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in SWCNT studies could also be used to exfoliate graphite into graphene. This is the reason, therefore, that many of the βgoodβ solvents originally used in SWCNT dispersion are also common in the more recent research on liquid-phase exfoliation of graphene. Towards the end of the last decade, a comprehensive experimental campaign was undertaken to analyse the effectiveness of numerous solvents to exfoliate and disperse graphene 18,62. Hernandez et al. 62 presented graphene concentration data for 40 different solvents. Cyclopentanone (CPO) provided the highest concentration, and N- Methyl-Pyrrolidone (NMP) was also a high performing solvent. The latter has become one of the most popular solvents in liquid-phase exfoliation studies to date. Water was a poor solvent for graphene exfoliation and dispersion, positioned 35 out of 40, and providing concentrations 1/8 of that achieved with CPO. This highlights one of the primary challenges in scaling-up solvent-based liquid- phase exfoliation. Many of the good solvents, such as NMP, have a high boiling point and are toxic. In an effort to correlate the experimental findings, and provide a physical interpretation behind the solvent ordering, the authors considered surface energy as a parameter 18 along with Hildebrand and Hansen solubility parameters 62. The overarching idea, linked to similar work on SWCNTs 64, is that suitable solvents can be selected through minimization of the enthalpy of mixing. The initial work proposed that matching solvent and graphite surface energy would minimize the energetic cost of exfoliation. The recommendation was the use of solvents with surface tension close to 40 mJ/m2. In a follow-on study, Hernandez et al. 62 suggested that although surface tension is a useful parameter to assess the overall interaction, the intermolecular interactions could be divided into at least three types based on the Hildebrand and Hansen solubility parameters. The cost of mixing solute-solvent was related to the Flory-Huggins interaction parameter, which scales as π β½ π 2. Here, R represents the Hansen solubility sphere between solute (1) and solvent (2), and π 2 = 4(πΏπ·1 β πΏπ·2)2 + (πΏπ1 β πΏπ2)2 + (πΏπ»1 β πΏπ»2)2. The dispersive, polar and hydrogen-bonding components are defined by πΏπ·, πΏπ and πΏπ» respectively 65. The cost of mixing is, therefore, reduced when small values of R are achieved. This is the commonly known βlike dissolves likeβ concept. 20PDF Image | graphene production via nonoxidizing liquid exfoliation
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