DETECTOR DE CRUCE POR CERO.

It was observed that in the case of amorphous solids, the amount of water taken up by the solid is much more than that expected from a 2-3 layer adsorption of water seen in the case of crystalline solids. This significant water uptake into such amorphous solids far in excess of what would be predicted from an adsorption to the surface as that seen in crystalline solids is termed absorption or sorption, to differentiate it from adsorption (Zografi, 1988). Water uptake in this case was correlated to sample mass rather than to surface area (particle size) as is the case in water adsorption for crystalline solids. It has been noted that water vapour can be adsorbed and desorbed reversibly from the surface of crystalline solids with no hysteresis between the sorption and desorption steps. An example mentioned by Ahlneck and Zografi (1990) whereby water vapour was repeatedly

adsorbed and desorbed on freshly crystallized sodium chloride up to its critical relative humidity of 76% (jdeliquescence point) and neither changes in the amounts of adsorbed water nor physical changes in the solid were seen. On the other hand in the case of water sorption into amorphous solids, there is usually significant hysteresis between the sorption and desorption isotherms (Zografi, 1988). The more polar the amorphous solid is, the higher the solubility of water in the solid, and hence the greater the extent of water absorption into the amorphous solid under any particular conditions of relative humidity and temperature.

1.3.2.1 WATER AS A PLASTICIZER:

When an additive lowers the Tg of a substance we speak of its plasticizing effect (Hancock and Zografi, 1994). Water, due to its very low Tg (-134 °C), its ability of hydrogen bonding and its small molecular weight can have a profound plasticizing effect. So a^lasticizeris basically a substance that has the ability to reduce the glass transition temperature of another amorphous substance when added to it. Zografi (1988) stated that, as soon as water penetrates into the amorphous solid structure, it acts as a plasticizer and reduces the glass transition temperature of the amorphous solid. From this it can be concluded that, each point on the sorption isotherm reflects the physical state of both the amorphous solid and water and how each have influenced one another under a certain set of conditions (temperature, RH). Thus in spite of the fact that the temperature of the experiment might be constant (T), the difference between this temperature and the glass transition temperature of the amorphous solid (T-Tg) will differ in conjunction with the difference in amount of water being sorbed. For an amorphous solid with a Tg value higher than experiment temperature, the more water is sorbed by the solid, the more the Tg will be suppressed until eventually Tg reaches a value close to or below T. Once this temperature is reached it will critically jeopardise the physical and chemical stability of this amorphous solid as the increased molecular mobility will be sufficient to allow amorphous solids to undergo solid-state chemical reactions and to support the crystallization of the amorphous regions. As water is dissolved in an amorphous solid, the plasticizer effect of water leads to an increase in free volume of the solid by reducing hydrogen bonding between adjoining molecules of the solid, with a corresponding reduction in its glass transition temperature (Ahlneck and Zografi, 1990). As free volume of the solid is significantly increased, the mobility of the molecules or segments of the

molecules in the solid is greatly increased. This increase in mobility is reflected in a decrease in the viscosity of the solid which is particularly significant if the solid is plasticized enough to pass from the glassy immobile state with lower free volume and higher viscosity to the rubbery liquid like state with more free volume and lower viscosity. Amorphous solids below their glass transition temperature (Tg) exist in the glassy state with less than 3% free volume and viscosities greater than about 10^^ poise (Zografi, 1988). Based on William-Landel-Ferry (WLF) equation, going just 20 °C above Tg will cause the viscosity to change from 10^^ poise at Tg to 10* poise with a very significant increase in the molecular mobility of the solid and water (Ahlneck and Zografi, 1990). When passing from the glassy to the rubbery state, diffusion of water molecules increases exponentially. Hysteresis in the sorption/desorption isotherm of amorphous solids upon exposure to water vapour, is an indication that absorbed water can result in conformational changes of polymer chains in the solid state and was explained as being due to the ability of water to act as a plasticizer when added to polymers or other amorphous solids (Zografi, 1988). Figure (1.10) shows the effect of water absorbed into an amorphous solid on the glass transition temperature of a typical amorphous solid having very high water solubility and a high Tg in the dry state. It can be seen that water with its very low Tg (-134 °C), increasingly and continuously lowers the Tg of the amorphous solid as its concentration in the solid increases. Combinations of the temperature effect with a change in RH effect show the possible changes in the amorphous solid from the glassy state to the rubbery state (Ahlneck and Zografi, 1990). Both an increase in temperature and an elevation in humidity result in increased molecular mobility of the amorphous solid, which can lead to a transition from the glassy to the rubbery state. The first case results in raising T to a level equal to or above Tg , whereas in the second case water causes lowering of Tg to a level equal to or lower than T. Poly(vinylpyrrolidone) is an amorphous solid whose interaction with water gives a typical behaviour as the one seen in Figure 1.10. It has been shown by Hancock and Zografi (1993) that there is a significant change in Tg of PVP at low water contents, with a levelling-off in the water plasticizing effect as the water content increases. This has been explained as being due to the strong interaction between water and the amorphous solid, leading to a marked plasticizing effect of water at low water contents. As the water content in the amorphous solid increases, the strength of the interaction between water and the amorphous solid is reduced, resulting in a lowering of the plasticizing efficiency

o f water. Eventually the affinity o f w ater to the am orphous solid is reduced to such an extent that w ater becom es preferentially associated with its own m olecules

r Tm

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Figure 1.10 Schematic solute-water state diagram illustrating the effect o f water

plasticization and its effect on Tg. Adapted from Ahlneck and Zografi, 1990.

Hancock and Zografi (1994) explained that if two am orphous solids are com pared in term s o f the extent o f water effect on suppressing the glass transition tem perature assum ing that both solids had the same Tg value, the one with the lower density will be plasticized the least by any certain am ount o f water. On the other hand, if two am orphous solids having the sam e density are com pared, the one with the lower Tg will be plasticized the least

1.3.2.2 ESTIMATING WATER EFFECT ON GLASS TRANSITION TEMPERATURE (Tg):

The m ost widely and easily applied equation to estim ate the effect o f adding a given weight fraction o f one am orphous solid to another am orphous solid on the glass transition tem perature is the G ordon-Taylor equation (G ordon and Taylor, 1952). It is assum ed that there is perfect volum e additivity at Tg and no specific interaction between the two com ponents is taking place and so simple mixing rules have been applied.

Tg mix = [(wi.Tgi) + (K.W2.Tg2)]/ [wi + (K.W2)] Equation. 1.5

W here:

Tg mix : Glass transition tem perature o f the m ixture o f the two com ponents. W| and W2 : W eight fractions o f com ponent 1 and com ponent 2 respectively.

Tg, and Xg2 : Glass transition temperatures of component 1 and component 2 respectively.

K: constant considered to be a measure of the relative free volume contributed to the