Unique interfacial adsorption behavior of a hydroxy group-containing amino acid surfactant

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Abstract

The interfacial adsorption properties of the novel hydroxy-group-containing amino-acid-type surfactant, N-alkanoyl-N-(2-hydroxyethyl)-β-alanine (Cn-EtOH-βAla, where n is the length of the alkyl chain; n = 8, 10, 12, 14, and 16; EtOH is the hydroxyethyl group; βAla is β-alanine), and the hydroxy-group-free conventional amino-acid-type surfactant, N-dodecanoyl-N-methyl-β-alanine (C12-Me-βAla), were investigated for comparison purposes in an alkaline solution. Cn-EtOH-βAla (where n = 10–14) possessed a significant minimum critical micelle concentration (CMC) in surface tension vs. concentration plots, contrary to the conventional surfactant C12-Me-βAla. The area occupied per molecule for C12-EtOH-βAla (0.316 nm2) was considerably decreased compared with C12-Me-βAla (0.683 nm2); this was attributed to hydrogen bonding between the hydroxy group and water/carboxylate ions in Cn-EtOH-βAla, which facilitated the dense adsorption and efficient orientation of the surfactant at the air/water interface. The relationship between the logarithm of the CMC and the n value of Cn-EtOH-βAla indicated that the surfactant exhibited nonionic-like behavior because the hydrogen bonding between the hydroxy groups and carboxylate ions suppressed the electrostatic repulsion between hydrophilic groups.

Introduction

Amino acids have been used as components in a variety of functional materials. Amino-acid-type surfactants are one such material and are obtained by the introduction of an alkyl chain to an amino acid. Among the amino-acid-type surfactants, the most representative are N-acyl-amino-acid-type surfactants, which have good foaming properties, are less sensitive to hard water than soap, are nonirritating to the skin, have good antibacterial and biodegradability properties. Accordingly, they are widely used in cosmetics, personal hygiene products, and food formulations [1].

Amino-acid-type surfactants were first synthesized in 1909 as a biological product; Bondi et al. were the first to synthesize N-acyl-alanine- and N-acyl-glycine-based amino-acid-type surfactants, and also report their biodegradation into amino and fatty acids [2]. Staudinger and Becker [3], and Heitmann [4,5] reported the details of their physicochemical properties in aqueous solution in 1937 and 1968, respectively. Thereafter, Takehara et al. conducted extensive studies on their physicochemical properties and applications in personal care products [[6], [7], [8], [9]], and also established a commercially applicable production process [10]. Other researchers have reported that increasing the alkyl chain length and hydrophobicity of the amino acid residue can decrease the surface tension and critical micelle concentration (CMC) of N-acyl-amino-acid-type surfactants [11] and that decreasing the size of the amino acid residue can increase the Krafft temperature of N-alkanoyl-amino-acid-type surfactants [12].

Recently, novel surfactants containing hydroxyl groups in their molecules have been developed with the purpose of improving the water solubility, micelle-forming capabilities in solution, and adsorption properties at the air/water interface of conventional surfactants. Particularly, Chatterjee et al. synthesized hexadecyl(2-hydroxyethyl)dimethylammonium bromide and hexadecyldihydroxyethylmethylammonium bromide, in which a methyl group of hexadecyltrimethylammonium bromide (CTAB) was substituted with a hydroxyethyl group;. The CMCs of these hydroxy-group-containing surfactants were demonstrated to be lower than that of CTAB [13]. Devi et al. have demonstrated that, in comparison with the corresponding hydroxy-group-free gemini surfactants, α,ω-bis(hydroxyethylmethylhexadecyl ammonium)alkanediyl bromide has a 10–100-times lower CMC value, improved micellar growth, and an increase in aggregation number [14,15]. Moreover, Zhao et al. synthesized the hydroxy-substituted gemini surfactants α,ω-bis(dimethyldodecylammonium)-2-hydroxybutanediyl bromide and α,ω-bis(dimethyldodecylammonium)-2,3-hydroxybutanediyl bromide and studied their viscoelastic properties in aqueous solution by steady-state and dynamic frequency sweep rheological measurements [16,17]. The hydroxy-substituted surfactants exhibited enhanced micellar growth compared with the unsubstituted α,ω-bis(dimethyldodecylammonium)butanediyl bromide. These results were attributed to the intermolecular hydrogen bonding. Li et al. investigated the phase behavior of a cationic gemini surfactant with a hydroxy group in the spacer, α,ω-bis(dimethyldodecylammonium)-2-hydroxypropanediyl bromide, in a protonic ionic liquid (ethylammonium nitrate (EAN)), thus highlighting the effects of the hydroxy group on micellization and lyotropic liquid crystal formation [18]. This hydroxy-group-containing gemini surfactant showed excellent micelle-forming ability in EAN and exhibited increased compactness at the air/EAN interface, compared with the corresponding hydroxy-group-free gemini surfactants.

