Rheology of α-Gel Formed by Amino Acid-Based Surfactant with Long-Chain Alcohol: Effects of Inorganic Salt Concentration.

Kumika Ichihara,Masaaki Akamatsu,Kenichi Sakai,Hideki Sakai,Tadashi Sugahara

doi:10.1021/acs.langmuir.1c00626

Kumika Ichihara, Masaaki Akamatsu + Show 3 more

Open Access

https://doi.org/10.1021/acs.langmuir.1c00626

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Abstract

Mixtures of surfactants, long-chain alcohols, and water sometimes yield lamellar gels with hexagonally packed alkyl chains. This assembly is called “α-gel” or “α-form hydrated crystal.” In this study, we characterized the rheological properties of α-gel prepared using disodium N-dodecanoylglutamate (C12Glu-2Na), 1-hexadecanol (C16OH), and water at different NaCl concentrations. The α-gel structure was assessed using small- and wide-angle X-ray scattering (SWAXS). The SWAXS measurements revealed that an increased NaCl concentration (0–200 mmol dm–3) resulted in a decreased d-spacing caused by the screening of electrostatic repulsion between lamellar bilayers. This led to an increased amount of excess water (i.e., the water present between the α-gel domains), and hence, the viscosity of the α-gel decreased in the range of the NaCl concentration. A further increase in the NaCl concentration (200–1000 mmol dm–3) resulted in decreased electrostatic repulsion between the α-gel domains and/or an increased number of α-gel domains (multilamellar vesicles). These effects increased the domain-to-domain interactions, leading to increased viscosity. Therefore, we concluded that the viscosity of the α-gel was controlled by the amount of excess water and the domain-to-domain interactions. Once the network structure collapsed under the strain, it was difficult to recover the original network structure. The low recoverability resulted from increased cohesion between the domains at high NaCl concentrations.

Highlights

Binary mixtures of surfactants and water or ternary mixtures of surfactants, long-chain alcohols, and water sometimes yield “lamellar gel” below their gel−liquid crystal phase transition temperatures (Tc)
We studied the effects of inorganic salts on the rheological properties of α-gels (α-form hydrated crystals)
NaCl concentration resulted in a decreased viscosity in the [NaCl] range of 0−200 mmol dm−3, and afterward, the viscosity increased in the [NaCl] range of 200−1000 mmol dm−3

Summary

Introduction

Binary mixtures of surfactants and water or ternary mixtures of surfactants, long-chain alcohols, and water sometimes yield “lamellar gel” below their gel−liquid crystal phase transition temperatures (Tc). The term “psycho-rheology” denotes the importance of rheological usability in such applications.[17−19] Yamagata and coworkers[20−24] discussed the rheological behavior of the ternary mixture of hexadecyltrimethylammonium chloride, 1-hexadecanol (C16OH), and water based on electron microscopy and electron spin resonance data. Their key finding was that the viscosity of the α-gel consisting of a lamellar network structure was higher than that of an α-gel consisting of multilamellar vesicular domains. The viscosity of the lamellar gel network (α-gel) formed by docosyltrimethylammonium methylsulfate was reported to be higher than that by docosyltrimethylammo-

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