## 4 Conclusions  
In this study, two new betaine surfactants (DCB and DSB) were prepared for the first time, and their chemical structures were characterized using \({}^{1}\)H-NMR and \({}^{13}\)C-NMR. The sulfobetaine exhibited higher thermal stability than the carboxyl betaine, and the initial thermal decomposition temperatures of DSB and DCB were 240 \({}^{\circ}\)C and 193 \({}^{\circ}\)C, respectively. DCB showed lower surface tension, mainly because the volume of the carboxyl group was smaller than that of the hydroxypropyl sulfonic group, and the arrangement of betaine molecules was closer after saturation adsorption on the liquid surface. In addition, the lower CMC of DSB is the result of solvation, because during micelle formation, the hydroxypropyl sulfonic group and the carboxyl group may have different solvation/desolvation characteristics. The interfacial tension measurements indicated that DSB had higher tolerance to divalent cations than DCB (Ca\({}^{2+}\): DCB tolerance range was 0.01-0.5% and DSB tolerance range was 0.01-1%; Mg\({}^{2+}\): DCB tolerance range was 0.01-0.2% and DSB tolerance range was 0.01-0.2%). In terms of reducing IFT, Na\({}^{+}\), Ca\({}^{2+}\), and Mg\({}^{2+}\) showed similar synergistic effects with betaine surfactant molecules. DCB and DSB solutions completely moistened the paraffin film according to the measurements of wetting ability and the lowest contact angles were 28.36\({}^{\circ}\) and 35.26\({}^{\circ}\), respectively. In conclusion, these two kinds of betaine surfactants have the potential to be used in high-temperature and -salt reservoirs.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app13074378/s1](https://www.mdpi.com/article/10.3390/app13074378/s1)]([https://www.mdpi.com/article/10.3390/app13074378/s1](https://www.mdpi.com/article/10.3390/app13074378/s1)), Figure S1: \({}^{1}\)H NMR spectra of 1-chloro-3-(4-dodecylphenoxy)propan-2-oil in CDCl\({}_{3}\); Figure S2: \({}^{13}\)C NMR spectra of 1-chloro-3-(4-dodecylphenoxy)propan-2-oil in CDCl\({}_{3}\); Figure S3: \({}^{1}\)H NMR spectra of 1-((3-(4-dodecylphenoxy)-2-hydroxypropyl)-dimethylammonio)acetate in CD\({}_{3}\)OD; Figure S4: \({}^{13}\)C NMR spectra of 1-((3-(4-dodecylphenoxy)-2-hydroxypropyl)-dimethylammonio)acetate in CD\({}_{3}\)OD; Figure S5: \({}^{1}\)H NMR spectra of 1-((3-(4-dodecylphenoxy)-2-hydroxypropyl)-dimethylammonio)-2-hydroxypropane-1-sulfonate in CD\({}_{3}\)OD. Figure S6: \({}^{13}\)C NMR spectra of 1-((3-(4-dodecylphenoxy)-2-hydroxypropyl)-dimethylammonio)-2-hydroxypropane-1-sulfonate in CD\({}_{3}\)OD.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & **0.05 g/L** & **0.1 g/L** & **0.15 g/L** & **0.2 g/L** & **0.25 g/L** & **0.3 g/L** \\ \hline DCB & 25 \({}^{\circ}\)C & 212 & 351 & 364 & 401 & 382 & 375 \\ emulsion stability time/s & 45 \({}^{\circ}\)C & 208 & 277 & 319 & 340 & 336  
**Author Contributions:** Conceptualization, Q.Z. and Z.W.; methodology, Q.Z.; software, L.Y.; validation, Q.Z., Z.W. and P.L.; formal analysis, Z.W. and P.L.; investigation, Q.Z. and Z.W.; resources, Z.S.; data curation, L.Y.; writing--original draft preparation, Q.Z.; writing--review and editing, Q.Z., Z.W. and P.L.; visualization, P.L.; supervision, L.Y.; project administration, Q.Z. and Z.S.; funding acquisition, Z.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development (No: 33550000-22-ZC0613-0028).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data used to support the findings of this study are included within the article.  
**Conflicts of Interest:** The authors declare no conflict of interest.