Revealing Sorption/Desorption Properties of Micro-Crystalline Cellulose by Means of STA

Introduction

Moisture may affect the properties of a wide range of active ingredients and excipients in terms of their stability, crystallinity, bio-availability, etc. One method for determining the influence of humidity on a substance’s behavior is dynamic vapor Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption (DVS), in which the mass changes in the sample are measured for different solvent vapor amounts, e.g., water vapor. [1] 

Such measurements can be carried out with an STA (simultaneous thermal analyzer) connected to a modular humidity generator (figure 1). In the following, a dynamic water Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption measurement was carried out on micro-crystalline cellulose (MCC, chemical structure in figure 2). This substance is used in tablet formulations as filler and binder. [2]

1) STA 449 F3 Nevio and modular humidity generator
2) Chemical structure of micro-crystalline cellulose

Measurement Conditions

The experimenal conditions are summarized in table 1.


Table 1: Conditions of the test

Device

STA 449 F3 Nevio connected with the humidity generator

Sample

Micro-crystalline cellulose

Sample mass

41.22 mg

Sample holder

Plate made of alumina, Ø 17 mm

Temperature program

Isothermal 44°C, nitrogen atmosphere, relative humidity (RH) increased

from 0 to 80%

Measurement Results

Figure 3 displays the measured sample mass and temperature during the experiment. 

The results demonstrate the strong hygroscopic nature of the micro-crystalline cellulose. The first increase in relative humidity from 0% to 20% (blue dashed curve) induces a mass increase of 4% (green curve). The subsequent steps show that the higher the relative humidity, the higher the mass gain. As soon as the humidity level decreases, the absorbed and/or adsorbed water is released, resulting in a mass loss. When a completely dry atmosphere is finally reached at the end of the measurement, the amount of water absorbed and/or adsorbed will have been quantitatively released. This can be confirmed by having arrived back at the initial sample mass (100%). 

Each change in relative humidity level is associated with a peak in the sample temperature curve (pink curve). This is due to the EsotermicoUna transizione o reazione è esotermica se il materiale genera calore exothermal and endothermal natures of the Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption and desorption of water, respectively.

3) Mass and temperature results for micro-crystalline cellulose in an atmosphere with different humidity levels (0%, 20%, 40%, 60% and 80%)

The mass gain and loss after reaching equilibrium are given in figure 4 for all measured relative humidity levels between 0% and 80%. The maximum mass increase amounts to 12% for a relative humidity of 80%. Microcrystalline cellulose exhibits Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption hysteresis, i.e., the water amount in the sample is higher during desorption than during Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption (see figure 4) but, ultimately, the starting point and ending point of the Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption/desorption cycle are identical.

This hysteresis phenomenon is typical for many porous materials. Chen et al [3] showed that the water-cellulose bonds formed during swelling of cellulose do not break upon desorption at the same chemical potential.

4) Difference in the moisture content of the sample between Sorption ProcessSorption is a physical and chemical process by which a substance (typically a gas or liquid) accumulates within another phase or on the phase boundary of two phases. Depending on the place of accumulation, a differentiation is made between absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary).sorption and desorption

Literature

  1. [1]
  2. [2]
  3. [3]