Molecular factors governing the viscosity peak of giant micelles in the presence of salt and fragrances

Abstract

Hypothesis

The formation of transient networks of giant micelles leads to a viscosity peak when salt is added to aqueous solutions of charged surfactants. It is the consequence of an increase of the packing parameter due to charge screening of the surfactant headgroups, leading to a continuous transformation of the aggregates from spherical to wormlike micelles, and finally to branched networks. It should therefore be possible to predict the macroscopic viscosity of entangled giant micelles by modelling the packing parameter at nanoscale.

Experiments

A thermodynamic model is presented with a minimum of adjustable parameters, where branched networks are considered to be built from three coexisting microphases: cylinders, endcaps, and junctions. We use spontaneous packing parameters, in which the whole molecular length instead of the commonly used hydrocarbon chain length is considered. Standard reference chemical potentials and subsequently the occurrence of each microphase can be explicitly derived at specific electrolyte concentrations. Effective micellar length of giant micelles can be obtained from the microphase composition and is subsequently used to calculate the viscosity.

Findings

The model successfully predicts position and intensity of the viscosity maximum observed in experimental salt curves of sodium laureth sulfate (SLES). The robustness of the model was further investigated for various types of added salts or fragrance oils that affect differently spontaneous packing parameters or interfacial bending energy. An excellent agreement of the simulated salt curves with experimental data was achieved.

Introduction

Giant micelles can form gel-like structures with particular properties that make them indispensable for use in a wide range of consumer and industry products, such as personal care applications, household cleaning agents, and oil well treatment fluids [1], [2], [3]. Their rheological properties are the consequence of a complex interplay between different micellar length and time scales, which is significantly influenced by temperature, the molecular structure, and the concentration of surfactants, co-surfactants, and other additives. The viscoelastic property of entangled giant micelles has been the subject of numerous studies that have provided a wealth of insights into mechanistic aspects at the molecular and supramolecular levels [4], [5], [6], [7]. Particularly interesting are surfactants with charged headgroups that show a strong response to the presence of counterions and where the addition of small amounts of electrolytes can lead to an increase in viscosity by several orders of magnitude. These interactions result in a non-monotonic trend as the ionic concentration is increased, referred to as the “salt curve” by application scientists and characterized by a viscosity that increases from values that are close to that of water to a pronounced peak, followed by a subsequent decrease. This characteristic behavior can be explained by the continuous topological transformation of the micelles from spherical, to elongated, to entangled wormlike shapes, and finally to branched cylinders decorated by endcaps. On a nanometer scale, these effects are the consequence of the increasing charge screening of the surfactant headgroups by the added electrolytes [8], [9], [10], [11], [12], [13], [14]. Testing the effect of adding a chaotropic, kosmotropic, or antagonistic salt thus represents a convenient formulation tool in the field of personal care applications and beyond. NaCl is routinely used to thicken aqueous solutions of sodium dodecyl sulfate (SDS) or sodium lauryl ether sulfate (SLES), the most commonly used surfactants for products such as shampoo, liquid hand soap, or shower gels [15].

The delicate balance of such formulations can be disrupted, however, by the simple addition of further ingredients, such as fragrance oils, making macroscopic behavior such as viscosity or viscoelasticity difficult to predict [16], [17]. In previous work [18], some of us investigated the effect of fragrance molecules on the viscoelastic property of solutions of SLES. Two independent mechanisms influencing the position and amplitude of the viscosity peak on the salt curve were identified and classified as a co-surfactant- and a co-solvent-type effect, respectively. The addition of fragrance molecules typically resulted in a linear combination of the two effects. The co-surfactant type of behavior tends to shift the peak of the salt curve to a lower salt concentration, whereas the co-solvent effect leads to a softening of the interface, which results in a decrease in the amplitude of the viscosity peak on the salt curve. The influence of fragrance molecules on the rheological behavior of giant micelles has also been investigated in other previous works [19], [20], [21], [22]. In two recent studies based on coarse grained dissipative particle dynamics (DPD) simulations it was shown how to predict radial distributions within a micelle [23] or to calculate scission free energy, which determines micellar length [24] in the presence and absence of fragrances. A further important step was made by Mandal et al. using coarse grained molecular simulations that could calculate directly both scission and branching free energies of cetyltrimethylammonium chloride micelles in presence of inorganic and organic salts [25]. They observed a non-monotonicity in scission energy of these model micelles versus salt concentration that could be correlated with the experimentally observed peak in viscosity. Kralchevsky and co-workers recently presented a self-consistent quantitative model for the growth of wormlike micelles for non-ionic surfactants [26]. The model is based on an accurate description of the different contributors to the micelle free energy: interfacial tension, steric repulsion of headgroups, and conformational free energy of the surfactant hydrocarbon chains in the micellar core. An excellent agreement between the expectation from theory at meso-scale and experimental values has been achieved. Nevertheless, to the best of our knowledge, no quantitative predictive model exists that translates the molecular organization at nanoscale of charged surfactant molecules that form giant micelles into macroscopic properties such as the viscoelasticity.

We present here a new thermodynamic model based on theoretical approaches of colloidal chemistry and polymer physics. It combines the concepts of molecular packing parameters, supramolecular curvature and elasticity of the surfactant film, and polymer physics extended to “living polymers” in the case of giant micelles. Emphasis is placed on the prediction of the salt curve of SLES. A quantitative approach to predicting both the position and the amplitude of the viscosity peak of giant micelles is shown. In addition, the influence of electrolytes and hydrophobic solutes, in particular fragrance molecules, is modeled and compared with experimental data.