Rheological modification of partially oxidised cellulose nanofibril gels with inorganic clays

This study aimed to quantify the influence of clays and partially oxidised cellulose nanofibrils (OCNF) on gelation as well as characterise their physical and chemical interactions. Mixtures of Laponite and montmorillonite clays with OCNF form shear-thinning gels that are more viscous across the entire shear range than OCNF on its own. Viscosity and other rheological properties can be fine-tuned using different types of clay at different concentrations (0.5–2 wt%). Laponite particles are an order of magnitude smaller than those of montmorillonite (radii of 150 Å compared to 2000 Å) and are therefore able to facilitate networking of the cellulose fibrils, resulting in stronger effects on rheological properties including greater viscosity. This work presents a mechanism for modifying rheological properties using renewable and environmentally-friendly nanocellulose and clays which could be used in a variety of industrial products including home and personal care formulations.

2. All the supporting information can be in one file (if possible). The citation of supporting information parts can be consistent such as " Figure SX or Table SX or Page SX in Supporting Information. Please insert page numbers for Supporting information file. Accordingly the figure captions can be revised.
The supporting information has been condensed into one file.
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Reviewer #1: 1. For comment 1, it still lack some experimental evidence or reference to confirm the mechanism for modifying rheological properties using clay, e.g. SEM or TEM to reveal the distribution of clay particles in OCNF gel, FTIR or Laser Raman or XPS to demonstrate the interactions between OCNF and clay platelets.
The authors appreciate the reviewer's concern. However, most of the above-mentioned techniques are inappropriate for this system and will not provide the information that the reviewer seeks. For example, the OCNF particles are too small to view in SEM and can only be visualised in TEM. However, the electron density disparity between clay particles and OCNF means that the distribution of clay within OCNF will be impossible to see using TEM as the OCNF will be essentially invisible. TEM images of OCNF are acquired by staining using a high electron density salt -however addition of salt will irreversibly alter any interactions present between clay particles and OCNF, thus destroying the information that is sought. OCNF samples for TEM are also typically diluted even further than in our solutions to avoid aggregation, allowing individual fibrils to be imaged on the grid -this would alter the interactions between any species present so they would not be representative of the distributions giving rise to the rheological behaviour reported. In addition, both SEM and TEM will require high vacuum, so the samples will need to be completely dried out. Even using supercritical drying, in such high-water-content systems, the structures are irreversibly altered by removal of water, meaning that the desired information on the distribution of clay particles within the OCNF network in the wet gels cannot be found. Thus, these two techniques are unsuitable to provide information on the distribution of clay particles in the OCNF gel network.
FTIR or laser raman techniques are useful techniques when a chemical reaction has occurred, altering the types of bonding present in the system so that patterns before and after the reaction can be compared. However here no chemical reactions occur between our components, and thus we do not expect large changes in the spectra using these techniques. In addition, the suspensions of clay and cellulose fibrils are rather dilute at maximum concentrations of 2wt% each, so the water signals dominate in the measurements of these systems, obscuring any tiny changes due to physical interactions between the components. FTIR spectra for the two separate components and their mixtures at equivalent concentrations in water are shown as an example below in Figure 1, for the highest concentrations studied, however this adds no useful information to characterise the interactions between the clay and OCNF in the wet gels -the spectra of the mixtures are clearly merely the sum of those from the two components. Thus so we do not feel that to adding them to the manuscript provides any useful information. We are also puzzled by the request for XPS measurements since this technique provides information about the elements present in a sample, specifically at the sample surface, so no bulk information on interactions can be obtained using this method -yet the interactions which modify the rheology in this system are occurring in the bulk. We also are not carrying out any chemical reactions in this system, simply physical mixing and so the particle compositions remain unchanged from those of the component materials, which are already well characterised by us and other researchers, as noted in the text. Additionally, measuring XPS on aqueous liquids requires highly specialised instrumentation since this technique also normally requires UHV conditions. Thus we are unable to carry out XPS measurements on our wet gel systems since we do not have access to such specialised equipment, however given it will only probe the liquid surface, it will also not provide the required information on interactions between clay and OCNF particles in the bulk.
The purpose of this paper was to demonstrate the effect of combining clay particles and OCNF on the viscosity of the formulation, and the rheological data is more than adequate for that. SAXS highlights that there are no direct interactions which altering the structures of the particles at the length scales measured, and thus that physical interactions on relatively large length scales are responsible for the changes in viscosity. Further characterisation of the underlying mechanisms is difficult due to the nature of the systems studied, and so is beyond the scope of the current paper.  2. For comment 6, it's means that it hard to understand Line 278 to 288. What is "The exponent (flow behaviour index)"," The base number (flow consistency index)". It is suggested to give more description of result, the equation or reference to calculate some index etc. to help readers to understand.