Short Communication
Rheological and rheo-SALS investigation of the multi-lamellar vesicle formation in the C12E3/D2O system

https://doi.org/10.1016/j.jcis.2011.10.057Get rights and content

Abstract

Usually in nonionic surfactant aqueous systems of the CnEm type, a lamellar phase occurs over a wide temperature and concentration range. For some CnEm surfactants, multi-lamellar vesicle (MLV) formation has been observed when the lamellar phase is subjected to shear flow.

This communication reports the shear flow behavior at different shear rate values of a CnEm (where “n” is 12 and “m” is 3) aqueous system at 34 °C. The typical transient viscosity behavior of the shear-induced MLV formation in C12E3/D2O at 50 wt% of surfactant has been observed. The MLV formation is confirmed by time-resolved rheo-small angle light scattering (SALS) experiments. The experimental data show an intermediate structure that has been attributed to a multi-lamellar cylinders (MLCs).

Highlights

► Shear-induced structures: shear-induced lamellar-to-MLV transition in 50 wt% C12E3/D2O system. ► Rheo-SALS and rheology elucidate the shear-induced transition. ► The SALS patterns give the size distributions of the MLVs obtained at different shear rates (10, 20, 40 s−1).

Introduction

Shear-induced structure is a relevant topic of research, and it has received a great deal of attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Multi-lamellar vesicles (MLVs) are common shear-induced structures that usually form from a lamellar phase under shear flow [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [24], [25], [26], [27].

Great attention has been focused on systems where the surfactant monolayer or bilayer is flexible [1], [2], [3], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [24], [25], like the lamellar structure of alkyloligoethylene oxide-type (CnH2n+1(OC2H4)mOH) nonionic surfactants. The surfactant monolayer is flexible when bending rigidity (κ) is of the order of the thermal energy, kBT. The bending moduli for the CnEm surfactants are still under discussion [28], [29], [30], but the values are always of the order of kBT.

The lamellar phase of CnEm surfactants is found to be unstable under shear flow, and the structure is converted into MLVs [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Moreover, Roux et al. [5], [6] proposed a dependence of the MLV radius (RMLV) on the applied shear rate (γ˙).RMLV4π(2κ+κ¯)ηsdγ˙cγ˙-0.5where κ¯ the saddle-splay modulus, ηs is the solvent viscosity, and d is the repeating distance between bilayers.

The development of rheo-tools (X-ray, neutron and light scattering, and NMR) allowed experimentalists to investigate the fluid structures under shear flow. The steady and intermediate states have been characterized using these new techniques. In C10E3/D2O system, an intermediate structure attributed to multi-lamellar cylinders (MLCs) has been observed [13], [14]. The curvature of the cylinders lies between those of planar bilayers and MLVs, which means that the formation of such multi-lamellar cylinders fits in the concept of membrane elasticity and curvature. On this background, the rheo-SALS technique may be applied to determine the MLV size distribution.

Nevertheless, in order to understand the relationship between membrane elasticity, spontaneous curvature, and MLV formation in the CnEm systems, more investigations are needed on surfactants with different value of “n” (unit number of the chain group) and “m” (unit number of the head group). In the present study, we have focused on 50 wt% of C12E3 (triethylene glycol dodecyl ether) dissolved in D2O, investigating the effect of the shear flow on the lamellar phase at 34 °C. The monolayer spontaneous curvature changes strongly with temperature in nonionic surfactant systems. It is worthy to note that in the vicinity of the phase boundaries of the lamellar phase, usually the membranes prefer the planar structure [12], [26], [27]. The temperature of 34 °C has been chosen to be in the middle of the lamellar phase region.

For the first time, we observed MLV formation in the C12E3 aqueous system and we also compared the MLV size distributions of C12E3 with the size distributions of some CnEm aqueous systems.

Rheological and rheo-Small Angle Light Scattering (SALS) techniques have been used to investigate the shear-induced structures in the C12E3/D2O system.

