Alkane/Alcohol mixed monolayers at the solid/liquid interface

In this work, we present the behavior of solid monolayers of binary mixtures of alkanes and alcohols adsorbed on the surface of graphite from their liquid mixtures. We demonstrate that solid monolayers form for all the combinations investigated here. Differential scanning calorimetry (DSC) is used to identify the surface phase behavior of these mixtures, and elastic neutron incoherent scattering has been used to determine the composition of the mixed monolayers inferred by the calorimetry. The mixing behavior of the alcohol/alkane monolayer mixtures is compared quantitatively with alkane/alkane and alcohol/alcohol mixtures using a regular solution approach to model the incomplete mixing in the solid monolayer with preferential adsorption determining the surface composition. This analysis indicates the preferential adsorption of alcohols over alkanes of comparable alkyl chain length and even preferential adsorption of shorter alcohols over longer alkanes, which contrasts strongly with mixtures of alkane/alkane and alcohol/alcohol of different alkyl chain lengths where the longer homologue is always found to preferentially adsorb over the shorter. The alcohol/alkane mixtures are all found to phase separate to a significant extent in the adsorbed layer mixtures even when molecules are of a similar size. Again, this contrasts strongly with alkane/alkane and alcohol/alcohol mixtures where, although phase separation is found for molecules of significantly different size, good mixing is found for similar size species.     

Relationship between properties of fluorinated graphite intercalates and matrix composition Part III. Intercalates with 1,2-dichloroethane

A series of PA-TD mixtures were prepared and their thermal properties were studied by DSC and thermal conductivity measurement. The phase diagram of the binary system was constructed, which showed an eutectic behavior for the solid-liquid equilibrium line. The eutectic composition of the binary system was at the mass fraction of TD near 0.7 with an eutectic temperature of about 29°C. At TD side, PA was partially miscible in the TD solid matrix and the solid phase transition of TD had an effect on the solidus line. The eutectic composition mixture could be viewed as a new phase change material with large thermal energy storage capacity.     

Determination and measurement of solid–liquid equilibrium for binary fatty acid mixtures based on NRTL and UNIQUAC activity models

The estimation of solid–liquid phase equilibrium is important for the design, development, and operation of many industrial processes because of application in many manufacturing fields such as cosmetic, pharmaceutic, and biotechnology industries. In this work, we measured solid–liquid phase equilibrium of six fatty acid binary mixtures using the DSC technique and developed thermodynamic approaches for binary fatty acid mixtures to estimate melting temperatures as a function of mole fraction in solid–liquid phase equilibrium. Derivation of NRTL and UNIQUAC activity models was developed to predict melting temperatures and latent heat to achieve eutectic points of undecylic acid, pentadecylic acid, margaric acid, and stearic acid six pairwise binary mixtures. The fatty acids eutectic mixtures are appropriate for heat water systems, phase clothes, concrete, and other similar applications. The results showed low deviations from experimental data measured in this study.

     

Adsorption and mixing behavior of ethers and alkanes at the solid/liquid interface

The behavior of binary mixtures of linear symmetrical ethers and alkanes adsorbed to a graphite surface from the bulk liquid mixtures is described on the basis of differential scanning calorimetry (DSC) data. Both the ethers and the alkanes are found to form solid monolayers when adsorbed from the liquid. In addition, the monolayer mixing behavior is addressed. The results indicate that there is good, essentially ideal, mixing in the monolayers for ethers and alkanes of the same overall chain length, where the chain length is equal to the total number of carbon and oxygen atoms in the molecule. However, a difference in chain length of more than one atom results in a variation of mixing behavior from nonideal mixing (for long pairs) to phase separation (for short pairs) on the graphite surface. Hence, we conclude that it is the relative chain lengths that control mixing behavior. The results are quantified using a regular solution model with a correction for preferential adsorption. The phase behavior of the mixed monolayers is also compared to the behavior of the bulk. Interestingly, we observe mixtures where the bulk and monolayer behavior are quite different, for example, phase separation in the bulk but essentially ideal mixing in the monolayer for mixtures of ethers and alkanes with the same chain lengths. At present, we attribute this mixing in the monolayer to dilution of the unfavorable ether oxygen-ether oxygen lone pair interactions by the coadsorbed alkanes. In addition, we find evidence for the preferential adsorption of the alkane over the ether. For example, heptane is preferentially adsorbed over dibutyl ether even though it contains two fewer atoms in the molecular chain. This contrasts with the preferential adsorption of alcohols over alkanes reported previously (Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21, 5085-5093).     

