Gels hysteresis

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

It was noted earlier (p. 115) that the upward swing in the Type IV isotherm characteristic of capillary condensation not infrequently commences in the region prior to the lower closure point of the hysteresis loop. This feature can be detected by means of an a,-plot or a comparison plot (p. 100). Thus Fig. 3.25(a) shows the nitrogen isotherm and Fig. 3.25(h) the a,-plot for a particular silica gel the isotherm is clearly of Type IV and the closure point is situated around 0 4p° the a,-plot shows an upward swing commencing at a = 0-73, corresponding to relative pressures of 013 and therefore well below the closure point. 

A typical example, from the extensive study by Kamakin on an alumina-silica gel, is shown in Fig. 3.32. When the mercury pressure was reduced to 1 atm at the end of the first cycle, 27 per cent of the intruded mercury was retained by the sample a second intrusion run followed a different path from the first, whereas the second extrusion curve agreed closely with the first. Change in f re structure of the kind described above could perhaps account for the difference between the two intrusion curves, but could not explain the reproducibility of the remainder of the loop. There is no doubt that hysteresis can exist in the absence of structural change. 

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

Fig. 4.26 Low-pressure hysteresis in the adsorption isotherm of water at 298 K on a partially dehydroxy la ted silica gel. O, first adsorption run (outgassing at 200°C) . first desorption A, second adsorption run (outgassing at 200°C) A. second desorption (after reaching p/p = 0-31) X, third adsorption run (outgassing at 25 C).

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

Expansion and contraction of siUca gel monoliths show hysteresis upon heating and cooling. As long as the sample was cycled below approximately 500°C, there was no hysteresis and the monolith was thermally stable (6,23). 

CaCl2 formed wide but relatively short strands without forming apparent continuous network structures in most cases (Fig. 6.13D). For i-carra-geenan, the addition of or Ca formed localized networks through side-by-side aggregation between helices, which was consistent with the thermal hysteresis between sol-to-gel and gel-to-sol transitions. However, this interhelical aggregation was not necessarily a prerequisite for gelation. 

T Tomari, M Doi. Hysteresis and incubation in the dynamics of volume transition of spherical gels. Macromolecules 28 8334-8343, 1995. 

Fig. 28. The degree erf swelling of a NIPA gel in pure water as a function of temperature. The curve on the left is an expanded graph, in which hysteresis erf the discontinuous volume transition can be observed...

Fig. 31. Temperature dependence for equilibrated volumes of NIPA gel including the Con A-DDS complex (DSS-gel, open circles), MP (MP-gel, filled circles), and free of both DSS and MP squares). The latter was prepared as a control sample. Hysteresis was observed in the volume changes of DSS-gel and the free-Con A gel on heating and cooling, indicating a discontinuous phase transition. The diameter of each gel in the collapsed state, determined at 50 °C, was do = 0.074 mm the volume of this gel is denoted by V0. The concentration of dry matter in the collapsed state was estimated from the preparation recipe to be 90wt%.

The adsorption of gases and vapors on mesoporous materials is generally characterized by multilayer adsorption followed by a distinct vertical step (capillary condensation) in the isotherm accompanied by a hysteresis loop. Studies of adsorption on MCM-41 have also demonstrated the absence of hysteresis for materials having pore size below a critical value. While this has been reported for silica gel and chromium oxide containing some mesopores, no consistent explanation has been offered [1], However, conventional porous materials, having interconnected pores with a broader size distribution, are generally known to display a hysteresis loop with a point of closure which is characteristic of the adsorptive. These materials have an independent method of estimating the pore size from XRD and TEM, that allows comparison with theoretical results. Consequently, we have chosen these materials to test the proposed model. 

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