Hydrogen Bonding and the Fragility of Supercooled Liquids and Biopolymers

Abstract

ABSTRACT. We review the behavior of a variety of liquids and polymers in which the presence of hydrogen bonding is of special importance to the liquid state properties. In particular we consider the case of a series of disubstituted benzenes in which appropriate changes of substitutents can systematically "turn on" hydrogen bonding between the molecules and the cases of ionic and non-ionic aqueous solutions in which change of water content changes the "fragility" in different directions. Finally we show how in hydrated biopolymers the presence of hydrogen bonding leads to the least fragile behavior yet observed in molecular systems. Where the p;esence of hydrogen bonding has previously been seen as conferring moderately "strong" liquid behavior on molecular liquids, we cite here cases in which either strong or fragile behavior can be seen, and an important case (water) in which there is apparently a transition from fragile to strong behavior as temperature is decreased. IntroductionAmong glass-forming liquids, hydrogen bonded liquids tend to be unusual with respect to their supercooling propensity and their behavior in the supercooled state. We have previously noted( I) that they tend to vitrify more readily than the general rule Tb/T m > 2.0( I ,2) would indicate (e.g. for the classical, widely studied, highly crystallization-resistant case of glycerol, Tb/T m = 1.7) and that in the supercooled liquid state they tend to have large activation energies and relatively small departures from Arrhenius behavior. Also, the liquid ranges, defined by the ratio Tb/T g(3,4), tend to be smaller than for molecular liquids of similar constitution but lacking H-bonds. Crudely, one can observe that for paraffins Tb/T g -4, for M.·C. Bellissent·Funel and J. C. Dore (eds.), Hydrogen Bond Networks, 3-22. © 1994 Kluwer Academic Publishers. 4 aromatics or paraffins with H-bonds , Tb / T g -3, while for aromatics with H-bonds , Tb!T g -->2. Some examples will be given in the course of this paper.In recent times it has become quite common to di scuss departures from Arrhenius behavior and other related propertie s (non-exponentiality and nonlinearity [or state dependence] of re laxation) in terms of the strong/fragile liquids classification, and it is this scheme which we will utilize here to systematize the discussion of several classes of hydrogen bonded liquids including polyalcohols and water, aqueous solutions and hydrated polymers . Strong/Fragile Liquid ClassificationThe strong/fragile classification scheme for liquids< 5,6) is based on a corresponding states representation of the liquid relaxation behavior, which is commonly represented by the widely available property, vi scosity (though generally it is preferable to represent the behavior by a structural relaxation time). The key notion involved is that the glass transiti o n temperature provides an appropriate scaling temperature for corresponding states comparisons. Insert: change in heat capacity observed at the glass transition temperature, also in normalized form....

Characterization of the Time Scales of Molecular Motion in Pharmaceutically Important Glasses

“…Generally, a large increase in C p conf is associated with fragile behavior in the strong/fragile classification scheme used to characterize glass-forming liquids . It is important to point out, however, that some exceptions to this correlation between the increase in C p conf at T g and fragility have been noted in the case of hydrogen-bonded liquids . For the three sugars, which might be expected to have similar hydrogen-bonding capabilities, sorbitol appears to be the most fragile on the basis of its very large value of C p conf at T g .…”

“…32 It is important to point out, however, that some exceptions to this correlation between the increase in C p conf at T g and fragility have been noted in the case of hydrogen-bonded liquids. 33 For the three sugars, which might be expected to have similar hydrogen-bonding capabilities, sorbitol appears to be the most fragile on the basis of its very large value of C p conf at T g . Indomethacin, having the smallest increase C p conf at T g , is apparently the least fragile of the four materials; however, it is considerably less hydrophilic and has a more limited capacity for hydrogen bonding.…”

“…Above T g , the temperature dependence of the configurational heat capacity is generally material dependent. ,,, The C p conf can increase linearly with temperature, decrease linearly with temperature, or follow a temperature dependence described by the hyperbolic relation 23,31,33 where K is a constant. Generally, eq 1 is the most accurate way to describe the temperature dependence of C p conf ; however, there are some situations where eq 1 is not an accurate description of experimental C p conf data. ,, …”

Properties of Citric Acid at the Glass Transition

“…The reduction of fragility due to the presence of water agrees with observations made by Angell on aqueous solutions of sucrose. 36,37 It is interesting to note in this regard that the heat capacity change at T g for the hydrated system is greater than that of the dry sample, which might appear to contradict this conclusion; a larger heat capacity change at T g often corresponds to greater fragility. 24,25 However, as suggested by Angell, [36][37][38] the presence of any reinforced hydrogen bonds produced by the addition of water can give rise to an incremental increase in heat capacity as the sample is heated through the glass transition region and cause the liquid structure to be less sensitive to temperature change.…”

Physics of amorphous solids

“…Typically, fragile liquids are those exhibiting a large increase in C p conf at T g . However, exceptions to this generality have been noted in the case of hydrogen‐bonded liquids 17. Recent articles have been published that relate changes in configurational heat capacity to pharmaceutical stability 18,19.…”