The surface free energy of mineral particles is a key factor affecting the bubble–particle attachment and mineralization process. In this study, the free surface energy was calculated in order to clarify the interaction mechanism between low-rank coal and dodecyltrimethylammonium chloride (DTAC) from the perspective of thermodynamics. Additionally, wetting heat was employed in order to reveal the change in the surface wettability of long-flame low-rank coal with different DTAC additions, i.e. 0, 15, 30, 45, and 60 g t−1. The wetting heat of long-flame low-rank coal gradually became less negative with the increase in the DTAC dosage, which indicated that the hydrophilicity of the low-rank coal’s surface was effectively restrained, corresponding to enhanced hydrophobicity. Furthermore, the effect of DTAC on the attachment time (induction time) between the coal particles and bubbles was investigated. The results illustrated that the attachment time between the coal particles and the bubbles decreased from 180 to 30 ms as DTAC dosage increased (DTAC dosage of 0–60 g t−1). That is, the attachment time decreased by up to 83.33% over the investigated range of DTAC dosages; this dynamic was consistent with the results of the calculation of the surface free energy and the measurement of the wetting heat. The mean zeta potentials became less negative with the DTAC addition in the presence and absence of diesel oil. Consequently, DTAC enhanced the flotation performance of long-flame low-rank coal in a flotation system; furthermore, the combustible recovery was significantly enhanced (by up to 11.29% points) and the concentrated ash value was less enhanced (by up to 0.20% points) as the DTAC dosage increased up to 60 g t−1.