Leon Cohen Æ Fernando Soto Æ Ana Melgarejo Æ David W. Roberts

DOI 1.17/s11743-8-169-3 ORIGINAL ARTICLE Performance of U-Sulfo Fatty Methyl Ester Sulfonate Versus Linear Alkylbenzene Sulfonate, Secondary Alkane Sulfonate and a-sulfo Fatty Methyl Ester Sulfonate Leon Cohen Æ Fernando Soto Æ Ana Melgarejo Æ David W. Roberts Received: 22 January 28 / Accepted: 18 March 28 Ó AOCS 28 Abstract Sulfoxidation of fatty acid methyl esters with SO 2, O 2 and ultraviolet light of appropriate wavelength, has led to the synthesis of methyl esters sulfonates or sulfoxylates, known as U-MES, because of the possible random position of SO 3 group in the alkyl chain. Aqueous solutions based on the sulfoxylated methyl ester of palmitic acid (U-MES C16) have been studied and compared to the leading types of surfactants used today: linear alkylbenzene sulfonate (LAS) secondary alkane sulfonate (SAS) and a- sulfo fatty methyl ester sulfonate (a-mes) with regard to solubility, performance and skin compatibility. The experimental results obtained indicate that U-MES C16 can be regarded as a potential component of detergent formulations and most likely also of body care products. Keywords Wetting Foaming properties Anionic surfactants Introduction Our research group has been working since 1993 on sulfoxylated fatty acid methyl esters, called here U-sulfo fatty methyl ester sulfonates (U-MES). Sulfoxidation is the addition of sulphur dioxide and oxygen in the presence of UV light of appropriate wavelength to some organic L. Cohen (&) F. Soto A. Melgarejo Escuela Politécnica Superior de Algeciras., Universidad de Cádiz, Avda Ramon Puyol, 1122 Algeciras, Spain e-mail: leon.cohen@uca.es D. W. Roberts Liverpool John Moores University, Liverpool, UK compounds, such as aliphatic hydrocarbons or fatty acid methyl esters. The sulfoxidation reaction proceeds through a radicalic mechanism unlike the a-mes traditional sulfonation, which proceeds through an electrophilic substitution. Due to the reaction mechanism, a characteristic of these compounds is that the sulfonate group is randomly distributed along the alkylic chain, this being the reason they are named U-MES. U-MES exhibit [1 3] properties that make them attractive as anionic surfactants, namely: Good water solubility, which makes them easy to include in liquid formulations. Very low viscosity of their aqueous solutions, which makes them easy to handle and pump. Very good wetting power Excellent water hardness stability, which allows them to be formulated in hard water regions. Excellent skin compatibility, which makes them potentially very good for hand dishwashing formulations and most likely for body care products. Expected excellent biodegradation. Good detergent power. In a previous paper [2], performances of U-MES with varying chain lengths were compared. Two main conclusions were drawn: the palmitic acid U-MES (U-MES C16) could be used as an LAS partner in detergent formulations and the C16 alkyl chain was the optimal length. Since then, progress has been made concerning the development of a procedure for the separation of reaction products [4] and analysis [5, 6]. These investigations have led to products with fewer impurities. It has therefore been judged worthwhile in the present work, to present the results of standard experimental tests,

that have allowed us to compare the performance of the most common commercial anionic surfactants such as LAS (linear alkylbenzene sulfonate), SAS (secondary alkane sulfonate) and a-mes (a-sulfo fatty methyl ester sulfonate) to U-MES C16. REACTOR OUTLET Hot water Separation Materials and Methods ME C16 (palmitic acid methyl ester). ME C16, (CE- 1695) from Procter & Gamble Chemicals, USA, with 99 wt.% of C16: alkyl chain U-MES C16 sodium salt. This product was obtained in our laboratory according to a modification of the methodologies described previously [1, 4]. Update. According to the water procedure described in Ref. [4] after the separation of the non-reacted methyl ester, a small amount of dissolved methyl ester and some fatty acid remains in the aqueous phase, and these have to be removed in order to improve the final product quality. An updated procedure, that we present for the first time, has been implemented. As mentioned in Ref. [4], the reactor outlet is mixed with an equivalent amount of water and the two phases are left to separate at 6 C in the oven. The upper layer is composed of unreacted methyl ester while the lower phase is the water solution containing mono and polysulfonic acids as well as some dissolved methyl esters and most likely a small amount of fatty acid. After separation of both phases, the unreacted methyl ester can be used for a new run and the water solution will follow further