Membranes based on graphite oxide (GO) are promising materials for the separation of polar liquids and gases. Understanding the properties of solvents immersed in GO is important for the development of various technological applications. Here, the molecular motions of the TEMPO nitroxide spin probe in acetonitrile intercalated into the GO inter-plane space were studied using electron paramagnetic resonance (EPR), including its pulsed version, and electron spin echo (ESE). For a sample containing 75% acetonitrile relative to equilibrium sorption at room temperature, ESE-detected stochastic librations were observed for TEMPO molecules above 135 K. Since these librations are an inherent property of molecular glasses, this fact indicates that intercalated acetonitrile forms a two-dimensional glass state. Above 225 K, an acceleration of stochastic librations was observed, indicating the manifestation of a glass-like dynamical cross-over. Continuous wave (CW) EPR spectra of TEMPO showed the absence of overall tumbling motions in the entire investigated temperature range of up to 340 K, indicating that the intercalated acetonitrile does not behave as a bulk liquid (the melting point of acetonitrile is 229 K). Dynamical librations of TEMPO molecules detected by CW EPR were found to accelerate above 240 K.
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