Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, β C , was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. β C has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization. JAOCS 74, 1213-1220 (1997).KEY WORDS: Binary mixture, differential scanning calorimetry, phase behavior, polymorphism, synchrotron radiation X-ray diffraction, triacylglycerol.Triacylglycerols (TAG) are employed in oils, margarine, cream, chocolate, and other edible fats products, and the fat structures are the major factors to determine their physical properties, such as texture, plasticity, morphology, and so on (1-4). Polymorphism is of the highest importance in the physical properties of TAG. Polymorphism of TAG has been elucidated at a molecular level for pure systems (5-7). In the practical systems, however, the fats are present in multicomponent states consisting of different types of TAG species. The first step in approaching the multicomponent systems is to study the binary systems of major TAG components, which reveal the nature of molecular interactions between the component materials at macroscopic and microscopic dimensions.Various types of phase diagrams of the binary mixtures of TAG were reviewed by Small (8), Knoester et al. (9), Timms (10), and Rossell (11). As a typical mixture system, tristearin-tripalmitin (SSS-PPP) mixtures have been studied by many researchers (11-13), where metastable α and β′ forms exhibited continuous solid solutions, yet eutectic nature was revealed in the most stable β form. The formation of the molecular compound was suggested in the mixture of sn-1,3-dipalmitoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-palmitoylglycerol (POP-OPO) (P, palmitoyl; O, oleoyl) by Moran (14) who indicated two binary systems, in juxtaposition, of POP/compound and compound/OPO. In this mixture, the compound was estimated to form at an equal concentration ratio. Most recently, Engstrom (15), Koyano et al. (16), and Minato et al. (17) reported precise analyses of the mixture systems of sn-1,3-distearoyl-2-oleoylglycerol and 1,2-distearoyl-3-oleoylglycerol (SOS/SSO) (S, stearoyl), sn-1,3-distearoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-stearoylglycerol (SOS/OSO), and sn-1,3-dipalmitoyl-2-oleoylglycerol and 1,2-dipalmitoyl-3-oleoylglycerol (POP/PPO), r...
Structural characterization of interfaces composed of soft materials (soft interfaces) helps in understanding their physical behavior. Neutron reflectometry is one of the most powerful tools to characterize interfacial structures with spatial resolution in nanometers. We have installed a novel horizontal-type time-of-flight neutron reflectometer SOFIA (SOFt Interface Analyzer) at the Japan Proton Accelerator Research Complex. The instrument is capable of accepting two downward neutron beams, at 2.21 and 5.71 to horizontal, which dedicate neutron reflectivity (NR) measurements over a wide range of neutron momentum transfer q (q ¼ (4p/k)sinh, where k and h are wavelength and incident angle, respectively). The accuracy of NR up to q ¼ 6 nm À1 was confirmed by measuring deuterated polystyrene (d-PS) thin films on a silicon (Si) wafer and multilayers of cadmium stearate prepared by the Langmuir-Blodgett method. NR at the deuterium oxide (D 2 O)/Si disk showed specular reflection down to 10 À6-10 À7 and q up to 2.0 nm À1 along the perpendicular to the sample surface, which improved the precise analysis of swollen polyelectrolyte brush structure at the D 2 O interface. Then, time-resolved in-situ NR measurements were carried out at 1-min intervals to observe interfacial mixing of d-PS on the PS brush surface during 398 K annealing, demonstrating that nonequilibrium behavior at the interfaces can be analyzed on the order of minutes.
Phase separation of giant vesicles composed of neutral saturated lipid, negatively charged unsaturated lipid, and cholesterol, is observed at different calcium concentrations. Confocal microscopy provides the information where the phase separation becomes distinct as the calcium concentration is increased. The negatively charged lipid domains tend to bud toward the interior of the vesicle. This budding is assumed to be due to an increase in the osmotic pressure, in cooperation with the spontaneous curvature change in the outer leaflet of the bilayer caused by the adsorption of calcium ions and charge screening effect. We interpret the effect of small cations on the phase separation based on the theoretical model with the Poisson-Boltzmann equation.
