566 JOURNAL OF COSMETIC SCIENCE Table II Mild Baby Care Detersive Formula of Selected Non-Ionic Surfactants With 7% SCI INCI name Trade name Percentage (%) Polysorbate 20 Tween-20 © i 0.00 Polysorbate 80 Tween-80 © 15.00 Sodium cocoyl isethionate Hostapon SCI 85G © or Jordapon CI Prill © 7 Thickener q.s. Preservatives q.s. Fragrance q.s. D.I. water To 100 It should be noted that as the formulation of cosmetics becomes more sophisticated, chemists will determine the geometric packing of micelies as standard practice, e.g., for spherical micelies, rods, and lameliar structures. It is known that as micelies proceed from spherical to lameliar formations, the density of micelies increases. As the density of micelies increases, the number of free CI- ions decreases. This would push the solubility equilibrium to the right. Thus it is very likely that control of micelie for- mation also affects SCI solubility. METHOD II: ION EXCHANGE Even though some specialty surfactants efficiently solubilize SCI in aqueous solution (method I), they are often more expensive than common surfactants such as sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES), which do not efficiently solubilize SCI. For example, an aqueous solution of 10% SLS or SLES can only solubilize less than 1% of SCI and remain clear. Making more expensive formulas with specialty surfactants defeats the advantage of using a relatively inexpensive material like SCI. In order to incorporate SCI in affordable detersive systems for the mass market, the ion exchange method was developed through detailed examination of equilibrium (12). The ion exchange method can prevent recrystallization of SCI by changing the total enthalpy of solvation from a positive to a negative value. Ion exchange is accomplished by adding ammonium ions from ALS and ALES, or triethanolamonium from TEALS and TEALES, all of which are significantly cheaper than the aforementioned specialty sur- factants. The presence of free ammonium ions (or triethanolamonium) disrupts the ability of Na + to associate with CI- ions and reform SCI. As was already stated, ACI and/or TEACI compounds have lower lattice energies and are more soluble. It thus becomes possible to keep more CI- ions available to form micelies, which clean surfaces. Having more CI- ions also favors interaction and substantivity to skin and keratin surfaces, allowing the user to experience the emolliency and moisturizing properties associated with this molecule. It should be noted the formulator may choose ACI as a secondary surfactant despite the generation of free CI- ions, which would normally shift equilibrium to the left. This is because the solubility contribution by the ammonium ion dominates the equilibrium the reduction in lattice energy by ammonium outweighs any increase in aqueous CI-. Monoalkyl phosphate (MAP) surfactants will serve the same purpose. MAP surfactants have been studied extensively for their excellent foaming properties, exceptional mild-

SOLUBILIZATION OF SCI 567 ness, and silky, talc-like afterfeel on skin or hair (6). Ammonium and triethanolamo- nium monoalkyl phosphate (AMAP and TEAMAP) are very soluble and able to increase the solubility of SCI. They are milder than ALS and ALES, but cost slightly more. In the same way, the ammonium salts of all the selected anionic surfactants in method I work much more efficiently than their corresponding sodium salts. The ion exchange method makes it possible to include SCI as a primary surfactant in many liquid detersive compositions. This greatly decreases irritation normally associated with ALS and/or ALES. It is anticipated that chemists can thus make use of the excellent properties of SCI to the consumer's benefit. Table III illustrates this method. METHOD III: EMULSIFICATION Introduction of a small quantity of an emollient compound increases the solubility of SCI in water. This is mainly due to the previously mentioned attractive interaction between hydrocarbon tails and other molecules and to geometric packing constraints. As the quantity of oil increases in an SCI/water solution, CI- will surround these droplets in the process of emulsification. As a result, the oil droplet becomes more stable as the emollient anchors the CI- ions like liquid cement. As the droplet becomes larger, the number of aqueous CI- ions that can associate with this body increases. Thus, stabili- zation via emulsified oil droplets and the uptake of CI- work to shift equilibrium to the right, once again favoring the solubilization of SCI. The most convenient way of employing this method is by using blends of emulsifying wax. These have non-ionic emulsifiers mixed with fatty esters or fatty alcohols. Those suitable for this application are: Polawax (emulsifying wax NF) from Croda self- emulsifying Kester Wax K-82 H (INCI: C-20-40 alkyl stearates, ceteareth 20, and PEG-14 stearate) from Koster Keunen, Inc. and Emulium Delta (INCI: cetyl alcohol, glyceryl stearates, PEG-75 stearate, ceteth-20, and steareth-20) from Gattefosse. One part of emulsifying wax NF will make three parts of SCI soluble in an emulsion. It is possible to have 30% SCI emulsified by 10% emulsifying wax NF to form an elegant facial cleanser. Since emollients are involved in this composition, only opaque liquid deter- sive systems can be formulated using method III. Table IV illustrates this application. CONCLUSION Three methods have been developed for solubilizing sodium cocoyl isethionate in aque- ous detersive systems based on the understanding of enthalpy of solubilization, equi- Table III Clear Economic Detersive Formula of ALS and/or ALES Surfactants With 10% SCI INCI name Percentage (%) Ammonium lauryl sulfate 7.00 Ammonium laureth sulfate 5.2 5 Cocamide MEA 1.75 Sodium cocoyl isethionate 10.0 Preservatives q.s. Fragrance q.s. D.I. water To 100%