Fabrication of ballpoint-ink via encapsulating inorganic pigments in microemulsion gels

The fabrication of ballpoint-ink might open up a new perspective on physico-chemical solubility thermodynamics. In this report, we present a method to encapsulate inorganic pigments, such as Fe(CNS)₃ (red), Fe₂Fe(CN)₆ (blue), CdS (yellow), and CuS (black) into w/o microemulsion gels. The area of w/o microemulsions was first determined by titrating surfactants Tween-60 into the given composition of water and cyclohexane in the pseudo-three phase diagram. Three prosperous phase areas were successfully mapped using this method, namely: microemulsion (w/o, and o/w) or (μE), lamellar liquid crystal (L_α), and hexagonal liquid crystal (H_α), respectively. The results show that inorganic pigments were well soluble in the w/o microemulsion gel of the Tween-60/cyclohexane/water system. The highest solubility of inorganic pigments in the microemulsion gel is 3.63 ± 0.05 mg g^-1 for the red pigment of Fe(CNS)₃, and the lowest is 2.92 ± 0.05 mg g^-1 for the yellow pigment of CdS. Hence, the solubility limit distribution for all pigments is 2.9 ± 0.05 <>3.63 ± 0.05 mg g^-1. The cation and anion size strongly affected the inorganic pigments' solubility in the w/o-microemulsion system. Some quantity of the ink-made of inorganic pigments encapsulated in the microemulsion gel has been inserted into empty ballpoint sleeves as prototypes. The resulting self-made inks demonstrated that the physical appearances of the ink could mimic the factory-made inks. Nevertheless, the self-made ink should be investigated further for long lifespan use, especially for long-term stability and corrosion resistance.