Characterizing Phase Transitions of Microfibrillated Cellulose Induced by Anionic and Cationic Surfactants

Abstract

Rheological modifiers are used to tune rheology or induce phase transitions of products. Microfibrillated cellulose (MFC), a renewable material, has the potential to be used for rheological modification. However, the lack of studies on the evolution in rheological properties and structure during its phase transitions has prevented MFC from being added to consumer, fabric, and home care products. In this work, we characterize surface-oxidized MFC (OMFC), a negatively charged colloidal rod suspension. We measure the rheological properties and structure of OMFC during sol-gel phase transitions induced by either anionic or cationic surfactant using multiple particle tracking microrheology (MPT). MPT tracks the Brownian motion of fluorescent probe particles embedded in a sample, which is related to the sample's rheological properties. Using MPT, we measure that OMFC gelation evolution is dependent on the charge of the surfactant that induces the phase transition. OMFC gelation is gradual in anionic surfactant. In cationic surfactant, gelation is rapid followed by length scale-dependent colloidal fiber rearrangement. Initial OMFC concentration is directly related to how tightly associated the network is at the phase transition, with an increase in concentration resulting in a more tightly associated network with smaller pores. Bulk rheology measures that OMFC forms a stiffer structure but yields at lower strains in cationic surfactant than in anionic surfactant. This study characterizes the role of surfactant in inducing phase transitions, which can be used as a guide for designing future products.

Figure 1

Bi-disperse MPT measures the ensemble-averaged MSD of probe particles in 0.63 wt % OMFC contacted with 5 wt % SDS to induce gelation. MSDs are measured simultaneously by (a) 0.5 μm (red) and (b) 2.0 μm (blue) probe particles in a single sample. The shade of the MSD markers indicates the time data are collected during gelation. For both particles, the magnitude and logarithmic slope of the MSD decreases as time increases. This indicates that the motion of both-sized probe particles becomes slower and more restricted. Measurements of particle static error are indicated on each graph in black.

Figure 2

Temporal bi-disperse microrheological measurements of the gelation of 0.63 wt % OMFC.5 wt % SDS is used to induce gelation. The logarithmic slope of the MSD, α, is plotted vs normalized time, t_n, for 0.5 and 2.0 μm probe particles. α values decrease as time increases, indicating that the system structure is growing into a network that is restricting probe particle motion. Cartoons of the likely colloidal fiber structures are provided.

Figure 3

Temporal bi-disperse MPT measurements of gelation of 0.32, 0.63, and 1.27 wt % OMFC. 5 wt % SDS is used to induce gelation. The logarithmic slope of the MSD, α, is plotted versus normalized time, t_n, for 0.5 and 2.0 μm probe particles. For all OMFC concentrations, α values decrease as time increases, indicating that the system structure is growing into a network. The system structure is more tightly associated at the highest OMFC concentration.

Figure 4

Time-cure superposition (TCS) analysis of bi-disperse MPT data measured with 2.0 μm particles of 0.32 wt % OMFC gelation in 5 wt % SDS. (a) Measured MSD data collected of the OMFC system as time increases. (b) MSDs are shifted into gel and sol master curves using lag time shift factor, a, and MSD shift factor, b. (c) Shift factors a and b diverge at the critical gelation time, t_c. (d) Fitting the logarithm of the shift factors versus the logarithm of the distance away from t_c, , calculates the scaling exponents y and z and the critical relaxation exponent n.

Figure 5

Temporal bi-disperse MPT measurements of the gelation of 0.63 wt % OMFC in contact with 5 wt % BDDAB. The logarithmic slope of the MSD, α, is plotted vs normalized time, t_n, for (a) 0.5 and (b) 2.0 μm probe particles. These measurements characterize rapid gelation of OMFC when BDDAB is used to induce gelation. After gelation, the OMFC fibers rearrange, which gives space for the smaller probe particles to increase their diffusivity while the larger particles remain trapped in the gel network. Cartoons are provided to illustrate the likely structure of the colloidal fibers.

Figure 6

Temporal bi-disperse MPT measurements of the gelation of 0.32, 0.63, and 1.27 wt % OMFC contacted with 5 wt % BDDAB. The logarithmic slope of the MSD, α, is plotted as a function of normalized time, t_n, for both 0.5 μm probe particles and 2.0 μm probe particles. For all OMFC concentrations and length scales, the change of α indicates that gelation is rapid and is followed by colloidal rearrangement measured on the smaller length scale. The system structure is more tightly associated at the highest OMFC concentration.

Figure 7

Average time for particles to start diffusing in the network after gelation of 0.32, 0.63, and 1.27 wt % OMFC in 5 wt % BDDAB. As the concentration of OMFC increases, more time is required to release larger-sized particles.

Figure 8

Bulk rheological measurements of 1.27 wt % OMFC immersed in 5 wt % SDS or BDDAB using (a) a time sweep at a constant frequency (1 Hz) and strain (0.1%) and (b) a strain sweep at a constant frequency (1 Hz).