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 (T_KP), 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 T_KP 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 (T_CP), 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 (T_KP and CMC) and when reaching full solubilization (T_CP). 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.