Historical perspectiveRheology of mixed solutions of sulfonated methyl esters and betaine in relation to the growth of giant micelles and shampoo applications
Introduction
Formation of large micellar aggregates of different morphology is most frequently observed in mixed surfactant solutions, in which the micelles are multicomponent and polydisperse in size [[1], [2], [3], [4], [5], [6], [7]]. Upon variation of solution's composition, high peaks in viscosity have been often observed [[8], [9], [10], [11], [12]]. Such concentration dependencies are of primary importance for various practical applications, e.g. for personal and household care detergency (e.g. shampoos and liquid detergents), because they allow one to control the micelle growth and formulation's viscosity [[13], [14], [15]]. The highest viscosities are observed in the presence of giant wormlike surfactant micelles. The high viscosity is due to the interplay of various processes and interactions that take place in such concentrated and internally structured solutions. First, the high aspect ratio of the long micellar aggregates gives rise to purely hydrodynamic interactions [16,17]. Second, de Gennes [18] identified the main relaxation mechanism for long linear polymers with reptation, which is related to the curvilinear diffusion of linear unbreakable molecules confined by their neighbors. Furthermore, in the case of long micellar aggregates (“living” polymers) Cates and coauthors [[19], [20], [21], [22], [23], [24], [25]] developed statistical theory, which accounts for micelle reversible scission and end-interchange processes. This theory predicts correctly the variations of zero-shear viscosity and other rheological parameters at not too high surfactant and salt concentrations [26]. Recently, Hoffmann and Thurn [27] took into account the energy of sticky contacts between micelles, which include contributions from the van der Waals, electrostatic, hydrophobic and hard-core interactions. Depending on the chemical nature of the component, whose concentration is varied, one could distinguish three types of viscosity peaks:
First, the variation of the mole fractions of the two basic surfactants (anionic and cationic, or anionic and zwitterionic) leads to a maximum in viscosity because of synergistic interactions between the two surfactants that promote growth of large self-assembled aggregates, usually – wormlike micelles, to the left of the maximum and diminishing of their size to the right of the maximum [10,[28], [29], [30], [31]]. At that, the maximal viscosity corresponds to the concentration domain with the longest entangled micelles. The synergism can be due to favorable headgroup interactions (e.g. in a catanionic pair) [32,33], as well as to a mismatch in the surfactant chainlengths [[34], [35], [36]].
Second, in systems containing ionic surfactants the dependence of viscosity on the concentration of added salt (the so called salt curve) often exhibits a high peak [28,[37], [38], [39], [40], [41], [42], [43]]. In this case, the peak could be explained with a transition from wormlike micelles to branched micelles [[44], [45], [46], [47], [48]]. The initial growth of wormlike micelles could be explained with the screening of the electrostatic repulsion between the surfactant headgroups by the electrolyte, whereas the subsequent transition to branched aggregates can be interpreted in terms of surfactant packing parameters and interfacial bending energy [49]. At high salt concentrations, the viscosity could drop because of phase separation due to the salting out of surfactant.
Third, viscosity peaks are observed upon the addition of amphiphilic molecules – cosurfactants, typically fatty acids and alcohols, which are used as thickening agents [[50], [51], [52], [53], [54]]. In this case, the peak can be due again to transformation of the wormlike micelles into branched [52] or ribbonlike and disklike [53,55] aggregates. Alternatively, the peak could be related to the onset of a phase separation of surfactant as a precipitate from droplets and/or crystallites (see Section 3.4). Peaks in viscosity have been observed also in catanionic systems as a function of temperature [56] and in zwitterionic systems as a function of pH [57].
With respect to micelle topology, here we follow the terminology originating from Ref. [45], viz. the entangled wormlike micelle is a linear aggregate with two endcaps and no junctions with other micelles; a branched micelle consist of several connected branches, each of them beginning at a junction and ending with an endcap, and finally, the multiconnected saturated network represents a bicontinuous structure, where all endcaps have been transformed into intramicellar junctions.
In a preceding paper [58], we reported that the viscosity in mixed solutions of sulfonated methyl esters (SME) and cocamidopropyl betaine (CAPB) significantly increases with the rise of the total surfactant concentration. Our goal in the present article is to further extend this study and to examine the effects of surfactant composition, added salt and thickening agents, including the possible appearance of peaks in viscosity that could evidence synergistic growth, micelle shape transformations or phase separation.
