Syndet cleansing bars: the better ‘soap’?

This fundamental statement was made by Heinrich Bertsch in his patent “Soap Preparation”. It proves that already more than 80 years ago proposals were made to overcome a wellknown drawback of classical soaps: Precipitation in water. There are two chemical reactions of soaps - the alkali salts of fatty acids - (Fig 1) which cause the precipitates. Both of them ”knock-out” typical surfactant properties like cleansing and foaming. Firstly, the reaction of soaps with magnesium and particularly calcium ions in hard water leads to water-insoluble precipitates (lime soap). Lime soap is difficult to remove from hard surfaces like wash basins. Secondly, even at neutral pH conditions, soaps to a great extent become fatty acids by protonation (Fig 2). Then they are (nearly) water-insoluble. This chemical reaction is the background of the statement given above:1 The instability of pure soaps in ‘acid waters’. The protonation is directly linked to the creation of an alkaline solution (Fig 2) and leads to a pH of soap solutions of about 9 – 11. For cosmetic applications this behaviour is considered the major drawback in using solid soap bars, as the pH of the skin is about 5.0 to 5.5. It is easily understandable that skin cleansing in a nonnatural pH-range may stress the skin. Therefore, for cosmetic applications there is a trend for pH-skin neutral products, which protect the skin by not damaging the natural acid layer. In this article we present some ‘insider’-tricks to create solid cleansing bars with neutral or even skinneutral pH-values when dissolved in water. 

Focus on soaps in aqueous solutions

The anionic carboxylate group gives soaps surfactant properties (anionic surfactants). The high hydrophilicity of the carboxylate group can lead to water-soluble soaps at room temperature. This depends on three surfactant parameters (alkyl chain length, alkyl saturation degree and counterion). A good measure to get information about the water solubility is the Krafft-temperature, as it is the temperature above which soaps become water-soluble. The counterion plays a major role: For instance, potassium laurate (C12) is easier water-soluble (Kraffttemperature: 10°C) than the sodium analogue (Krafft-temperature: 25°C). Additionally, with a longer alkyl chain the Krafft-temperature rises: For sodium palmitate (C16) it is 60°C.2 These data seem to be ‘knock-out’ criteria for the use of sodium soaps for instance in cold water. The secret to lower the Krafft-temperature and pave the way to water solubility is to use suitable sodium soap mixtures. For instance, a mixture of 50% sodium laurate and 50% sodium oleate (C18, 1-fold unsaturated) is water-soluble in cold water (Krafft-temperature below 0°C).3 Recently, another solution was found: By using the counterion choline - an amine-based biomolecule - even palmitate soaps become water-soluble at room temperature (Krafft-temperature about 10°C).2