Estimating the Adsorption Efficiency of Sugar-Based Surfactants From QSPR Models

Estimating the Adsorption Efficiency of Sugar-Based Surfactants From QSPR Models

Théophile Gaudin (INERIS / Université de Technologie de Compiègne, Compiègne, France), Patricia Rotureau (INERIS, Verneuil-en-Halatte, France), Isabelle Pezron (Université de Technologie de Compiègne, Compiègne, France) and Guillaume Fayet (INERIS, Verneuil-en-Halatte, France)
DOI: 10.4018/IJQSPR.2019040102


Adsorption efficiency, measured as the surfactant concentration at which the surface tension of the aqueous solution decreases by 20 mN/m, characterizes their affinity for surfaces and interfaces, which is crucial for a cost-effective use of surfactants. In this article, the first Quantitative Structure-Property Relationship models to predict this efficiency were proposed based on a dataset of 82 diverse sugar-based surfactants and using different types of molecular descriptors. Finally, an easy-to-use model was evidenced with good predictivity assessed on an external validation set. Moreover, it is based on a series of fragment descriptors accounting for the different structural trends affecting the efficiency of sugar-based surfactants. Due to its predictive capabilities and to the structure-property trends it involves, this model opens perspectives to help the design of new sugar-based surfactants, notably to substitute petroleum-based ones.
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In the context of development and use of more environmentally-friendly products, efforts are nowadays in progress towards the design of new surfactants issued from renewable resources to substitute petroleum-based surfactants that constitute most part of the market. Among them, sugar-based surfactants are an important subfamily of bio-based surfactants (contributing up to 40% of biosurfactant consumption (Anbu, 2017)), characterized by their polar head constituted by carbohydrates such as glucose, maltose or sucrose, and their derivatives. For this reason, sugar-based surfactants can be obtained from renewable resources such as starch (Kjellin & Johansson, 2010), and are often biocompatible and easily biodegradable (Matsumura, Imai, Yoshikawa, Kawada, & Uchibor, 1990). Therefore, they are commonly considered among the most promising alternatives to conventional petroleum-based non-ionic surfactants (Hill & LeHen-Ferrenbach, 2009), particularly regarding soft detergents or personal care products, cosmetics and pharmaceutical formulations (Rojas, Stubenrauch, Lucia, & Habibi, 2009).

The main performance characteristics of surfactants that are used to select surfactants in application formulations relate to their effectiveness and their efficiency. The target effectiveness, i.e., the maximum performance to reach, is the maximum lowering of the surface tension in aqueous solution. This maximum is almost reached when the concentration in surfactants favors their aggregation to form micelles in the solution. This concentration is the critical micelle concentration (CMC). So, effectiveness is characterized by measuring the surface tension at CMC, denoted γCMC (Rosen, 1976).

Efficiency (Rosen, 1974) represents the amount of surfactant needed to reach a given performance. In surfactant solutions, two phenomena are in competition, adsorption at solvent/air interface and aggregation into micelles. While CMC is more related to aggregation, adsorption efficiency (commonly simply named efficiency) is measured as the quantity of surfactants needed to decrease the surface tension of the aqueous solution by 20 mN/m, in negative logarithmic unit by convention (-log C20, denoted pC20) (Rosen & Kunjappu, 2012). This level of reduction of surface tension is expected as generally sufficient to characterize a quasi-saturated interface (Rosen, 1974).

In applications, a high efficiency enables to reduce the amount of surfactant used (Fisher, Zeringue, & Feuge, 1977; Rosen, 1974). Indeed, using efficient surfactants allows to limit the amount of surfactant in formulation for a same target application and, as a consequence, limit their cost and, in some cases, avoid their possible toxicity.

If CMC and pC20 characterize different aspects of the behavior of surfactants in solution (aggregation for CMC and adsorption for pC20), they are both related to the hydrophobic effect (Tanford, 1979), that minimizes the contact between solvent and alkyl chains. So, it is not surprising to note that some common trends have been highlighted between the molecular structure and both the CMC (in log) and the pC20 in the experimental literature (Rosen & Kunjappu, 2012). For instance, in general, pC20 was observed to increase with alkyl chain length (Rosen, 1974; Zhu, Rosen, Vinson, & Morrall, 1999), and to slightly decrease with polar head size, which also affects CMC (Crook, Fordyce, & Trebbi, 1963; Myers, 2006; Rosen & Kunjappu, 2012).

The relationship between the molecular structure and pC20 was quantified by Rosen (Rosen, 1974) under a thermodynamic formalism by considering the free energies of transfer of the CHn groups and of the polar head from solvent to interface. This approach has notably been used to evidence structural trends of a series of conventional surfactants (without any sugar-based surfactants), like the increase of efficiency with the length of the alkyl chain or its decrease in the case of ionic surfactants.

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