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

Introduction

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 γ_CMC (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 CH_n 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.