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defined foam was formed by means of a foam dispenser, the amount of liquid draining from the foams was measured on a time dependent basis. This is an indication of the amount of liquid in the foam. The results show that sarcosinates and more particularly glutamates provide a stable foam with a high water content that is stable over a longer period (Fig 7). CAPB and coco glucoside provided the fastest water loss and thereby may be described as being the most unstable and driest foam. A wet foam consistency as demonstrated by amino acid-based surfactants allows a particularly easy application of the product on skin and leaves a pleasant skin-feel. It was also demonstrated that the relatively unstable foam of a 0.3% coco glucoside solution (active content of 3 g/l) was noticeably stabilised by an equivalent amount of sodium cocoyl glutamate (1:1) and xanthan gum (1%) and gave the best stability under these test conditions.
Optical investigation of foam ability and foam stability
A scientific analysis of liquid foams can be performed with the Dynamic Foam Analyser DFA100 (Krüss).10
This optical
instrument measures foam ability of liquids and foam stability based on precise and reproducible measurements of foam height. Furthermore, the apparatus allows the determination of foam structure on the
basis of a time-dependent bubble size distribution. The measuring principle is shown in Figure 8. For foam measurement, 0.3% active surfactant solution (active content of 3 g/l) were prepared, which roughly corresponds to the concentration of a shower gel on the skin after foaming. Coco glucoside and sodium cocoyl glutamate (SC 25 NKW) were measured as pure substances and as an equivalent mixture (1:1). Furthermore, a shower gel formulation (Formulation A) with sodium cocoyl glutamate (Perlastan SC 25 NKW), sodium lauroyl sarcosinate (L- 30) and a typical SLES/CAPB mixture were compared (Fig 9). The second formulation (Formulation B)
is a ‘green’ and certifiable formulation whereas sodium cocoyl glutamate (SC 25 NKW) and coco glucoside were used. This formulation was compared with a market product which had a good foaming standard surfactant combination including SLES/CAPB. In Figure 10, foam stability of raw materials sodium cocoyl glutamate (SC 25 NKW), coco glucoside and their equivalent mixture is illustrated. After the foaming process (20 seconds of constant stirring), all solutions resulted in a foam height between 10-12 cm, whereby the liquid is completely absorbed in the foam lamellae. Over time, foam stability and thus foam height decreases by collapsing foam lamellae and liquid drainage of the foam. As shown by the
Ross-Miles test, it was found that coco glucoside has the least stable behaviour and the fastest decrease in foam height whereas the sodium cocoyl glutamate foam height remains constant almost for twice as long. The foam stability is improved by equal mixture of both substances in comparison to pure coco glucoside and the resulting foam height is effectively increased by this foaming method. Even in the two Formulations A and B the amino acid-based surfactants provide good performance (Fig 11a). Although the market product with SLES/CAPB formulation presents a good foam height of 12 cm, its foam is less stable and collapses after 320 sec. In addition to SLES/CAPB, Formulation A also contains a high proportion of SC 25 NKW and L-30 as secondary surfactants in the formulation. In this combination there is a visible stabilising of the foam in comparison to pure SLES/CAPB formulation, whereby foam height remains in the same range. Thus, the surfactants can be used as secondary SLES/CAPB formulations while texture becomes much finer and milder without a noticeable reduction of foam height. The ‘green’ formulation B based on coco glucoside and SC 25 NKW as surfactants shows lower foam height. However, this should also be viewed on the quality of the foam structure and its stability. Additionally, the initial stages of foam formation were noted as
Figure 12: Formulation B: Bubble size distribution of the foamed sample after 2 min and 14 min (active content of 3 g/l).
Figure 13: Market product (with SLES/CAPB): Bubble size distribution of the foamed sample after 2 min and 14 min (active content of 3 g/l). September 2018 PERSONAL CARE EUROPE
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