Temperature dependence of the damping constant and the order parameter close to the lambda phase transitions in ammonium halides

This study gives our calculation for the damping constant, using the expressions derived for an Ising pseudospin-phonon coupled system in the ammonium halides (NH(4)Cl and NH(4)Br). For this calculation of the damping constant, we use the temperature dependence of the order parameter calculated from the molecular field theory. We predict here the damping constants for the v(5) (174 cm(-1)) and v(5) (177 cm(-1)) Raman modes of NH(4)Cl and NH(4)Br, respectively, below T(lambda) and compare them with our observed bandwidths measured as a function of temperature for those phonon modes at zero pressure. Our predictions agree well with the observed bandwidths below T(lambda) for those modes in both ammonium halide structures, in particular for NH(4)Br. Some discrepancy that occurs below T(lambda) for the v(5) (174 cm(-1)) mode of NH(4)Cl, is explained in terms of the model studied here.     

Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

Calculation of the brillouin frequencies close to phase transitions in NaNO2.

We calculate here the Brillouin frequencies of the L-mode [010], [001] and [100] of NaNO2 for the phase transitions from the paraelectric phase to the sinusoidal anti-ferroelectric phase near the Neel temperature (TN = 437.7 K) and to the ferroelectric phase near the critical temperature (TC = 436.3 K) in this crystalline system. For calculating the frequencies. we use the thermal expansivity data for the phase regions considered, under the assumption that the mode Gruneisen parameter determined for each mode remains constant across the phase transitions. Our calculated frequencies agree well with the observed frequencies for the modes studied in NaNO2.     

Exoelectron emission during polymorphic phase transitions of some ammonium salts

Results of experimental investigations of the photostimulated exoelectron emission (EEE) accompanying polymorphic phase changes in ammonium chloride, ammonium bromide and ammonium iodate are reported for the first time. The temperature-dependences of the intensity of photostimulated EEE from reagent grade powder materials were measured in air at atmospheric pressure, the exoelectrons being detected with an open air point counter with saturated ethanol quenching vapour. The DTA control measurements were performed at the same heating rate, with reagent grade Al₂O₃ as a reference. A comparison of the results of the EEE and DTA measurements shows clearly that the polymorphic phase transitions of the investigated materials are accompanied by peaks of the EEE intensity, thereby allowing the detection of solid-solid phase transitions in inorganic compounds with the EEE technique.

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Zusammenfassung In dieser Arbeit wird zum ersten Mal über die Untersuchung der photostimulierten Exoelektronenemission (EEE) berichtet, die die polymorphen Phasenübergänge von Ammoniumchlorid,-bromid und -jodat begleitet. Die Temperaturabhängigkeit der Intensität der photostimulierten EEE wurde an analysenreinen pulverisierten Proben bei Normaldruck in Luft untersucht, wobei die Detektion der Exoelektronen mit einem offenen Spitzenzähler und äthanol als Löschdampf diente. Als Bezug dienten DTA-Messungen mit analysenreinem Al₂O₃ als Referenz und mit der gleichen Aufheizgeschwindigkeit. Der Vergleich von EEE- und DTA-Ergebnissen zeigt, dass die polymorphen Phasenübergange der untersuchten Substanzen von einer verstärkten Exoelektronenemission begleitet sind. Dies ermöglicht die Anwendung der Exoemissionstechnik zur Detektion von Fest-FestPhasenumwandlungen anorganischer Verbindungen.

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Work sponsored by the Polish Ministry of Science and Education within the Central Project of Fundamental Research CPBP 01.08.A4.     

The thermodynamic properties of solutions and phase equilibria in the water-2-butanol-sodium chloride system

Fragments of the phase diagram of the H₂O-2-C₄H₉OH-NaCl system were studied experimentally at 298 and 313 K. The thermodynamic properties of sodium chloride in three-component solutions with ionic strengths up to 1.9 mol/kg and alcohol content in the solvent 4.97 and 10 wt % were measured at 298 and 323 K by the electromotive force method with ion-selective electrodes. The eNRTL (electrolyte Non-Random Two-Liquids) model parameters correctly describing the results of electrochemical measurements of the partial properties of NaCl and phase equilibria in the water-2-butanol-sodium chloride ternary system and binary subsystems constituting it were determined. The isothermal sections of the phase diagram of the H₂O-2-C₄H₉OH-NaCl system were calculated using the method of convex hulls implemented in the TernAPI package.     

Thermal characteristics of solid-solid phase transitions in long-chain dialkyl ammonium salts

A series of symmetrical dialkyl ammonium salts, DC_nX, has been prepared and characterized with respect to temperature and enthalpy of solid-solid phase transitions, temperature of melting, thermal stability as well as the reversibility of the phase transitions. The number of carbon atoms, C_n, was varied between 8 and 18 and as anions X halides, nitrate, chlorate, perchlorate and hydrogen sulphate had been chosen. In dependence on chain length and anion type transition temperatures from 20 to 100 °C were observed. Mass specific solid-solid transition enthalpies reach values of 185 J g⁻¹, which makes this class of substances attractive for heat storage applications. The influence of anion type on the transition enthalpies is explained in terms of packing requirements, hydrogen bond network formation and contributions from hindered anion rotation.     

