Negative Excess Free Energies Research Articles

Interaction of promethazine hydrochloride with TX-165 in aqueous, NaCl and urea media: a tensiometry and FTIR analysis

An experiment using surface tension was conducted to investigate the interactions of promethazine hydrochloride (PMT), with TX-165, at the interface in different ratios and solvents (water/50 mmol/kg NaCl, and 500 mmol/kg urea) at 298.15 K. A thorough FTIR study of the employed pure and mixed system was also conducted in the aqueous solution. PMT is a medication employed to manage and cure allergic conditions. Usually, NaCl media improved the surface properties of singular and mixed components over aqueous media, resulting in synergistic/attractive interactions among ingredients. TX-165’s composition at mixed monolayer ( ), surface excess concentration (Γ max), pC 20, activity coefficient, and so on were evaluated. The achieved activity coefficient values are attained below one in each case, indicating the attractive interactions amongst ingredients. A negative Gibbs energy of adsorption was obtained all through, indicating spontaneity in the process. A negative excess free energy ( ) was found at the mixed interface of PMT + TX-165, showing a higher surface stability of mixed monolayer than pure component monolayer. FTIR spectroscopy is used to study the spectrum of single and mixed amphiphiles to validate the interaction between PMT and TX-165. This study suggested that TX-165 surfactant might behave as an able antidepressant drug delivery agent.

Micelle formation and physico-chemical variables for the sodium dodecyl sulfate/cetylpyridinium chloride and their mixture in aqueous propranolol hydrochloride drug solvent: Conductivity and theoretical analysis

Herein, study of both single and mixed micelle development of an anionic surfactant sodium dodecyl sulfate (SDS) and a cationic surfactant cetylpyridinium chloride (CPC) in propranolol hydrochloride (PPH) drug medium has been performed using a conductometric technique at different temperatures and compositions. The critical micelle concentration (CMC) along with degree of ionization (g) were assessed from the specific conductivity (κ) versus [surfactant] plots. Within the concentration of surfactant studied, a single CMC was achieved in the situations of pure and mixed surfactant micellization. The CMC and g values have been noticed reliant on the employed drug concentrations, variations of mole fractions of the mixture of surfactants and experimental temperature. For the micellization of single SDS/ CPC surfactants in aq. PPH drug medium, PPH drug causes delayed micelle development which has been detected as the upsurge of CMC values of SDS/CPC in PPH drug media. For the association of SDS/ CPC surfactants in aq. PPH drug medium, the CMC values experienced a lessening with the growth of study temperature. At a fixed temperature, the experimental CMC values, of the SDS + CPC mixture, endure a decline through the upsurge of mole fractions of CPC (α1), attain a minimum value and the CMC values tend to enhance at and above α1= 16.2 × 10-5. The ideal critical micelle concentrations (CMCid) are detected always as higher values than experimental CMC up to the α1= 10.8 × 10-5. The -β values and |β| > |ln(CMC1/ CMC2)| for the present study reveal that the interaction among the amphiphiles under analysis is synergistic. The negative free energy (-ΔGmO) of micellization were obtained throughout the study. The enthalpy (ΔHmO) and entropy (ΔSmO) of micellization reveal the presence of hydrophobic interaction as the principal type of interaction among the studied constituents. The negative excess free energy (ΔGex) values inferred that stable mixed micelles are formed for the mixture of SDS + CPC in aq. PPH drug medium. The micellar mole fractions (both experimental and ideal values), activity coefficients of components in mixed micelles as well as enthalpy–entropy compensation parameters were also measured and illustrated. It is possible that the knowledge gained from the current analysis may be used to enhance pharmaceutical formulations, drug delivery carriers, along with drug act proficiency.