Miscibility and segregated structures in mixed Langmuir monolayers

The phase behavior and morphology of segregated structures are considered for mixed Langmuir monolayers, which comprise a type of supramolecular polymer having a complex internal structure mixed with a long chain fatty acid. We fabricated two different series of mixed monolayers from a polyglutamate (PG) copolymer having 30% octadecyl ester side chains and 70% methyl ester side chains and fatty acids. These mixed monolayers deposited on a solid substrate were studied by pressure-area diagram measurements, X-ray analysis, and atomic force microscopy. Stearic acid (STA) and hexacosanoic acid (HCA) with alkyl chain lengths of 17 and 25 carbon atoms, respectively, were used as low molecular weight components. For the mixture PG:STA, where the length of the STA molecules is comparable to the length of the PG side chains, we observed the formation of partially miscible monolayers. These mixtures exhibit a nanometer scale domain morphology formed by the STA molecules dissolved in the outer shell of the PG monolayer. In contrast, for the PG:HCA mixture we observed a strong tendency for microphase separation and the formation of well-defined submicron segregated structures in the monolayers. Lateral compression of the mixed monolayers to a point close to the collapse pressure promotes microphase separation in both types of mixed monolayers with the formation of anisotropic surface morphology and oriented domains.     

Fine structure and phase transition behavior of fluorinated comb copolymers

The fine structure in the solid state and phase transition behavior of newly synthesized comb copolymers having fluorocarbon and hydrocarbon side‐chains were investigated by temperature controlled wide angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). From the WAXD profiles, two kinds of short spacing peaks based on the formation of the subcell for fluorinated and hydrogenated side‐chains were confirmed at 5.0 and 4.1 Å, respectively. Furthermore, two kinds of endothermic peaks, which corresponded to melting peaks of both side‐chain crystals, appeared in heating process of the DSC thermograms. From these experimental findings, the phase separation structure having the independently packed immiscible side‐chain crystalline was formed in the whole polymer crystal. In addition, it was found that these comb polymers formed highly ordered (double) layer structure estimated using WAXD and small angle X‐ray scattering (SAXS). These fluorinated comb copolymers form a monolayer on the water surface and their transferred film with phase‐separated structure at nanometer size on solid. There were hydrogenated domains at 10–20 nm diameter scales in these phase separated surface structure of monolayers. From these experimental results, these copolymer monolayers are expected to be used as a new molecular device such as nanolithography based on the surface patterning of polymer nanomaterials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 416–425, 2006     

Thermodynamic and real-space structural evidence of a 2D critical point in phospholipid monolayers

The two-dimensional phase diagram of phospholipid monolayers at air-water interfaces has been constructed from Langmuir compression isotherms. The coexistence region between the solid and fluid phases of the monolayer ends at the critical temperature of the transition. The small-scale lateral structure of the monolayers has been imaged by atomic force microscopy in the nm to microm range at distinct points in the phase diagram. The lateral structure is immobilized by transferring the monolayer from an air-water interface to a solid mica support using Langmuir-Blodgett techniques. A transfer protocol that ensures preservation of the structure during the transfer has been established. The lateral structure reflecting the density fluctuations has been visualized and quantitatively characterized as the monolayer passes through a series of first-order phase transitions and ultimately approaches a critical point. The critical behavior inferred from the thermodynamic as well as the structural data is found to be consistent with the 2D Ising universality class. Additional results are presented demonstrating the presence of striped phases and coexisting domains in binary mixtures.     

Compatibility and Stability Studies of Propranolol Hydrochloride Binary Mixtures and Tablets for TG and DSC-photovisual

This study demonstrates the thermalanalysis applications in compatibility and stability studies of the propranolol binary mixture sand tablets A and B. The propranolol binary mixtures were prepared in the laboratory and compared to the fully formulated tablets using the thermogravimetric (TG) and calorimetric(DSC) methods. DSC of binary mixtures showed similar phase transition to propranolol drug. The tablets phase transition decreased and there was no detectable significant interaction in propranolol–lactose mixture and tablets. The DSC-photovisual test revealed an interaction similar to the Maillard reaction. The TG isothermal study showed a difference in the profile between the drug and tablets due excipients quality and problems in manufacture process. The kinetic parameters indicated a lower stability for the tablets than propranolol drug. The thermal techniques thermally differentiated the propranolol preparations demonstrating the importance in the design development of pharmaceuticals solid-dosage form. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of gel-phase microdomains and lipid rafts in lipid monolayers on the electron transfer of a lipophilic redox probe: dioctadecylviologen

