Rudolf Clausius (1822-1888), whose inquiries into the concept of entropy had a lasting influence on the young Planck.
  Thermodynamics was the subdiscipline of physics to which Max Planck took a special interest as a researcher. He focused on the two main laws of thermodynamics and, in particular, the concept of entropy. Taking up the thread from his doctoral and habilitation theses, he concentrated his early efforts on a thorough examination of the consequences of the second law and the significance to physics of the concept of entropy. He investigated the conditions of equilibria in physical and chemical processes, in particular how bodies transform between the solid, liquid, and gaseous states. He also included a study of electric processes in solutions (the theory behind electrolysis). Important results were made in providing a thermodynamical explanation for the laws governing the lowering of the freezing point and rise of the boiling point of a dilute solution.

Planck's thermodynamic studies were substantial contributions to the theoretical underpinnings and conceptual foundations of physical chemistry. These papers made his fame as an important physicist of the day and are interspersed throughout the course of his working life. In 1910 he arrived at the final phenomenological formulation of Nernst's heat theorem, and in 1934 in one of his last scientific papers he articulated the Le Chatelier-Braun principle in its final form.

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  "Planck's papers distinguish themselves very favorably from those of the majority of his colleagues in that he seeks to carry through the strict consequences of thermomechanics constructively, without adding other hypotheses, and carefully sunders the secure from the doubtful ... his papers ... clearly reveal him as a man of original ideas who is breaking his own paths (and) that he has a comprehensive overview of the various sections of science."
Excerpted from the appointment proposal by the Faculty of Philosophy at the University of Berlin, 1888.