## 4 Coupled Thermographic and Spectral Analyses  
The Raman spectra of the RDP or the brine contain peaks characteristic of the chemical bonds constituting them and their concentration. In this study, we focused on the peak at 930 cm\({}^{-1}\), directly related to RDP. We monitored its intensity over the course of the experiment to better understand its development. We therefore traced the evolution of the minimum temperature reached by the RDP over time. To achieve this, however, we required a radiative correction of the thermal data by applying Equation (1) [36].  
\[T_{measured}^{4}=\varepsilon_{sample}\times T_{sample}^{4}+\left(1-\varepsilon _{sample}\right)\times T_{environment}^{4} \tag{1}\]  
where \(T_{measured}\) corresponds to the corrected thermal value, \(\varepsilon_{sample}\) is the ice emissivity (0.97) [37], \(T_{sample}\) is the measured temperature of the sample during the test, and \(T_{environment}\) is the mirror temperature during the test.  
The top curve in Figure 8A shows the evolution of the significant peak of the RDP at 930 cm\({}^{-1}\) over time, and the bottom curve presents the evolution of the minimum temperature of the RDP over time.  
Inflections in both Raman intensity and temperature occurred simultaneously. Between 0 and 188 s, the temperature and Raman intensity increased as ice melted and concentrated brine formed. Between 188 and 503 s, the 930 cm\({}^{-1}\) peak intensity plateaued,  
Figure 8: Coupled thermographic and spectral analyses; (**A**) maximum intensity of a Raman peak at 930 cm\({}^{-1}\) and minimum temperature during the test with 0.05 mL of the RDP sodium formate (NaFo) at \(-10\) °C; (**B**) RDP with rhodamine B melting on ice at \(-10\) °C; and (**C**) a 2D representation of the RDP de-icing phenomenon.  
whereas the temperature slowly decreased because of a locally saturated brine and latent dissolution heat. After 503 s, the Raman peak of the RDP reached maximum efficacy, coinciding with the minimum temperature. All available RDP was used, allowing for additional ice melting and diluting of the brine and causing a decrease in Raman peak intensity. Subsequently, the efficiency decreased, and the minimum temperature rose, although the product remained effective throughout the test.  
Note that the minimum temperature rises at the start of this test, whereas it falls in the ice melting test (Figure 5). This can be explained by the camera angle and the number of grains. In the ice melting test, the camera is positioned above the Petri dish and there are several grains present in the test. As a result, the minimum temperature observed can be a very precise point in a fairly large area. In the Raman spectrometer test, the camera is tilted to observe the interface between the RDP and the ice, and focuses on a single grain, so that the minimum temperature observed is more precise and the measurement area much smaller.  
To visually understand this phenomenon without spectroscopic support, we ran an observational test at \(-10\)\({}^{\circ}\)C. Solid RDP powder was mixed with rhodamine B, which is used in ice penetration [21] and ice undercutting [22] tests and does not alter RDP properties. Unfortunately, compaction of RDP was not possible when mixed with rhodamine B; however, this did not affect the visual observations made to understand the phenomenon.  
The RDP started to create brine 3 min after being placed on the ice surface (displayed as a sharper pink color; Figure 8B). This intense melting state continued until approximately 8 min and then gradually diminished (turning faded pink). From 0 to 200 s, the RDP produced an exothermic reaction and melted to form brine (Figure 8C), a similar pattern to that observed previously [12]. Between 200 and 500 s, heat exchange occurred between the RDP and the ice, resulting in the dissolution of the RDP. From 500 s until the end of the test, only brine remained, which had limited heat exchange with the ice.