Surfactant in laundry detergent by Vis-NIR spectroscopy

Significance of the Topic

Rapid and accurate quantification of surfactants is essential for quality control in detergent manufacturing. Surfactants determine the cleaning efficacy against both polar and nonpolar soils, and their concentration directly affects product performance and environmental impact. Conventional titration methods are time-consuming and require chemical reagents, whereas Vis-NIR spectroscopy offers a faster, reagent-free approach.

Objectives and Overview of the Study

The aim of this study was to establish a Vis-NIR spectroscopic method for the determination of anionic surfactant concentration in liquid laundry detergents. A calibration model was developed and validated against a reference HPLC method, focusing on achieving high accuracy, minimal sample preparation, and rapid results.

Methodology and Instrumentation

Thirty-seven commercial detergent samples with varying surfactant levels were measured directly without any dilution or chemical treatment. Spectral data in the Vis-NIR range were acquired using a RapidLiquid Analyzer fitted with a 1 mm quartz cuvette. Preprocessing and multivariate calibration were performed using Vision Air Complete software to relate spectral variations to surfactant concentration.

Used Instrumentation

• XDS RapidLiquid Analyzer (Metrohm No. 2.921.1410)
• NIRS Quartz Cuvette, 1 mm Pathlength (Metrohm No. 6.7401.200)
• Vision Air 2.0 Complete Software (Metrohm No. 6.6072.208)

Main Results and Discussion

The Vis-NIR calibration model achieved a coefficient of determination (R²) of 0.97. The standard error of calibration (SEC) was 2.20 mmol/100 g, and the standard error of cross-validation (SECV) was 2.38 mmol/100 g, indicating a robust predictive performance with minimal overfitting. Spectral features in the 1,200–1,700 nm range showed strong correlation with surfactant concentration, enabling reliable quantification across the sample set.

Benefits and Practical Applications

• Analysis time under one minute per sample
• No reagents or manual sample preparation steps
• High-throughput capability ideal for QA/QC in production
• Non-destructive testing, preserving samples for additional analyses

Future Trends and Opportunities

The integration of Vis-NIR spectroscopy into inline process analyzers could facilitate real-time monitoring and control of surfactant dosing in detergent production. Additionally, the method can be extended to quantify other formulation components—such as bleaching agents and enzymes—enabling comprehensive quality assessment in a single measurement.

Conclusion

Vis-NIR spectroscopy provides a rapid, accurate, and reagent-free alternative to traditional titration methods for surfactant determination in liquid detergents. The developed method reduces analysis time by approximately ten minutes per sample and supports efficient laboratory workflows.

References

• Metrohm AG. Surfactant in Laundry Detergent by Vis-NIR Spectroscopy. Application Note AN-NIR-074.