Determination of surfactants from foam-formed products

In this Master’s thesis, determination of surfactant residues from solid foam-formed samples was studied. The residues of an anionic surfactant, sodium dodecyl sulphate (SDS), and a non-ionic surfactant Tween 20 (polyoxyethylenesorbitan monolaurate), were determined from foam-formed hand sheets. The measured surfactant residues were compared with the theoretical values calculated from the water content of the sample before drying phase and the surfactant dosage. In addition to analysis of surfactant residues, determination of 4-dodecylbenzenesulphonic acid (4-DBSA, anionic surfactant) in water solution by UV spectrophotometry was studied. The literature part considers the chemistry of surfactants and introduces the relatively new concept of papermaking, foam forming. The main focus in the literature part is in the analysis methods of surfactants. Sample pre-treatment methods and the use of spectrophotometry, chromatography and atomic spectroscopy in the determination of surfactants are discussed. In addition, the chemical legislation, toxicological concerns of surfactants and the regulations concerning paper and board in food contact materials are discussed.

To determine surfactant residues, two different determination methods were used for both surfactants. SDS residues were determined by spectrophotometry and by inductively coupled plasma optical emission spectrometry (ICP-OES). Tween 20 residues were determined by spectrophotometry and by high-performance liquid chromatography equipped with a diode array detector (HPLC-DAD). Ultrasound-assisted nitric acid digestion was used as a sample preparation method for the ICP-OES analysis. Before the chromatographic determination, Tween 20 was extracted from the sample by methanol in Soxhlet extraction. In the spectrophotometric determination, SDS and Tween 20 were extracted by water.

The results showed that surfactant residues increased as the surfactant dosage increased and when the dry matter content of the sample (before the last drying stage) decreased. ICP-OES and spectrophotometric methods gave very similar results for SDS residues. The results indicated that SDS was effectively extracted from the fibre network by water. The amount of SDS residues was affected by water hardness, the temperature of the water-fibre suspension and the addition of a non-ionic surfactant or cationic starch. When the water hardness was °dH = 5, the measured SDS residues were clearly higher than the theoretical values. This is due to the formation of insoluble calcium dodecyl sulphate Ca(DS)₂ that remains in the fibre network. The amount of SDS residues decreased when water hardness decreased, temperature was raised or a non-ionic surfactant (Tween 20) was added. The addition of cationic starch resulted in the increase of SDS residues.
Tween 20 could be determined by both HPLC-DAD and spectrophotometry. HPLC-DAD gave higher results compared to the spectrophotometric determination, because Tween 20 was not fully extracted by water. When water, ethanol and acetone were compared as extraction solvents in Soxhlet extraction, the results showed clearly that more Tween 20 was extracted by ethanol and acetone than water. The amount of Tween 20 residues was not observed to be higher than theoretically estimated.