List of ingredients, heuristics, and property models:
Emollients: A proper combination of three or more emollients of a high, medium and low spreading types provides the complete profile for a well-performing product.57 The residual film of the emollient helps to lubricate and reduce the friction exerted on the skin surface.58 The greasiness value (\(\gamma)\) of a mixture of emollients was estimated here as a weighted average from individual values.59 Typical greasiness values for skin moisturizers are in the range from 2.0 and 2.4 on a five-point scale.The RHLB value of the emollients was obtained directly from the ingredient provider or other sources.2 The list of the emollients, grouped by its spreading properties, is available in Table C1 of the Supplementary Material.
Emulsifiers: Generally, a suitable combination of at least two nonionic surfactants should be used at a minimum level of 2% and up to 5% in a skin moisturizer.7,60 The final HLB value of the combination should be between 8 and 15 to assure a stable O/W emulsion. This value is generally achieved by combining one part of a medium HLB (9-15) emulsifier, three to six parts of a high HLB emulsifier (>16), and two to six parts of a low HLB emulsifier (<8). Also, a total surfactant/oil ratio of 1:4 to 1:6 should be used to stabilize the resulting emulsion.61 The classical empirical HLB model is here adopted; namely, the HLB of the surfactant mixture should approximately match the HLB required by the oil mixture: HLB = RHLB. The list of the emulsifiers used in this study is available in Table C-2 in the Supplementary Material.
Thickeners: Thickeners commonly used in skin moisturizers are water-dispersible polymers and fatty alcohols. Typically, they are used simultaneously in skincare emulsions, as these formulations are more prone to destabilization.61 The list of the thickeners used in this study is in Table C-3 in the Supplementary Material.