Impact of ceftriaxone sodium on tetradecyltrimethylammonium bromide & cetyltrimethylammonium bromide mixture: Conductivity and theoretical investigation

The formation of mixed micelle of tetradecyltrimethylammonium bromide (TTAB) & cetyltrimethylammonium bromide (CTAB) mixture has been carried out in appearance of ceftriaxone sodium trihydrate (CTS) drug having different concentrations over a temperature range of 298.15 to 323.15 K, keeping a break of 5 K. The CMC of pure and mixture of surfactants were observed to be reliant on the concentration of drug. The CMC values of TTAB + CTAB mixture lie in between that of pure TTAB & CTAB. The larger deviation of CMC of TTAB & CTAB mixture from the corresponding ideal CMC (CMCid) at higher mole fraction (α1) of CTAB signify stronger interaction amid TTAB and CTAB at higher bulk α1 of CTAB. The negative excess free energy (ΔGexRub) values reveal the stable form of micelle and the stability was attained to be enhanced by the rise of α1 of CTAB. The interaction parameter (β) also reveals the attractive interaction amongst the studied species of surfactants. The micellar mole fraction (X1Rub), as well as its ideal (X1id) values, experience an augmentation by the rise of α1, which reveal that the contribution of CTAB increase with the raise of α1. The Gibbs free energy of micellization (ΔGm0) reveals a spontaneous association of both pure and mixed systems. The enthalpy (ΔHm0) and entropy (ΔSm0) of micellization reveal that the micellization is exothermic in nature, while the recommended forces are hydrophobic interaction, along with weak electrostatic interactions. The values of intrinsic enthalpy also reveal the micelle’s stability, thereby, formed. Degree of dissociation (g), compensation temperature, etc., were also reported.

The Assembly of the Mixture of Two Ionic Surfactants in Polyols Media at Variable Temperatures: Combined Conductivity and Theoretical Investigations

AbstractHerein, the aggregation phenomena of the two cationic amphiphiles, the hexadecyltrimethylammonium bromide (CTAB), and tetradecyltrimethylammonium bromide (TTAB), has been investigated in the presence of monosaccharide (glucose) and disaccharide (maltose) at varying compositions and temperatures. The critical micelle concentration (cmc), degree of ionization (g), and several thermodynamic properties have been estimated applying the conductivity measurement technique. To explore the consequence of additives on the mixed micellization of TTAB + CTAB, glucose as monosaccharide having two concentrations (1% & 5% (w/w) glucose) and maltose as disaccharide containing single concentration (1% (w/w)) have been employed. Together with the mixture of TTAB and CTAB, investigation of the physicochemical parameters of pure amphiphiles in glucose and maltose solutions was quantified. The efficiency of the additives in depressing the cmc values followed the order: 1 (w/w) % maltose >5 (w/w) % glucose >1% (w/w) glucose. The ideal cmc (cmcid), activity coefficients, and β values suggested the synergistic attraction (attractive force) between these two surfactants. By utilizing the Rubingh model, the evaluated micellar mole fractions of CTAB (α1) coupled with the ideal mole fraction () imply the increase of the contribution of CTAB in the mixed micellization with rising the α1 values. The values of the negative excess free energy () implied the stability of the mixed micelle formed. The changes in standard enthalpy (ΔH°m) and changes in standard entropy (ΔS°m) of micellization reveal the presence of different modes of interactions like electrostatic, ion‐dipole, H‐bonding, and hydrophobic interactions among the studied components.

Activity Coefficients of Dimethyl-β-cyclodextrin in Aqueous Solutions

Abstract The activity coefficients of aqueous solutions of dimethyl-β-cyclodextrin were measured by vapor pressure osmometry at 25, 35, and 45±0.1°C. The activity coefficient decreased with increase of concentration at each temperature. Both the excess enthalpy and the excess entropy were positive. The negative excess free energy originates from the large excess entropy term, indicating that dimethyl-β-cyclodextrin is a hydrophobic solute.

