Interfacial properties of novel surfactants based on maleic and succinic acid for potential application in personal care
Introduction
The self-assembly of amphiphilic molecules show an imperative role in natural and industrial applications [1], [2], [3], [4], [5]. The driving force for self-assembly stems from the interplay between hydrophobic and hydrophilic interactions of the amphiphile. Depending on the molecular architecture and solution conditions (presence of salt, co-surfactant, temperature, etc.), surfactants form numerous aggregates of well-defined shapes and sizes beyond the critical micelle concentration [6], [7], [8], [9], [10]. Anionic surfactants have gained much consideration from the scientific community in the past few years and are expected to further increase owing to their unique physicochemical properties [11], [12], [13], [14], [15]. This compels the improvement of new anionic surfactants possessing a high surface activity, excellent adsorptive ability, and is mild to skin by judiciously tuning their molecular structure. Besides the continual task of finding means to reduce the manufacturing cost for prevailing surfactants, appeal for ‘greener’ products is the dominant driving force for surfactants development.
Numerous different classes of surfactants can be synthesized from simple amino acid building blocks [16], [17], [18]. These surfactants have been comprehensively explored with respect to dermatological properties, surface activity and foaming properties and serves as a vital asset in personal care formulations. Hemiesters and hemiamides of maleic acid and succinic acid with different chain lengths (C8-C16) of the hydrophobic alkyl group were synthesized by Abele et al. and were used as surfactants in the emulsion polymerization of styrene and butyl acrylate [19], [20]. However, they did not report comprehensive interfacial properties of these surfactants owing to their low solubility in pure water. The succinates have an analogous structure to maleates, but they do not contain any reactive double bonds. Hence it would be interesting to explore their comprehensive surface properties.
In this context, we have synthesized hemiesters and hemiamides with lauryl chain length and hemiesters of maleic acid and succinic acid with hexadecyl chain length by adopting the method reported by Abele et al. [19] and finally neutralizing them with aqueous sodium bicarbonate solution. Physicochemical properties of these surfactants were explored for the first time which could be highly beneficial for their application in personal care formulations. These surfactants displayed intriguing interfacial properties but lower water solubility. However, we found that overall solubility of these surfactant could be increased in the existence of hydrotropes (sodium p-toluene sulfonate; NaPTs) which were structurally similar to surfactants (but possess a weaker hydrophobic character) and have many practical applications [21], [22]. On the basis of various theoretical and experimental efforts, it is widely accepted that there is a formation of a complex between hydrotropes and additives, which would then show a higher aqueous solubility.
This paper will encompass fundamental information relating to synthesis, basic characteristics, and the physicochemical properties of novel surfactants based on maleic and succinic acid like sodium lauryl succinate (C12SE), sodium lauryl maleate (C12ME), sodium lauryl succinamide (C12SA), sodium lauryl maleamide (C12MA), sodium hexadecyl succinate (C16SE) and sodium hexadecyl maleate (C16ME) in pure water and aqueous NaPTs solution. The chemical structures of the prepared surfactants were established by FTIR and 1H NMR spectroscopy. Measurements of critical micelle concentration (CMC) made by surface tension and size distribution of the micelles was derived using dynamic light scattering (DLS) techniques. In addition, the foaming behavior was investigated and the protein/lipid solubilization by surfactants was examined. Viscosity measurements were performed by using Brookfield viscometer.
Section snippets
Materials
Surfactants used in the study like sodium lauryl succinate, sodium lauryl maleate, sodium lauryl succinamide, sodium lauryl maleamide, sodium hexadecyl succinate and sodium hexadecyl maleate were synthesized (were pure as confirmed by NMR measurements in supporting information) in the laboratory. Sodium p-toluene sulfonate (>98%, Sigma), Zein protein (>98%, Sigma) and stearic acid (>98%, TCI) were used as received. All experiments were carried out using Milli-Q water. Samples were allowed to
Synthesis of novel surfactants
Synthesis of surfactants was started with maleic or succinic anhydride as starting materials. Opening of these cyclic anhydrides either with long chain alcohols or amines result in the formation of hemiesters or hemiamides [19], [20]. Later, the carboxylic acid group was converted to respective sodium salts by treating with sodium bicarbonate to get required surfactants of interest. Synthesized surfactants were pure, as per the NMR recorded for the intermediates; hemiester carboxylic acids and
Adsorption at the air–liquid interface
Surface tension (γ) measurements were executed to explore and estimate the surface active properties of as-synthesized surfactants in pure water. Longer alkyl chain surfactants were insoluble in water because of their high hydrophobicity. The equilibrium surface tension versus concentration plot obtained for sodium lauryl succinate (C12SE) and sodium lauryl succinamide (C12SA) were depicted in Fig. 1a, and for sodium lauryl maleate (C12ME) and sodium lauryl maleamide (C12MA) was presented in
Micellar size and charge
Micellar size and micellar charge was evaluated for each surfactant based on dynamic light scattering (DLS) and zeta potential measurements at CMC in pure water and NaPTs solution (1 wt%). The obtained results were reported in Table 2. We have also included widely utilized conventional surfactants in personal care like anionic sodium lauryl sulfate (SLS), zwitterionic cocamidopropyl betaine (CAPB) and nonionic decyl glucoside (DG) for comparison.
The results from DLS clearly indicates that
Foaming behavior
Foam creating power of the liquid or foamability was referred to the capability of the surfactants to accomplish low surface tension in a short time when a fresh interface was created. The foam stability of a surfactant solution was defined as the alteration in foam volume which was primarily controlled by the drain speed and intensity of the interfacial film. Foam construction and permanency were imperative in numerous applications [26], [27], [28]. The surfactant molecules adsorb to the
Viscosity measurements
Measurement of the rheological properties is one of the dominant tools to characterize the properties of different surfactant solutions, which in many cases are non-Newtonian. Viscoelastic properties have been reported in anionic and mixed-surfactant systems comprising of cationic–anionic and anionic–zwitterionic surfactants [29], [30]. Because of the electrostatic interactions between anionic surfactant headgroups, the growth of micelles in the solution is inadequate. But with inclusion of
Skin mildness of surfactants
Stratum corneum (SC), the uppermost cover of the epidermis, affords a vital barrier function in intact skin. Surfactants reduce the barrier properties of the SC proteins leading to their swelling and denaturation, though the molecular mechanisms involved have not yet been entirely revealed [31]. The propensity of surfactants to interact with model proteins has also been associated with their severity toward human skin. Thus, higher the tendency of a surfactant to interact; higher was its
Conclusions
Synthesis and physicochemical properties of novel surfactants based on maleic and succinic acid like sodium lauryl succinate, sodium lauryl maleate, sodium lauryl succinamide, sodium lauryl maleamide, sodium hexadecyl succinate and sodium hexadecyl maleate has been described. The chemical structures of these surfactants were established by FTIR and 1H NMR spectroscopy. We have shown that solubility of these surfactants could be improved in the presence of hydrotrope like NaPTs. It was also
CRediT authorship contribution statement
Devi Sirisha Janni: Conceptualization, Methodology. Gajendra Rajput: Visualization, Investigation. Niki Pandya: Data curation. Gayathri Subramanyam: Supervision, Project administration. Dharmesh Varade: Supervision, Writing – review & editing.
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
This work was supported by Seed Grant from Ahmedabad University and UGC-DAE (CRS-M-304).
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