Hydration regions

When neutralization is complete, the inner layer of intrinsic water assumes a cylindrical form along the length of the polyion with a diameter of 0-5-0-7 nm (Ikegami, 1964). The outer second cylindrical hydration region has a diameter of 0-9-1-3 nm (Figure 4.8). 

Figure 4.8 Cylindrical and spherical hydration regions around poly(acrylic acid) at various degrees of neutralization (or charge densities). Based on Ikegami (1964).

Counterions can affect the strocture of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. 

Large bound monovalent cations, e.g. tetrabutylammonium ions, are too large to penetrate any of the hydration regions. However, the smaller lithium, sodium and potassium ions are able to penetrate the outermost hydration region of the neutralized polyacid and this is accompanied by volume increases (Figure 4.9). These cations are probably not site-bound but are mobile in the outer cylindrical region of hydration (Figure 4.10). 

Even greater disruption is encountered in the case of trivalent cations (Figures 4.9,4.10). They completely penetrate both hydration regions and destroy the structure of water around the polyion. This amounts to complete desolvation. The same is true of bound hydrogen ions which are localized. 

Divalent and trivalent ions can precipitate PAA, and this phenomenon is related to the loss of a hydration region. Such precipitation is to be distinguished from salting-out effects which occur with high concentrations of monovalent ions. 

Although all divalent ions precipitate PAA when the degree of dissociation, a, approaches 10, there are differences when a = 0-25 (Figure 4.11). Small amounts of barium and calcium ions precipitate PAA at this low a value, whereas magnesium ions do not. These differences are not to be attributed to differences in the amounts of counterions bound, for condensation theory (Section 4.2.3) predicts that all divalent counterions are bound to polyanions to the same extent (Imai, 1961). Therefore, differences must arise from differences in solubility between the various polyacrylates. At low degrees of neutralization barium polyacrylate has low solubility, while magnesium polyacrylate is very soluble. This is related to the extent of disruption of hydration regions as cations are bound to polyions. 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

FIGURE 4.2 Pressure-temperature diagrams, (a) Methane + water or nitrogen + water system in the hydrate region, (b) Hydrocarbon + water systems with upper quadruple points, (c) Multicomponent natural gas + water systems, (d) Hydrocarbon + water systems with upper quadruple points and inhibitors. 

For systems with two quadruple points, the hydrate region is bounded by line I-H-V at conditions below Qi, line Lw-H-V between Qi and Q2, as well as line Lw-H-Lhc at conditions above Q2. Hydrates can form at lower temperatures and higher pressures to the left of the region enclosed by the three lines in Figure 4.2b to the right, no hydrates are possible. Upper quadruple point Q2 is often approximated as the maximum temperature of hydrate formation, because line Lw-H-Lhc is almost vertical however see data in Chapter 6 for exceptions. 

Yamane, K. Aya, I., Solubility of Carbon Dioxide in Hydrate Region at 30 MPa, in Proc. International Conference on Technology for Marine Environment Preservation, Tokyo, Sept 24-29, 2, 911 (1995). 

Drilling rate The drilling rate also decreases in the hydrate region relative to that in a fluid-saturated sediment, but not significantly different from that of ice... 

Core temperatures upon recovery on the catwalk were variable. Small areas of low temperatures (6-8°C versus other parts of the core at 11-13°C) were interpreted as indicating areas where endothermic hydrate decomposition decreased the core temperature. Cores evolved large amounts of gas, which was considered responsible for low core recovery—from a norm of > 80% to 20-60% in the hydrate region. 

Low chlorinity zones were coincident with zones of anomalously low recovered core temperatures on the ship catwalk. For example, while some of the background core temperatures were at 10-12°C, cores in suspected hydrate regions had temperatures as low as 1°C, perhaps caused by endothermic dissociation of hydrate. The extrapolated geothermal gradient of 33.5°C/km yielded a temperature of 18.3°C at the BSR (440 mbsf), well within the temperature stability field of methane hydrate. 

Hydrate was inferred by low temperature observations, interstitial-water low chloride values, and velocity and resistivity logs. Most of the indirect indicators were very similar to those in the earlier Sites 994 and 995. Increases in resistivity (by 0.2 2m) and acoustic velocity (by 0.2 km/s) were marked in the hydrate region. In some cases, the temperature (-2.1°C) was less than the ice point due to endothermic hydrate dissociation. 

At all three sites, the six indirect indicators were found as listed in the Site 997 discussion. The similarity of the indicators in the three sites is exemplified by the chlorinity anomalies in the hydrate regions of Figure 7.22b. There is a minimum of approximately 1.4 vol%, 1.7% and 2.1% gas hydrate at Sites 994, 995, and 997, respectively assuming a low chlorinity baseline, and a sediment porosity of 50%. The amount of gas hydrate appears to increase from the ridge flank (Site 994) to the ridge crest (Site 997) with various indicators shown in Table 7.11. 

The ocean cools the fluids as they flow, including both produced water (here assumed to be salt-free) and condensed water that is always salt-free. At about 9 miles the flowing hydrocarbons and water enter the hydrate region (to the left of the line marked hydrate formation curve ), remaining in the uninhibited hydrate envelope until mile 45. Such a distance may represent several days of residence time for the water phase (which flows slower than the hydrocarbon phases) so that hydrates would undoubtedly form, were no inhibition steps taken. 

Figure 8.1 Typical offshore flowline system with intrusion into hydrate region. (From Notz, P.K., in (First) International Conference on Natural Gas Hydrates, Ann. N.Y. Acad. Sci., 715, 425, 1994. With permission.)...

Figure 8.6 Dog Lake line burial, inlet heating, and insulation removes system from hydrate region. (From Todd, J.L., et al., Reliabilty Engineering—Gas Freezing and Hydrates, Texaco Company Hydrate Handbook (1996). With permission.)...

Figure 8.14 Temperature changes as a result of depressurization (1) isenthalpic rapid expansion as through a valve, and (2) very slow depressurization, as in a large-volume pipeline. Note that for the rightmost case, a fluid system can be expanded into the hydrate region, as calculated by the methods in Section 4.2.1.1 and the programs of CSMGem on the CD accompanying this book.

Setting in hydrocarbon environment Unaffected by CO2 Flash-sets by carbonation CBS is most useful in gas hydrate regions... 

Adding cenospheres and Styrofoam up to 10wt%, its thermal conductivity can be lowered to half that of conventional cement. When Ceramicrete-based permafrost sealant was cured in a CO2 environment, it set well, and when stored in CO2 for a week, it did not show any deterioration. Sugama and CarcieUo [8] predict that these sealants are durable up to 20 years in a downhole environment, compared to conventional cements that last for only a year. Unlike conventional cements, because CBS are neutral in pH and are not affected by downhole hydrocarbon gases, they are ideal for use in the gas hydrate regions in arctic climates. 

As noted above, normalization of the carboxylate ionization is likely the source of the reaction heat observed in region IV of the heat capacity isotherm. The carboxylate ionization process must contribute to the enthalpy isotherm in the low-hydration region. 

Amide hydrogen exchange in protein powders depends weakly on water activity, and its hydration dependence is complete within the low-hydration region (0.15 A). Apparently, the rate-determining step for the exchange of buried hydrogens is not much influenced by the protein surface. This is unexplained. 

Figure 2 The phase diagram of CH +H2O. Note that the non-stoichiometric hydrate region replaces the previously proposed vertical line for stoichiometric hydrate concentration...

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