To express $\sqrt{-18}$ as a complex number, we first observe that $\sqrt{-1} = i$. Thus, we can write $\sqrt{-18}$ as $\sqrt{18} \cdot \sqrt{-1}$. Since $\sqrt{18} = 3\sqrt{2}$, we have $\sqrt{-18} = 3\sqrt{2}i$. Therefore, $\sqrt{-18}$ expressed as a complex number with the imaginary unit $i = -1$ is $3\sqrt{2}i$.
