Type II condensation

In type ii condensations three types of molecular species are present ... 

In subsequent treatment, a condensation polymer formed from a monomer of the type A-B will be referred to as type I polymer and the polymerization will be called type I condensation, while a polymer formed from a mixture of A—A and B—B type monomers will be termed a type IIpolymer and the process referred to as type II condensation. 

This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Equation XVII-78 turns out to ht type II adsorption isotherms quite well—generally better than does the BET equation. Furthermore, the exact form of the potential function is not very critical if an inverse square dependence is used, the ht tends to be about as good as with the inverse-cube law, and the equation now resembles that for a condensed him in Table XVII-2. Here again, quite similar equations have resulted from deductions based on rather different models. 

Condensers. Several types of condensers are widely used. Fig. II, 56, 16 is an improved form of Liebig s con-... 

Type 111 units are a combination of the Type I and Type II where part is in spiral flow and part is in cross flow. This SHE can condense and subcool in a single unit. 

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

Here the phenomenon of capillary pore condensation comes into play. The adsorption on an infinitely extended, microporous material is described by the Type I isotherm of Fig. 5.20. Here the plateau measures the internal volume of the micropores. For mesoporous materials, one will first observe the filling of a monolayer at relatively low pressures, as in a Type II isotherm, followed by build up of multilayers until capillary condensation sets in and puts a limit to the amount of gas that can be accommodated in the material. Removal of the gas from the pores will show a hysteresis effect the gas leaves the pores at lower equilibrium pressures than at which it entered, because capillary forces have to be overcome. This Type IV isotherm. 

Rg. II, 56, 17 (Davies types) and Fig. II, 56, 18 (double coil type) are examples of efficient double surface condensers. Fig. II, 56, 19 depicts a screw type of condenser (Friedrich pattern) the jacket is usually 10, 15 or 25 cm. long and the cone and sockets are. 619 or 624 this highly efficient condenser is employed for both reflux and for downward distillation. 

Type II, typical of macroporous solids where the prevailing adsorption processes are the formation of a monolayer at low relative pressures, followed by gradual and overlapping multilayer condensation as the pressure is increased. 

Existence of a large amount of mesopores usually results in the appearance of capillary condensation hysteresis loop. Type II AIs transform to type IV, and type III AIs transform to type V Type VI AIs are characteristic to low-temperature adsorption of some noble gases over energetically homogeneous surfaces. 

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

As discussed in Section 1.4.2.1, the critical condensation pressure in mesopores as a function of pore radius is described by the Kelvin equation. Capillary condensation always follows after multilayer adsorption, and is therefore responsible for the second upwards trend in the S-shaped Type II or IV isotherms (Fig. 1.14). If it can be completed, i.e. all pores are filled below a relative pressure of 1, the isotherm reaches a plateau as in Type IV (mesoporous polymer support). Incomplete filling occurs with macroporous materials containing even larger pores, resulting in a Type II isotherm (macroporous polymer support), usually accompanied by a H3 hysteresis loop. Thus, the upper limit of pore size where capillary condensation can occur is determined by the vapor pressure of the adsorptive. Above this pressure, complete bulk condensation would occur. Pores greater than about 50-100 nm in diameter (macropores) cannot be measured by nitrogen adsorption. 

As the temperature falls below the ice frost point, water condenses out as ice, forming large particles (Fig. 12.21). These are known as Type II PSCs. They are formed at lower temperatures corresponding to the frost point of water ( 188 K for stratospheric conditions), or possibly 2-3 K below that (Tabazadeh et al., 1997). They are much larger than Type I PSCs, of the order of 5-50 jum in diameter, and consist mainly of... 

Toon and Tolbert (1995) suggest that if Type I PSCs are primarily ternary solutions rather than crystalline NAT, the higher vapor pressure of HN03 over the solution would in effect distill nitric acid from Type I to Type II PSCs, assisting in denitrification of the stratosphere. This overcomes the problem that if Type II PSCs have nitric acid only by virtue of the initial core onto which the water vapor condenses, the amount of HN03 they could remove may not be very large. The supercooled H20-HN03 liquid layer observed by Zondlo et al. (1998) clearly may also play an important role in terms of the amount of HNO, that can exist on the surface of these PSCs. 

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