Different types of phase transitions at charged interfaces: A thermodynamic analysis

A thermodynamic treatment of surface phase transitions due to orientational and structural effects is presented. It is shown that thermodynamics predict only two types of surface phase transitions: separation of the interface into two new phases and a kind of two-dimensional condensation. The first type takes place when both adsorbate and solvent molecules coexist at the interface. The new phases may be concentrated surface solutions of adsorbate in solvent and vice versa or pure adsorbate and a concentrated surface solution of adsorbate in solvent. In the last case, the adsorbate may be expelled in the form of a surface precipitate or micelles. The second type of surface phase transition occurs only at saturated interfaces with adsorbate molecules. Orientational and structural effects do not lead to independent transitions, as in bulk phases, but coexist in the two types of surface phase transitions. Rigorous relationships, which describe the transition region, are also developed and discussed.     

Bulk properties and hydration numbers of ammonium nitrate and ammonium chloride in solution: A structural and thermodynamic analysis

The apparent volumes of the salts in the systems H₂O-NH₄Cl (298 K) and H₂O-NH₄NO₃ (273 K, 298 K, and 323 K) are reproduced with an accuracy of 0.03–0.01 cm³/mol by the equation ? = ?⁰ + Aw ₂ ^0.5 + Bw ₂, where w ₂ is the salt content (mass fractions). The study shows that there is a correspondence between the critical (for determining the hydration number) structural parameters-the intrinsic volume of the electrolyte and the volume of water in ion hydration shells-and the limiting (at w ₂ = 1) partial molar volumes of the components. The hydration numbers at infinite dilution are 6.9 for NH₄Cl at 298 K and 9.1, 6.7, and 6.4 for NH₄NO₃ at 273 K, 298 K, and 323 K. The water volume in ion hydration shells decreases in the sequence: No ₃ ^? , Cl^?, and NH ₄ ⁺ . The hydration numbers decrease with increasing salt concentration. The study shows that within a simpler model ? = ?⁰ + aw ₂ ^0.5 , the hydration numbers are temperature independent.     

Raman scattering study of the low temperature phase transitions in ammonium nitrate

The phase changes in polycrystalline ammonium nitrate and its fully deuterated analogue have been studied by Raman scattering. The phase V and IV transition was found to be associated with the softening and intensity decrease of a NO₃^? librational mode. The Raman line-width data suggest that the VII to V phase transition involves a change in the degree of orientational order of the ions without incurring a change in lattice structure.     

Analysis of the Raman intensities near the phase transitions in ammonium halides

This study concentrates on the temperature dependence of the Raman intensities for the lattice modes in ammonium halides (NH(4)Cl and NH(4)Br) close to phase transitions. We predict their intensities using the results of a shell model for the Raman polarizability within the framework of an Ising pseudospin-phonon coupled model. From our observed Raman intensities of those phonon modes studied here, we extract the values of the critical exponent for the order parameter in these crystalline systems. The exponent values indicate that the Raman intensities show a logarithmic divergence at higher pressures in NH(4)Cl, whereas they predict a lambda-type phase transition at zero pressure in NH(4)Br.     

Temperature dependence of the Raman bandwidths for the lattice and internal modes in ammonium halides close to the lambda-phase transitions

We study here the temperature dependence of our Raman bandwidths for the upsilon(5) lattice modes of NH(4)Cl (T(lambda)=241 K) and NH(4)Br (T(lambda)=235 K) and for the upsilon(2) internal mode of NH(4)Cl close to the lambda-phase transitions. Our Raman bandwidths of those modes are studied using a power-law formula on the basis of the soft mode-hard mode coupled model. From our analysis, we extract the values of the critical exponent beta for the order parameter of the ferro-ordered (NH(4)Cl) and the antiferro-ordered (NH(4)Br) phases. Our beta values are close to zero which exhibits a logarithmic behaviour in the vicinity of the lambda-phase transitions in NH(4)Cl and NH(4)Br. It is indicated here that the ammonium halides (NH(4)Cl and NH(4)Br) undergo a weakly first order or nearly second order phase transition close to the lambda transition point.     

