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The logarithmic relaxation process and the critical temperature of liquids in nano-confined states

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ABSTRACT

The logarithmic relaxation process is the slowest of all relaxation processes and is exhibited by only a few molecular liquids and proteins. Bulk salol, which is a glass-forming liquid, is known to exhibit logarithmic decay of intermediate scattering function for the β-relaxation process. In this article, we report the influence of nanoscale confinements on the logarithmic relaxation process and changes in the microscopic glass-transition temperature of salol in the carbon and silica nanopores. The generalized vibrational density-of-states of the confined salol indicates that the interaction of salol with ordered nanoporous carbon is hydrophilic in nature whereas the interaction with silica surfaces is more hydrophobic. The mode-coupling theory critical temperature derived from the QENS data shows that the dynamic transition occurs at much lower temperature in the carbon pores than in silica pores. The results of this study indicate that, under nano-confinements, liquids that display logarithmic β-relaxation phenomenon undergo a unique glass transition process.

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The temperature dependence of B1(T) for salol in different states.Bulk salol and salol confined in Carbon pore (39 ± 1 Å) (a), Carbon pore (56 ± 1 Å) (b), Silica pore (40 ± 1 Å) (c) and Silica pore (60 ± 1 Å) (d). The linear fitting lines are extrapolated consistently to get the mode-coupling crossover temperature, Tc.
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f4: The temperature dependence of B1(T) for salol in different states.Bulk salol and salol confined in Carbon pore (39 ± 1 Å) (a), Carbon pore (56 ± 1 Å) (b), Silica pore (40 ± 1 Å) (c) and Silica pore (60 ± 1 Å) (d). The linear fitting lines are extrapolated consistently to get the mode-coupling crossover temperature, Tc.

Mentions: The parameters f(Q, T), H1(Q, T), H2(Q, T) and τβ(T) are obtained by fitting the curves in the measured time range. The Q dependence of the parameter H1(Q, T) is shown in Fig. 3. The solid line is the fitting line by H1(Q, T) = B1(T)Qb, where b can take a value between 1 and 2 for small Qs. The fitted B1(T) values are plotted as a function of temperature in Fig. 4. The Q and temperature dependence of other parameters are provided in the supplementary information (Figures 1S, 2S, and 3S). The temperature dependence of B1(T) is extrapolated consistently to get the MCT crossover temperature, Tc, also known as MCT critical temperature. According to MCT at this temperature long range liquid like molecular motion freezes and the transport mechanism becomes more like that of the hopping process in a solid41. The Tc of bulk salol is 255 ± 2 K, in reasonable agreement with the previous findings4042. As expected, this value is about 40 K above the calorimetric glass-transition temperature Tg (218 K), it is universally observed that the Tc is approximately 1.2Tg43. The present QENS data of salol confined in 39 ± 1 Å and 56 ± 1 Å carbon pores show the Tc are 204 ± 2 K and 214 ± 2 K, respectively. As the pore size decreases Tc also decreases and this observation is similar to that of other reported results1. However, in the silica pores with similar pore sizes the freezing temperature did not show an appreciable change. The freezing temperature obtained from the present QENS data of salol confined in 40 ± 1 Å and 60 ± 1 Å silica pores are 234 ± 2 K and 229 ± 2 K, respectively. The freezing temperature of salol in the confining geometry is shifted ~50 K and ~20 K to lower temperatures in the carbon pore (39 ± 1 Å) and silica pore (40 ± 1 Å), respectively. These results show that the microscopic glass-transition temperature is about 30 K lower in 39 ± 1 Å hydrophilic carbon pores than 40 ± 1 Å hydrophobic silica pores. This reduction in the critical temperature is unusual, for example, by confining water in a hydrophilic porous silica matrix MCM-41-S (25 Å pore diameter), Faraone et al. reported the phenomenon of a dynamic crossover at TL ≈ 225 K44. In contrast, water confined in a hydrophobic substrate exhibits a lower dynamic crossover temperature at TL ≈ 190 K13.


