Demicellization thermodynamics of amphiphilic carbon dots via isothermal titration calorimetry

Abstract

Amphiphilic carbon dots C₁₂₁₄-ACDs were synthesized from alkyl glucoside APG1214, and had shown good surface activity and high oil removal efficiency. In this study, we have employed ITC to study the demicellization processes of C₁₂₁₄-ACDs and its precursor APG1214 within the temperature range from 288 K to 313 K. The enthalpy change of demicellization ΔH_demic for C₁₂₁₄-ACDs is negative at the lower temperatures as -7.37 J g⁻¹ at 288 K and -4.31 J g⁻¹ at 293 K, gradually decreases to approximate zero around the room temperature, and becomes positive with the increase of temperature as 5.02 J g⁻¹ at 303 K, 10.18 J g⁻¹ at 308 K and 16.68 J g⁻¹ at 313 K. The temperature dependence of ΔH_demic demonstrates that the micellization processes of C₁₂₁₄-ACDs are dominated by the hydrophobic force. The ΔH_demic for APG1214 is more negative than C₁₂₁₄-ACDs below room temperature as -17.20 J g⁻¹ at 288 K and -8.90 J g⁻¹ at 293 K, but becomes more positive than C₁₂₁₄-ACDs above room temperature as 20.97 J g⁻¹ at 313 K. The heat capacity changes ΔC_p,demic for APG1214 and C₁₂₁₄-ACDs have been calculated as the values of 1.517±0.027 J g⁻¹ K⁻¹ and 0.958±0.056 J g⁻¹ K⁻¹ respectively. The smaller value of ΔC_p,demic for C₁₂₁₄-ACDs indicates that the relatively hydrophobic surface is reduced after polymerization of APG1214. The effects of NaCl and CaCl₂ on the demicellization processes of C₁₂₁₄-ACDs have also been investigated. The values of CMC increase slightly, but the values of ΔH_demic decrease significantly in the saline solutions. The results indicate that the effects of NaCl and CaCl₂ are mainly attributed to the reduction of water activity and disturbance on the hydration of hydrophobic groups by ions.