A variable-temperature nuclear magnetic resonance method for determination of Krafft point and CMC of surfactants.

Understanding physicochemical properties is crucial for rationally designing surfactants. Particularly the Krafft point (TKP), below which surfactants become insoluble and lose their applicability, has been of interest for industry. Conventional methods, such as visual observation, conductivity, or differential scanning calorimetry (DSC), detect the TKP indirectly through macroscopic changes in turbidity, conductivity or heat flow, which may lack precision and sensitivity, and do not directly measure or quantify surfactant concentration directly. We demonstrate a straightforward, variable-temperature nuclear magnetic resonance (VT-NMR) method to determine the Krafft point, the clearing point temperature (TCP), and the critical micelle concentration (CMC) of surfactants precisely and reproducibly by measuring the ratio of the integrals of surfactant and H₂O as a function of the temperature. In contrast to conventional methods, VT-NMR directly measures and quantifies dissolved surfactant concentration, connecting the Krafft phase transition to the temperature-dependent solubilization. This approach provides the solution-phase concentrations of surfactant, and changes in this concentration signal the where the solubility reaches the CMC (TKP and CMC) and when reaching full solubilization (TCP). Solid-state NMR results confirm that the remainder is crystalline solid. We used six different surfactants with largely varying CMCs and Krafft points to illustrate the scope of the method; all values agree well with literature. The addition of 1-dodecanol to SDS demonstrates the applicability of the method for impure mixtures or formulations. In addition, it was shown that this method can be performed on a benchtop NMR, providing a cost-effective and accessible platform.