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Mechanisms of Nonexponential Relaxation in Supercooled Glucose Solutions: the Role of Water Facilitation

Molinero, Valeria and Çağin, Tahir and Goddard, William A., III (2004) Mechanisms of Nonexponential Relaxation in Supercooled Glucose Solutions: the Role of Water Facilitation. Journal of Physical Chemistry A, 108 (17). pp. 3699-3712. ISSN 1089-5639. doi:10.1021/jp036680k.

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Concentrated sugar solutions are prototypical glass formers with a wide application in food technology and cyropreservation, but the microscopic mechanisms underlying these processes remain obscure. To uncover these microscopic details, we study the structure and dynamics of binary glucose−water mixtures by means of atomistic and coarse grain molecular dynamics simulations. From atomistic simulations, we find that water in glucose forms extended clusters that percolate above a water concentration of ∼18 wt % at T = 340 K. This percolation threshold and structure is very well reproduced with a coarse grain model even though it lacks of directional interactions. Using the coarse grain model, we present a detailed study of the translational dynamics of the 12.2 wt % water mixture in the temperature range T/T_g = 1.5−1.05 and for times up to 0.65 μs. These coarse grain studies lead to a glass transition temperature of 239 ± 25 K in excellent agreement with the experimental value of 240 K. The water diffusion coefficient obtained from these calculations has an activation energy of 35−38 kJ/mol, which compares very well with the 31 kJ/mol obtained experimentally for the 25 wt % water−glucose mixture. Both water and glucose show nonexponential relaxation, although the nonexponentiallity is more pronounced for water. We find that water diffusion in supercooled glucose proceeds by two mechanisms:  (i) continuous diffusion and (ii) discrete jumps on the order of 3 Å. The contribution of the jump mechanism increases with supercooling. On the other hand, the continuous diffusion component of water diffusion decreases at lower temperatures until it becomes negligible for T < 1.2T_g. At this point rare jump events with characteristic times above 10 ns are the only mechanism of water relaxation. The decrease of the extent of the continuous diffusion to water mobility with lowering temperatures is associated with the freezing of the sugar matrix. In the deep supercooled regime, at T/T_g = 1.05 water moves in an almost translationally frozen glucose matrix, and displays a broad distribution of waiting times between jumps. Contrary to water, the mechanism of glucose translation does not involve big jumps even at the lowest temperatures analyzed. Rather the center of mass of the glucose molecules translate through a continuous diffusion mechanism with a distribution of characteristic times. We analyze the mobility of water molecules as a function of the water−water connectivity and find that the mobility of the water molecules increases with their water coordination. The lower the temperature the more important the effect of water coordination in water mobility. The distribution of mobilities associated with different water local environments constitutes a structural contribution to the heterogeneous, nonexponential dynamics in the binary mixture.

Item Type:Article
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URLURL TypeDescription
Çağin, Tahir0000-0002-3665-0932
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2004 American Chemical Society. Received: September 8, 2003; In Final Form: December 9, 2003. Publication Date (Web): April 2, 2004. The facilities of the Materials and Process Simulation Center used in this research are supported by ONR-DURIP, ARO-DURIP, SIR−IBM, and NSF (CHE), and additional support is provided by DOE-ASCI, DOE-FETL, ARO-MURI, ONR-MURI, NIH, NSF, General Motors, ChevronTexaco, Seiko-Epson, the Beckman Institute, and Asahi Kasei.
Funding AgencyGrant Number
Office of Naval Research (ONR)UNSPECIFIED
Army Research Office (ARO)UNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Caltech Beckman InstituteUNSPECIFIED
Issue or Number:17
Record Number:CaltechAUTHORS:20170315-154452450
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Official Citation:Mechanisms of Nonexponential Relaxation in Supercooled Glucose Solutions:  the Role of Water Facilitation Valeria Molinero, Tahir Çaǧın, and William A. Goddard, III The Journal of Physical Chemistry A 2004 108 (17), 3699-3712 DOI: 10.1021/jp036680k
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:75161
Deposited By: Tony Diaz
Deposited On:16 Mar 2017 16:44
Last Modified:15 Nov 2021 16:31

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