Molecular order and dynamics of water in hybrid cellulose acetate–silica asymmetric membranes

ABSTRACT

In this work ²H NMR spectroscopy and ¹H NMR relaxometry and diffusometry were used to characterise water order and dynamics in cellulose acetate/silica asymmetric membranes. Two hydrated membranes were characterised allowing the identification of extra ordering of the water molecules and the presence in each membrane of up to two spectral components with different degrees of order and different $T_1$ values. The mechanism behind this order increase was ascribed to the rapid exchange of the water molecules between the pore walls and its interior. $T_1$ relaxometry dispersions allowed for the identification of the relevant mechanisms of pore-confined water motion, with rotations mediated by translational displacements (RMTD) as the dominant mechanism in the low frequency region. Using the RMTD low cut off frequency along with the in situ directly measured diffusion constant it was possible do determine characteristic lengths of correlated water motion in both membranes studied, which fall in ranges compatible with typical pore dimensions in similar membranes.

GRAPHICAL ABSTRACT

KEYWORDS:

• Cellulose acetate
• asymmetric membranes
• proton relaxometry
• dynamic processes
• molecular dynamics

Acknowledgements

The authors wish to acknowledge the Portuguese Fundação para a Ciência e a Tecnologia – FCT projects UID/CTM/01540/2013, UID/CTM/50025/2013, and PTDC/CTM-BIO/6178/2014. The authors also acknowledge COST Action CA15209 EURELAX-European Network on NMR Relaxometry (http://www.cost.eu/COST_Actions/ca/CA15209).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The NMR spectrometers are part of The National NMR Facility (PTNMR), supported by Fundação para a Ciência e a Tecnologia [ROTEIRO/0031/2013 – PINFRA/22161/2016] (co-financed by FEDER through COMPETE 2020, POCI, and PORL and FCT through PIDDAC).