Hydration enthalpy phase transition

This section is concludedby discussing another type of phase transition, known as solvation, which refers to the dissolution of a gaseous substance in a liquid solvent. We already met this concept when we were deriving the enthalpy of reaction 2.1 (see figure 2.1 and sections 2.4 and 2.5) the solution enthalpy of gaseous O2 in water, AS]n7/(2), can also be called the solvation enthalpy of O2 in water. Note that when the solvent is water, the word solvation is often replaced by hydration. 

Another result shown in Table 3.15 is the slight shift of the main phase transition towards lower temperatures. Similar results have been found for water ethanol dispersions of DPPC [445,453,454]. The strong influence of ethanol on the enthalpy of the main phase transition of DMPC water-ethanol dispersions shown in Table 3.15 is similar to the substantial increase in this enthalpy in the case of water-ethanol dispersions of DPPC [445,453]. Thus, a correspondence is found for the temperature of the chain-melting phase transitions in the cases of foam bilayers and the fully hydrated water-ethanol dispersions of DMPC. 

The lamellar gel-lamellar liquid-crystalline (L - L ) phase transition, frequently also referred to as (chain-)melting, order-disorder, solid-fluid, or main transition, is the major energetic event in the lipid bilayers and takes place with a large enthalpy change. It is associated with rotameric disordering of the hydrocarbon chains, increased headgroup hydration, and increased... 

Calorimetric studies of surface and nanoparticle energetics fall into several classes enthalpies of wetting and hydration/dehydration, heat capacity measurements, thermal analysis of coarsening and phase transition, and enthalpy differences by solution calorimetry. These methods measure different quantities, suffer from different potential difficulties, and are generally regarded as complementary. 

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

Pure solid + fluid phase equilibrium calculations are challenging but can, in principle, be modeled if the triple point of the pure solid and the enthalpy of fusion are known, the physical state of the solid does not change with temperature and pressure, and a chemical potential model (or equivalent), with known coefficients, for solid constituents is available. These conditions are rarely met even for simple mixtures and it is difficult to generalize multiphase behavior prediction results involving even well-defined solids. The presence of polymorphs, solid-solid transitions, and solid compounds provide additional modeling challenges, for example, ice, gas hydrates, and solid hydrocarbons all have multiple forms. 

The lower concentration limit of existence of some phases may be found by plotting the enthalpy per gram of sample against concentration and extrapolating to zero enthalpy. In n-decanephosphonic acid-water and n-dodecanephosphonic acid-water systems, the lower limit of existence of the waxy solid was determined by plotting the enthalpy associated with the hydrated crystals-waxy solid transition, and that of the low-temperature lamellar mesophase (La) by plotting the enthalpy associated with the waxy solid-L transition [54]. 

Modem and comprehensive investigations of physicochemical properties of citric acid solutions actrtally starts only in 1938 when Marshall published paper entitled A Phase Study of the System Citric Acid and Water [122]. For the first time systematic thermodynamic data were determined in the 10-70 °C temperature range. They included values of enthalpies of hydration and crystallization, determination of the citric acid monohydrate to anhydrous transition point and decomposition pressmes of the hydrate. Marshall measured also solubility of citric acid as a function of temperatirre, densities and vapoirr pressmes of water over satirrated solutions. After a long pause, only in 1955, we meet with an extremely important paper of Levien [123] A Physicochemical Study of Aqueous Citric Acid Solutions. It contains resrrlts of isopiestic measrrrements (activity and osmotic coefficients), the enthalpy of solution, electrical conductivities, densities, viscosities, partial... 

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