Yu. B. Grunin

Microstructure of cellulose: NMR relaxation study

The models of structural organization of cellulose are reviewed within the framework of the semi-crystalline concept. Special attention is given to two crystalline modifications of cellulose: Iα and Iβ. The results of low-resolution 1H NMR studies of the supramolecular structure of cellulose are considered. The mathematical model describing the line shape of the solid-state NMR spectrum of cellulose is presented. A scheme for the structure of cellulose microfibrils that fits most of the known model concepts is proposed. It is shown that this scheme can be used to determine the major supramolecular parameters of this polymer.

Features of the sorption of water vapor and nitrogen on cellulose

Molecular-kinetic parameters of adsorptives, i.e., water (at 300 K) and nitrogen (at 77 K) vapors, are calculated and compared at the initial steps of their adsorption by cellulose. The role of the dipole structure of water molecules is considered upon their interaction with active centers of cellulose, forming heterogeneous electric fields in its pores. The effect of the temperature of the adsorptive and the sizes of its molecules on activation penetration through narrowings of the micropores dominant in absolutely dry cellulose due to the mobility of its structure is determined. The development of a porous system upon water adsorption is demonstrated according to 1H NMR. It is concluded that low-temperature nitrogen adsorption on cellulose yields rather limited information on its structure and adsorption properties.

Study of the Structure and Sorption Properties of Some Types of Cellulose

The pulsed NMR, isopiestic and pycnometric methods are applied to investigate the physicochemical properties of softwood [GOST (State Standard) 9571-89] and hardwood [GOST 28172-89] celluloses, as well as flax cellulose [GOST 10330-76, 9394-76], both in the initial state and subjected to chemical treatment. Possible changes occurring in the supramolecular structure of a polymer are analyzed. The structural and sorption characteristics of the investigated samples are calculated. Their water absorption capacities are determined.

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T1) and spin–spin (T2) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T1 and T2 on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.

Features of the structural organization and sorption properties of cellulose

Modern concepts about the structural organization of cellulose and the character of formation of its inter- and intermolecular hydrogen bonds are analyzed. A scheme for the structure of the cellulose microfibril that supposes the presence of slit-shaped micropores in its structure is proposed. The proton magnetic relaxation study of sorption processes reveals that, at a moisture content of 7–10%, the filling of cellulose micropores occurs with a concomitant increase in their lateral sizes and decreases in the degrees of crystallinity of samples.

Pulsed NMR method for determining the thermodynamic characteristics of adsorption processes in biopolymers

The efficiency of the pulsed NMR method in studying phenomena in the interphase layer of biopolymer-water systems within the framework of the Gibbs thermodynamic theory was examined. Relationships between the nuclear magnetic relaxation times and the chemical potential and surface tension coefficient at the interphase interface of the adsorption system were established. How the thermodynamic and relaxation parameters change during the formation of the adsorption Gibbs layer on various cellulose samples was demonstrated. An analysis of the state of the water in the adsorption layer was performed, and its mean thickness was determined.

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of Water Molecules in Active Carbons. 1. Relation between the Spin-Spin Relaxation of Adsorbed Water Molecules and Structural Parameters of Microporous Active Carbons

Relation between the spin-spin nuclear magnetic relaxation time T2 of adsorbed water molecules and parameters of microporous structure of carbon adsorbents is disclosed. The pattern of dependences of T2 on the relative pressure and the number of water molecules per one primary adsorption site (PAS) is governed by the pore sizes and the number and nature of PASs. At a complete micropore filling, the T2 value depends on the volume density of PASs in active carbons. In the absence of PASs in the micropores, T2 is equal approximately to 21 ms. The larger the volume density of PASs, the smaller the number of water molecules per one PAS at the complete filling of micropores; i.e., the looser the packing of water molecules. The results of studying active carbons by the pulsed 1H NMR method agree well with the data of the adsorption method.

Proton magnetic relaxation study of the thermodynamic characteristics of water adsorbed by cellulose fibers

The possibility of determining the thermodynamic parameters that characterize the sorption properties of cellulose and the state of water associated with its fibers is demonstrated using modern concepts of the structure of this vegetable polymer and methods based on theories of adsorption and NMR relaxation in heterogeneous systems.

Supramolecular Reorganizations in Cellulose during Hydration

Modern views of the structural organization of cellulose microfibrils were analyzed. A mechanism whereby an additional capillary-pore system forms in cellulose during its moistening was proposed. With the moisture reaching 8–10% in cellulose, its micropores were found to be filled and to increase in cross size and specific surface, while the crystallinity decreased. The parameters of the supramolecular structure and capillary-pore system of cotton cellulose were determined in the context of the microfibril structure model.

Energy characteristics of adsorbed water in active carbons according to the NMR relaxation data

Nuclear magnetic relaxation measurements were used to determine activation energy Eact of the motion of water molecules adsorbed in active carbons. The Eact value was found to depend on the filling of active carbon pores due to changes in the state of water molecules under adsorption. It was established that the Eact = f(p/ps) plots, where p/ps is the relative pressure of water vapor observed for microporous active carbons (FAS-1, 2, N-15, SKT-6A), are similar in form to the corresponding plots of changes in water adsorption heats. In particular, we concluded that the plateau in the Eact = f(p/ps) dependences, as in the case of adsorption heats, reflects the volumetric filling of active carbon micropores with water. We show that a linear function describes the increase in Eact values for water upon the complete filling of micropores with an increase in the volume of adsorbed water clusters per one primary adsorption center (W0/am). We establish that, for water in the FAS-3 sample, the deviation of Eact values from this linear function was due to the contribution from the vapor phase in the mesopores (x0 = 0.7−1.2 nm) that make up a considerable part of the active carbon’s porous system.