月桂胺盐酸晶体相变的红外光谱研究

在290-365K的温度区间内考察了月桂胺盐酸盐晶体红外光谱随温度的变化规律。结果表明:月桂胺盐酸盐在339K发生了固-固结构相变, 该相变的预相变起始点为327K。在327K以下的低温相中, 晶体中分子的碳氢链以高度有序的全反式构象存在, 极性头部的三个N-H-Cl氢键是不等价的。在327-339K的中间过渡相, 碳氢链中出现了旁式构象, 分子链间相互作用减弱。不等价的N-H-Cl氢键的差异减小。在339K以上的高温相中, 分子旁式链构象的增多导致了分子链横向堆积无序性的明显增加, 三个N-H-Cl氢键已变得等价。     

Effects of dioctadecyl dimethyl ammonium chloride on the rheological behavior of behenyl trimethyl ammonium chloride/1-hexadecanol/water ternary system

The effects of dioctadecyl dimethyl ammonium chloride (DODAC) on the rheological properties of ternary systems consisting of behenyl trimethyl ammonium chloride (C22TAC), 1-hexadecanol (C16OH), and water are studied to improve the long-term stability and to establish the preparation method of cosmetic products. The basic ternary systems behave as solids at low stresses, due to the formation of lamella liquid crystals called alpha gel. The additions of DODAC to ternary systems cause the rupture of alpha gel structures and transformation to vesicles. The structures of molecular assemblies are confirmed through particle size distribution, differential scanning calorimetry, and freeze fracture electron microscopy. The vesicle structures formed in coexistence of single-chain surfactant and double-chain surfactant are highly stable. Because the quaternary systems are considered to be constructed by the mixtures of alpha gel and vesicles at the appropriate concentrations of DODAC, the rheology can be controlled by the structural balance between them.     

Comparative thermodynamic studies of aqueous glutaric acid, ammonium sulfate and sodium chloride aerosol at high humidity

Aerosol optical tweezers are used to simultaneously characterize and compare the hygroscopic properties of two aerosol droplets, one containing inorganic and organic solutes and the second, referred to as the control droplet, containing a single inorganic salt. The inorganic solute is either sodium chloride or ammonium sulfate and the organic component is glutaric acid. The time variation in the size of each droplet (3-7 microm in radius) is recorded with 1 s time resolution and with nanometre accuracy. The size of the control droplet is used to estimate the relative humidity with an accuracy of better than +/-0.09%. Thus, the Kohler curve of the multicomponent inorganic/organic droplet, which characterizes the variation in equilibrium droplet size with relative humidity, can be determined directly. The measurements presented here focus on high relative humidities, above 97%, in the limit of dilute solutes. The experimental data are compared with theoretical treatments that, while ignoring the interactions between the inorganic and organic components, are based upon accurate representations of the activity-concentration relationships of aqueous solutions of the individual salts. The organic component is treated by a parametrized fit to experimental data or by the UNIFAC model and the water activity of the equilibrium solution droplet is calculated using the approach suggested by Clegg, Seinfeld and Brimblecombe or the Zdanovskii-Stokes-Robinson approximation. It is shown that such an experimental strategy, comparing directly droplets of different composition, enables highly accurate measurements of the hygroscopic properties, allowing the theoretical treatments to be rigorously tested. Typical deviations of the experimental measurements from theoretical predictions are shown to be around 1% in equilibrium size, comparable to the variation between the theoretical frameworks considered.     

Polymerization of tetraallyl ammonium chloride

The radical polymerization of tetraallyl ammonium chloride (TAAC) was carried out in water using azo-initiator as compared to that of diallyl dimethyl ammonium chloride (DADMAC); the rate of polymerization was quite low for TAAC, around one-third of DADMAC. Kinetic discussion revealed the importance of degradative chain transfer in the polymerization of TAAC. The cyclopolymerizability of TAAC was estimated kinetically as the ability of 5-membered monocyclic radical to form a bicyclic ring, giving the cyclization constant of 21 mol/L at [M] = 2 mol/L. Gelation occurred at around 20% conversion.     

Spectroscopic intensities as measures of order parameter close to order-disorder transitions

The infrared and Raman intensities can be related to the order parameter close to order-disorder phase transitions in crystal systems. In the present study this relationship has been obtained and compared with our experimental results for ammonium halides.Our predictions give satisfactory agreement with the observations in these crystals.It is proposed that the FTIR techniques can be appropriately used to evaluating the order parameter in various crystal systems.     

Phases and phase transitions in a nematic lyotropic system with a biaxial phase

NMR spectroscopy and optical microscopy have been used to study phase transitions and structure in nematic lyotropic mesophases formed by potassium laurate, decylammonium hydrochloride and water. The different mesophases obtained in a well-defined composition range have been characterized, by deuterium NMR, following the evolution of the D₂O spectra as a function of temperature and of the orientation of the samples with respect to the magnetic field. Wide ranges of biaxiality have been found and the asymmetry parameter of the averaged electrical field gradient tensor on the deuterium of the D₂O molecule has been determined. The presence of the different mesophases has always been confirmed by observing oriented samples under the polarizing optical microscope.     

Chlorination of uranium metal to uranium trichloride using ammonium chloride

Journal of Radioanalytical and Nuclear Chemistry - Given that the most feasible option for fabricating chloride-based molten salt fuel for molten-salt reactors (MSRs) is to use uranium/transuranic...