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Enzymatic degradation of cellulosic biomass is a well-established route for the sustainable production of biofuels, chemicals, and materials. A strategy employed by nature and industry to achieve an efficient degradation of cellulose is that cellobiohydrolases (or exocellulases), such as Cel7A, work synergistically with endoglucanases, such as Cel7B, to achieve the complete degradation of cellulose. However, a complete mechanistic understanding of this exo–endo synergy is still lacking. Here, we used single-molecule fluorescence microscopy to quantify the binding kinetics of Cel7A on cellulose when it is acting alone on the cellulose fibrils and in the presence of its synergy partner, the endoglucanase Cel7B. To this end, we used a fluorescently tagged Cel7A and studied its binding in the presence of the unlabeled Cel7B. This provided the single-molecule data necessary for the estimation of the rate constants of association kONand dissociation kOFFof Cel7A for the substrate. We show that the presence of Cel7B does not impact the dissociation rate constant, kOFF. But, the association rate of Cel7A decreases by a factor of 2 when Cel7B is present at a molar proportion of 10:1. This ratio has previously been shown to lead to synergy. This decrease in association rate is observed in a wide range of total enzyme concentrations, from sub nM to μM concentrations. This decrease in kONis consistent with the formation of cellulase clusters recently observed by others using atomic force microscopy.

Degradation of starch granules by a psychrophilic α-amylase, AHA, from the Antarctic bacterium Pseudoalteromonas haloplanktisTAB23 was facilitated by C-terminal fusion to a starch-binding domain (SBD) from either Aspergillus nigerglucoamylase (SBDGA) or Arabidopsis thalianaglucan, water dikinase 3 (SBDGWD3) via a decapeptide linker. Depending on the waxy, normal or high-amylose starch type and the botanical source, the AHA-SBD fusion enzymes showed up to 3 times higher activity than AHA wild-type. The SBD-fusion thus increased the density of enzyme attack-sites and binding-sites on the starch granules by up to 5- and 7-fold, respectively, as measured using an interfacial catalysis approach that combined conventional Michaelis–Menten kinetics, with the substrate in excess, and inverse kinetics, having enzyme in excess, with enzyme-starch granule adsorption isotherms. Higher substrate affinity of the SBDGAcompared to SBDGWD3was accompanied by the superior activity of AHA-SBDGAin agreement with the Sabatier principle of adsorption limited heterogenous catalysis.

In nature, enzymatic degradation of recalcitrant polysaccharides such as chitin and cellulose takes place by a synergistic interaction between glycoside hydrolases (GHs) and lytic polysaccharide monooxygenases (LPMOs). The two different families of carbohydrate-active enzymes use two different mechanisms when breaking glycosidic bonds between sugar moieties. GHs employ a hydrolytic activity and LPMOs are oxidative. Consequently, the topologies of the active sites differ dramatically. GHs have tunnels or clefts lined with a sheet of aromatic amino acid residues accommodating single polymer chains being threaded into the active site. LPMOs are adapted to bind to the flat crystalline surfaces of chitin and cellulose. It is believed that the LPMO oxidative mechanism provides new chain ends that the GHs can attach to and degrade, often in a processive manner. Indeed, there are many reports of synergies as well as rate enhancements when LPMOs are applied in concert with GHs. Still, these enhancements vary in magnitude with respect to the nature of the GH and the LPMO. Moreover, impediment of GH catalysis is also observed. In the present review, we discuss central works where the interplay between LPMOs and GHs has been studied and comment on future challenges to be addressed to fully use the potential of this interplay to improve enzymatic polysaccharide degradation.

Interfacial enzyme reactions are common in Nature and in industrial settings, including the enzymatic deconstruction of poly(ethylene terephthalate) (PET) waste. Kinetic descriptions of PET hydrolases are necessary for both comparative analyses, discussions of structure–function relations and rational optimization of technical processes. We investigated whether the Sabatier principle could be used for this purpose. Specifically, we compared the kinetics of two well-known PET hydrolases, leaf-branch compost cutinase (LCC) and a cutinase from the bacterium Thermobifida fusca(TfC), when adding different concentrations of the surfactant cetyltrimethylammonium bromide (CTAB). We found that CTAB consistently lowered the strength of enzyme–PET interactions, while its effect on enzymatic turnover was strongly biphasic. Thus, at gradually increasing CTAB concentrations, turnover was initially promoted and subsequently suppressed. This correlation with maximal turnover at an intermediate binding strength was in accordance with the Sabatier principle. One consequence of these results was that both enzymes had too strong intrinsic interaction with PET for optimal turnover, especially TfC, which showed a 20-fold improvement of kcatat the maximum. LCC on the other hand had an intrinsic substrate affinity closer to the Sabatier optimum, and the turnover rate was 5-fold improved at weakened substrate binding. Our results showed that the Sabatier principle may indeed rationalize enzymatic PET degradation and support process optimization. Finally, we suggest that future discovery efforts should consider enzymes with weakened substrate binding because strong adsorption seems to limit their catalytic performance.

