Type III

These compounds have been obtained by the addition of cyclooctadiene to an equimolar mixture of /i3-allyl or /d -crotyl-(2,4-pentanedionato)palladiuni(II) and tetrafluoroboric acid in methylene chloride-ether solution.1 If silver tetrafluoroborate is on hand, the slight modification described below obviates the need to prepare the /3-diketonate complex as an intermediate. 

The dimer di-Ju-dichloro-bis(/i3-allyl)dipalladium (1.83 g., 5.0 mmoles) and silver tetrafluoroborate (1.95 g., 10.0 mmoles) in methylene chloride (50 ml.) are stirred for 15 minutes. 1,5-Cyclooctadiene (2.0 ml.) is added and stirring continued for a further 2 minutes. The mixture is filtered and the residue washed with methylene chloride (two times with 10 ml.). Ether (150 ml.) is added to the combined filtrate and washings to give a white or greyish precipitate. This is filtered, washed with ether (three times with 50 ml.), and dried in air. The solid is dissolved in methylene chloride (75 ml.), and this solution is 

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Organosilicon polymers. Silicon resembles carbon in certain respects and attempts have been made to prepare polymers combining carbon and silicon units in the molecule with the object of increasing the heat resistance of polymers. It has been found that the hydrolysis of a dialkyl-dichlorosilicane or an alkyltrichlorosilicane, or a mixture of the two, leads to polymers (Silicones), both solid and liquid, which possess great thermal stability. Thus dimethyldichlorosilicane (I) is rapidly converted by water into the silicol (II), which immediately loses water to give a silicone oil of the type (III) ... 

Isotherms of Type 111 and Type V, which are the subject of Chapter 5, seem to be characteristic of systems where the adsorbent-adsorbate interaction is unusually weak, and are much less common than those of the other three types. Type III isotherms are indicative of a non-porous solid, and some halting steps have been taken towards their use for the estimation of specific surface but Type V isotherms, which betoken the presence of porosity, offer little if any scope at present for the evaluation of either surface area or pore size distribution. 

When c is less than 2 but still positive, the BET equation results in a curve having the general shape of a Type III isotherm (cf. Fig. 2.1, Curve A and Fig. 2.3). 

As explained in Section 2.13, the use of iz,-plots makes it possible to avoid the involvement of either n or when an alternative adsorptive is being used for evaluating the surface areas of a set of related solids. It is then no longer necessary to exclude the use of isotherms having a low value of c, consequently the method is applicable even if the isotherm of the alternative adsorptive is of Type III (cf. Chapter 5). Calibration of one sample by nitrogen or argon adsorption is still required. 

Type III and Type V Isotherms The Special Behaviour of Water... 

Both Type III and Type V isotherms are characterized by convexity towards the relative pressure axis, commencing at the origin. In Ty )e III isotherms the convexity persists throughout their course (Fig. 5.1(a), whereas in Type V isotherms there is a point of inflection at fairly high relative pressure, often 0-5 or even higher, so that the isotherm bends over and reaches a plateau DE in the multilayer region of the isotherm (cf. Fig. 5.1 (b)) sometimes there is a final upward sweep near saturation pressure (see DE in Fig. 5.1(b)) attributable to adsorption in coarse mesopores and macropores. 

Types III and V isotherms arc characteristic of weak gas-solid interactions, the Type III isotherm being given by a nonporous or macroporous solid and the Type V isotherm by a mesoporous or microporous solid. 

Type III end Type V isotherms speciel behaviour of weter... 

Type III (and Type V) isotherms may originate through the adsorption of either nonpolar or polar molecules, always provided that the adsorbent-adsorbate force is relatively weak. 

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

The strength of dispersion interaction of a solid with a gas molecule is determined not only by the chemical composition of the surface of the solid, but also by the surface density of the force centres. If therefore this surface density can be sufficiently reduced by the pre-adsorption of a suitable substance, the isotherm may be converted from Type II to Type III. An example is rutile, modified by the pre-adsorption of a monolayer of ethanol the isotherm of pentane, which is of Type II on the unmodified rutile (Fig. 5.3, curve A), changes to Type III on the treated sample (cf. Fig. 5.3 curve B). Similar results were found with hexane-l-ol as pre-adsorbate. Another example is the pre-adsorption of amyl alcohol on a quartz powder... 

Fig. 5.2 Type III isotherms, (a) n-hexane on PTFE at 25°C (b) n-octane on PTFE at 20 C (c) water on polymethylmethacrylate at 20°C (d) water on bis(A-polycarbonate) (Lexan) at 20°C. The insets in (c) and (d) give the curves of heat of adsorption against fractional coverage the horizontal line marks the molar heat of liquefaction. (Redrawn from diagrams in the original papers, with omission of experimental points.)...

Type III and Type V isotherms special behaviour of water... 

As already pointed out, a Type III isotherm results from the BET equation when the value of c is less than 2 (p. 46). For c = 3, the isotherm is no longer strictly of Type III, but the point of inflection, at about 0Olp°, is barely perceptible, and at first glance the isotherm appears to be a genuine Type 111—a fact of some consequence because the value c 3 is relatively common amongst isotherms which are apparently of Type III. 

In applying the BET procedure to Type III isotherms, c = 1 constitutes a special case insertion of c = 1 into the standard BET equation (2.12) leads to the simplified equation... 

When the values of the BET monolayer capacity calculated from Type III isotherms are compared with independent estimates (e.g. from nitrogen adsorption) considerable discrepancies are frequently found. A number of typical examples are collected in Table 5.1. Comparison of the value of the monolayer capacity predicted by the BET equation with the corresponding value determined independently (columns (iv) and (v)) show that occasionally, as in line 6, the two agree reasonably well, but that in the majority... 

One must conclude therefore that the BET procedure for evaluation of monolayer capacity is not applicable to a Type III (nor by implication, to a Type V) isotherm. 

An outstanding feature of the adsorption of water vapour on silica is its sensitivity to the course and subsequent treatment of the silica sample, in particular the temperature to which it has been heated. Figure 5.15 shows the strong dependence of the isotherm for a particular silica gel on the temperature of its heat treatment the isotherm is progressively lowered as the temperature increases, especially above 400°C, and the shape changes from Type II for the lower temperatures to Type III for 600°C, 800°C and 1000°C. 

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

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