Surface Properties of Colloidal Quantum-Confined One-Dimensional Lepidocrocite Titanates: Insights into their Ion-Induced Gelation

The surfaces of 1D layered lepidocrocite-structured titanates (1DLs) are negatively charged due to an oxygen-to-titanium atomic ratio >2. This, and their layered structure, allow for facile ion exchange and high colloidal stability, demonstrated by ζ-potentials of ≈ -85 mV at their unadjusted pH of ≈10.4. This is nearly maintained across a 20 to 70 °C temperature range, with only a slight decrease in stability. The acid resistance of 1DL solids (little dissolution until pH 1) is demonstrated through inductively coupled plasma mass spectrometry. The Fourier transform infrared spectra of the dried 1DLs are also discussed. From a fundamental charge perspective, these materials offer an ion exchange capacity of ≈1.8 mmol/g, nearly <i>twice</i> that of highly charged clays or Nafion. As a Brønsted-Lowry base, they readily adsorb protons onto their heterogeneous surfaces, as illustrated by an isothermal adherence to the Freundlich model. 1DLs have two p<i>K</i>_a values, one at pH ≈10.9 and the other at ≈3.2, and can be protonated to their point of zero charge (≈pH 6.8) before they destabilize. With the understanding of the acid/base properties of 1DLs, cation-stabilized hydrogel-like solids were formed using H⁺, Li⁺, Na^+, K⁺, Mg^2+, Ca²⁺, Ba²⁺, and Fe³⁺. A gelation mechanism is proposed that relies on cation exchange being the driving force for water removal from between adjacent 1DLs. The rheological properties of the soft H₃O⁺-cross-linked gel-like solids show a more than 1000-fold increase in the viscosity of the 1DL colloidal suspensions compared to before gelation.