treatment consisting in adding the same amount of methanol as the volume of water. The mixture is then transferred to a liquid liquid extractor as the one depicted in Ref. [4] and the extraction is carried out during 8 h with a non polar solvent such as n-hexane. In the hexane phase all the dissolved fatty acid and methyl ester are recovered, while the water methanol solution is distilled under a slight vacuum and all the methanol recovered. The remaining water solution is then neutralized with sodium hydroxide and concentrated as desired. As mentioned above, basically, the modification consisted of the use of a mixture of methanol water 5/5 v/v and hexane, to perform the extraction of dissolved impurities. The detailed analytical procedure is depicted below in Scheme 1. LAS sodium salt. A commercial sodium sulfonate sample derived from Petresul 55 acid manufactured by Petroquímica Española, Spain. SAS sodium salt. A commercial cut of C14 C17 sample of Hostapur SAS 93 from Clariant Germany. Methanol Water Phase Non reacted addition Methyl ester a-mes C16 C18. A commercial sample of C16 C18 (5/5 wt%) concentrated powder with 97% monosulfonate purity, from Desmet Ballestra, Italy. Results and Discussion Solubility Hexane Extraction Methanol-Water Hexane Phase Phase Neutralization Hexane recovery Methanol removal Scheme 1 Analytical Scheme work Turbidity points, defined as the temperature at which an aqueous solution of surfactant becomes turbid on cooling, were measured for the different surfactants as seen in Fig. 1. Turbidity points were determined by cooling, in a thermostated bath, solutions of defined surfactant concentration until they became cloudy or turbid. For the same surfactant concentration, the lower the turbidity point temperature the higher the solubility in water, since the surfactant solution remains clear at lower temperatures. It must be remembered that turbidity point is the immediate previous step to crystallization. In Fig. 1, the best results correspond to SAS and LAS, while U-MESC16 has acceptable solubilities. All a-mes solutions were cloudy, because these products are not water soluble at laboratory temperatures (18 C). Therefore, in this case, cloud points were measured after heating the solutions until they became clear and then cooling them. U-MES C16, SAS and LAS are all double chain surfactants, in each case consisting of mixtures of isomers, where the lengths of the two chains vary. In SAS and LAS both chains are n-alkyl, but in U-MESC16 the chains are

3 25 2 1 5-5 -1 Temp. ( C) Surf.Conc. 25% 2% % Φ-MES C16 Solubility (Turbidity Point) LAS SAS αmes Fig. 1 Solubility at different surfactant concentrations different from each other, one being n-alkyl and one being terminated by a CO 2 Me group. The hydrophobicity of SAS, LAS and U-MES C16 is lower than would otherwise be expected (water solubility higher), because of the water sharing effect between the two chains, described and modeled quantitatively in [7, 8]. The magnitude of the water sharing effect is different for each isomer. LAS is a mixture of C1 C13 homologues where each homologue is a mixture of different isomers with a phenyl ring attached to the alkyl chain at different positions. SAS is a mixture of C14 C17 homologues, each homologue being a mixture of different isomers, depending on the position of the SO 3 hydrophilic group. Apparently SAS has a higher water solubility than LAS and this may be due to the fact that SAS isomers are more soluble than LAS ones because of the presence in the latter of the six carbon atoms of the benzene ring. U-MES C16 is less soluble than LAS and SAS, probably because, among other factors, as it is already well known, a pure homologue is less soluble in water than a mixture of homologues with the same molecular weight, because of synergic interactions between homologues and isomers in the mixture. As mentioned above we have to bear in mind that LAS and SAS are both mixtures of homologues with a significantly higher number of isomers than the single U- MES C16 homologue. In order to explain the very low water solubility of a- MES C16 C18, we have to consider that it is a mixture of two linear single chain surfactants (C14 and C16) 1, each with a CH(CO 2 Me)SO 3 head group. In this case the CO 2 Me group does not reduce the hydrophobicity of the carbon chain as in U-MES, being within the hydration 1 We don t count the C attached to the SO 3 group and we don t count the C s of the CO 2 Me group. sphere of the SO 3 group which is at the a carbon. Moreover the ester group hinders the sulfonate group and decreases its interaction with water. Stability to Water Hardness The determination of the stability to water hardness ions was carried out according to UNE-55-57-72, the