Quarter-salt formation defining the anomalous temperature dependence of the aqueous solubility of sodium monododecyl phosphate

The salts of monoalkyl phosphates (MAPs) have been identified as a class of inherently mild surfactants for use in household and personal products. They represent an anionic species intermediate in terms of pKa between the sulfates and the carboxylates and are analogous to the carboxylates in that they form acid-salts (which are here termed quarter-salts)-hydrogen-bonded dimers consisting of an undissociated MAP acid and an MAP monosalt. These complexes precipitate from solutions of the monosalt over a range of lower MAP concentrations giving rise to an unusual solubility/temperature relationship. The solubility of monosodium monododecyl phosphate (NaC(12)MAP) increases with temperature up to 0.01 M at approximately 60 degrees C, which corresponds to the conventional Krafft point as shown by the appearance of micelles in solution. The solubility then increases further to approximately 0.04 M as the solubility temperature declines from 60 to 38 degrees C. The transition between these two trends is characterized by a rather sharp temperature maximum in the solubility curve. In a third stage, the solubility then rises rapidly with very small change of temperature. This unusual overall behavior is shown to correspond with three distinct solid-phase compositions for the precipitates at temperatures below the solubility curve. At the lowest concentrations and up through the Krafft Point, the solid phase has been identified as the stoichiometric quarter-salt. Over the declining temperature portion of the solubility curve, the supernatant solution coexists with a macroscopic mixture of separate quarter-salt and monosalt solids. In the high-concentration third region the solid phase is exclusively the MAP monosalt. The coprecipitation of quarter-salt and monosalt from the monosalt solution occurs reversibly in the declining portion of the solubility curve and is accompanied by an increase in pH. The four phase system (solution, vapor, and two pure solid phases) retains one degree of freedom according to the phase rule since the system is in effect three component in that region.     

On the anomalous temperature dependence of ferroelectric Sc*. liquid crystals

A biaxial elastic model for the temperature dependence of S_c* helical pitch is proposed on the basis of a phenomenological theory of S_c* liquid crystals accompanied by a biaxial molecular ordering playing an important role for the anomalous behaviour of the helical pitch near the S_c*-S_A transition point. In the S_c*. phase of p-(n-decyloxybenzylidene)-p-amino-(2-methylbuthyl)-cinnamate (DOBAMBC), a qualitative agreement with the observed helical pitch is found by choosing a few material parameters concerned with a biaxial elasticity and an anisotropy of the elastic constants.     

Investigation of solubility of microcrystalline cellulose in aqueous NaOH

A cost‐effective and environmentally friendly method to dissolve microcrystalline cellulose (MCC) has been utilized. A detailed investigation of the effects of cellulose amount and solvent (aqueous NaOH) concentration on MCC solubility has been presented. In the experiments, NaOH solutions with concentrations ranging from 3.7 to 18.6 wt% have been employed to dissolve MCC with various weights. The results show that an optimal NaOH concentration range can always be found to give the best solubility of MCC having a certain weight. The solubility monotonically decreases with either the decreasing or increasing of NaOH concentration away from the optimal concentration range. In addition, the optimal concentration range of NaOH for dissolving cellulose has been shown to shrink as the amount of MCC increases. Copyright © 2005 John Wiley & Sons, Ltd.     

The anomalous temperature dependence of enzyme-catatlyzed reactions

Breaks or curvatures in the plot of log v_br vs. 1/T have been described in the literature for a number of reactions. If sufficiently small temperature intervals are used in the investigation of the temperature dependence (STI method) a trend line is obtained instead. It is then possible to distinguish regions of linear behavior and regions of anomalous behavior. The effects of e. g. the pH, the ionic strength, and added salts as well as of the size of the enzyme and substrate molecules on the anomalies were examined. The anomalous temperature dependence of the reaction rate was found to be a particularly sensitive indicator of the reaction event on the enzyme. The course of the reaction is evidently affected even by relatively small changes in conformation, particularly in the region of the active center.     

Metal oxide solubility behavior in high temperature aqueous solutions

The solubility behavior of metal oxides in sub- and super-critical aqueous solutions is quantified using thermodynamic concepts. Three physicochemical phenomena are discussed: (1) metal oxide solid phase stability; (2) metal oxide dissolution reaction equilibria; and (3) metal ion hydroxocomplex formation. Thermochemical properties of metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., elements having atomic numbers between 22 (Ti) and 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted at General Electric and elsewhere.Presented at the Second International Symposium on Chemistry in High Temperature Water, Provo, UT, August 1991.     

