Evaluation of microscale thermophoresis-based methods for critical micelle concentration determination using the RGBfastCMC model

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Critical micelle concentration (CMC) is an important physicochemical parameter used for surfactant characterization. However, methods for its determination remain poorly explored within the concepts of Green and White Analytical Chemistry. In this work, the RGBfastCMC model was developed as a dedicated extension of the RGBfast model, integrating analytical performance, environmental impact, and practical applicability using criteria adapted specifically to equilibrium-related measurements. The main objective was to retrospectively evaluate microscale thermophoresis (MST) and stop-flow microscale thermophoresis (SF-MST) as emerging approaches for CMC determination, thereby complementing our previous studies on MST-based methodologies. Conductometry and capillary electrophoresis (CE) were included as reference techniques. Sodium dodecyl sulphate was used as a model surfactant. All investigated methods provided CMC values consistent with literature data, ranging from 7.03 to 8.05 mM. MST-based methods exhibited substantially higher sensitivity toward the micellization breakpoint, and their greenness assessment based on quantitative empirical indicators demonstrated clear advantages in low reagent consumption, low chemical risk (ChlorTox values), and reduced carbon footprint compared to CE. The final whiteness (W) scores indicated that conventional MST provided the most balanced overall profile (W=61), followed by SF-MST (W=56), conductometry (W=49), and CE (W=44). The proposed RGBfastCMC model represents the first dedicated WAC-oriented framework for the evaluation of physicochemical methods for CMC determination and, by relying on transparent quantitative criteria and experimentally derived indicators, reflects the principles of Good Evaluation Practice. The approach can be readily adapted to other equilibrium-based analytical methodologies.