Interactions in mixed monolayers between dioleoyl-l-phosphatidylcholine and 11-cis retinal. A comparison with all-trans retinal

The surface pressure isotherms of 11-cis retinal, diolcoyl-l-phosphatidylcholine and seven mixtures of these components covering the whole range of molar fractions have been studied at the nitrogen-water interface at 20.5°C. For all the mixtures, an almost constant collapse pressure at 44 ± 1 mN m−1 is observed. The surface pressure isotherms also show an apparent collapse pressure at 14, 16, 20, 24, and 28 mN m−1 at a molar fraction of dioleoyl-l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, and 0.600, respectively. Below this apparent collapse pressure, it is shown that the components are completely miscible at the interface. However, this pressure represents the onset of the rejection of 11-cis retinal from the monolayer, this rejection appearing to be total at the true collapse pressure. The miscibility region of the isotherms is studied in terms of the additivity rule. Negative deviations to ideality are observed and negative excess free energies of mixing are calculated for all the systems at four different surface pressures. A comparison between 11-cis retinal and all-trans retinal mixtures with dioleoyl-l-phosphatidylcholine shows that the two isomers have about the same thermodynamics of interaction with this phospholipid.

Thermodynamic functions of biopolymer hydration. II. Enthalpy–entropy compensation in hydrophilic hydration processes

AbstractThe thermodynamic functions of biopolymer hydration were investigated by multitemperature vapor pressure studies. Desorption measurements were performed that allowed determination of reversible isotherms in the hydration range of 0.1 to 0.3–0.5 g H2O/g dry polymer. These isotherms are accessible to thermodynamic interpretation and are relevant to the interaction of water with biopolymers in their solution conformation. The results obtained on a series of different biopolymers (lysozyme, α‐chymotrypsin, apo‐lactoferrin, and desoxyribonucleic acid), show the following common features of interest: (1) The differential excess enthalpies (ΔHe) and entropies (ΔSe) are negative, and exhibit pronounced anomalies in a well‐defined low‐humidity range (approx. 0.1 g H2O/g dry polymer). These initial extrema are interpretable by structural changes, induced in the native biopolymer structures by water removal below a critical degree of hydration. (2) The ΔHe and ΔSe terms exhibit statistically significant linear enthalpy–entropy compensation effects in all biopolymer–water systems investigated. The compensation temperatures \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta = \overline {\Delta H} ^e /\overline {\Delta S} ^e $\end{document} are approximately identical for all biopolymers, ranging from 360 to 500 K. The compensation effects are attributable to phase transitions of water molecules between the bulk liquid and the inner‐sphere hydration shell of native biopolymers. (3) The negative excess free energies (ΔGe) decrease monotonically with increasing water content and are close to zero at 0.3 to 0.5 g H2O/g polymer. This result indicates that only transitions between the bulk liquid and the inner‐sphere hydration shell are associated with significant net free energy effects. The outer‐sphere hydration water is thermodynamically comparable to bulk water. The importance of the proportionality factor \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta $\end{document} in the control of the free energy term is discussed.

Interactions in mixed monolayers between dioleoyl- l-phosphatidylcholine and all- trans retinal

The surface pressure isotherms of all- trans retinal and dioleoyl- l-phosphatidylcholine at the nitrogen-water interface were studied at 20.5°C. The extrapolated limiting areas per molecule are 49 and 83 Å 2 molecule −1, respectively. The properties of seven mixtures of all- trans retinal and dioleoyl- l-phosphatidylcholine, covering the whole range of molar fractions were also measured. The results show that an almost constant collapse pressure is observed at 44 ± 1 mN m −1 for all the mixtures studied. Furthermore, an apparent collapse pressure is also observed at 16, 18, 22, 26, 32, and 35 mN m −1 at a molar fraction of dioleoyl- l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, 0.600, and 0.800, respectively. An analysis of the results in terms of the surface phase rule shows that below the apparent collapse pressure, the components are completely miscible. Negative deviations from ideality were observed in the region of miscibility for all the systems, as shown by the negative deviations to the additivity rule and the negative excess free energies of mixing calculated for all the systems at 5, 7, 10, and 13 mN m −1. Furthermore, it is shown that above the apparent collapse pressure, all-trans retinal is rejected from the monolayer, the rejection being total at the collapse pressure.

