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Levels and codes of independent variable used for response surface analysis.
| Factors | Codes | Levels | ||
|---|---|---|---|---|
| −1 | 0 | 1 | ||
| Ultrasound time (min) | A | 40 | 50 | 60 |
| Ethanol concentration (%) | B | 40 | 50 | 60 |
| SDS content (%) | C | 1.5 | 2 | 2.5 |
| Liquid material ratio (v/w) | D | 15:1 | 20:1 | 25:1 |