Isothermal titration calorimetry (ITC) is a widely used method to determine binding affinities and thermodynamics in ligand–receptor interactions, but it also has the capability of providing detailed information on much more complex events. However, the lack of available methods to analyze ITC data is limiting the use of the technique in such multifaceted cases. Here, we present the software ANISPROU. Through a semi-empirical approach that allows for extraction of quantitative information from complex ITC data, ANISPROU solves an inverse problem where three parameters describing a set of predefined functions must be found. In analogy to strategies adopted in other scientific fields, such as geophysics, imaging, and many others, it employs an optimization algorithm which minimizes the difference between calculated and experimental data. In contrast to the existing methods, ANISPROU provides automated and objective analysis of ITC data on sodium dodecyl sulfate (SDS)-induced protein unfolding, and in addition, more information can be extracted from the data. Here, data series on SDS-mediated protein unfolding is analyzed, and binding isotherms and thermodynamic information on the unfolding events are extracted. The obtained binding isotherms as well as the enthalpy of different events are similar to those obtained using the existing manual methods, but our methodology ensures a more robust result, as the entire data set is used instead of single data points. We foresee that ANISPROU will be useful in other cases with complex enthalpograms, for example, in cases with coupled interactions in biomolecular, polymeric, and amphiphilic systems including cases where both structural changes and interactions occur simultaneously.

Alginates, serving as hydrocolloids in the food and pharma industries, form particles at pH < 4.5 with positively charged proteins, such as β-lactoglobulin (β-Lg). Alginates are linear anionic polysaccharides composed of 1,4-linked β-d-mannuronate (M) and α-l-guluronate (G) residues. The impact of M and G contents and pH is investigated to correlate with the formation and size of β-Lg alginate complexes under relevant ionic strength. It is concluded, using three alginates of M/G ratios 0.6, 1.1, and 1.8 and similar molecular mass, that β-Lg binding capacity is higher at pH 4.0 than at pH 2.65 and for high M content. By contrast, the largest particles are obtained at pH 2.65 and with high G content. At pH 4.0 and 2.65, the stoichiometry was 28–48 and 3–10 β-Lg molecules bound per alginate, respectively, increasing with higher M content. The findings will contribute to the design of formation of the desired alginate–protein particles in the acidic pH range.

Amar Annus, The Overturned Boat: Intertextuality of the Adapa Myth and Exorcist Literature (Helsinki: The Neo-Assyrian Text Corpus Project, 2016). XII + 148 pp. $59.95 (pbk). ISBN-13 978-952-10-9491-0.

Isothermal titration calorimetry (ITC) is a technique that allows us to accurately determine the thermodynamic parameters that characterize a binding interaction between two molecular systems. However, this technique has not had a wide application in petroleum science. This study is an attempt to determine the adhesion of different fluids onto a rock surface through ITC experiments. Two artificial brines with different ionic composition were titrated into chalk powder, and then crude oil was added to those systems in order to mimic the processes that take place in an oil reservoir. In addition, the wettability alteration process associated with smart water flooding was investigated from a thermodynamic point of view. The results from the ITC experiments suggest that the interaction between smart water and chalk + brine + oil systems is both exothermic and endothermic. The exothermic heat response indicates chemisorption of sulfate (SO42–) onto the mineral lattice, whereas the endothermic response proved the substitution of carboxylate complexes from the chalk surface by magnesium (Mg2+). The ITC results also show that the performance of diluted seawater seems to be higher than smart water with increased sulfate concentration. This is due to dynamic processes like brine dilution resulting in an increased osmotic pressure.

