(167g) Rheological Study of Clay-Zwitterion Interaction: A New Perspective on Functionalization of Clay with Zwitterions

The increasing use of non-sustainable materials in technology such as transportation, energy, medicine, and manufacturing has negatively impacted the environment and human well-being. In response to these challenges, almost all technologies are propelled to search for a more efficient, cleaner, sustainable material. The development of cost-effective materials is crucial in moving towards a more sustainable society. Clay is a prominent example of such exemplary material that has an abundant natural supply, is non-toxic, easily recyclable, and the reasonable price makes it a highly desirable material for use in sustainable technologies and environmentally friendly projects. Clay is a naturally occurring material with a long history of human application dating back thousands of years. The demand for clay continued even in modern times. In modern technological development, the application of clay has diversified in various industrial uses, such as pharmaceutics, agriculture, petroleum and gas, ceramics, cosmetics, paints, and coating, etc. Among the clay minerals, bentonite clay, a member of the smectite clay family, has attracted much attention because of its unique properties, such as large surface area, high cation exchange capacity, good swelling properties, and high thermal stability. These properties of bentonite clay make it a suitable material for a wide range of applications, including sensing, drug delivery, gas sorption, energy storage, and many more.

Bentonite clay has a unique layered structure composed of one alumina octahedra sandwiched between two silica tetrahedra. The layered structure stack on top of each other and are held by a Vander Waals bond and weak cation-to-oxygen linkage. The bentonite clay layer has a space between them that can hold small cations and water molecules because of the negative charge on its surface. The negative charge, along with exchangeable cations within its layers, allows the organic interaction to modify it for various applications. Incorporating organic molecules into the interlayer spacing of clay imparts new functional properties to the clay. Alkylammonium ions, peptides, and amino acids are commonly used organic molecules or surfactants for modifying bentonite clay. The interaction and intercalation of these organic molecules into the clay layer led to alterations in the rheological properties of the clay. Rheology helps in understanding the material flow and deformation under stress and force, which in turn helps to predict the properties of the final product.

In this study, we have studied the rheological properties of clay-zwitterion interaction. A zwitterion is a molecule with net zero charges that contains both positively and negatively charged functional groups within its structure. The zwitterionic molecule used in this experiment is trimethyl glycine, commonly called betaine. Betaine contains quaternary ammonium and a carboxylic group as positive and negative functional groups, respectively. Furthermore, we also synthesized the longer carbon chain length betaine molecules in our laboratory to study how varied betaine molecules with a different number of carbon chains interact with bentonite clay and impact its rheological properties. Furthermore, the pH of the slurry also affects the interaction between the clay particles as it alters the surface charge of the clay platelets, which in turn changes the rheological properties. Therefore, we have also examined the rheological properties of clay-zwitterion interaction at different pHs. In situ ATR-FTIR was used to confirm the functionalization of betaine inside the clay gallery.

The zwitterion functionalized clay slurries were prepared by mixing 20-weight percent of bentonite clay in deionized water. An equimolar quantity of varied carbon chain length zwitterions was added to the clay slurry. To allow complete intercalation and functionalization, the slurries were kept in a mechanical shaker overnight. The prepared zwitterion functionalized clay slurries were then named BB-0, BB-1, BB-3, and BB-5, where BB-0 represents the clay slurry without zwitterion functionalization, whereas BB-1, BB-3, and BB-5 represent the clay slurry functionalized with C1-, C3-, and C5- zwitterion respectively. The rheological data was collected using TA instruments Discovery HR-20 rheometer. The flow sweep, amplitude sweep, frequency sweep, and thixotropy were carried out to study the rheological properties of zwitterion functionalized clay slurry.

The structure of the procured and synthesized betaine and the rheological data are provided in the figure attached separately. The clay slurry with and without betaine functionalization showed a shear thinning behavior, meaning the viscosity changes with the shear rate. This is a typical phenomenon in clay slurries. The clay particles are randomly oriented at rest. When the shear stress is applied, the clay particles orient themselves in the direction parallel to the applied shear stress. This makes it easier for the clay particles to slide over one another. This phenomenon is observed in flow sweep with decreasing viscosity at a higher shear rate. The higher viscosities of the zwitterion (betaines) functionalized clay samples (BB-1, BB-3, and BB-5) suggested the aggregation caused by the intercalation of betaine molecules. The interactions between the betaine molecules and clay caused increased agglomeration, resulting in a higher viscosity. Similarly, the amplitude sweep test showed higher linear viscoelastic reason (LVR) for BB-0 and BB-1. This suggests that the stiffness of the clay slurry is less pronounced with increasing carbon chain betaines. This might be because of the different orientations of the betaine molecules inside the clay gallery. The amplitude sweep test suggested that the clay functionalized with smaller carbon chain betaine has more desirable characteristics from the structural point of view because of increased storage modulus without affecting the LVR. Furthermore, the frequency sweep, time sweep, and thixotropy experiments also provided, helpful information about the stability of the functionalized clay slurries.

Overall, we used rheology to study the interaction between the clay and zwitterions (betaines) and how these interactions impact the structure and properties of bentonite clay. Our findings provide valuable information about the effects of altering the number of carbon atoms, the chain length, and pH on the rheological properties of functionalized clays. These results provide valuable information in the scientific community and industries utilizing functionalized clays for various applications.