Spanish standard equivalent to the widely used stability test (DIN 5395 or ISO-163/79). In the Spanish test, five surfactant solutions with increasing concentrations are tested for each water hardness level. In this test, a number ranging from 1 to 5 is assigned to surfactant solutions with varying calcium concentrations, depending on the apparent turbidity and precipitate formation: five denotes clear solutions, four opalescent, three turbid, two precipitate, one precipìtate excess. According to the latter, 25 is the maximum value for each hardness level. As seen in Fig. 2, U-MES C16 shows the highest tolerance to water hardness (numbers are the sum of the five surfactants concentrations tested for each water hardness level). Due to the above depicted structure of U-MES C16, SAS and LAS molecules, crystallization of the calcium salt is more inhibited with U-MESC16 than it is with SAS and LAS. Concerning a-mes, since distilled water solutions of a- MES are turbid even at low surfactant concentrations, we cannot measure absolute stability but the relative one, (meaning that, even though the stability number is necessary low, its value doesn t change when water hardness increases) and as seen in Fig. 2, a-mes is not affected by a water hardness increase from to 65 ppm. The reason why the single chain surfactant a-mes is reasonably 25 2 1 5 Points 2 19 19 STABILITY to WATER HARDNESS 14 14 1 11 11 11 11 Φ- MES C16 LAS P-55 SAS αmes C16-C18 8 3 45 6 Fig. 2 Water hardness tolerance according to UNE-55-57-72

hardness tolerant (at the higher hardness concentrations it is more tolerant than SAS and LAS) is because there is some hindrance or shield effect of the CO 2 ME group, which results in a weaker interaction with calcium ions, in particular with 2Ca ++. On the other hand, Satsuki [9] claims that LAS and most likely SAS, are very sensitive to calcium concentration because of the production of a liquid crystalline calcium salt which is insoluble. a and probably U MES form a calcium salt that exists in a metastable state, so that might be the reason why methyl ester sulfonates in general, can be regarded as ones of the most hardness tolerant surfactants. Foaming Power seconds 35 3 25 2 1 5 4 65 5 21 WETTING POWER 1 g/l 7 228 3 3 3 3 3 3 Φ-MES C16 LAS P-55 SAS α MES C16- C18 12 Foam height. The Ross Miles test was conducted at 49 C. and 1 g/l of surfactant at three different levels of water hardness expressed as ppm of CaCO 3. Experimental results are represented in Fig. 3. U-MESC16 gives the best results together with LAS. A water hardness increase has less effect on both U-MESC16 and a-mes than on LAS and SAS. This can be explained based on stability to water hardness of U-MESC16 and a- MES (which reaches a maximum at 1 g/l and 4 ppm), compared to SAS and LAS. Foam stability. According to the Ross Miles test, foam stability is given by the foam height variation after 5 min. The results show that all the surfactants show a similar variation after 5 min. Fig. 4 Wetting time (Draves Test) at 2 8C as a function of water hardness Wetting Power The Draves test was conducted and 1 g/l of surfactant concentration used at three water hardness levels. According to the experimental results plotted in Fig. 4, U- MES C16 is by far the best wetting agent, specially at a higher water hardness. According to Rosen, since the rate of wetting is a function of the surface tension of the wetting front, the wetting power of a surfactant is a function of the concentration of molecularly dispersed material at the front, so we can utilize the same argument used for dishwashing, in the sense that more monomers exist in solution. mm 8 7 6 5 4 3 2 1 64 56 53 Initial Foam Height 1 g/l 7 56 53 72 23 12 ppm 4 ppm 65 ppm 23 31 22 Dishwashing The method used gives a stability index for foam generated using a certain type of soil, in our case a mixture of olive oil 75% and lard 25%, at a definite surfactant concentration [1] and 49 C. A correlation exists between the stability index and a hypothetical number of dishes washed according to the typical dishwashing manual test. A choice has been made so that the value 5 is an indication when the number of dishes washed is above 5. The results plotted in Fig. 5, show that the best performance is reached with U-MES C16 at medium water hardness. This probably reflects the higher CMC of U-MES C16 due to the lower micellar radius (particularly compared to a-mes), which means that more monomer is in solution to replenish the foam. Φ-MES C16 LAS P-55 SAS α MES C16- C18 Fig. 3 Foam height (t = ) at 49 8C as a function of water hardness Skin Compatibility Zein test. The Zein test, a widespread screening in vitro test for the evaluation of skin irritancy of surfactants, was used.