Novel enhanced applications of QSPR models: Temperature dependence of aqueous solubility

A model developed to predict aqueous solubility at different temperatures has been proposed based on quantitative structure–property relationships (QSPR) methodology. The prediction consists of two steps. The first one predicts the value of k parameter in the linear equation , where S_w is the value of solubility and T is the value of temperature. The second step uses Random Forest technique to create high‐efficiency QSPR model. The performance of the model is assessed using cross‐validation and external test set prediction. Predictive capacity of developed model is compared with COSMO‐RS approximation, which has quantum chemical and thermodynamic foundations. The comparison shows slightly better prediction ability for the QSPR model presented in this publication. © 2016 Wiley Periodicals, Inc.     

On the modeling of calcium sulfate solubility in aqueous solutions

The electrolyte–NRTL model used in a previous work (B. Messnaoui, T. Bounahmidi, Fluid Phase Equilibr. 237(1–2) (2005) 77–85), was extended for modeling of α-hemihydrate and gypsum solubilities in the complex system Ca²⁺–H⁺–SO₄²⁻–HSO₄⁻–H₂PO₄⁻–H₃PO₄–H₂O, at wide range of temperature and P₂O₅ concentration. The chemical equilibrium constant was evaluated as function of temperature according to the Gibbs–Helmholtz equation. The temperature dependence was taken into account in the expression of the standard state heat capacity of ionic, molecular and cristalline salts species. The standard-state heat capacity of Ca²⁺at 298.15 K is calculated to be 27.30 J mol⁻¹ K⁻¹. It is also shown that the experimental data agree with the predicted values of gypsum and anhydrite solubilities in water, at high temperature, by using only the values of parameters ${\tau }_{{\text{H}}_2\text{O}-(\text{C}{\text{a}}^{2+},\text{S}{{\text{O}}_4}^{2-})}$, ${\tau }_{(\text{C}{\text{a}}^{2+},\text{S}{{\text{O}}_4}^{2-})-{\text{H}}_2\text{O}}$ which were calculated, at 298.15 K from data of gypsum solubilities in phosphoric acid solutions.     

A theoretical study on anomalous temperature dependence of pKw of water

pH, with its well-known value of 7 at ambient condition, is a most basic property of water, with wide implications in chemistry and biology. The pH value is determined by the tendency of autoionization of water molecules into ion pairs, H(+) and OH(-), and is expected to vary extensively with the water condition, which determines the stability of the ion pairs. When temperature rises from the normal to the supercritical region, the pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for OH(-), is very difficult to reproduce. In the present study a molecular simulation is performed to understand this peculiar temperature dependence. The imbalance between the ion-water and the water-water molecular interaction strengths and the concomitant water density enhancement in the hydration shell, observed in the supercritical liquids, serve to put a subtle balance to produce this temperature dependence of the pH value. It is found that the large charge transfers from H(+) and OH(-) to the surrounding water molecules take place. In these transfers, not only water molecules in the neighboring hydration shell but also those in the outer hydration shell play a significant role. The coordination number of water molecules around OH(-) is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.     

The temperature dependence of the solubility of carbon dioxide in several extraction solvents

The solubility of CO₂ in the extraction solvents 1-methyl-2-pyrrolidone, 4-formylmorpholine, γ-butyrolactone, sulfolane and 1,4-dioxane have been measured over the temperature range 303. 15–333.15 K at a partial pressure 1 a1 atm. using the falling-film flow technique. For all systems, the solubility was found to decrease with increasing temperature. Agreement with solubilities predicted by the Redlich-Kwong equation of state for binary systems was within ±3.0%     

Polarized absorption and anomalous temperature dependence of fluorescence depolarization in cylindrical J-aggregates

We study chiral J-aggregates of the amphiphilic dye 1A that are spontaneously and asymmetrically generated from achiral dye monomers. These aggregates occur in two types. One type possesses a threefold split J-absorption band and forms micrometer-sized superhelices. The other type has a twofold split J-absorption band and forms smaller nanoparticles. We show that the analysis of optical experiments with polarized light in terms of an exciton model yields strong indications that the smaller aggregates have a cylindrical structure as well; the lower exciton band is polarized along the cylinder axis, while the higher band is polarized perpendicular to it. Our analysis allows for an estimate of the number of molecules around the circumference of the cylinder. Fluorescence–polarization excitation spectra at room temperature confirm the cylinder model. At low temperature, these spectra reveal a surprising loss of fluorescence polarization upon excitation in the higher exciton band. Possible explanations for this observation are discussed.     

