers with similar type of molecular structure. Statistical parameters of LC textures based on gray scale intensity as a function of temperature are computed and discussed. Download full text:Check if you have access through your login credentials or your institution Username Password

Facile control of surfactant lamellar phase transition and adsorption behavior.

This study sets out to investigate the effect of the presence of small water-soluble additives on the tunability of the surfactant gel-to-liquid crystalline (Lβ–Lα) phase transition temperature (Tm) for a bilayer-forming cationic surfactant and the phase behavior of such surfactant systems on dilution. This is strongly driven by the fact that this type of cationic surfactant has many interesting unanswered scientific questions and has found applications in various areas such as consumer care, the petrochemical industry, food science, etc. The underlying surfactant/additive interactions and the interfacial behavior of lamellar surfactant systems including the surfactant deposition on surfaces can provide new avenues to develop novel product formulations. We have examined dioctadecyldimethyl ammonium chloride (DODAC) in the presence of small polar additives, with respect to the phase behavior upon dilution and the deposition on silica. Differential scanning calorimetry (DSC) is used to track the transition temperature, Tm, and synchrotron and laboratory-based small and wide-angle X-ray scattering (SAXS and WAXS) were used to determine the self-assembled surfactant structure below and above the Tm. DSC scans showed that upon dilution the additives could be removed from the surfactant bilayer which in turn tuned the Tm. A spontaneous transition from a liquid crystalline (Lα) phase to a gel (Lβ) phase on dilution was demonstrated, which indicated that additives could be taken out from the Lα phase. By means of in situ null ellipsometry, the deposition of a diluted surfactant Lβ phase upon replacement of bulk solution by deionized water was followed. This technique enables time-resolved monitoring of the deposited surfactant layer thickness and adsorbed amount, which allows us to understand the deposition on surfaces. Robust layers at least one bilayer-thick were deposited onto the surface and shown to be irreversibly adsorbed due to poor surfactant solvency in water. The thickest layer of surfactant deposited after dilution was found for mixtures with small amounts of additive since high amounts might lead to a phase-separated system.

Cooperative Adsorption of Trehalose to DPPC Monolayers at the Water-Air Interface Studied with Vibrational Sum Frequency Generation.

A combination of surface tension, surface-specific vibrational spectroscopy, and differential scanning calorimetry experiments was performed to examine the ability of lipid films to enrich interfacial organic content by attracting soluble, neutral saccharides from bulk aqueous solution. This "cooperative adsorption" hypothesis has been proposed as a possible source of the high organic fractions found in sea spray aerosols and is believed to be responsible for cryoprotection in some organisms. Experiments described in this work show that the neutral disaccharide trehalose (Tre) is drawn to lipid films composed of dipalmitoylphosphatidylcholine (DPPC), a saturated lipid that is a major component of most eukaryotic cells. The effects of Tre on DPPC monolayer structure and organization were tested with tightly packed monolayers in the two-dimensional solid phase (40 Å2/molecule) and more expanded monolayers in the two-dimensional liquid condensed phase (55 Å2/molecule). Surface tension data show that DPPC monolayer behavior remains largely unchanged until Tre bulk concentrations are sufficiently high (≥50 mM). In contrast, surface-specific vibrational sum frequency spectra show that when Tre bulk concentrations are ≥10 mM, DPPC monolayers in their liquid condensed state (55 Å2/molecule) became more ordered, implying relatively strong noncovalent interactions between the two species. Tre also induces changes in DPPC bilayer behavior as evidenced by a gel-to-liquid crystalline phase transition temperature that increases with increasing Tre concentration. This result suggests that Tre associates with the DPPC headgroups in very specific ways leading to partial dehydration. Together, these results support the cooperative adsorption mechanism under some circumstances, namely, that there is a minimum aqueous phase Tre concentration required to induce observable structural changes in a lipid monolayer and that these effects are most pronounced with DPPC monolayers in their liquid condensed state compared to those of a tightly packed two-dimensional solid.

Enhanced chain packing achieved via putative headgroup ion-triplet formation in binary anionic lipid/cationic surfactant mixed monolayers

Lipid/surfactant miscibility was investigated in monolayers composed of binary mixtures of dipalmitoylphosphatidylglycerol (DPPG) and dihexadecyldimethylammonium bromide (DHDAB). Langmuir monolayers formed from biomimetic DPPG/DHDAB mixtures based on the anionic:cationic lipid ratios observed in the bacterium Staphylococcus aureus (7:3 and 1:1) were examined alongside those of the pure amphiphiles and a surfactant rich 3:7 mixture. Using a combination of GIXD, TRXF and IRRAS, DPPG/DHDAB 1:1 monolayers were found to form a more stabilised condensed phase compared to pure DPPG, which was composed entirely of electrostatically neutral ion pairs, analogous to the so-called catanionic amphiphiles spontaneously formed by single-chain surfactants with opposing headgroup charges. Despite the lack of lateral charge repulsion the ion paired phase of DPPG/DHDAB exhibited slightly looser chain packing that was observed for DPPG indicating a significant steric effect on packing geometry caused by ion pair formation. Surprisingly, the 7:3 mixture of DPPG/DHDAB formed a completely condensed phase, with no isotherm transitions, in which the chain packing was significantly closer than was found for either DPPG or the totally ion paired monolayer. It is postulated that this mixture forms a distinct DPPG/DHDAB/DPPG ion triplet phase in which the overall negative charge is delocalised across the headgroups. Vesicles composed from the 7:3 mixture formed highly stable dispersions with an increased gel to liquid crystalline phase transition temperature with respect to its pure components. Increasing the proportion of DHDAB above 50 mol% led to demixing between the condensed ion paired phase and the more fluid surfactant, as was clearly observed in epifluorescence images taken of the surface films.

Thermal Processing and Interactions of Ethyl Formate in Model Astrophysical Ices Containing Water and Ethanol

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) have been used to investigate the interactions between ethyl formate and water and ethyl formate and ethanol in model astrophysical ices adsorbed on a graphitic model grain surface. Experiments show that the ethyl formate forms hydrogen bonds to both water and ethanol via the oxygen lone pairs. This leads to the observation of shifts in the vibrational wavenumber of the C═O and C—O—C modes of ethyl formate, which can potentially be used to identify the environment of this complex organic molecule in astronomical observations. TPD data show that the interaction of ethyl formate with water is stronger than that with ethanol with an additional species being observed in the TPD spectrum corresponding to the desorption of ethyl formate directly bonded to the water ice surface. The desorption energy of ethyl formate adsorbed on water ice was found to be 48.5 kJ mol–1, compared to 43.2 kJ mol–1 for pure ethyl formate monolayers. Ethyl formate also traps in water ice and undergoes volcano desorption at the water amorphous to crystalline phase transition temperature. In contrast to the water, ethanol has very little effect on the desorption of ethyl formate with the two species behaving independently even in a codeposited ice.