6 DISHWASHING TEST.5 g/l 6 ZEIN TEST 564 526 N Dishes 5 4 3 2 1 5 5 5 5 5 Φ MES C16 22 13 LAS P-55 The test is based on the solubilization of the water-insoluble zein protein by surfactants. Solubilization of zein is measured through the determination of nitrogen content of the solubilized protein, giving the so-called Zein number. According to Kaestner and Frosch [11], the ability of a surfactant to dissolve the water-insoluble zein protein correlates very well with in vitro test data. Irritation of human skin is due to the formation of a complex between the protein present in the skin and the surfactant. According to Seibert and Bolsterdorf [12] the extent to which a combination of anionic surfactant and protein takes place depends on many factors. Only surfactant monomers or submicellar species penetrate membranes, whereas micelles would presumably be too large to penetrate. Apparently a reduction of critical micelle concentration (CMC) results in lower levels of free monomers; therefore, surfactants with high CMC penetrate faster. The nitrogen content of the solubilized protein gives the so-called Zein number which classifies anionic surfactants into: \2 mg N/1 ml: nonirritant; 2 4 mg N/ 1 ml: moderate irritant; [4 mg N/1 ml: strong irritant. The results plotted in Fig. 6, show that U-MES C16 sodium salt is nonirritant up to 2.5 wt% and can be compared favorably to LAS, SAS and a-mes and gives similar results to AES [1]. We see that a-mes is much less irritant than LAS and SAS, which can be explained by the shield effect of the CO 2 Me group as mentioned earlier. Even if U-MES C16 has a higher CMC than a-mes, its lower Zein number may reflect weaker hydrophobic binding to the protein, due to 12 5 5 SAS 14 6 α MES C16-C18 ppm 4 ppm 65 ppm Fig. 5 Number of washed dishes at 49 8C as a function of water hardness mgn2/1ml 5 4 3 2 1 63 the presence of the ester linkage in the hydrophobe portion of U-MES C16. In a-mes, the ester linkage is within the hydration sphere of the sulfonate group and leaves intact the hydrophobic binding of the chain. References 19 19 169 255 Φ-MES C16 LAS P-55 SAS α MES C16- C18 1. Cohen L, Trujillo F (1998) Synthesis, characterization and surface properties of sulfoxylated methyl esters. J Surfactants Detergents 1:335 341 2. Cohen L, Trujillo F (1999) Performance of sulfoxylated fatty acid methyl esters. J Surfactants Detergents 2:363 365 3. Cohen L, Soto F, Luna MS (21) Sulfoxylated methyl esters as potential components for liquid formulations. J Surfactants Detergents 4:147 4. Cohen L, Soto F, Luna MS (21) Separation and extraction of U-methyl ester sulfoxylate: new features. J Surfactants Detergents 4:73 74 5. Cohen L, Soto F, Luna MS, Roberts DW, Saul CD, Lee K, Williams E, Bravo JE (22) Derivatization GC-MS, LSIMS and NMR analysis of sulfoxylated methyl esters. Tenside Surfactants Detergents 39:78 83 6. Cohen L, Soto F, Luna MS, Pratesi CR, Cassani G, Faccetti L (23) Analysis of sulfoxylated methyl esters: sulfonic acid composition and isomers identification. J Surfactants Detergents 6:1 4 7. Roberts DW (2) Aquatic toxicity are surfactant properties relevant? J Surfactants Detergents 3:39 3 8. Roberts DW (24) Environmental risk assessment of surfactants: quantitative structure activity relationships for aquatic toxicity. In: Zoller U (ed) Handbook of detergents: part B environmental impact. Dekker, New York, pp 271 298 9. Satsuki T et al (1998) Effect of calcium ions on detergency. Tenside Surfactants Detergents 35:112 117 1. Soler J (1984) Proceedings of the XV Jornadas del Comité Español de la Detergencia. Barcelona, Spain, pp 139 169 11. Kaestner W, Frosch PJ (1981) Fette-Seiffen und Anstrichmittel 83:33 12. Seibert K, Bolsterdorf D (1988) Magnesium surfactants as contribution to mildness, Proceedings of the 2nd CESIO World Surfactant Congress, Paris, pp 646 662 85 136 18 164 288 Surf. Conc.,5% 1% 2,5% Fig. 6 Zein test versus surfactant concentration. Zein protein concentration : 2 g/4 ml

Author Biographies Dr. Leon Cohen received his Ph.D. in chemistry at Sevilla University. In 1994, he earned the EURCHEM designation. He worked for PETRESA from 197 to 1996. Since 1989 he has been a Professor of Chemical Engineering at the University of Cadiz, where he has led the research group on Surface Activity and Detergency since 1993. He is the author of more than 25 papers, more than 4 contributions to Congresses, and has four patents related to detergency. Dr. Fernando Soto received his M.Sc. in chemistry at the Sevilla University and his Ph.D. in Chemical Engineering in 21 at Cadiz University. He has been a Professor of Chemical Engineering at the University of Cadiz, since 1979. He has been a member of the research group on Surface Activity and Detergency since 1994. Ana Melgarejo received her B.Sc. in Chemical Engineering at Cadiz University in 27. Dr. David W Roberts received his Ph.D. in Chemistry from Manchester University, UK, in 1965. He is a Fellow of the Royal Society of Chemistry and has the EURCHEM designation. He worked for Unilever Research from 1967 to 23. Since 23 he has been a consultant in Manufacturing and Toxicological Chemistry and is an honorary researcher at John Moores University in Liverpool. He is the author of more than 1 papers in the fields of surfactant science and toxicology.