DSC study of phase transitions of cephalin pseudo-binary systems in excess water

The gel-liquid crystal phase transitions of the pseudo-binary systems of cephalins DMPE and DHPE in excess water were studied by differential scanning calorimetry. The phase diagram of the pseudo-binary systems has been given. The experiments showed that the partial phase separation in gel phase might occur at least at the mole fractions of DHPE below 0.1. The analysis by the model of ideal solution showed that both the cephalins were non-ideally miscible both in the gel phases and in the liquid crystal phases. The analysis by the model of regular solution showed that all the non-ideality parameters in the gel phases were larger than those in the liquid crystal phases at the same temperature. All the non-ideality parameters were not constant, but rather dependent on temperature.     

AC calorimetry at the first order phase transition point

A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C₂₀H₄₂). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.

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Zusammenfassung Es wird ein Modell für die AC-Kalorimetrie (ACC) von Phasenumwandlungen erster Ordnung vorgeschlagen. Das Modell wird mit den Ergebnissen von ACC in Nähe des Schmelzpunktes des n-Paraffins C20H42 verglichen. Die beobachtete Frequenzabhängigkeit von ACC stimmt mit dem Modell überein. Im Phasenumwandlungspunkt kann eine harmonische Komponente der Temperaturmodulation mit einer Frequenz festgestellt werden, die doppelt so hoch wie die Heizfrequenz ist. Es wird gezeigt, daß die harmonische Komponente anhand des vorgeschlagenen Modelles erklärt werden kann.

     

A new approach to the calorimetric investigation of physical and chemical transitions

The theoretical basis for Modulated DSC is described and the additional information in can give over conventional DSC illustrated for some polymers.     

Two calorimetric glass transitions do not necessarily indicate immiscibility: The case of PEO/PMMA

Differential scanning calorimetry has been used to examine blends of a poly(ethylene oxide) (PEO), M_n = 300 g/mol, and a poly(methylmethacrylate) (PMMA), M_n = 10,000 g/mol, across the complete composition range. The relatively low molar mass of the PEO minimizes interference from crystallization. In the midrange of composition, ～25–70% PEO, two broad, but distinct, glass transitions are resolved. These are interpreted as distinct glass transitions of the two components, as anticipated by the self‐concentration model of Lodge and McLeish. The composition dependence of the observed transitions is well described by the self‐concentration approach, using lengthscales of approximately two‐thirds of the Kuhn length. The results are compared with previous measurements on PEO/PMMA blends and other miscible systems. The principal, general conclusion is that one should actually expect two glass transitions in a miscible polymer blend or polymer solution; the rule of thumb that two transitions indicate immiscibility is incorrect. Furthermore, attempts to rationalize two transitions on the basis of incomplete segmental mixing, or other unspecified “nanoheterogeneity,” may not be justified in many cases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 756–763, 2006     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.

     

Equilibrium and non-equilibrium phase transitions studied by adiabatic calorimetry

An adiabatic calorimetry was used for some investigations of equilibrium and non-equilibrium phase transitions. For one of the substances studied (4,4′-di-n-heptyloxyazoxybenzene) it was possible to determine temperature dependence of an order parameter and number of clusters of high temperature phase in a region of a phase transition. For another substance (liquid 3,4 dimethylpiridine) an anomaly on the specific heat curves was interpreted as being responsible for a decay of molecules’ clusters. Non-equilibrium phase transitions were investigated for some liquid crystal substances. The process of transformation between metastable and stable phases was described quantitatively. The conclusions obtained concern the stability of metastable phases.     

Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Model for phase coexistence in phase transitions

We propose a general model for describing the phenomena of phase coexistence in relation to pressure induced phase transformations by means of the T–P distribution in statistical thermodynamics. Using the well‐known B1–B2 transition in NaCl as a prototype, we demonstrate how phase coexistence gives rise to the changes in the bulk modulus and the equation‐of‐state across the transition. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Thermal analysis of phase transitions in microemulsions

Differential scanning microcalorimetric measurements on phase transitions in water-oil-surfactant mixtures are presented, demonstrating that this method is highly sensitive towards small heat changes connected with structural transitions in the samples. The values for the latent heat of phase transitions are determined and the results are compared with predictions from mean field theory, emphasizing the role calorimetric experiments can play to identify the most important contributions to the free energy describing the mixtures. Doing this, the present status of the understanding of temperature dependent phase transitions in microemulsions is reviewed.     

