Study of the environmental responsiveness of amino acid-based surfactant sodium lauroylglutamate and its foam characteristics

https://doi.org/10.1016/j.colsurfa.2016.05.097Get rights and content

Highlights

  • The bulk phase aggregation and interfacial behavior of LGS were studied.

  • The foam stability of LGS changed with environmental conditions was investigated.

  • The gel could be formed in foam film and its strength influenced on the foam stability.

Abstract

In this paper, the bulk phase aggregation and interfacial adsorption properties as well as foam characteristics of a kind of amino acid-based surfactant sodium lauroylglutamate (LGS) under the variety of external conditions such as temperature and pH were investigated. The variation of the protonation degree of the head group of the surfactant as a function of pH was analyzed by integrating determination results of acid-base titration and FT-IR (ATR) spectra. At neutral pH, the LGS solution had the minimum surface tension and could form more stable foam. The mechanism about how the pH affects the foam stability of LGS was discussed thoroughly. TEM images of the LGS foam films and the bulk phase solutions showed that gel network association structure was formed not only in bulk phase in specific temperature range and suitable pH, but also in the foam films. The variation of the strength of the gel formed in foam film under different external conditions has obviously different influence on foam stability.

Introduction

Aqueous foam has got widely utilization in daily life and industrial applications, such as detergents, mineral floatation, detergent, and enhanced oil recovery [1], [2], [3]. In the varied applications, foams remaining stable for a long time (e.g., oil exploitation) and those could be destroyed rapidly on demand (e.g., radioactive materials recovering) are both probably required [4], [5], [6]. Being recognized as a thermodynamically unstable colloid system, the stability of foam could be influenced by many factors in the following processes after being generated, such as liquid drainage, bubble coalescence, gas diffusion, etc. [7], [8], [9], [10], [11], [12], [13]. The effect of environmental conditions on the stability of foam is always an important issue to be considered, especially for occasions in which the properties of foam stabilizers might be influenced by some of the environmental conditions, and thereby the foam properties could be switching controlled by adjusting these conditions [5], [14].

Surfactant is the most popular type of aqueous foam stabilizers, though diverse kinds of substances have been used to stabilize aqueous foam, such as particles, water soluble polymers, etc. [13], [15], [16], [17], [18], [19], [20], [21]. Amino acid-based surfactants have become increasing popular nowadays, not only because they are human friendly and less toxic comparing with traditional surfactants, but also in account of some of the excellent properties derived from the special chemical structures [22], [23], [24]. For example, according to previous reports, the N-acylamino acid-based surfactants have nice solubility and a good detergency, so they have high potential in skincare and cleanser products [24]. A few kind of acid-based surfactants have even been used commercially in the field of personal cares [24], [25].

Because the polar head-group and the hydrophobic tail of N-acylamino acid-based surfactant are connected by an amide band, which can act as both hydrogen bond donor and hydrogen bond acceptor, so pH plays an important role in variation of the self-assembly behavior of amino acid-based surfactants in bulk phase solution, and amino acid-based surfactant was known as a type of surfactants with environmental responsiveness, and aroused people’s great interest [26], [27], [28]. Daisuke et al. studied the self-assembly of N-Lauroyl-l-glutamate in water solution and found that a cubic phase was formed in the region between the isotropic micellar and the hexagonal phase in phase diagram [29]. Romain et al. investigated the bulk phase behavior of a series of amino acid-based surfactants having two carboxyl groups separated by a spacer of one, two or three carbon atoms [30]. Kenichi et al. synthesized a gemini surfactant, 12-GsG-12 (dodecanoylglutamic acid anhydride with alkyl diamines), by reacting dodecanoylglutamic acid anhydride with alkyl diamines and found that α-gel was formed in this system [31]. By far, most of the investigations of N-acylamino acid-based surfactants focused on the self-assembly in bulk phase, however there is rare report on interfacial behavior and foaming properties of amino acid-based surfactant, which is actually extremely important in many applications. Meanwhile, studies about environmental responsiveness of amino acid-based surfactants being used as foam stabilizer are scarce.

In this paper, the bulk phase aggregation, the interfacial adsorption properties and the foam characteristics of a kind of amino acid-based surfactant, Sodium lauroylglutamate (LGS), under the variety of pH and temperature were investigated. The mechanism was discussed thoroughly by combining experimental determination and theoretical molecular simulation methods.

Section snippets

Materials

Sodium lauroylglutamate (LGS) was provided by Nanjing Huashi Chemical Co. Ltd. and used as given. Hydrochloric acid (∼37.5 wt%, AR) was purchased from Tianjin Guangcheng Chemical Corp. The pH of the surfactant solution was adjusted by adding HCl solution (1 M) into the solution and measured by using Mettler-Toledo Sevenexcellence with a specific pH electrode. The chemical structures of LGS with different neutralization degree are shown in Fig. 1.

Samples preparation

The LGS stock solution with pH 10.0 was firstly

Effect of pH on the neutralization degree and surface activity of LGS

For amino acid-based surfactants, their physical-chemical properties are highly dependent on pH, because the degree of protonation influences its properties heavily [30]. 25 ml LGS (0.05 M) solution was titrated by HCl (0.1 M) at 25.0 °C, the variation of pH as a function of volume of titrated HCl solution was shown in Fig. 2a. The aggregation of the surfactants which can have an influence on the pKa did not take into accounts. The pKa1 and pKa2 value of LGS were determined to be about 2.1 and 4.5

Conclusion

The effect of pH and temperature on the bulk phase and interfacial characteristics of amino acid-based surfactant Sodium lauroylglutamate (LGS), especially the foam properties, were investigated in this paper. It was found that LGS was protonated in different degree under various pH values, the interfacial activity and bulk phase aggregation behavior of LGS changed correspondingly with the variation of pH and temperature. Gel could be formed in the LGS solutions and also in the foam film under

Conflict of interest

The authors declare no competing financial interest.

Acknowledgments

This funding of National science fund of China (No. 201473103) and National Municipal Science and Technology Project (No. 2008ZX05011-002) are gratefully acknowledged.

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