Journal of O leo Sience C opyright © 2020 by Japan O il C hem ists’ Sciety doi : 10.5650/jos.ess20159 J . O l e o S i . 69, (11) 1411-1416 (2020) Coacervation in Cationic Polyelectrolyte Solutions with Anionic Amino Acid Surfactants Kenji Aramaki1＊ , Yasutaka Shiozaki1, Shuhei Kosono2, and Naoaki Ikeda2 1 Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN 2 Research Institute for Bioscience Products & Fine Chemicals, Ajinomoto Co., Inc., 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki-shi 210-8681, JAPAN A bstract: C oacervates form ed by cationic polyelectrolytes and anionic surfactants are utilized to im prove the user’s tactile perception of sham pooing hair during w ashing and after drying. In this study, w e investigated the form ation and structure of coacervates in aqueous system s containing anionic am ino acid surfactants. T he phase behaviors at constant tem perature w ere investigated in aqueous system s com bining cationic polyelectrolyte JR -400 w ith potassium cocoyl glutam ate (C oG luK ) or potassium cocoyl glycinate (C oG lyK ) for a qualitative depiction of coacervate form ation. T he com position range of coacervate form ation varied w ith the hydrophilic group of the surfactant. T he surface tension w as m easured at different surfactant concentrations and constant polyelectrolyte concentration. T he surface tension behavior revealed the critical association concentrations and critical m icelle concentrations, indicating that coacervate w as produced via com plex form ation through electrostatic interaction betw een opposite charges. O ptical m icroscopy and sm all-angle X -ray scattering m easurem ents revealed that the coacervates w ere com posed of fibrous aggregates, a few m icrons thick, and those form ed in the C oG lyK system had thicker f i b e r s . K e y w o r d s : c o a c e r v a t i o n , o l y m e r - u r f a c t a n t n t e r a c t i o n , m i n o c i d u r f a c t a n t , h a s e e h a v i o r , s m a l l - n g l e - a y c a t t e r i n g 1 Introduction bricating effect5, 6）and promoting the adhesion of the func- When an ionic surfactant and a polyelectrolyte with op- tional substances in shampoo, such as silicone oil, to hair7－9）. posite charges are mixed in an appropriate ratio, precipita- It is known to improve the user’s tactile perception while tion may occur as a complex coacervation. Fundamental washing and after drying hair. Cellulose derivatives have investigations in the fields of solution chemistry and colloid long been used as cationic polyelectrolytes in shampoos. chemistry focused on such complex coacervation（hereafter High-performance and low-cost alkyl sulfate salts or alkyl simply referred to as coacervation）have long been report- sulfate ester salts are widely used as anionic surfactants. ed. The phase behavior in ternary water/surfactant/poly- However, there are concerns about the use of such petro- electrolyte systems is well understood1）, and the effects of leum-based surfactants from the perspectives of skin irrita- various parameters, such as charge densities of surfactant tion and environmental impact. micelles and polyelectrolytes, molecular weights of poly- Acyl amino acid salts, which consist of fatty acids and mers, surfactant/polymer mixing ratio, and concentration, various amino acids, have high biodegradability and are, on coacervate formation have been reported2, 3）. The mo- therefore, considered environment-friendly green surfac- lecular mechanism of coacervation formation4）has addition- tants. They are mainly used in cosmetics as they cause low ally been clarified. skin irritation. In particular, anionic acyl glutamate and gly- Hair shampoos often contain a cationic polyelectrolyte cinate salts have been used in various cosmetics, and there and an anionic surfactant. Such a combination of oppositely is a vast literature on the dissolution states of, and molecu- charged molecules leads to coacervate formation during lar assembly formation in, aqueous solutions10－18）. rinsing in the process of hair washing. The coacervate gen- However, no studies on coacervation in combination with erated adheres to the hair surfaces, thereby providing a lu- cationic polyelectrolytes have been reported yet. In the ＊Correspondence to: Kenji Aramaki, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN E-mail: [email protected] Accepted July 20, 2020 (received for review June 10, 2020) J o u r n a l f l