Topic
Standard molar entropy
About: Standard molar entropy is a research topic. Over the lifetime, 1586 publications have been published within this topic receiving 29886 citations.
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TL;DR: A molecular picture of temperature-induced disorder in octadecylsilane stationary phases is proposed, with disorder originating at the distal carbon and propagating toward the proximal carbon.
Abstract: Raman spectroscopy is used to examine the effects of temperature, surface coverage, nature of the alkylsilane precursor (octadecyltrichlorosilane, methyloctadecyldichlorosilane, or dimethyloctadecylchlorosilane), and surface grafting method (surface or solution polymerized) on alkyl chain conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Conformational order is assessed using the intensity ratio of the antisymmetric and symmetric v(CH2) modes as well as the frequency at which these Raman bands are observed. Conformational order increases with surface coverage. Temperature-induced surface phase changes are observed between 258 and 343 K for this homologous series of stationary phases that are demonstrated to adhere to the Clapeyron equation for a simple first-order transition. Phase changes are discussed in terms of variation of the molar enthalpy, molar entropy, and molar volume of the stationary phase, all of which depend on surface coverage. For the limited range of systems investigated, a correlation between stationary-phase preparation (surface versus solution polymerized and nature of the silane precursor) and extent of alkyl chain order is not clearly observed.Instead, akyl chain order is largely dependent on bonding density. A molecular picture of temperature-induced disorder in octadecylsilane stationary phases is proposed, with disorder originating at the distal carbon and propagating toward the proximal carbon.
46 citations
TL;DR: The experimental results rule out a two-isoenzyme (with varying heat sensitivity) model and the effect of pH on the activity of the PPO, and the very slight differences between the values suggest the existence of only one site on the molecule for both substrates.
Abstract: The optimal temperature of palmito (Acanthophoenix rubra) polyphenol oxidase (PPO) is 30 degrees C. The Arrhenius activation energy was calculated to be 5.41 kJ mol(-1). Standard enthalpy of the reaction is -60.99 kJ mol(-1). At 25 degrees C, standard free energy and standard entropy were, respectively, 16.75 kJ mol(-1) and -260.87 J mol(-1) K(-1). The enzyme heated at temperatures above 30 degrees C loses its activity. Fifty percent inhibition is reached in 18 min at 70 degrees C, in 8 min at 75 degrees C, and in 2.5 min at 80 degrees C. The kinetics of the thermal irreversible denaturation of this enzyme is characterized by two steps: N leads to X(Td) leads to D, where N represents the native form, X represents an intermediate form, the structure of which depends on the deactivation temperature Td, and D is the completely denatured form of the enzyme. Our experimental results rule out a two-isoenzyme (with varying heat sensitivity) model. The thermodynamic parameters of the irreversible denaturation of the intermediate form were 102.70 and 97.10 kJ mol(-1) for activation enthalpy and activation energy, respectively, and 16.85 J mol(-1) K(-1) for activation entropy at 60 degrees C. Furthermore, this paper describes the effect of pH on the activity of the PPO. Studies were carried out with 4-methylcatechol and pyrogallol as substrates. The pH profile was not a function of the nature of the substrate assayed. The pH optimum was 5.2. The plot of logVmax app vs pH indicates that the oxidation of the substrates depended of the ionization of two groups in the enzyme-substrate complex with apparent pK values of 3.06 and 7.29 and 3.44 and 7.12, respectively, for 4-methylcatechol and pyrogallol. The very slight differences between the values suggest the existence of only one site on the molecule for both substrates.
46 citations
TL;DR: In this paper, the thermodynamics of mixed micellization of amitriptyline hydrochloride (AMT) with ionic liquid-type imidazolium gemini surfactant ([C10-4-C10im] Br2), was investigated at different mole fractions and temperatures by surface tension measurements.
Abstract: The thermodynamics of mixed micellization of amitriptyline hydrochloride (AMT) with ionic liquid-type imidazolium gemini surfactant ([C10-4-C10im] Br2), was investigated at different mole fractions and temperatures by surface tension measurements. The deviation of the critical micelle concentration (CMC) from the ideal critical micelle concentration (CMC*), micellar mole fraction (
$$X_{1}^{m}$$
) from ideal micellar mole fraction (
$$X_{1}^{\text{ideal}}$$
), the values of interaction parameter (
$$\beta$$
) and activity coefficients (
$$f_{i}$$
) (for both mixed micelles and mixed monolayer) explained the non-ideal behavior (i.e., synergistic behavior) of binary mixtures. The excess free energy (∆G
ex) for the AMT-[C10-4-C10im] Br2 binary mixtures explained the mixed micelles stability in comparison to micelles of [C10-4-C10im] Br2 and pure AMT. Interfacial parameters, i.e., Gibbs surface excess (
$$\varGamma_{\hbox{max} }$$
), minimum head group area at air/water interface (
$$A_{\hbox{min} }$$
), free energy of micellization (
$$\Delta G_{m}^{o}$$
), and standard Gibbs energy of adsorption (∆G
ads
) were also evaluated for the systems. The standard entropy of adsorption (∆S
ads
) was found higher than the standard entropy of micellization (∆S
) at all mole fractions of AMT (α
1).
46 citations
TL;DR: It is shown that heat capacity is strongly linearly correlated with formula volume for large sets of minerals, for ions in general, and for ionic liquids and that the results demonstrate that the Neumann-Kopp rule is widely valid for Ionic materials, but the smaller heat capacity contribution per unit volume for ionsic liquids is noted and discussed.
Abstract: Thermodynamic properties, such as standard entropy, among others, have been shown to correlate well with formula volume, Vm, thus permitting prediction of these properties on the basis of chemical formula and density alone, with no structural detail required. We have termed these procedures “volume-based thermodynamics” (VBT). We here extend these studies to ambient isobaric heat capacities, Cp,m, of a wide range of materials. We show that heat capacity is strongly linearly correlated with formula volume for large sets of minerals, for ionic solids in general, and for ionic liquids and that the results demonstrate that the Neumann–Kopp rule (additivity of heat capacity contributions per atom) is widely valid for ionic materials, but the smaller heat capacity contribution per unit volume for ionic liquids is noted and discussed. Using these correlations, it is possible to predict values of ambient (298 K) heat capacities quite simply. We also show that the heat capacity contribution of water molecules of c...
46 citations
TL;DR: In this paper, a self-consistent analysis of the liquid and vapor-phase equilibria based on a simple solution treatment of the Ga + P liquid is presented, showing that the deviations from ideality in the Ga-rich solutions results primarily from a small positive excess entropy of mixing.
46 citations