Equilibrium, apparent water, vapour

Chemically bound water is most reasonably defined as including that present in interlayer spaces, or more firmly bound, but not that present in pores larger than interlayer spaces. As will be seen in Chapter 8, the distinction between interlayer space and micropores is not sharp water adsorbed on surfaces of pores further blurs the definition. From the experimental standpoint, the determination is complicated by the fact that the amount of water retained at a given RH depends on the previous drying history of the sample and on the rate at which water is removed. An approximate estimate is obtained by equilibrating a sample, not previously dried below saturation, with an atmosphere of 11% RH (F12,F13,F14). Saturated aqueous LiCl HjO gives the required RH (partial pressure of water vapour = 2,7 torr at 25°C). To achieve apparent equilibrium in a reasonable time (several days), the sample must be crushed and the system evacuated the salt solution should be stirred, at least intermittently. Young and Hansen (Y5) found the composition of the C-S-H in C3S paste thus... 

As is demonstrated by the results summarized in Tables I and II, introduction of water vapour into the carrier gas led to a decrease in the retention times of all the sorbates, compared with those obtained in the dry carrier gas. An analogous decrease in the retention times when using water vapour in the carrier gas was also described in e.g. the paper by Guillemin and Millet [18]. The authors employed much higher water vapour contents (above 10%) and classical adsorbents (silica gel, alumina, porous polymers). With cyclodextrins, a perceptible decrease in the retention occurs even for 0.8% water vapour in the carrier gas. This phenomenon can be explained by partial adsorption of water molecules on the stationary phase surface, in a similar manner to common adsorbents. However, the experimental results, especially those obtained with a-CD as the stationary phase (Table I), indicate an effect on the equilibrium in the formation of the sorbate-cyclodextrin inclusion complex, in favour of the free guest. A great excess of water vapour over the sorbate apparently leads to competitive inclusion of water molecules and thus to faster desorption of the guest. 

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