*2.4. Measurement of Water Self-Di*ff*usion Coe*ffi*cient at 20 MHz*

Diffusion measurements of water molecules are carried out on a Bruker Minispec (Bruker, Billerica, MA, USA) mq 20 bench-top relaxometer using a pulsed-field gradient spin echo (PFG-SE) sequence [48]. As previously described [42], the water self-diffusion coefficient, D, is calculated by linear regression of the echo attenuation versus the tunable parameter of the PFG-SE sequence, k, as showed in the following Equation (2):

$$\mathbf{k} = (\gamma \mathbf{g} \boldsymbol{\delta})^\* (\boldsymbol{\Delta} - \boldsymbol{\delta}/\mathbf{3}) \tag{2}$$

where γ is the proton's gyromagnetic ratio (equal to 42.58 MHz T−<sup>1</sup> ), δ is the length of the two gradients (set equal to 0.5 ms), g is the strength of the two gradients (varied between 0.5 and 2 T m−<sup>1</sup> ), ∆ is the delay between the two gradients (set equal to 7.5 ms).