Polymorphic crystallization of SOS (sn-1,3-distearoyl-2-oleoyl glycerol) has been studied with a time-resolved synchrotron radiation X-ray diffraction method (SR-XRD) using pure samples. SOS has five polymorphs: α, γ, β‘, β2, and β1. Emphasis was placed on analyzing the events of melt-mediated transformation, in which the crystallization of more stable forms (γ, β‘, and β2) was induced by rapid melting of the less stable forms. This mode of crystallization is closely related to a long running dispute concerning a “memory effect” possibly occurring in the polymorphic crystallization of triacylglycerols from liquid. Two types of thermal processes were applied: (a) a rapid heating soon after the least stable α form was crystallized from liquid and (b) a rapid heating after thermal annealing of the α form for 20 min before its melting. The SR-XRD measurements were monitored at a time interval of 10 s. The main results are as follows: (1) the formation of lamellar ordering of SOS occurred more rapidly than that of subcell packing, as exhibited in the earlier occurrence of SR-XRD long spacing spectra in comparison to the short spacing spectra, (2) the α-melt mediation without thermal annealing unveiled the formation of two types of liquid crystalline structures having long spacing values of 5.1 and 4.6 nm, (3) the γ form directly occurred in the α-melt mediation involving the thermal annealing, which generated γ-embryos in the solid phase of α to serve as seed materials, and (4) the formation of the second most stable β2 form of SOS was obtained after quite long incubation of the β‘-melt mediation, yet the most stable β1 did not crystallize. The present study has given deeper insights of the polymorphism of SOS in comparison to previous studies with optical, calorimetric, and conventional XRD techniques: in particular the presence of the liquid crystalline form and the formation of γ-embryos were directly observed for the first time.
The phase behavior of thesn‐1,3‐dipalmitoyl‐2‐oleoylglycerol (PPP‐POP) binary mixture system was studied by powder X‐ray diffraction with synchrotron radiation and by differential scanning calorimetry. The results showed that the immiscible phases were observed in metastable and in the most stable forms. In particular, synchrotron X‐ray diffraction enabled us to reveal the monotectic nature of α as a kinetic phase behavior. The equilibrium phase diagram of the PPP‐POP mixture is divided into two regions. In POP concentration ratios below 40%, solid‐state transformation from α to β was observed, indicating that the α‐β transition of PPP was promoted in the presence of POP. By contrast, the polymorphic transition proceeds from α to β through the occurrence of the intermediate β′ form at POP concentration ratios above 50%.
Phase transition is observed between one-phase disordered phase and an ordered phase with multilamellar (onion) structures in an off-critical mixture of D2O and 3-methylpyridine (3MP) containing a salt at 85mM. The salt consists of hydrophilic cations and hydrophobic anions, which interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by D2O. The onion phase is realized only for small volume fractions of 3MP (in D2O-rich solvent).PACS numbers: 82.45. Gj, 61.20.Qg, 64.75.Cd, 81.16.Dn Binary mixtures of water and organic solvent have been used extensively to study universal aspects of critical behavior and phase separation dynamics. However, not enough attention has yet been paid to unique ion effects in such mixtures, where preferential hydration around each ion should affect the composition fluctuations [1,2]. Salts composed of small hydrophilic ions can drastically change the phase behavior even at small concentrations [3,4,5,6,7]. More strikingly, many groups have observed long-lived heterogeneities (sometimes extending over a few micrometers) in one-phase state [8,9] and a third phase visible as a thin plate formed at a liquid-liquid interface in two-phase state [10]. These observations were reproduced in different experiments using solvents and salts of high purity, so they should be regarded as ioninduced supramolecular aggregates. We also comment on a mixture of water+isobutylic acid (IA), where IA partly dissociates into H + and Butyrate − ions. There, the mobility of H + ions much decreases at large IA contents in one-phase state [11] and the third phase also appears around an interface in two-phase state [10]. Similar phenomena have often been observed in various soft matters such as polymers, gels, colloids, and mixtures containing surfactants. Though not well understood, the solvation (ion-dipole) interaction among ions and polar molecules should play a major role in these phenomena together with the Coulombic interaction among charges. [1,2] Recently, the solvation effect on phase behavior was * Electronic address: hideki.seto@kek.jp theoretically studied in polar mixtures including the case of antagonistic ion pairs composed of hydrophilic and hydrophobic ions [12]. Such cations and anions interact differently with the composition fluctuations, leading to a charge-density-wave phase for sufficiently large solvation asymmetry even at small salt concentrations. In a smallangle neutron scattering (SANS) experiment, Sadakane et al. [13] found a peak at an intermediate wave number Q m (∼ 0.1Å −1 ) in a near-critical mixture of D 2 O and 3-methylpyridine (3MP) containing sodium tetraphenylborate NaBPh 4 at 100 mM. The volume fraction of 3MP, denoted by φ 3MP , was chosen to be 0.3...
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