Qualification of droplet morphology in hair conditioner by Vis-NIR spectroscopy

Importance of the topic

Hair conditioners are complex emulsions in which droplet size directly influences product performance, stability and sensory properties. Traditional methods for assessing droplet morphology, such as microscopy or laser diffraction, are time-consuming and require specialized sample preparation. Rapid, non-destructive techniques like Vis-NIR spectroscopy offer a promising alternative for quality control in cosmetic manufacturing.

Objectives and overview of the study

This application note demonstrates how Vis-NIR spectroscopy can be employed to

• Differentiate between unprocessed and processed hair conditioner formulations.
• Qualify droplet size distributions against predefined thresholds.

The study evaluates spectral methods for both identification and qualification of droplet morphology in two intermediate products with size ranges of 5–20 µm and 5–50 µm.

Methodology and used instrumentation

Sample preparation involved filling disposable vials with unprocessed (A) and processed (B) conditioner batches. Spectra were collected in diffuse reflectance mode over 400–2500 nm using a Metrohm NIRS DS2500 Analyzer with variable spot size. Data analysis and model development were carried out in Vision Air Complete software.

Identification and qualification methods

Two chemometric workflows were established:

• Identification: 2nd-derivative spectra and wavelength regions 416–1080 nm and 1120–2480 nm. Pre-treatment with segment = 10 nm and gap = 0 nm.
• Qualification: 2nd-derivative spectra in 1380–1404 nm range with the same pre-treatment. Classification thresholds were defined to determine whether droplet size lies below or above the specification limit.

Main results and discussion

Raw spectra of both products revealed overlapping baselines but distinct features after 2nd-derivative processing. The identification model correctly assigned unknown samples to the respective formulations. Qualification models reliably flagged samples with droplet sizes outside the target range. Both processed and unprocessed samples were evaluated with high speed and reproducibility.

Benefits and practical applications of the method

• Rapid, reagent-free quality control requiring no complex sample preparation.
• Immediate, automated classification and sizing results within seconds.
• Integration into production lines for continuous monitoring of emulsion quality.

Future trends and applications

Advances in portable Vis-NIR instrumentation and machine learning algorithms are expected to further enhance sensitivity and robustness. Prospects include real-time in-line monitoring, extension to additional cosmetic formulations, and combination with other spectroscopic modalities for multi-parameter quality assessment.

Conclusion

This feasibility study confirms that Vis-NIR spectroscopy, combined with derivative pretreatment and chemometric modelling, provides a fast, non-destructive approach for identifying hair conditioner formulations and qualifying droplet morphology. The method can supplement or replace traditional sizing techniques, streamlining quality control workflows in cosmetic production.

Used instrumentation

• Metrohm NIRS DS2500 Analyzer (400–2500 nm)
• Metrohm NIRS DS2500 Iris sampling accessory
• Vision Air 2.0 Complete software (Chemometric model development)