e o c i e n c e S S N 1 4 - 9 5 7 r i n t S S N 1 4 - 3 5 2 n l i n e h t t p : / / w w w . s t a g e . s t . o . p / b r o w s e / j o s / http://m c. anusriptcentral.om /jjocs 1411 K . A ram aki, Y. Siozaki, S K osono et al. present study, therefore, the amino acid surfactants, i.e., absence of coacervate was visually observed. potassium cocoyl glutamate（CoGluK）or potassium cocoyl glycinate（CoGlyK）, were separately combined with the 2.4 Optical microscopy cationic polyelectrolyte, JR-400, in which a substituent A differential interference microscope（CX31-P, Olympus, containing a quaternary ammonium salt was grafted to a Japan）was used to perform simple polarization observa- cellulose chain. The formation and structure of coacervates tions at room temperature（25±5℃）. were investigated in the different aqueous systems. 2.5 Conductivity measurement A total of 25 g of water was taken in a 50 mL glass con- tainer, and the mother liquor（1.41 wt.％ surfactant 2 Experimental aqueous solution）was added with a micropipette to adjust 2.1 Materials the concentration of the solution. The temperature of the A cationically modified cellulose ether, JR-400（Dow solution was set at 25℃ using a constant-temperature Chemical, USA）, was used. Its molecular structure is illus- water bath, and the electrical conductivity at each concen- trated in ref. 21. Its molecular weight, degree of substitu- tration was measured using an electrical conductivity mea- tion, and charge density were approximately 500000 g mol－1, suring device（DS-72, Horiba, Japan）and an electrical con- 0.27, and 1000 g mol－1, respectively19－21）. CoGlyK and ductivity cell（3552-10D, cell constant of 0.976 cm－1）. CoGluK were obtained from Ajinomoto Co., Inc.（Japan）. The degree of carboxylic acid neutralization is 1 for 2.6 Surface tension measurement CoGlyK and CoGluK, and their typical molecular structures The surface tension was measured using the Wilhelmy are shown in Fig. 1. The carbon number（n）in the cocoyl method（K100, Krüss, Germany）. The plate used was a group varied in the range 7–17, and the average n was roughened platinum plate designed for K100. 11.322）. Deionized water, obtained from an Elix 3（Nihon Millipore）water purification system, was used as the 2.7 Small- and wide-angle X-ray scattering（SWAXS） solvent. A small-angle X-ray scattering（SAXS）device（SAXSess, Anton Paar, Austria）was used. The X-rays（Cu-Kα, wave- 2.2 Sample preparation length λ＝0.1542 nm）used to irradiate the samples were A predetermined composition of reagents was weighed, generated by a sealed glass X-ray tube and collimated by a placed in a 20 mL screw-capped bottle, and uniformly block optical system. A glass capillary sample cell（1 mm mixed using a vortex mixer or magnetic stirrer or shaker. sample thickness）was used. The scattered X-rays were de- The sample compositions, WP and WS, were determined tected by an image plate, which was read using Cyclone using Equations（1）and（2）, respectively. （Perkin Elmer, USA）and a 1D spectrum was obtained using SAXSquant software（Anton Paar, Austria）. The mea- / , 1 Wp＝P（S＋P） （ ） surements were performed by setting the parameters of the X-ray generator at 40 kV and 50 mA. The X-ray irradia- / , 2 WS＝（ S＋P）（W＋S＋P） （ ） tion time was 10 min. where S, P, and W indicate the mass of surfactant, poly- electrolyte, and water, respectively. 2.3 Visual observation 3 Results and Discussion The samples prepared were placed in an incubator set at 3.1 Phase behavior 25℃ for several hours to several days, and the presence or Figure 2 shows the partial phase diagrams of the water/ Fig. 1 Typical molecular structures of（1）CoGlyK, and（2）CoGluK. 1412 J . O l e o S i . 69, (11) 1411-1416 (2020) C oacervation w ith Anionic Am ino Acid Srfactants Fig. 2 Phase diagrams of（a）water/CoGlyK/JR-400 system, and（b）water/CoGluK/JR-400 system at 25℃. Shaded areas indicate coacervation regions. surfactant/JR-400 systems in the dilute region（WS＜0.1）. Table 1 State of dilution. In both the systems, the coacervation region was formed S P when the polyelectrolyte and surfactant were mixed, and 10 7 . 5 2. 1 [ w t .％] furthermore, the coacervates were formed from high to low solute concentrations. In addition, the coacervation region C o G l y K T T T T T exists around the charge neutralization compositions that CoGluK T T T C C is approximately calculated as mass ratios, 33/67 and 41/59 T : T r a n s p a r e n t , : o a c e r v a t i o n .