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.     

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.     

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.     

On the temperature dependence of rate constants

The temperature dependence of rate constants to be expected from a simple general theory is compared with the usual Arrhenius from: k = α exp(−ϵ/k_BT. The meaning of the parameters α and ϵ is 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.     

On the temperature dependence of the activation enthalpy for sucrose hydrolysis

Recent interest in the measurement and the interpretation of heat capacities of activation has led to more rigorous methods to detect temperature-varient activation parameters. One such method devised by Blandamer and co-workers was applied to the sucrose hydrolysis data of Moelwyn-Hughes and Leininger and Kilpatrick. Both sets of investigators claimed that the energy of activation for sucrose inversion decreased with increasing temperature, while Heidt and Purvis and more recently, Kuhler's group contend that the energy of activation is constant. It was found that neither the data of Moelwyn-Hughes nor of Leininger and Kilpatrick warrant the inclusion of a temperature-dependent activation enthalpy.     

Estimation of aqueous solubility in drug design

The solubility of drugs in water is of central importance in the process of drug discovery and development from molecular design to pharmaceutical formulation and biopharmacy. The ability to estimate the aqueous solubility and other properties of a promising lead compound affecting its pharmacokinetics is a prerequisite to rational drug design, although it has received much less attention than the prediction of drug-receptor interactions. In this review, methods for the estimation of aqueous solubility of organic compounds are described and limited to approaches, which might be used in the early stage of drug design and development.     

Apparent solubility of hydroxyapatite in aqueous medium and its influence on the morphology of nanocrystallites with precipitation temperature

Two differing wet-chemical synthesis routes, Ca(OH)2 + H3PO4 and CaCl2 + Na3PO4/NaOH, were used to prepare hydroxyapatite (HA) at various temperatures ranging from 30 to 95 degrees C. The electrical conductivity of the solution was measured at regular intervals of time during H3PO4 and Na3PO4 addition to the suspension/solution containing Ca2+ ions. The rate of change of conductivity is used to note the end point of the reaction. X-ray diffraction of the dried, precipitated particles revealed HA as the predominant phase, and the FTIR spectroscopy studies indicated the presence of CO3(2-) groups which substituted PO4(3-) groups in the HA lattice (B-type). FESEM observations revealed that the aspect ratio of the particles decreased with increasing precipitation reaction temperature in one system [Ca(OH)2 + H3PO4] and in the other system it increased with increasing temperature. The changes in the morphology with temperature were analyzed through conductivity measurements and the thermochemical properties of the reaction systems. Conductivity measurements showed that the concentration of dissolved ions at the end point of the reaction between Ca(OH)2 and H3PO4 increased, indicating an increased apparent solubility of HA with increasing temperature, whereas the end-point conductivity did not increase noticeably in the other reaction system.     

Support vector machines for the estimation of aqueous solubility

Support Vector Machines (SVMs) are used to estimate aqueous solubility of organic compounds. A SVM equipped with a Tanimoto similarity kernel estimates solubility with accuracy comparable to results from other reported methods where the same data sets have been studied. Complete cross-validation on a diverse data set resulted in a root-mean-squared error = 0.62 and R(2) = 0.88. The data input to the machine is in the form of molecular fingerprints. No physical parameters are explicitly involved in calculations.