J. Phys. Chem. B 2007, 111, 12643-12648
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ARTICLES Molecular Packing in Amorphous Carbohydrate Matrixes Sam Townrow,† Duncan Kilburn,† Ashraf Alam,† and Job Ubbink*,‡ H. H. Wills Physics Laboratory, UniVersity of Bristol, Tyndall AVenue, Bristol BS8 1TL, U.K., and Nestle´ Research Center, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland ReceiVed: June 22, 2007; In Final Form: August 14, 2007
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 maltopolymermaltose blends are equilibrated at a range of water activities between 0 and 0.75 at 25 °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 antiplasticization observed before. At higher water contents, corresponding generally to water activities above 0.11 at 25 °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.
Introduction Amorphous carbohydrates in the glassy state are key constituents of many dehydrated foods,1,2 and they are also of importance as encapsulation matrixes for flavors,3 pharmaceutics,4 and various other bioactive ingredients.5 The properties of amorphous carbohydrates are strongly dependent on a number of factors including the water content of the matrix. Water is a strong plasticizer of amorphous carbohydrates, and the glass transition temperature (Tg) of carbohydrates decreases rapidly with increasing water content.6,7 In previous studies, we have explored the concept of molecular packing to explain several hitherto puzzling phenomena observed in the physics of amorphous carbohydrates8 and to provide a link to developments in polymer physics.10 The molecular packing in the glassy state appears to have a profound effect on numerous physical properties of carbohydrate matrixes, such as the diffusion of small penetrants such as oxygen,11 the stabilization of sensitive biological materials by glassy state encapsulation and hydrogen bonding,12 and the sorption of water in the glassy state.10 We have demonstrated that important aspects of the molecular packing in amorphous and crystalline carbohydrate matrixes can be elucidated using * Corresponding author. Tel: +41-21-785 9378. Fax: +41-21-785 8554. E-mail:
[email protected]. † University of Bristol. ‡ Nestle ´ Research Center.
Positron Annihilation Lifetime Spectroscopy (PALS).13,14 PALS is a technique which has been widely used to study the free volume holes in condensed matter.15 Whereas in previous studies highly polydisperse maltooligomer mixtures were used,8,13 here we investigate the molecular packing of bidisperse matrixes of amorphous carbohydrates consisting of a fractionated maltopolymer supplemented with various amounts of the disaccharide maltose by combining PALS with specific volume measurements. The combination of a ternary system of controlled composition with PALS and specific volume measurements allows us to probe the molecular packing at both molecular and macroscopic length scales. Because of the importance of water as a plasticizer for amorphous carbohydrates, the maltopolymer-maltose blends are equilibrated at a range of water activities between aw ) 0 and 0.75 at 25 °C 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. Experimental Section Solvent Casting of Matrixes. LAB2490, a maltopolymer with a fairly narrow molecular weight distribution (weightaverage molecular weight Mw ) 1.2 × 104 Da, Mw/Mn ) 2.2, where Mn is the number-average molecular weight), was prepared by acid hydrolysis of starch followed by chromato-
10.1021/jp074884l CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007
12644 J. Phys. Chem. B, Vol. 111, No. 44, 2007
Townrow et al.
graphic separation; the LAB2490 was a kind gift of Roquette Fre`res (Lestrem, France). The degree of branching of LAB2490 is 6.8% (number of R 1f6 linkages on total number of R 1f4 and R 1f6 linkages), as determined from 1H NMR. D(+)Maltose (purity >99%, maltotriose content
