PSI - Issue 26

Chiara Bertolin et al. / Procedia Structural Integrity 26 (2020) 147–154 Bertolin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3

out of the wood and eliminate most of the species physisorbed during the storage of the sample, especially adsorbed water. Vacuum was successively maintained until constant weight was reached; successively, then water vapor was introduced (or removed) at a slow constant rate to keep quasi-equilibrium conditions while the changes in weight were recorded. Samples were considered to have reached ‘equilibrium’ when the weight changes were less than 0.05% over 40 minutes. For details on the experimental procedure and time using the microbalance with wooden samples (see Bertolin et al., 2020). The M ads ( i . e . adsorption mass) values were calculated based on the initial weight of the outgassed sample. Sorption data were furtherly elaborated in order to calculate the volume of adsorbed water (i.e. V ads [cm 3 /g of dry wood]) expressed as a function of relative humidity (%) using the GAB equation (eq.1,2): = 1 0 0 (1− 1 0 0 )[1+( −1) 1 0 0 ] (1) 0 = 1 0 0 (2) where V m is the monolayer capacity, C and k are the adsorption GAB parameters, and p and p 0 are the partial water vapor experimental pressure and its value at saturation, respectively. C GAB has been calculated by the GAB best fit equation i . e. C in eq.1, from a physical-chemical approach it can be defined by eq. 3: = 0 ( 0 − ) (3) where C 0 is an entropic factor, R is the ideal gas constant, T the absolute temperature and H 0 , H n the molar sorption enthalpies of the mono- and multi-layers, respectively. The correcting factor k GAB , (k from the GAB best fit equation 1) as originally expressed by Guggenheim (Lesar et al., 2009) (eq. 4) has a larger entropic content than C GAB and describes a less structured state of the sorbate in multilayers (on the top of the monolayer) than in its pure bulk liquid state (Wolf et al., 1984; Timmermann, 2003; Bertolin et al., 2020).

0 p p = 

k

1

(4)

GAB

n

Similarly, to equation (3), k GAB can be expressed as in eq. 5:

0 exp RT  −  =     n l H H k

k

(5)

GAB

where k 0 represents the entropic factor and H l is the molar sorption enthalpy of the bulk liquid. 3. Discussion of the results

3.1 Adsorption-desorption isotherms

For most of the tested samples the obtained C GAB values resulted higher than 5.76; the only exception is represented by E whose C GAB value was found to be 4.28. Parallel, for all the specimens k GAB falls between 0.64 and 0.82. Fig. 1 resumes values of the calculated GAB parameters. The V m values represent the minimum water monolayer (here below V m is reported as V m ads to highlight the GAB fit of the adsorption phase of the isotherm) on the sample surface and bound to the active hydrophilic site. It can give information on alterations of the material service life before and after the treatments since the deposited coating can modify the availability of reactive sites on the sample’s surfaces. As shown in Fig. 1, V m ads values tend to decrease for treated samples except for E, probably because occurring reactions lead to the formation of new OH sites.