Evidence for structural changes in garnet caused by calcium substitution

Recent work on the pyrope-grossular and almandine grossular solid solution series has shown that at concentrations of Ca up to 15mol.% the garnets show negative excess volumes and negative excess free energies of mixing. It has been suggested that this results from a change in structure caused by Ca ordering. This note describes a preliminary X-ray study of such a garnet and concludes that there is a structural change, possibly to space group I 213, caused by Ca substitution and that further work must be done to see if this is caused by ordering.

Surface viscosities and pressure-area isotherms of mixed monolayers of hexadecoxy ethanol with docosanoxy propanol and docosanoxy ethanol with hexadecoxy propanol at 25°C

Surface pressure (π)-area ( A) and surface viscosity (μ)-π isotherms on water substrate of mixed monolayers of hexadecoxy ethanol (C 16H 33OC 2H 4OH) with docosanoxy propanol (C 22H 45OC 3H 6OH) and docosanoxy ethanol (C 22H 45OC 2H 4OH) with hexadecoxy propanol (C 16H 33OC 3H 6OH) were studied as a function of mole fraction at 25°C. The positive and negative deviations observed from additivity rule for molecular areas and fluidities (φ) for these systems and high negative excess free energies of mixing (Δ G E) suggested that the systems are nonideal and miscible. The data are indicative of intermolecular interaction between the two components. At high film pressures and at 1:1 molar ratio, the mixed monolayers exhibited higher stability which has been observed from area-composition curves and Δ G E values. μ 0, μ ∞, τ, Δ F, Δ F-Δ F 1, and w i-k (0.94 r i ) have been calculated for all the mixed monolayers studied. Nearly equal values of Δ F and w i-k (0.94 r i ) suggest that similar relaxation phenomena are involved in both pure and mixed monolayers. Δ F of mixed monolayers is found to be a weighted average of the pure constituents.

High temperature thermodynamic properties of solid ta-w alloys

The partial molar and integral molar thermodynamic properties of solid Ta-W alloys have been determined between 1050 and 1300 K using galvanic cells with a thoria-based electrolyte. The activities in this system exhibit negative deviations from ideality, but the magnitude of the negative deviation is less than that observed previously for the Ta-Mo alloys. The negative excess free energies of mixing are due mainly to the enthalpies of mixing. The observed small negative excess entropies of mixing can be attributed to vibrational contributions and/or short range ordering in these alloys.

Statistical thermodynamics of mixtures of molecules of different sizes

The central problem in the theory of mixtures is the calculation of the free energy of mixing of molecules of different sizes. An explicit calculation of this free energy is made for a mixture in which all intermolecular potentials are of the form uαα(r)=(σαα/r)n, where σαα is a distance characteristic of the interaction of two molecules of species α, and where σαβ= 1/2(σαα+σββ) when α≠β. This result follows from a solution of Percus-Yevick integral equation for the pair distribution function of a mixture of hard spheres. The form of the free energy provides a criterion by which existing theories can be judged, and it is shown that an approximation of the type originally suggested by van der Waals is superior to approximations based on the concept of random mixing. Molecules that differ only in size mix with a small and negative excess free-energy. The recommended approximation is confirmed by comparison with experiment.

Dissociation pressures of GaAs, GaP and InP and the nature of III–V melts

Direct measurements of the dissociation pressures of GaAs, GaP and InP have been made as a function of temperature by the quartz Bourdon gauge technique. The pressures at the stoichiometric melting points were found to be: for GaAs, 1·00 ± 0·02 atm. at 1238°C; for GaP, 35 ± 10 atm. at 1467°C ± 3°C; and for InP, 21± 5 atm. at 1058°C ± 3°C. These data together with the phase diagram data for the III—V compounds indicate that the melts of these compounds are neither ideal nor regular solutions but show negative excess free energies which are dependent on composition and/or temperature. This result is interpreted as indicative of association of the group V species in these melts.