Gel-phase microdomains and lipid rafts form spontaneously in monolayers of lipid mixtures of dioleoylphosphatidylcholine (DOPC), palmitoylsphingomyelin (PSM) and cholesterol (Chol), self-assembled on mercury. The influence of microdomains on the electron transfer properties of 2 mol% dioctadecylviologen (DODV), incorporated in these lipid monolayers, was investigated by cyclic voltammetry. In pure DOPC, the DODV molecules tend to aggregate, giving rise to strong attractive lateral interactions. With an increase in the PSM mole fraction in DOPC/PSM binary mixtures, the edges of the resulting gel-phase microdomains act as docking sites for the DODV molecules, decreasing lateral interactions and modifying the DODV redox properties. A similar behavior is shown by lipid rafts formed by adding Chol to the above binary mixtures. By varying the DOPC/PSM molar ratio, the midpoint between the peak potentials of the DODV reduction and oxidation peaks shifts in parallel with the surface dipole potential of the lipid mixture. This behavior indicates that the formal (half-reduction) potential of a redox pair, as measured versus a given reference electrode, may include a surface dipole potential if one or both members of the redox pair are embedded in a medium different from the bulk phase containing the reference electrode.     

Comb-like ionic complexes of cationic surfactants with bacterial poly(gamma-glutamic acid) of racemic composition

Comb-like ionic complexes, nATMA . PG(DL)GA, were prepared from microbial poly(gamma-glutamic acid), with a nearly racemic configuration, and alkyltrimethylammoniun bromides, with linear alkyl chains containing an even number of carbon atoms, n, ranging from 12 to 22. The complexes had a nearly stoichiometric composition, displayed thermal stability up to temperatures above 200 degrees C and were insoluble in water but soluble in organic solvents. In the solid state, they were arranged in a regularly layered structure with the alkyl side chain crystallized for n > or = 18. Heating above melting entailed a contraction in the interlayer distance which varied from 1 to 10% depending on the value of n. Comparison with data reported on similar complexes obtained from nearly enantiomerically pure poly(gamma-glutamic acid) revealed an overall behavior very similar for the two series but with specific significant differences concerning side chain crystallinity and dimensional response to temperature.     

Thermal analysis of hydrogen bonded benzoic acid liquid crystals

Novel homologous series of supramolecular hydrogen bonded liquid crystals have been investigated. Hydrogen bonds are formed between p-n octyloxy benzoic acid and various p-n alkyloxy benzoic acids whose carbon chain length varied from pentyl to dodecyl. These complexes are characterized by Fourier transform infrared spectroscopy, polarizing optical microscopy (POM), and differential scanning calorimetry (DSC). Phase diagram is constructed from POM and DSC data. The order of the phase transitions is determined by Navard and Cox ratio (N _R). Characteristic phases like nematic, smectic C, and smectic F are identified. A new smectic ordering observed in this series is investigated by constructing phase diagram obtained from two binary mixtures of the present homologs. Inter-digitation of lamellar layers is observed to be one of the reasons for the occurrence of new smectic ordering. Optical tilt angle in smectic C phase is fitted to a power law. The magnitude of exponent of the power law is found to concur with the Mean Field theory predicted value.     

Mixing behavior of binary insoluble phospholipid monolayers. Analysis of the mixing properties of binary lecithin and cephalin systems by application of several surface and spreading techniques