Cellobiohydrolases (CBHs) make up an important group of enzymes for both natural carbon cycling and industrial deconstruction of lignocellulosic biomass. The consecutive hydrolysis of one cellulose strand relies on an intricate pattern of enzyme–substrate interactions in the long, tunnel-shaped binding site of the CBH. In this work, we have investigated the initial complexation mode with cellulose of the most thoroughly studied CBH, Cel7A from Hypocrea jecorina(HjCel7A). We found that HjCel7A predominantly produces glucose when it initiates a processive run on insoluble microcrystalline cellulose, confirming the validity of an even and odd product ratio as an estimate of processivity. Moreover, the glucose released from cellulose was predominantly α-glucose. A link between the initial binding mode of the enzyme and the reducing end configuration was investigated by inhibition studies with the two anomers of cellobiose. A clear preference for β-cellobiose in product binding site +2 was observed for HjCel7A, but not the homologous endoglucanase, HjCe7B. Possible relationships between this anomeric preference in the product site and the prevalence of odd-numbered initial-cut products are discussed, and a correlation between processivity and anomer selectivity is proposed.

The interaction of the bile salts taurocholate, taurodeoxycholate, taurochenodeoxycholate, glycocholate, glycodeoxycholate, and glycochenodeoxycholate present in man, dog, and rat with α‐cyclodextrin and 2‐hydroxypropyl‐α‐cyclodextrin was investigated by mobility shift affinity capillary electrophoresis. The cyclodextrins are applied as excipients for solubilisation of drug substances with poor aqueous solubility. Accurate determination of stability constants is challenging for weak analyte–ligand interactions such as the conjugated bile salt α‐cyclodextrin interactions. A new approach for correction of medium effects due to the high additive concentrations in the background electrolyte was introduced. The use of prostaglandin A1as an interacting marker molecule offered a more satisfactory approach for correction than the commonly employed methods based on viscosity or current ratios. The interacting marker was chosen over a non‐interacting marker to avoid the difficult validation of the non‐interacting properties. The investigated bile salts all interacted with α‐cyclodextrin and 2‐hydroxypropyl‐α‐cyclodextrin. Stability constants ranging from 14 to 95 M−1were obtained with slightly higher affinities toward the substituted cyclodextrin. Molecular modelling demonstrated that the interaction between the two species involves the side chain of the bile salt. All together, these results indicate minor bile salt‐mediated displacement of substances from α‐cyclodextrin complexes in the small intestine.

β-Sheet proteins are particularly resistant to denaturation by sodium dodecyl sulfate (SDS). Here we compare unfolding of two β-sandwich proteins TNfn3 and TII27 in SDS. The two proteins show different surface electrostatic potential. Correspondingly, TII27 unfolds below the critical micelle concentration via the formation of hemimicelles on the protein surface, whereas TNfn3 only unfolds around the critical micelle concentration. Isothermal titration calorimetry confirms that unfolding of TII27 sets in at lower SDS concentrations, although the total number of bound SDS molecules is similar at the end of unfolding. In mixed micelles with the nonionic detergent dodecyl maltoside, where the concentration of monomeric SDS is insignificant, the behavior of the two proteins converges. TII27 unfolds more slowly than TNfn3 in SDS and follows a two-mode behavior. Additionally TNfn3 shows inhibition of SDS unfolding at intermediate SDS concentrations. Mutagenic analysis suggests that the overall unfolding mechanism is similar to that observed in denaturant for both proteins. Our data confirm the kinetic robustness of β-sheet proteins toward SDS. We suggest this is related to the inability of SDS to induce significant amounts of α-helix structure in these proteins as part of the denaturation process, forcing the protein to denature by global rather than local unfolding.

We use the thermodynamic behaviour of 1-propanol (1P) as a probe in ternary 1P–salt–H2O systems to elucidate the effect of a salt on the molecular organization of H2O. For salts, we have chosen CaCl2, NH4Cl, and (CH3)4NCl (TMAC). Having fixed the counter anion at Cl−, we compare here mainly the effects of chosen cations on H2O. Together with an earlier study on NaCl, we found that Ca2+, Na+, and NH4+are hydrated by a number of H2O molecules and leave the bulk H2O away from the hydration shell unperturbed. The hydration numbers were found to be 6.4 ± 1.6, and 1.2 ± 0.4, for Ca2+and NH4+, respectively with the hydration number 5.2 for Na+, the result of a simulation study, chosen as a reference. Thus, a salting out (also referred to as structure making, stabilizing, or kosmotropic) tendency would decrease in the order; Ca2+> Na+> NH4+. TMA+, on the other hand, showed a more hydrophilic characteristics than the probing 1P. Thus, TMA+is rather chaotropic (salting in, structure breaking or destabilizing) within this methodology.

