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Using dielectric relaxation spectroscopy (DRS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and viscometry, we have investigated the hydration behavior, static structures, Brownian dynamics, and mechanical properties of aqueous solutions of alcohol ethoxylate (C12E15) and methyl ester ethoxylate (C12MEE), hereafter abbreviated as AE and MEE, respectively, in which we especially focus on the effects of the endcaps of these nonionic surfactants. We find that AE and MEE exhibit fairly different phase behaviors in water: AE produces liquid crystalline phases at w (surfactant weight fraction) > 0.35, whereas MEE retains a liquid phase in an extremely wide concentration range (w < 0.7) at ambient temperature. The structure factor deduced from SAXS intensities using a generalized indirect Fourier transformation technique and the effective hydration number evaluated from the negative excess bulk water relaxation amplitude revealed by DRS unambiguously demonstrate that hydration water molecules, exhibiting about 4-times-slower collective reorientational dynamics than that of bulk water, contribute to the excluded volume of the micelles. The blocked terminal hydrogen-bond donor/acceptor site of MEE leads to smaller hydration number of MEE than compared to that of AE, and consequently the lower excluded volume of the MEE micelles. The effective micellar volume fraction, ϕ(eff), should be defined by incorporating such different hydration effects. Importantly, voluminosity, defined as the micellar volume fraction per unit mass, is clearly a decreasing function of w, demonstrating progressive dehydration at a higher w. The collective diffusion constants determined by DLS for the AE and MEE micelles show a monotonous increase up to ϕ(eff) ≈ 0.5, as expected for the hard spheres. Low-shear-rate viscosities follow a Krieger-Dougherty model in the identical micellar packing fraction range. All static, dynamic, and mechanical properties of these micellar solutions can be explained in a consistent and quantitative manner only when the excluded volume of hydration water molecules is properly taken into account.

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Introduction Viscosity is an important factor for the texture and quality of cosmetics and liquid products 1-11). In particular, stability during storage and usability of these products is greatly influenced by viscosity. Recently, addition of functional particles (e.g. fragrance capsules) and changing flow patterns with shear have been implemented to achieve a higher performance in liquid products. It is advantageous that the dispersion flow patterns change reversibly from the stable and steady state to the flow state with the use of shear. For the realization of these characteristics, structural viscosity should be rendered to dispersions, which consist of flow patterns in which the viscosity decreases with increasing shear rate. However, it is difficult to control the reversible nature of dispersion flow patterns. Generally, polymers and surfactants are used to control viscosity. An effective polymer used for increasing viscosity strongly binds to the water molecules and has an excluded volume in aqueous solution. When the polymer concentration exceeds the threshold, the excluded volume of the polymer begins to overlap and interpenetrate each other. Thus, polymers increase the viscosity three-dimensionally in the whole aqueous solution 12, 13) .

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