Lithium intercalation with phase transitions in model systems of electrode materials for lithium power sources

A comparison was made for Li⁺ chemical diffusion coefficients (D _Li) in graphite as calculated by mathematical models of Li⁺ intercalation under constant potential into semirestricted and restricted kinetic systems with mobile phase boundary and into a single-phase system. Close D _Li values were calculated by means of double-phase models. The double-phase model produces 6–7-fold D _Li coefficient as compared to the values of the single-phase model.     

A differential scanning calorimetric study of phase transitions in polysubstitutedn-alkylammonium halides

A DSC investigation of severaln-alkylammonium salts is reported. The results of this investigation can be used to help decide which of several crystalline forms should be studied by X-ray diffraction for the best correlation between the liquid and solid states.

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Zusammenfassung Es wird über eine DSC-Untersuchung verschiedenern-Alkylammoniumsalze berichtet. Die Ergebnisse dieser Untersuchung können zur Entscheidung beitragen, welche von mehreren Kristallformen durch Röntgendiffraktion untersucht werden sollten um die beste Korrelation zwischen den flüssigen und festen Zuständen zu erzielen.

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Résumé On communique les résultats d'une étude par analyse calorimétrique différentielle (DSC) de divers sels den-alkylammonium. Les résultats de cette étude donnent des indications pour le choix des formes cristallines qui nécessitent d'être étudiées par diffraction des rayons X afin d'obtenir la meilleure corrélation entre les états liquides et solides.

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Visiting Professor of Chemistry, 1973–1974, Hebrew University, Jerusalem, Israel.

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This investigation was partially supported by the National Science Foundation. The hospitality of the Department of Inorganic and Analytical Chemistry of the Hebrew University is gratefully acknowledged. The authors thank Prof. I. Mayer and A. S. Kertes of the Hebrew University for suggesting this problem and for kindly furnishing samples of then-alkylammonium halides.     

Successive phase transitions of <Emphasis Type="Italic">p</Emphasis>-methylbenzyl alcohol crystal studied by X-ray and adiabatic calorimetry

Summary Crystal structures of the room-temperature (RT) and low-temperature (LT) phases of p-methylbenzyl alcohol were reexamined by single-crystal X-ray diffraction method while paying special attention to detect structural disorder in the RT phase involved in successive structural phase transitions at 179 and 210 K. In the RT phase at 250 K, positional disorder of oxygen atoms was detected in contrast to the previous structure report. The structure of the LT phase coincided to the previous one. Heat capacities were measured by adiabatic calorimetry below 350 K, which covers the structural phase transitions and fusion at 331.87 K. The structural phase transitions were of first-order and required long time for completion. The combined magnitude of entropies of transition was ca. 5 J K^-1 mol^-1, a part of which can be ascribed to the positional disorder observed in the structure analysis. Standard thermodynamic functions are tabulated below 350 K.     

Interface influenced phase transitions of alkylcyanobiphenyl liquid crystals A calorimetric study

Abstract

Using an AC-calorimetry technique, we report measurements of the heat capacity and the phase shift between the applied heating power and the resulting thermal oscillations on the thermotropic liquid crystal series of alkylcyanobiphenyl (nCB) above the tricritical point. Specifically, we studied the first order phase transition smectic A to isotropic as a function of substrate, under atmospheric pressure. Different combinations of sapphire, Kapton type 200H polyimide film and air, are used to vary systematically the interfaces encountered by a liquid crystal. The calorimeter uses a sapphire disk, 10 mm in diameter and 0·13 mm thick, above which the samples are placed. A second sapphire disk, or a disk-shaped Kapton film, are used to sandwich the liquid crystal. Air interface samples are droplets allowed to spread naturally on the chosen substrate. The chosen geometry is such that interface effects appear to be maximized. Striking features found in the heat capacity and the phase shift studies with 10CB and 12CB will be presented for the different interfaces.     

Photoinduced phase transitions and helix untwisting in the SmC* phase of a novel cinnamoyl-based liquid crystal

A photoinduced phase transition and helix untwisting in a new liquid crystal forming the SmC* phase were studied in detail. The compound consists of a cinnamoyl photosensitive fragment with C?=?C double bond capable of photoisomerisation and photocycloaddition. It was shown that ultraviolet (UV) irradiation (365 nm) induces an extreme decrease in phase transitions temperatures (SmC*–SmA*, SmA*–N*, N*–I). Vertically aligned samples in the SmC* phase cause selective light reflection in the visible spectral range. The light action results in a noticeable helix untwisting that causes a shift in the selective light reflection peak to the long-wavelength spectral region. The temperature dependence of spontaneous polarisation P _s was measured and it was found that UV irradiation induces a decrease in the values of P _s. Photo-optical phenomena taking place in the liquid crystal are attributed to the formation of photoproducts having low anisometry, which disrupts mesophases.     

