Multilayer water

The addition of dissociable solutes to water disrupts its normal tetrahedral structure. Many simple inorganic solutes do not possess hydrogen bond donors or acceptors and therefore can interact with water only by dipole interactions (e.g. Figure 7.5 for NaCl). Multilayer water exists in a structurally disrupted state while bulk-phase water has properties similar to... 

The moisture present in zone I (Figure 7.10) is the most tightly bound and represents the monolayer water bound to accessible, highly polar groups of the dry food. The boundary between zones I and II represents the monolayer moisture content of the food. The moisture in zone II consists of multilayer water in addition to the monolayer water, while the extra water added in zone III consists of the bulk-phase water. 

Kent and Meyer (1984) made broad-band dielectric measurements in the frequency range 0.3-16 GHz on various hydrated protein powders, including hemoglobin and bovine serum albumin. Comparison of relaxation spectra measured at 20-80°C suggested that at high hydration there is more than one state of multilayer water. 

Lioutas et al. (1986) measured the 0 and resonances of lysozyme powders and solutions, in experiments like those carried out for H by Fullerton et al. (1986). They similarly interpreted discontinuities in the NMR response in terms of three populations of water 20 mol of water per mol of protein (corresponding to 0.025 h) with a correlation time of 41 psec, 140 mol of water (0.17 h) with a correlation time 27 psec, and 1400 mol of water (1.7 h) with a correlation time 17 psec. The differences between these results and those of Fullerton et al. (1986) indicate the difficulty of estimating water correlation times. Lioutas et al. (1987) extended these results by analyzing H resonance data through comparison with the sorption isotherm. D Arcy-Watt analysis of the sorption isotherm gave 19 mol of tightly bound water per mol of lysozyme, 148 mol of weakly bound water, and 2000 mol of multilayer water. These classes plus two more types, corresponding to water in solutions... 

About 300 water molecules are sufficient to cover the lysozyme surface. This is a remarkably small amount of water. Calculations based on the crystal structure of lysozyme show that the surface is about 6000 A in area (Lee and Richards, 1971 Shrake and Rupley, 1973). Thus, each water covers, on average, about 20 A, which is twice the projection of a water molecule packed as in the liquid. Since 20 is the most area a water molecule can cover and maintain hydrogen bonding, there can be no multilayer water. Moreover, whatever arrangements there are at the surface must integrate simply into the bulk water namely, there is no B shell of disordered water, required to interface the water adjacent to the protein surface with the bulk solvent. 

Water at 4 X or greater distance is not in contact with protein atoms and represents multilayer water. We believe that multilayer water is not detected in thermodynamic measurements. Apparently water can be localized through hydrogen-bonding to other water at the protein surface and yet display thermodynamic properties indistinguishable from the bulk solvent (see concluding section below). 

Winkler (144-148) has studied the proton NMR relaxation time of water on y-Al203 and found that the correlation time = 1.2 x 10 second lies between that of water (149) (3.5 x 10 second) and ice (150) (2 X second). Winkler observed two regions of behavior, one in micropores with radii less than 1000 A, and another in macropores with larger radii. A rapid exchange occurred between the various layers of water molecules in micropores. Ebert (151,152) used dielectric constant measurements to distinguish between monolayer and multilayer water coverages. 

In some cases, both NMR and DSC techniques have been used to determine the amount of nonfreezable water. For example, pulsed NMR relaxation data for the hydrated copolymer poly(A-vinyl-2-pyrrolidone/methyl methacrylate) allow one to estimate the relative fractions of three distinguishably different types of water [147] (1) tightly bound (type B) at specific polymer sites, such as carbonyl groups (2) more loosely bound (type A) that is more moderately influenced by the polymer matrix, for example, multilayer water and water in interstices and, in samples with water content in excess of about 76 wt%, (3) bulk-like water that freezes at the vicinity of 273 K. Both type A and type B, which have nearly the same energy, are nonfreezable in the accepted sense of the term but undergo glasslike transitions at 170-200 K. NMR is sensitive to both type A and type B, whereas DSC is sensitive only to type A and correspondingly predicts a lower estimate for the amount of nonfreezable water [147]. 

Another proportion of bound water (3 2% of the total water content) exists at water activities ranging from 0.2 to 0.7. This water occupies the remaining first-layer sites and forms several layers around the monomolecular layer. In these layers mutual hydrogen bonds between water molecules already dominate, but there are also interactions between water molecules and ions or dipoles. Some water molecules penetrate into the capillary pores in the food structures by physical sorption. This water has the limited function of a solvent and the main proportion of this water does not freeze at 40 °C. The boundary between the first and second category water is the value of water activity of about 0.25. This water is known as multilayer water. 

Classification of water is also possible on the basis of thermodynamic properties. The binding enthalpy of water of the first category (vicinal water) is about —4 to —6kJ/mol, of the second category of water (multilayer water) approximately 1-3 kj/mol and of the third category of water (condensed water) around -0.3 kj/mol. 

The sorption isotherm of a hypothetical food with high water content is shown in Figure 7.41, while the same isotherm over a narrow range of water contents and indicating the existence of vicinal water, multilayer water and condensed water is shown in Figure 7.42. 

Figure 7.42 Detail of general sorption Isotherm w/d = grams of water per gram of dry matter, a = water activity, A = vicinal water, B = multilayer water, C = condensed water.

The importance of sorption isotherms is generally in the evaluation of the water content in food at which the adverse effects on the food quahty can be minimised. This is usually the moment when other layers are formed around the monomolecular layer of water (vicinal water), which is the instant when the multilayer water arises. Most adverse events in storage of foods with medium and low water content, such as crystallisation of amorphous sugars (e.g. lactose in powdered or condensed milk), agglomeration of powder materials, stickiness or re-crystallisation of water and formation of large crystals in frozen products (e.g. in ice cream) relates to water content, its activity and storage temperature, and are therefore... 

Hariri, H.H., Lehaf, A.M., Schlenoff, J.B. Mechanical properties of osmotically stressed polyelectrolyte complexes and multilayers water as a plasticizer. Macromolecules 45, 9364-9372 (2012). doi 10.1021/ma302055m... 

Two interrelated topics that bear most directly on the description of the hydration shell—i.e., the bound water layer(s)—are the definition of the shell and its thickness. The problem of how the bound water can be sufficiently precisely defined is discussed elsewhere [11,37,51] and we shall not pursue it further here. It is clear, however, that the extent to which water is affected by a nearby surface is a function of the distance between them, namely the thickness of the hydration shell. Second-layer water (and, obviously, multilayer water) is much less perturbed than the water adjacent to the surface. We have used several methods to evaluate the thickness of the interphasal water layer in system A (as revealed by the low-temperatme behavior of water) [2,11] and found it to be about 0.5 nm. Virtually the same value has been assessed for the thickness of the bound water layer on many organic and inorganic substrates [37,52-57]. As 0.5-0.6 nm is the thickness of two water molecules [45], we may envisage two monolayers of interphasal (or boimd) water that are loosely associated with the substrate. We have shown that Aw/eo = 3 for system A at a total water content of 30 wt%. 

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