Plasticization, antiplasticization, and molecular packing in amorphous carbohydrate-glycerol matrices.

The molecular packing of amorphous maltodextrin-glycerol matrices is systematically explored by combining positron annihilation lifetime spectroscopy (PALS) with thermodynamic measurements and dilatometry. Maltodextrin-glycerol matrices are equilibrated at a range of water activities between 0 and 0.54 at T = 25 °C to analyze the effect of both water and glycerol on the average molecular hole size and the specific volume of the matrices. In the glassy state, glycerol results in a systematic reduction of the average molecular hole size. In contrast, water interacts with the carbohydrate matrix in a complex way. Thermodynamic clustering theory shows that, at very low water contents the water molecules are well dispersed and are closely associated with the carbohydrate chains. In this regime water acts as an antiplasticizer, whereby it reduces the size of the molecular holes. Conversely, at higher water contents, while still in the glassy state, water acts as a plasticizer by increasing the average hole volume of the carbohydrate matrices. This plasticization-dominated mechanism is likely to be due to the interplay between the ability of water to form hydrogen bonds with the hydroxyl residues on the carbohydrate chains and its mobility, which is significantly decoupled from the bulk mobility of the matrix. Our findings are of key importance for the understanding of the effect of glycerol on the biostabilization performance of these carbohydrate matrices, as it provides a first insight on how molecular packing can relate to the dynamics in such matrices.

Specific volume-hole volume correlations in amorphous carbohydrates: effect of temperature, molecular weight, and water content.

The specific volume and the nanostructure of the free volume of amorphous blends of maltose with a narrow molecular weight distribution maltopolymer were systematically studied as a function of temperature, water content, pressure, and blend composition. Correlations between the hole free volume and the specific volume were investigated in the glassy and rubbery phases and in solution using positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) measurements, with the aim to provide a consolidated mechanistic understanding of the relation between changes in molecular packing and at the molecular level and the behavior of the specific volume at the macrolevel. Both specific volume and hole volume show a linear dependence on the temperature, but with a slope which is higher in the rubbery state than in the glassy state. As a function of temperature, the hole volume and the specific volume are linearly related, with no discontinuity at the glass transition temperature (T(g)). In the glassy state, both the specific volume and the hole volume decrease nonlinearly with the addition of maltose to the maltopolymer matrix, due to a more efficient molecular packing. For variations in carbohydrate composition, a linear dependence between the hole volume and the specific volume was again observed. The role of water was found to be significantly more complex, with increasing water content causing an increase in density in both the glassy and rubbery phases indicating that water exists in a highly dispersed state with a significantly lower specific molar volume than in bulk water. At very low water contents, the hole volume and the specific volume both decrease with increasing water content, which suggests that water acts as both a hole filler and a plasticizer. In the glassy state at slightly higher water contents, the specific volume continues to slowly decrease, but the hole size passes through a minimum before it starts to increase. This gives rise to a negative correlation between the hole volume and the specific volume which has not previously been observed and which can be interpreted in terms of water molecules which are dispersed within the glassy carbohydrate matrix and which thereby influence the hydrogen bonding between the carbohydrate molecules.

Molecular packing in amorphous carbohydrate matrixes.

The molecular packing of bidisperse matrixes of amorphous carbohydrates consisting of a fractionated maltopolymer supplemented with various amounts of the disaccharide maltose is investigated by combining Positron Annihilation Lifetime Spectroscopy (PALS) with specific volume measurements. The maltopolymer-maltose blends are equilibrated at a range of water activities between 0 and 0.75 at 25 degrees C in order to investigate the effect of water content and carbohydrate molecular weight distribution on the size of the molecular free volume holes in both the glassy and rubbery states. In the rubbery state, the size of the intermolecular holes is only very weakly dependent on the carbohydrate molecular weight, provided that the carbohydrate blends are analyzed at the same water content. In contrast, in the glassy state, significant differences in the size of the free volume holes are observed between the various blends at constant water content. Both the specific volume and the hole volume decrease with increasing maltose content, initially rapidly up to a maltose content of about 40 wt % on total carbohydrate. In addition, we find that the role of water as a plasticizer and matrix constituent is a complex one. At very low water contents, water acts by filling the free volume holes between the carbohydrate molecules. This hole-filling mechanism could well be related to the phenomenon of anti-plasticization observed before. At higher water contents, corresponding generally to water activities above 0.11 at 25 degrees C, water conversely increases the average hole volume in the carbohydrate matrixes, most likely caused by water interfering with the hydrogen bonding between the carbohydrate molecules, leading to a local expansion of the molecular packing.

Organization and mobility of water in amorphous and crystalline trehalose.

The disaccharide trehalose is accumulated by microorganisms, such as yeasts, and multicellular organisms, such as tardigrades, when conditions of extreme drought occur. In this way these organisms can withstand dehydration through the formation of an intracellular carbohydrate glass, which, with its high viscosity and hydrogen-bonding interactions, stabilizes and protects the integrity of complex biological structures and molecules. This property of trehalose can also be harnessed in the stabilization of liposomes, proteins and in the preservation of red blood cells, but the underlying mechanism of bioprotection is not yet fully understood. Here we use positron annihilation lifetime spectroscopy to probe the free volume of trehalose matrices; specifically, we develop a molecular picture of the organization and mobility of water in both amorphous and crystalline states. Whereas in amorphous matrices, water increases the average intermolecular hole size, in the crystalline dihydrate it is organized as a confined one-dimensional fluid in channels of fixed diameter that allow activated diffusion of water in and out of the crystallites. We present direct real-time evidence of water molecules unloading reversibly from these channels, thereby acting as both a sink and a source of water in low-moisture systems. We postulate that this behaviour may provide the overall stability required to keep organisms viable through dehydration conditions.

Carbohydrate polymers in amorphous states: an integrated thermodynamic and nanostructural investigation.

The effect of water on the structure and physical properties of amorphous polysaccharide matrices is investigated by combining a thermodynamic approach including pressure- and temperature-dependent dilatometry with a nanoscale analysis of the size of intermolecular voids using positron annihilation lifetime spectroscopy. Amorphous polysaccharides are of interest because of a number of unusual properties which are likely to be related to the extensive hydrogen bonding between the carbohydrate chains. Uptake of water by the carbohydrate matrices leads to a strong increase in the size of the holes between the polymer chains in both the glassy and rubbery states while at the same time leading to an increase in matrix free volume. Thermodynamic clustering theory indicates that, in low-moisture carbohydrate matrices, water molecules are closely associated with the carbohydrate chains. Based on these observations, we propose a novel model of plasticization of carbohydrate polymers by water in which the water dynamically disrupts chains the hydrogen bonding between the carbohydrates, leading to an expansion of the matrix originating at the nanolevel and increasing the number of degrees of freedom of the carbohydrate chains. Consequently, even in the glassy state, the uptake of water leads to increased rates of matrix relaxation and mobility of small permeants. In contrast, low-molecular weight sugars plasticize the carbohydrate matrix without appreciably changing the structure and density of the rubbery state, and their role as plasticizer is most likely related to a reduction of the number of molecular entanglements. The improved molecular packing in glassy matrices containing low molecular weight sugars leads to a higher matrix density, explaining, despite the lower glass transition temperature, the reduced mobility of small permeants in such matrices.