Accordingly, in this study, we developed a novel amino-acid-type surfactant by introducing a hydroxy group in a conventional N-acyl amino-acid-type surfactant to augment its properties and performance. Furthermore, we reported the interfacial adsorption behavior in alkaline solution of a novel hydroxy-group-containing amino-acid-type surfactant, N-alkanoyl-N-(2-hydroxyethyl)-β-alanine (Cn-EtOH-βAla, where n is the length of the alkyl chain and can be 8, 10, 12, 14, or 16; Fig. 1), and compared it to that of a conventional amino-acid-type surfactant without a hydroxy group, i.e., N-dodecanoyl-N-methyl-β-alanine (C12-Me-βAla, Fig. 1).

Section snippets

Materials

The hydroxy-group-containing amino-acid-type surfactant Cn-EtOH-βAla (n = 8, 10, 12, 14, 16) and the hydroxy-group-free amino-acid-type surfactant C12-Me-βAla were kindly supplied by NOF CORPORATION (Tokyo, Japan). Cn-EtOH-βAla was initially purified via silica gel column chromatography (chloroform/methanol = 100:0–90:10), then washed with hexane, and finally recrystallized using ethyl acetate to obtain a product with a high degree of purity. The structure was evaluated using proton nuclear

Acid dissociation constant (pKa)

The dissociation constant (pKa) of protonated C12-EtOH-βAla was determined as 7.92 by acid-base titration, thus indicating that the carboxy group was dissociated at pH > pKa. Therefore, the adsorption properties of surfactant C12-EtOH-βAla were investigated in alkaline solution at pH 12–13, at which the carboxy group is completely dissociated. All measurements were conducted in borosilicate glass containers to avoid silicate dissolution.

Krafft temperature (TK)

The Krafft temperature (TK) of an ionic surfactant

Conclusions

In this study, the interfacial adsorption behavior of the novel hydroxy-group-containing amino-acid-type surfactant Cn-EtOH-βAla (n = 8, 10, 12, 14, 16) in alkaline solution was investigated, and compared with that of the conventional hydroxy-group-free amino-acid-type surfactant C12-Me-βAla. The surface tension of the hydroxy-group-containing C12-EtOH-βAla decreased with increasing surfactant concentration and presented a low minimum at the CMC, despite its high purity; thus, we concluded that

CRediT authorship contribution statement

Shiho Yada: Investigation, Writing - original draft, Writing - review & editing. Miyako Wakizaka: Investigation. Hiroshi Shimosegawa: Resources, Validation. Hiroya Fujita: Resources, Validation. Munehiro Yamada: Project administration. Yukako Matsue: Resources, Validation. Tomokazu Yoshimura: Conceptualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Mr. Motonari Mizuta and Mr. Satoshi Matsuo from the NOF Corporation for their helpful discussions. Additionally, we would like to thank Editage (www.editage.jp) for their English language editing.

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