The effect of temperature and calcium ions on the denaturation of a recombinant α-amylase from Bacillus halmapalus α-amylase (BHA) has been studied using calorimetry. It was found that thermal inactivation of BHA is irreversible and that calcium ions have a significant effect on stability. Thus an apparent denaturation temperature (Td) of 83 °C in the presence of excess calcium ions was observed, whereas Td decreased to 48 °C when calcium was removed. The difference in thermal stability with and without calcium ions has been used to develop an isothermal titration calorimetric (ITC) procedure that allows simultaneous determination of kinetic parameters and enthalpy changes of the denaturation of calcium-depleted BHA. An activation energy EA of 101 kJ/mol was found for the denaturation of calcium-depleted BHA. The results support a kinetic denaturation mechanism where the calcium-depleted amylase denatures irreversibly at low temperature and if calcium ions are in excess, the amylase denatures irreversibly at high temperatures. The two denaturation reactions are coupled with the calcium-binding equilibrium between calcium-bound and -depleted amylase. A combination of the kinetic denaturation results and calcium-binding constants, determined by isothermal titration calorimetry, has been used to estimate kinetic stability, expressed in terms of the half-life of BHA as a function of temperature and free-calcium-ion concentration. Thus it is estimated that the apparent EA can be increased to approx. 123 kJ/mol by increasing the free-calcium concentration.

Glycerol is accumulated in response to environmental stresses in a diverse range of organisms. Understanding of favorable in vivo effects of this solute requires insight into its interactions with biological macromolecules, and one access to this information is the quantification of so-called preferential interactions in glycerol-biopolymer solutions. For model membrane systems, preferential interactions have been discussed, but not directly measured. Hence, we have applied a new differential vapor pressure equipment to quantify the isoosmotic preferential binding parameter, Γμ1, for systems of unilamellar vesicles of DMPC in aqueous glycerol. It is found that Γμ1 decreases linearly with the glycerol concentration with a slope of −0.14±0.014 per molal. This implies that glycerol is preferentially excluded from the membrane-solvent interface. Calorimetric investigations of the same systems showed that the glycerol-DMPC interactions are weakly endothermic, and the temperature of the main phase transition increases slightly (0.16°C per molal) with the glycerol concentration. The results are discussed with respect to a molecular picture which takes into account both the partitioning of glycerol into the membrane and the preferential exclusion from the hydration layer, and it is concluded that the latter effect contributes about four times stronger than the former to the net interaction.

The excess chemical potential, partial molar enthalpy, and volume of 1-propanol were determined in ternary mixtures of 1-propanol–glycerol–H2O at 25°C. The mole fraction dependence of all these thermodynamic functions was used to elucidate the effect of glycerol on the molecular organization of H2O. The glycerol molecules do not exert a hydrophobic effect on H2O. Rather, the hydroxyl groups of glycerol, perhaps by forming clusters via its alkyl backbone with hydroxyl groups pointing outward, interact with H2O so as to reduce the characteristics of liquid H2O. The global hydrogen bond probability and, hence, the percolation nature of the hydrogen bond network is reduced. In addition, the degree of fluctuation inherent in liquid H2O is reduced by glycerol perhaps by participating in the hydrogen bond network via OH groups. At infinite dilution, the pair interaction coefficients in enthalpy were evaluated and these data suggest a possibility that the interaction is mediated through H2O.

Excess chemical potential, μ1PE, partial molar enthalpy, H1PEand volume of 1-propanol, V1PE, were determined as a function of mole fraction of 1-propanol, x1P, in mixed solvents of aqueous 2-propanol with various initial mole fraction of 2-propanol, x2P0. The 1-propanol–1-propanol interaction functions, H1P–1PE≡ N(∂H1PE/∂n1P), and V1P–1PE≡ N(∂V1PE/∂n1P), were evaluated by graphical differentiation. The x1P-dependence of all these quantities indicates that 1-propanol and 2-propanol modify the molecular organization of H2O in the same and additive manner in the water-rich region. The additive effect of 1-propanol and that of 2-propanol are in the ratio of (0.07/0.08).

The eutardigrade Adorybiotus (Richtersius) coronifer survives cryptobiosis for years. During entrance into anhydrobiosis this species accumulates the disaccharide trehalose reaching a maximum content of 2.3% d.w.