DSC study of phase transition of anhydrous phospholipid DHPC and influence of water content

The phase transition behavior of 1,2-di-n-heptadecanoyl phosphatidylcholme (DHPC)with and without water has been studied by use of differential scanning calorimetry It was found by experiment that the glass transition occurred at first during the first heating of a sample of DHPC without water and then the sample underwent melting as an ordinary crystal.Therefore the sample of DHPC without water was a glassy crystal However,the DHPC sample crystallizing from melt was an ordinary crystal From the relationship between the total melting enthalpy Qf of freezable water and the water content h,it was concluded that the water contained in the DHPC samples might exist in three states recognizable thermodynamically.The water in the first state was an unfreezable water It was the water bound directly with the head groups of the phospholipid,i.e.the primary hydration water Every head group might bind seven such molecules of water.The water in the second state was the secondary hydration water,us melt ing point was     

Simple and accurate ac calorimeter for liquid crystals and solid samples

We have designed, built and calibrated a microcomputer-controller automated a.c. calorimeter for liquid crystals and solid samples, in the temperature range from 30 to 250°C, in which we enhance several features of previous ones reported in the literature. We have incorporate the lock-in amplifier model 5302 from EG&;G in our set-up, which permits to extend the frequency range to 1 mHz. This allows the performance of suitable frequency sweeps in order to choose the optimum operating frequency for low thermal conductivity materials. The calorimeter has been calibrated with the 8OCB liquid crystal compound. The resolution obtained forC _p values was better than 0.1% and the absolute erroer around 5%. As applications, we show the ferroelectric transition of the triglicerine sulphate (TGS), the Nematic-Smetic A transition of the 8OCB and a study in a binary system of ferroelectric liquid crystals, SCE9-SCE10.     

Temperature scanning X-ray diffraction at phase transitions of biologically related lipid assemblies

Summary Based upon the results of ac calorimetry and temperature scanning X-ray diffraction in the phospholipid/cholesterol system, the phase diagram was constructed by taking into account the ripple structure. From the analysis of the cholesterol concentration dependence of the modified ripple structure the cholesterol-rich state which lies in the higher cholesterol concentration than 20 mol% in the phase diagram is proposed. It is proposed that this is a fundamental complex that appears generally in lipid/cholesterol systems.     

A calorimetric study of spontaneous polymerization in acrylamide systems containing metal nitrates

A calorimetric study of the kinetics of spontaneous polymerization of acrylamide in systems containing Cr(III) and Er(III) nitrates has been carried out. At 20–25 °C, the process is characterized by a considerable induction period (20–40 min), a fast increase of the polymerization rate, up to its maximum value (5–10 min), and its subsequent slow decrease to insignificant values. It has been established that the composition of the reaction mixture significantly affects the recorded polymerization rates, and practically does not change the time parameters and the integral degrees of conversion of the process, which are 20 and 80 % for the nitrates of Cr(III) and Er(III), respectively. The effect of the addition of water or inhibitors of radical reactions on the process has been studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1070–1074, June, 1993.     

A differential scanning calorimetric study of phase transitions in Na2SO4

The results of a differential scanning calorimetric study of the transitions involving the various polymorphs of Na₂SO₄ are described. They are discussed in the light of their crystal structures and also in conjunction with more recently published data regarding these transitions.

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Zusammenfassung Die Ergebnisse einer mit Differentialabtastkalorimetrie durchgeführten Untersuchung der Übergänge verschiedener Polymorphe von Na₂SO₄ werden beschrieben. Sie werden im Hinblick ihrer Kristallstrukturen und im Zusammenhang mit verschiedenen unlängst veröffentlichten Daten bezüglich dieser Übergänge diskutiert.

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Résumé On décrit les résultats de l'étude, effectuée par analyse calorimétrique différentielle, des transitions mettant en jeu les diverses formes polymorphes de Na₂SO₄. Les résultats sont discutés en tenant compte des données récemment publiées sur ces transitions.

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Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) the use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) the peculiar properties of natural surfactants saponins, which form extremely viscoelastic surface layers; and (3) the main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to fully explore these opportunities.