First-order volume phase transition

First, a strong volume change can be excited by a large spectrum of different physical and chemical factors such as temperature, electrical voltage, pH, concentration of organic compounds in water, and salt concentrations. The possibility of a first-order volume phase transition in gels was suggested by K. Dusek and... 

Certain polyelectrolyte hydrogels display first-order volume phase transitions, with hysteresis, in response to external stimuli 2,7). Hydrophobic polyelectrolyte hydrogels, in particular, may undergo discrete transitions in response to changes in external pH. Such hydrogels have been considered as chemically-sensitive mechanical switches 8). 

The SmS semiconductor to metal transition was later verified by the direct observation of a discontinuous change in the optical reflectivity at 6 kbar (Kirk et al., 1972). This is consistent with a first order magnetic phase transition which was directly verified by magnetic susceptibility measurements under pressure by Maple and Wohlleben (1971). In the collapsed phase the susceptibility of SmS showed no magnetic order down to 0.35 iC and was almost identical to the susceptibility of SmBa (see fig. 20.10 of volume 2). Bader et al. (1973) measured the heat capacity (fig. 11.16) and electrical resistivity (fig. 11.17) of SmS under pressure. They found a large electronic contribution to the heat capacity ( y = 145 mJ/mole-K ) and a resistivity reminiscent of SmB. Mossbauer isomer shift measurements of SmS under pressure by Coey et al. (1976) reveal the transition from a Sm isomer shift at zero pressure to an intermediate value at pressures above 6 kbar (fig. 11.18). The isomer shift of SmS above 6 kbar was found to be about the same as the isomer shifts for chemically collapsed Smo.77Yo.23S and SmBo at zero pressure. 

Before the volume phase transition was experimentally demonstrated in synthesized gels, its existence was theoretically predicted by Dusek and Patterson [4]. They suggested that the volume phase transition of gels is similar to the coil-globule transition of polymer chains and could be regarded as a first-order phase transition. 

In this volume, we will apply the principles developed in Principles and Applications to the description of topics of interest to chemists, such as effects of surfaces and gravitational and centrifugal fields phase equilibria of pure substances (first order and continuous transitions) (vapor + liquid), (liquid 4-liquid), (solid + liquid), and (fluid -f fluid) phase equilibria of mixtures chemical equilibria and properties of both nonelectrolyte and electrolyte mixtures. But do not expect a detailed survey of these topics. This, of course, would require a volume of immense breadth and depth. Instead, representative examples are presented to develop general principles that can then be applied to a wide variety of systems. 

The swelling curves show that the gel undergoes a sharp, but continuous volume phase transition (Fig. 1). By decreasing the cross-linking density, a first-order or discontinuous volume phase transition has also been observed. 

Hirotsu S (1993) Coexistence of phases and the nature of first-order phase transition in poly-7V-isopropylacrylamide gels. Adv Polym Sei 110 1-26 Hirotsu S (1994) Static and time-dependent properties of polymer gels around the volume phase transition. Phase Trans 47(3-4) 183-240... 

On cooling through Tc in the range 0.10 < x < 0.30, there is a first-order volume contraction associated with a discontinnons increase in the volume fraction of the more conductive FV and FM phases (Radaelli et al., 1996 Huang et al., 1998) where electrons tunnel from Mn(III) to Mn(IV) neighbors in a time tj, <. In contrast, the transition from the paramag-... 

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