The molecular mechanisms underlying the adaptations to water loss developed in several tardigrade species remain poorly understood. It seems, however, that the binding of the disaccharide trehalose to membranes and other cellular components at low water contents is important for the tolerance to extreme drought. Trehalose is thus thought to replace interfacial- or “bound” water and enhance the conformational stability of labile macromolecules. To gain further insight into this we investigate here thermodynamic properties of water bound to the protein lysozyme at low water content (<100 water molecules pr. protein). It appears that this surface water has a higher enthalpy and higher entropy than the bulk liquid. These observations call for re-evaluation of the term “bound water” since “bound” carries the connotation of a low-energy, ordered (i.e. low-entropy) state.

We measured vapour pressures of aqueous acetonitrile (abbreviated as ACN) at 6, 20, and 37°C, from which excess chemical potentials of ACN (µ EACN) were calculated. We also determined excess partial molar enthalpies of ACN (HEACN) at 6, 20, 30, 37, and 45°C. From these data, excess partial molar entropies of ACN (SEACN) were calculated at 6, 20, and 37°C. Using density data by Benson's group, excess partial molar volumes of ACN (VEACN) were evaluated. The response function data by the same group were also used to evaluate amplitude and wavelength of mean-square fluctuations in terms of volume, entropy, and cross between volume and entropy. All the above quantities and their dependence on the mol fraction of solute, i.e., the effect of additional solute on the above quantities were used to study the effect of acetonitrile on the molecular organization of H2O. It was found that acetonitrile works as a stronger structure-making solute than methanol. Rather its effect on H2O is about the same as that of propan-1-ol.Key words: aqueous acetonitrile, interaction functions, fluctuations.

The association of ethanol with unilamellar dimyristoyl phosphatidylcholine (DMPC) liposomes of varying cholesterol content has been investigated by isothermal titration calorimetry over a wide temperature range (8–45°C). The calorimetric data show that the interaction of ethanol with the lipid membranes is endothermic and strongly dependent on the phase behavior of the mixed lipid bilayer, specifically whether the lipid bilayer is in the solid ordered (so), liquid disordered (ld), or liquid ordered (lo) phase. In the low concentration regime (<10mol%), cholesterol enhances the affinity of ethanol for the lipid bilayer compared to pure DMPC bilayers, whereas higher levels of cholesterol (>10mol%) reduce affinity of ethanol for the lipid bilayer. Moreover, the experimental data reveal that the affinity of ethanol for the DMPC bilayers containing small amounts of cholesterol is enhanced in the region around the main phase transition. The results suggest the existence of a close relationship between the physical structure of the lipid bilayer and the association of ethanol with the bilayer. In particular, the existence of dynamically coexisting domains of gel and fluid lipids in the transition temperature region may play an important role for association of ethanol with the lipid bilayers. Finally, the relation between cholesterol content and the affinity of ethanol for the lipid bilayer provides some support for the in vivo observation that cholesterol acts as a natural antagonist against alcohol intoxication.

We determined excess chemical potential, partial molar enthalpy, entropy and volume, heat capacity, isothermal compressibility and thermal expansivity for aqueous butan-1-ol in the water-rich region up to the phase separation boundary at 25°C. The latter three response functions were used to calculate the mean-square fluctuation densities, which signifies the amplitude, or the intensity, of fluctuations in volume, entropy or cross (entropy–volume) fluctuations. Furthermore, we calculated the (mean-square) normalized fluctuations that are indicative of the wavelength, or the extensity, as well as the amplitude of respective fluctuations. The behaviour of these thermodynamic quantities were compared with those obtained earlier in this laboratory for aqueous methanol, ethanol, propan-1-ol, tert-butanol (tert-butyl alcohol), and 2-butoxyethanol. We conclude that in the water-rich region of aqueous butan-1-ol, mixing scheme I is operative as in other alcohols, whereby butan-1-ol molecules enhance the hydrogen bond network of H₂O in their immediate vicinities with concomitant reduction of hydrogen bond probability in the bulk H₂O away from solutes. However, before reaching the phase separation boundary there was no signature indicative of the transition of mixing scheme observed in other aqueous alcohols. Thus, in aqueous butan-1-ol phase separation occurs directly from mixing scheme I, without going through mixing scheme II, which we argued earlier to be a preparation stage for phase separation for the other alcohols.