The logarithmic relaxation process and the critical temperature of liquids in nano-confined states
The temperature dependence of B1(T) for salol in different states.Bulk salol and salol confined in Carbon pore (39 ± 1 Å) (a), Carbon pore (56 ± 1 Å) (b), Silica pore (40 ± 1 Å) (c) and Silica pore (60 ± 1 Å) (d). The linear fitting lines are extrapolated consistently to get the mode-coupling crossover temperature, Tc.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037365&req=5

f4: The temperature dependence of B1(T) for salol in different states.Bulk salol and salol confined in Carbon pore (39 ± 1 Å) (a), Carbon pore (56 ± 1 Å) (b), Silica pore (40 ± 1 Å) (c) and Silica pore (60 ± 1 Å) (d). The linear fitting lines are extrapolated consistently to get the mode-coupling crossover temperature, Tc.
Mentions: The parameters f(Q, T), H1(Q, T), H2(Q, T) and τβ(T) are obtained by fitting the curves in the measured time range. The Q dependence of the parameter H1(Q, T) is shown in Fig. 3. The solid line is the fitting line by H1(Q, T) = B1(T)Qb, where b can take a value between 1 and 2 for small Qs. The fitted B1(T) values are plotted as a function of temperature in Fig. 4. The Q and temperature dependence of other parameters are provided in the supplementary information (Figures 1S, 2S, and 3S). The temperature dependence of B1(T) is extrapolated consistently to get the MCT crossover temperature, Tc, also known as MCT critical temperature. According to MCT at this temperature long range liquid like molecular motion freezes and the transport mechanism becomes more like that of the hopping process in a solid41. The Tc of bulk salol is 255 ± 2 K, in reasonable agreement with the previous findings4042. As expected, this value is about 40 K above the calorimetric glass-transition temperature Tg (218 K), it is universally observed that the Tc is approximately 1.2Tg43. The present QENS data of salol confined in 39 ± 1 Å and 56 ± 1 Å carbon pores show the Tc are 204 ± 2 K and 214 ± 2 K, respectively. As the pore size decreases Tc also decreases and this observation is similar to that of other reported results1. However, in the silica pores with similar pore sizes the freezing temperature did not show an appreciable change. The freezing temperature obtained from the present QENS data of salol confined in 40 ± 1 Å and 60 ± 1 Å silica pores are 234 ± 2 K and 229 ± 2 K, respectively. The freezing temperature of salol in the confining geometry is shifted ~50 K and ~20 K to lower temperatures in the carbon pore (39 ± 1 Å) and silica pore (40 ± 1 Å), respectively. These results show that the microscopic glass-transition temperature is about 30 K lower in 39 ± 1 Å hydrophilic carbon pores than 40 ± 1 Å hydrophobic silica pores. This reduction in the critical temperature is unusual, for example, by confining water in a hydrophilic porous silica matrix MCM-41-S (25 Å pore diameter), Faraone et al. reported the phenomenon of a dynamic crossover at TL ≈ 225 K44. In contrast, water confined in a hydrophobic substrate exhibits a lower dynamic crossover temperature at TL ≈ 190 K13.

View Article: PubMed Central - PubMed

ABSTRACT

The logarithmic relaxation process is the slowest of all relaxation processes and is exhibited by only a few molecular liquids and proteins. Bulk salol, which is a glass-forming liquid, is known to exhibit logarithmic decay of intermediate scattering function for the β-relaxation process. In this article, we report the influence of nanoscale confinements on the logarithmic relaxation process and changes in the microscopic glass-transition temperature of salol in the carbon and silica nanopores. The generalized vibrational density-of-states of the confined salol indicates that the interaction of salol with ordered nanoporous carbon is hydrophilic in nature whereas the interaction with silica surfaces is more hydrophobic. The mode-coupling theory critical temperature derived from the QENS data shows that the dynamic transition occurs at much lower temperature in the carbon pores than in silica pores. The results of this study indicate that, under nano-confinements, liquids that display logarithmic β-relaxation phenomenon undergo a unique glass transition process.

No MeSH data available.


Related in: MedlinePlus