*2.1. Neutron Scattering Method and SANDALS*

The total neutron scattering experiment is an important method for the study of the atomic structure of liquids. In neutron scattering, the observed neutron structure factor (*F(Q)*) and the atomic structure of the sample have the relationship:

$$F(Q) = \sum\_{a=1}^{N} c\_a b\_a^2 + \sum\_{a=1, \notin \mathcal{S} \ge a} (2 - \delta\_{a\beta}) c\_a c\_\beta b\_a b\_\beta \{4\pi \rho \int\_0^\infty r^2 (g\_{a\beta}(r) - 1) \frac{\sin(Qr)}{Qr} dr\} \tag{1}$$

where *Q* and *r* are the momentum transfer and the distance between the two atoms in the sample; *ci* and *bi* (*i = α*, *β*) are the quantity ratio and neutron scattering length of *i* species; *δαβ* is the Kronecker *δ* function; *ρ* is the atomic number density of the sample; and *gαβ (r)* is the radial distribution function (hereinafter referred to RDF), which reflects the microscopic atomic structure of amorphous matter.

$$g\_{a\beta}(r) = \frac{n\_{a\beta}}{4\pi r^2 \, dr \rho\_{\beta}}\tag{2}$$

where *nαβ* is an average number of β atoms around an *α* atom contained in a spherical shell with *r* radius and *dr* thickness; *ρβ* is the average number density of *β* atoms in the sample; and *gαβ(r)* describes the number density change of the *β* atom as the function of the distance from the *α* atom, which reflects the interaction between atoms *α* and *β*. We used *g(1)αβ(r)* to mark the first peak of *gαβ(r)*. It represents the closest, most probable distance between atoms *β* and *α*. The peak height indicates the ratio of the atomic number density of atom *β* to its average value at this position.

The neutron total scattering experiments were performed at SANDALS in ISIS (Didcot, UK) and Multi-Physics Instrument (MPI) in CSNS (Dongguan, China). The scattering vector (Q) range was from 0.1 Å−<sup>1</sup> to 50 Å−1. This means that the instrument could only measure the micro-structure of samples from ~0.1 Å to ~30 Å. Therefore, we chose trehalose as the model molecule for cellulose. The size of trehalose is ~11.6 Å; we could, thus, use a neutron total scattering instrument to observe its microscopic atomic structure in alkali/urea aqueous solution.

#### *2.2. EPSR Simulations*

The empirical potential structure refinement (EPSR) is a program developed by Prof. Soper to explore the most probable, all-atom structure of an experimental sample based on neutron scattering [11]. EPSR is essentially a Monte Carlo simulation program. In EPSR, there are two kinds of potential energy: reference potential and experimental potential. The reference potential is taken from the molecular dynamics simulation force field, which is used to realize the basic structural constraints of the simulation system, such as molecular structure, the minimum distance between atoms, etc.; the experimental potential is the structural data observed in the neutron scattering experiment [11,12]. After the experiment, we used EPSR simulation to reconstruct the atomic structure of the experimental sample.