The detailed miscibility analysis of binary phospholipid monolayers requires the application of a variety of spreading and surface techniques which often yield complementary results. Testing the equilibrium state of the binary monolayer by long-time experiments is also of great importance. Studies of the compression and spreading behavior of binary monomolecular systems form a basis for the determination of binary monomolecular phase diagrams. Within these plots different phase regions occur which permit clear statements regarding the miscibility state. Additional knowledge of the miscibility properties (phase diagrams) of the binary bulk systems is required. From the analogy of the properties of the bulk systems, the miscibility state of the monolayers is also determined by the temperature, and we can classify the monolayers of binary lecithin and cephalin systems into systems of complete miscibility, partial miscibility and complete immiscibility. In addition to the differences in the chemical structure of the mixing components, the film states in the monolayer and the miscibility behavior of the bulk systems are also influencing factors. If one of the components does not produce a spreading pressure, miscibility gaps occur in the phase diagram of the phospholipid monolayer. The miscibility gap, expressed by a constant spreading pressure, indicates complete immiscibility within this concentration range. If both components produce spreading pressures, and condensed and liquid-expanded film states within the considered temperature range, partial miscibility of the components becomes probable. The most effective parameter is then the difference in the chemical structure of the components. When both components produce spreading pressures and condensed films, the chemical structure of the mixing phospholipid compounds within their hydrophilic and hydrophobic parts is of essential importance. Depending on the differences in the chemical structures of their chains and their head groups in the case of binary phospholipid monolayers, the following possibilities result: complete miscibility, partial miscibility and complete immiscibility of the lecithins and the cephalins. Complete miscibility within the binary phospholipid monolayer takes place in the case of identical head-group structure and where there are only small differences in the chain length of the fatty acid groups. With increasing hydrocarbon chain length differences, partial miscibility or even complete immiscibility can occur within the monolayer. Chemical differences in the head-group structure of the mixing components have a similar influence. In the case of binary lecithin/cephalin mixtures, the differences in the head-group structure affect the miscibility behavior more than the chain length differences do in the case of lecithin/lecithin and cephalin/cephalin mixtures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Patterns of solid–fluid phase equilibria: new possibilities?

Thermodynamic phase space patterns of solid–liquid–vapor (slg) behavior in binary systems are reviewed. The van der Waals equation of state is used in combination with a common mathematical artifice for the solid–phase fugacity function to map out the slg loci for a model binary homologous series of solvent+solute mixtures as a function of the solute characterization. The computational results suggest the possible existence of a richer thermodynamic phase space topography than previously envisioned.     

Thermal Analysis of Binary Mixtures of Imidazolium,Pyridinium, Pyrrolidinium,and Piperidinium Ionic Liquids

Ionic liquids (ILs) are being widely studied due to their unique properties, which make them potential candidates for conventional solvents. To study whether binary mixtures of pure ionic liquids provide a viable alternative to pure ionic liquids for different applications, in this work, the thermal analysis and molar heat capacities of five equimolar binary mixtures of ionic liquids based on imidazolium, pyridinium, pyrrolidinium, and piperidinium cations with dicyanamide, trifluoromethanesulfonate, and bis(trifluoromethylsulfonyl)imide anions have been performed. Furthermore, two pure ionic liquids based on piperidinium cation have been thermally characterized and the heat capacity of one of them has been measured. The determination and evaluation of both the transition temperatures and the molar heat capacities was carried out using differential scanning calorimetry (DSC). It was observed that the thermal behavior of the mixtures was completely different than the thermal behavior of the pure ionic liquids present, while the molar heat capacities of the binary mixtures were very similar to the value of the average of molar heat capacities of the two pure ionic liquids.     

氢键缔合主链超分子液晶聚合物:间隔基及扩展介晶元长度的影响研究

采用慢挥发溶剂组装方法制备了一系列羧酸/吡啶氢键缔合的主链型超分子复合物,并采用FTIR,DSC以及偏光显微镜等对其相转变与热致液晶相行为进行了比较研究.研究表明,分别具有较短的6或10个亚甲基的烷烃间隔基的双苯甲酸衍生物4',4'-二羧酸-1,6-二酚氧基己烷(C6-2COOH)和4',4'-二羧酸-1,10-二酚氧基癸烷(C10-2COOH)的系列复合物具有较高的熔点和清亮点,一般都只出现结晶近晶相和多晶型转变现象.而具有柔顺性较好的四甘醇醚链间隔基的4,4'-二羧酸-α,ω-二酚氧基四甘醇醚(C8O4-2COOH)得到的系列复合物均在降温过程生成单致的流体近晶SA和/或向列N液晶相.可见,间隔基增长,相转变温度降低,最终导致真正的流体液晶相的产生.另一方面,对于从同一种二元羧酸得到的组装体系,从4,4'-联吡啶(4,4'-BPy)、4,4'-联吡啶乙烯撑(4,4'-BPyE)到对苯二酚二异烟酸酯(p-PhBPy),由于双键或酯基的引入,可变形性和极性增大,刚性依次减弱,尽管中心核部分持续长度增大,所得复合物的各向同性化温度降低,形成流体液晶相的趋势增强,液晶有序性降低,流动性增加.指出了早期文献报道的一些不一致甚至矛盾的结果.通过不同系列的对比研究,得出的一些规律性对氢键组装尤其对羧基/吡啶氢键缔合超分子体系设计与构筑具有一定指导意义.     