Theoretical consideration of the anomalous temperature dependence of the surface tension of pure liquid gallium

The surface tension of pure liquid gallium in the temperature range 303–503 K (303 K is the melting point) was previously measured using the noninvasive method of capillary wave spectroscopy (CWS). The result of this experiment showed that the value of surface tension increases from 303 to 345 K indicating a negative surface excess entropy (S ^σ), and decreases linearly from ~345 to 503 K confirming a negative slope, and thus a positive S ^σ. This unusual behavior of Ga is not known for other liquid metals such as Bi, Pb, Hg, Sn and Al. The reported experimental behavior is modeled here. A theoretical equation for calculating the surface tension of liquid Ga, based upon formulating a proper partition function that includes the rotational part, is derived and described. The theory predicted no maximum in the temperature-dependence of the surface tension, as seen in the experiment, where the analysis was done over a large temperature range (325–503 K). The value obtained from this mathematical expression indicates that the temperature variation of surface tension has no positive slope within the temperature range 303–345 K. At T > 345 K, the surface tension shows the usual linear temperature-dependence with a negative slope. Therefore, the equation is only applicable for the latter temperature range. A comparison between the theoretical and experimental values of surface tension of liquid Ga is discussed.     

The temperature dependence of the micellisation of chlorpromazine hydrochloride in aqueous solution

The association characteristics of the weakly associating drug chlorpromazine hydrochloride have been examined over the temperature range 10–35 °C by means of conductimetric measurements. Critical micelle concentrations (cmc) have been determined by the application of a recently developed numerical method [Pérez-Rodríguez et al. (1998) Langmuir 14:4422] especially designed for the analysis of the association pattern in highly polydisperse systems of low aggregation number. The cmcs determined in this manner are used in combination with the mass-action model to obtain the thermodynamic parameters of the micellisation process, in particular the surface and hydrophobic contributions to the free energy. The use of exact forms of equations for the thermodynamics of micellisation applicable to systems of low aggregation number leads to values of the enthalpy of micellisation in reasonable agreement with experimentally determined values. Received: 6 January 2000/Accepted: 9 February 2000     

On the temperature dependence of vibrationally induced intensity

The temperature dependence of vibrationally induced transitions is investigated in the general case in which both BO and HT mechanisms are active and it is shown that this can give information about the relative strength of the two mechanisms.     

On the temperature dependence of non-radiative deactivation processes

It is shown that the fluorescence quantum yield depends on temperature rather than on the viscosity of the solvent. The intersystem crossing rate can be described as a sum of a temperature independent and a temperature dependent term. For most of the molecules investigated in this paper the latter can be assumed to have the Arrhenius form. With anthracene, however, significant deviations from the Arrhenius behaviour are observed. With a more general expression involving the density of states the temperature dependence of the fluorescence quantum yield can be satisfactorily described.     

The temperature dependence of the solubility of oxygen in sea water according to the pulsed NMR data

The temperature dependences of the solubility of oxygen in distilled and sea water were obtained over the temperature range 0–90°C on the basis of proton spin—lattice relaxation measurements in degassed and not degassed samples. The temperature dependence of O₂ solubility is approximated by the sum of exponential functions according to not only relaxation measurement results but also literature data. The temperature dependence of the solubility of O₂ in distilled water can be written as the sum of two exponential functions with the predominance of the first component over the low-temperature interval. The best approximation for sea water has the form of the sum of three exponential functions, the relative contributions of the second and third components being approximately equal. The conclusion was drawn from the temperature dependences obtained that the main mechanism of the solution of oxygen in water was the filling of ice-like formation voids.     

Characteristic property in aqueous solutions: effect of iceberg formation of water surrounding solute on the solubility(or cmc) and its peculiar temperature dependence.

The solubilities and the peculiar temperature dependence of hydrophobic substances in water were coherently explained in 1977 taking into account 1) the strong solidification tendency of water, 2) the large enthalpy of solution evident from the low solubilities, 3) the large entropy of dilution due to the small solubility, and 4) the large negative enthalpy and entropy of iceberg formation. As far as the author knows, there is no paper contradicting this interpretation. The present paper reviews the thermodynamic reasoning and experimental evidences confirming the interpretation. The phenomena are now much better understood due to new experimental evidences.     

Concentration and temperature dependence of surface parameters of some aqueous salt solutions

The effect of concentration and temperature on surface tension of aqueous solutions of four different salts has been studied. These include the chloride salts of lithium, sodium, potassium and ammonium. The study was conducted at five different concentrations i.e. 0.1, 0.5, 1.0, 1.5 and 2.0 M, and five different temperatures i.e. 10, 15, 20, 25 and 30 °C. By analyzing the results, the concentration and temperature dependence of surface excess concentration and thermodynamic parameters of surface formation such as enthalpy, entropy, and surface pressure have been calculated.