The sulfonated methyl esters (SME) are produced from renewable palm-oil based materials [[59], [60], [61]] and have been promoted as alternatives to the petroleum-based surfactants [62]. SMEs exhibit a series of useful properties, such as excellent biodegradability and biocompatibility; excellent stability in hard water; good wetting and cleaning performance, and skin compatibility [60,[63], [64], [65], [66], [67], [68], [69]]. The SME surfactants are produced typically with even alkyl chainlengths, from C12 to C18. Here, they will be denoted CnSME, n = 12, 14, 16, 18. Information for the adsorption and micellar properties of CnSME can be found in Refs. [[70], [71], [72]]. So far, there is a single study on the rheology of micellar SME solutions without cosurfactants (like CAPB) and thickeners, but in the presence of nanoparticles [73].
A complete systematic study on all possible combinations of the basic surfactants, cosurfactants (including fatty alcohols of various chainlengths) and salts exceeds the scope of the present article. Here, we focus on the most significant and interesting effects observed in our experiments with an emphasis on the interpretation of the obtained rheological data on the basis of theoretical models, in order to achieve a better understanding of the underlying molecular processes and phenomena.
In Section 2, we briefly describe the ingredients in the investigated systems and the used experimental methods. In Section 3, we report and systematize data from many rheological experiments in steady shear regime, which demonstrate the existence of strong synergism in the SME + CAPB system with respect to the rise of viscosity; effects of various additives on the synergistic maxima; salt curves with and without added fatty alcohol, and the effect of the concentration of thickeners – fatty alcohols and cocamide monoethanolamine (CMEA). Section 4 is dedicated to rheological experiments in oscillatory regime and to theoretical interpretation of the obtained results for the storage and loss moduli, G′ and G″. Part of the data exhibit standard rheological behavior and are interpreted in terms of the Maxwell viscoelastic model and the reptation-reaction model proposed by Cates [19] and developed in subsequent studies [[20], [21], [22], [23], [24], [25]]. The latter model successfully explains both the Maxwellian behavior of micellar systems and the deviations from it. However, many of the investigated systems exhibit nonstandard rheological behavior. We have demonstrated that the rheological data for such systems can be theoretically described and analyzed in terms of an augmented version of the Maxwell model.
The paper could be useful for a broad audience of researchers, who are interested in synergistic effects in mixed micellar solutions and in their theoretical interpretation.
Section snippets
Materials
In our study, we used two kinds of sulfonated methyl esters (α-sulfo fatty acid methyl ester sulfonates, sodium salts, denoted also α-MES), which are products of the Malaysian Palm Oil Board (MPOB) and KLK OLEO. The first one is myristic sulfonated methyl ester (C14SME) 98%, M = 344.34 g/mol, with critical micelle concentration CMC = 3.68 mM [70].
The second one, which will be denoted C16,18SME, represents a mixture of 85 wt% palmitic (C16SME) and 15 wt% stearic (C18SME) sulfonated methyl esters
Types of flow curves
Fig. 2 illustrates the existence of two types of η-vs.- dependencies (flow curves): regular (Fig. 2a and b) and irregular (Fig. 2c and d). The experimental data are obtained with mixed solutions of C14SME + CAPB and C16,18SME + CAPB at two total surfactant concentrations, 8 and 12 wt%, in the presence of various additives: fatty alcohols, CMEA and NaCl. In our experiments, the weight fraction of CAPB, w, in the mixed surfactant solutions has been varied; w is defined as follows:
Experimental results for G' and G"; comparison with the Maxwell model
In the case of experiments with rotational rheometer in oscillatory regime, sinusoidal oscillations of the strain are imposed:where γa is amplitude, t is time and ω is angular frequency. Our experiments were carried out at γa = 0.02, whereas ω was varied. As a rule, the measured stress, σ(t), is phase-shifted, so that it can be expressed in the form:where G′ and G″ are the storage and loss moduli, respectively; G′ and G″ are independent of t, but they depend on ω;
Conclusions
The mixing of anionic and zwitterionic surfactants in aqueous solutions is known to promote the synergistic growth of giant micelles, which is detected as a significant rise of viscosity [1,10,29,31,39,42,[50], [51], [52], [53], [54]]. In the present paper we investigated this phenomenon for mixed solutions of sulfonated methyl esters, SME, and CAPB, potential ingredients for personal-care formulations. The effect of SME is compared with those of a standard anionic surfactant, SDS. Moreover,
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.
Acknowledgments
All authors gratefully acknowledge the support from KLK OLEO. VY, KD and PK acknowledge the support from the Operational Programme “Science and Education for Smart Growth”, Bulgaria, grant number BG05M2OP001-1.002-0012. GR acknowledges the financial support received from the program “Young scientists and postdoctoral candidates” of the Bulgarian Ministry of Education and Science, MCD No 577/17.08.2018.
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