FORMULATING FOR MILDNESS 111
Syndet cleansing bars: the better ‘soap’?
Dr. Alexander T. Wagner, Torsten Krohn - Zschimmer & Schwarz, Germany
Flashback to the 1930s: “The principal object of the invention is to provide a soap composition which is totally stable in the presence of hard water, strongly alkaline or acid waters, producing no insoluble precipitates whatever”.1
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 well- known 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 non- natural 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 skin- neutral pH-values when dissolved in water.
O O– + OH2 Figure 1: Soap (sodium laurate).
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 (Krafft- temperature: 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
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 Beyond the Krafft-temperature, the alkaline pH-range created by soaps in solutions leads to a good water solubility. A 1% aqueous solution of sodium laurate, for instance, has a pH-value of 10. So, at this pH less than 1% of the soap molecules are protonated (Fig 2). Those fatty acids are solubilised by the non-protonated soap molecules: The corresponding solution is stable and clear. By reducing the pH, more and more soap molecules are protonated until the remaining soap molecules are no longer able to solubilise the increasing amount of fatty acids: The solution will become cloudy and unstable. The apKA (apparent pKA
value) of a soap4 These data seem is an
experimentally accessible parameter providing information about the protonation degree of a soap: It is the pH- value at which the molecular ratio between soap and the protonated version (fatty acid) is 1 : 1. For sodium laurate the apKA about 7.5,5
is meaning that at this pH-value
there are 50% soap molecules (surfactants) and 50% lauric acid molecules. The longer the alkyl chain, the higher the apKA sodium palmitate it is about 8.7,5
: For so even in
this alkaline pH-range there is a 1 : 1 mixture between soap and fatty acid. The consequence is: Although the increasing pH-value caused by soaps can easily be
O OH
Figure 2: Protonation of soaps (left): Creating fatty acids (right) and an alkaline solution (OH-). April 2019
+ OH – O O- Na+
PERSONAL CARE EUROPE
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188
