Liquid laundry detergents contain fabric softeners, bleaching agents, surfactants, as well as enzymes. Out of these, the surfactant is the most important factor for the cleaning effect, as it breaks down the interface between polar and nonpolar compounds. This allows the detergent to be effective against greases as well as stains from soil or drinks.
Quantification of surfactant content is most commonly performed by primary analyses (e.g., two-phase potentiometric titration). Disadvantages include manual sample preparation steps such as dilution and pH adjustment, and the method itself is time-consuming. In contrast, Vis-NIR spectroscopy has a time-to-result of less than 1 minute and does not require any sample preparation or chemicals for high quality data.
A total of 37 samples with varying surfactant content were provided by a customer. The Vis-NIR spectra (Figure 2) were acquired on a Metrohm NIRS XDS RapidLiquid Analyzer equipped with 1 mm quartz cuvette (Figure 1). The samples were measured as-is, without any sample preparation steps. Data collection and model development was carried out with the Vision Air complete software package.
|XDS RapidLiquid Analyzer||2.921.1410|
|NIRS 1mm quartz cuvette||6.7401.200|
|Vision Air 2.0 Complete||6.6072.208|
The obtained graph (Figure 3) displays a high correlation (R2 = 0.97) between the values predicted by NIRS and the reference method. The nearly perfect ratio of the SEC and SECV illustrates the validity of the model.
|Figures of merit||Value|
|Standard error of calibration||2.20 mmol/100 g|
|Standard error of cross-validation||2.38 mmol/100 g|
The results presented herein show that the Vis-NIR method is excellently suited for the fast quantification of surfactant concentration in detergents. Using Vis-NIR for this application saves 10 minutes per sample compared to other methods (Table 3).
|Parameter||Method||Time to result|
|Surfactant (anionic)||Potentiometric titration||∼10 min (adding solutions, stirring, pH-adjustments, determination)|