Nanoorganization and Phase Behavior of Mixtures of Nicotinic Acid with Dodecylbenzenesulfonic Acid

Abstract

Mesomorphically ordered structures and phase behavior of the mixtures of nicotinic acid (NICA) and dodecylbenzenesulfonic acid (DBSA) were investigated by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and polarized optical microscopy (POM). The POM observations revealed that the NICA–DBSA mixtures spontaneously formed liquid crystalline phases, although both NICA and DBSA were not liquid crystalline molecules. The NICA–DBSA mixtures formed ordered lamellar structures in DBSA‐rich mixtures and hexagonal cylinder structure in NICA‐rich mixtures. The mesomorphically ordered structures and optical anisotropy were caused by hierarchical interactions in the NICA–DBSA mixtures. The phase diagram divided into five regions—optically isotropic disordered phase, optically isotropic lamellar phase, optically anisotropic lamellar phase, optically anisotropic cylinder phase, and crystalline solid phase—is drawn by summarizing the XRD and POM results.     

Crystal nucleation in binary hard sphere mixtures: a Monte Carlo simulation study

We present calculations of the nucleation barrier during crystallization in binary hard sphere mixtures under moderate degrees of supercooling using Monte Carlo simulations in the isothermal-isobaric semigrand ensemble in conjunction with an umbrella sampling technique. We study both additive and negatively nonadditive binary hard sphere systems. The solid-fluid phase diagrams of such systems show a rich variety of behavior, ranging from simple spindle shapes to the appearance of azeotropes and eutectics to the appearance of substitutionally ordered solid phase compounds. We investigate the effect of these types of phase behavior upon the nucleation barrier and the structure of the critical nucleus. We find that the underlying phase diagram has a significant effect on the mechanism of crystal nucleation. Our calculations indicate that fractionation of the species upon crystallization increases the difficulty of crystallization of fluid mixtures and in the absence of fractionation (azeotropic conditions) the nucleation barrier is comparable to pure fluids. We also calculate the barrier to nucleation of a substitutionally ordered compound solid. In such systems, which also show solid-solid phase separation, we find that the phase that nucleates is the one whose equilibrium composition is closer to the composition of the fluid phase.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures V. Mixtures of bimolecular smectics and linear mesogens

Two binary mixtures, prepared from either 4-cyanophenyl 4-hexadecyloxybenzoate (Ib) or 4-nitrophenyl 4-hexadecyloxybenzoate (Ic), with 4-carboxyphenyl 4-hexadecyloxybenzoate (Ia), were thermally characterized to construct the phase diagrams for the two systems Ia/Ib and Ia/Ic. Due to differences in the smectic layering between the individual components of each system, phase separation took place either in the solid or in the mesophase. A mathematical relation was derived to calculate the composition of each phase in the solid state. Another two phase diagrams were constructed for binary mixtures of the linear dimer molecule, 4-hexadecyloxybenzoic acid (III) with either the acid Ia or its isomeric derivative, 4-hexadecyloxyphenyl 4-carboxybenzoate (II).     

Solid-fluid and solid-solid phase equilibrium in a model of n-alkane mixtures

Solid-fluid and solid-solid phase equilibrium for binary mixtures of hard sphere chains modeling n-hexane, n-heptane, and n-octane has been calculated using Monte Carlo computer simulations. Thermodynamic integration was used to calculate the Gibbs free energy and chemical potentials in the solid and fluid phases from pure component reference values. A multiple stage free energy perturbation method was used to calculate the composition derivative of the Gibbs free energy. Equation of state and free energy data for the fluid phase indicate ideal solution behavior. Nonideality is much more significant in the solid phase with only partial solubility of shorter chains in the longer chains and essentially no solubility at the other end of the composition range. The miscibility decreases with increasing chain length difference between the components. For the model of n-hexane/n-octane mixtures solid--solid phase separation has been observed directly in some of the simulations, with the components segregating between the layers of the solid structure. The behavior is similar to that seen in some binary n-alkane mixtures with longer chain lengths but comparable chain length ratios between the components. Such phase separation, although indicated thermodynamically, is not seen directly in the simulations of the n-heptane/n-octane mixture due to the difference in the pure component crystal structures.