The vapor pressures pthe excess partial molar enthalpies of glycerol HGlyEthe densities dand the thermal expansivities αpof aqueous glycerol were measured at 25°C. From the vapor pressure data, the excess chemical potential of H2O µWEwas calculated, assuming that the partial pressure of glycerol pGlyis negligibly small. The excess chemical potential of glycerol µ GlyEwas estimated by applying the Gibbs–Duhem relation and these data were used to calculate the excess partial molar entropies SGlyE. From the density data, the excess partial molar volumes of glycerol VGlyEand from the thermal expansivity data, the normalized cross fluctuations SVΔ, introduced by us earlier, were evaluated. While the detailed manner in which glycerol modifies the molecular arrangement of H2O in its immediate vicinity is yet to be elucidated, the hydrogen bond probability in the bulk H2O away from solute molecules is reduced gradually as the glycerol composition increases to the point where putative presence of icelike patches is no longer possible. Thereupon, a qualitatively different mixing scheme seems to set in.

This paper describes ice formation in two freeze-tolerant eutardigrades Adorybiotus coronifer and Amphibolus nebulosus, both commonly found in Arctic areas, based on a study by differential scanning calorimetry, and results are compared with similar investigations on freeze-tolerant insects. Under Arctic conditions A. coronifer typically inhabits drought-resistant mosses and overwinters in a frozen or dry state. A. nebulosus inhabits moist mosses or algae and overwinters as frozen cysts or, in few cases, as eggs or adults. Both species show a moderate capacity for supercooling, with crystallization temperatures (Tc) of -6° to -7°C. No differences in Tc were observed between summer- and winter-acclimatized A. coronifer or between active and encysted A. nebulosus. Distinct differences in the heat stability of their ice-nucleating activity indicate that the compounds responsible for nucleation are chemically different in the two species. Ice formation progresses rapidly in both species, and crystallization of water probably ceases within 1 min of the nucleation. Ice constitutes 80–90% of total body water, even at temperatures just below Tc, in both species. Winter-acclimatized A. coronifer build up about 10% less ice than summer-acclimatized animals, and a similar reduction was induced by cold-acclimation of summer animals in the laboratory. No winter reduction of the melting point could be detected in A. coronifer, indicating little, if any, accumulation of low molecular weight cryoprotectives. We suggest that the reduction in ice content is functionally related to increased amounts of water kept unfrozen due to interactions with macromolecules.

Apparently contradictory statements about the thermodynamics of aqueous protein solutions and of hydrophobic effect are quoted and discussed. Some credibility is found in the divergent points of view and it is pointed out that they focus attention on different aspects of the complicated conditions in aqueous solutions, some of which are more important than others for the stability of protein conformations. The importance of characteristics of solvent water is emphasized, in particular (1) the strong mutual cohesion of water molecules, and (2) structural changes of water induced by (nonpolar) solute molecules. It is stressed that consideration of only one of these effects and an inexpedient choice of standard states are origins of confusion in the literature about aqueous systems. A simple approach to hydrophobic effects considering both of the above mentioned effects, is proposed.

The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)-H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H2O. The values of the excess partial molar Gibbs energies for both DMSO and H2O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H2O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H2O molecules from exposure to the nonpolar alkyl groups of DMSO.

Excess partial molar enthalpies, HAE, of methanol, ethanol, and 1- and 2-propanols in aqueous solutions were measured directly, accurately, and in small increments in mole fraction at 25 °C. From these data, the solute–solute enthalpic interactions, HAAE≡N(∂HAE/∂nA), were evaluated for each alcohol. These data indicate that three distinctively different mixing schemes, I, II, and III exist, as was the case for aqueous 2-butoxyethanol previously studied in our laboratory. The transition from mixing scheme I to II appears to take place gradually within a small composition range. As the hydrophobic moiety becomes smaller from 2-butoxyethanol to methanol, the locus of the transition moves to a higher value in mole fraction of the alcohol. At the same time, the range of transition becomes wider and the solute–solute enthalpic interaction weaker. Keywords: excess partial molar enthalpies in aqueous solutions, methanol, ethanol, 1-propanol, 2-propanol, enthalpic interaction, transition of mixing scheme.

Liquid–solid phase diagram was determined in the water-rich region of aqueous 2-butoxyethanol. A solid addition compound was suggested with the composition = 0.0260, which melts incongruently at 269.5 K. The mixing scheme boundary separating two regions in the single liquid phase domain (Chem. Phys. Lett., 217, 245 (1994)) was found to cross the incongruent melting point of the above solid addition compound.

The normalized fluctuations were reformulated in a unified manner, keeping the number of molecules in each coarse grain fixed. The normalized fluctuations were evaluated for aqueous 2-butoxyethanol using the existing data of κT, Cp, αp, and Vm. The partial molar fluctuations of 2-butoxyethanol were then calculated. These quantities added some more evidences to the mixing schemes in aqueous 2-butoxyethanol conjectured earlier in this laboratory.