We have performed rheological measurements in duplicate, using a stress-controlled rheometer equipped with a Couette geometry (gap 1 mm). In addition, the rheometer has been equipped with a solvent evaporation trap. For depolarized rheo-SALS experiments, an Anton Paar Physica MCR 301 equipped with a glass cone-plate geometry (gap 0.188 mm) has been used in the rate-controlled mode. The laser beam with a wavelength of 658 nm illuminated the sample vertically down between crossed polarizers. Scattering patterns have been captured using a CCD camera located below the screen on which the scattered light had been directed.

The transient viscosity for a 50 wt% C12E3/D2O system at 34 °C, as a function of time, at different shear rate values, is shown in Fig. 1. The typical viscosity behavior of the MLV formation appears at 1.0 s−1, and it is also evident a shear thickening behavior (increase in the steady-state viscosity with the shear rate) of the system. This shear thickening phenomenon can be related to a continuous MLV formation [12].

Fig. 2 reports the steady-state viscosity (η) and the shear stress (σ) as a function of the γ˙ within 0.1–100 s−1 range at 34 °C. The shear rate dependence of the viscosity has been divided into three regions. In region I, the first shear thinning appears at lower shear rate. This regime corresponds to the orientation or alignment of the lamellar phase in the flow direction. In region II, a shear thickening appears between 0.5 and 2.0 s−1, while at high shear rates (region III) there is again a shear thinning, which can be described by a power law relation ηγ˙-0.75±0.05 or σγ˙0.26±0.05. Similar value of the exponent has been obtained by Fujii and Richtering [16]. Investigating the C10E3/D2O system, Fujii and Richtering [16] suggested that the MLVs in the shear thickening regime (region II) are “imperfect-MLVs” respect to densely packed-MLVs in the shear thinning regime (region III). In present study, we observed similar rheological results for C12E3/D2O system, consequently the same interpretation of the experimental data can be used.

The results from a depolarized rheo-SALS experiment performed at γ˙ = 10 s−1 are shown in Fig. 3, where SALS patterns and rheological data have been recorded simultaneously.

Initially the viscosity is low, as expected for a parallel orientation of planar layers, which is confirmed by the depolarized SALS pattern that also show a finger print of a lamellar phase [14]. The viscosity increases immediately and after about 700 s the slope in the viscosity versus time changes and a small reproducible plateau is observed, as found for C10E3 at 300 s [12], [13]. At the plateau, the SALS scattering intensity is mainly found in the neutral direction. This pattern in depolarized rheo-SALS set-up, suggests that there is a continuous structure in the flow direction, while the refractive index is modulated perpendicular to the flow. The same pattern has been already observed in previous studies by Zipfel et al. [13] and Nettesheim [14], who explained this scattering with multi-lamellar cylinders (MLCs) structure. After this intermediate plateau region, the viscosity continues to increase and the SALS pattern evolves into a four-lobe pattern (1700 s) a characteristic signature of MLVs [17], [19]. At 2500 s a strong structure factor intensity is observed indicating a narrow MLV size distribution [11], [18].

The radii of the MLVs formed at 10, 20 and 40 s−1 can be estimated from the position of the maxima of the intensity distribution in the SALS patterns (Fig. 4A). For the densely packed MLV state, the relative maximum intensity corresponds to a structure factor peak. Kosaka et al. [26] recently suggested a FCC packing of the MLV and the observed structure factor peak would correspond to the strong 1 1 1 reflection, with which we obtain RMLV=3.9/qMAX. RMLV values, calculated by using this equation are plotted in Fig. 4B as a function of the shear rate. The values are compared to the shear thinning law (Eq. (1)) suggested by Roux et al. [5], [6] RMLVcγ˙-0.5 (dashed line), which gives a reasonable description of the data.

For the first time in C12E3/D2O system, at 50 wt% of surfactant, the MLV formation has been observed. A minimum shear rate (1 s−1) is required to obtain the MLV formation, and if this minimum is exceeded, the process is strain controlled. The transition from planar lamellae to MLVs follows a similar path that was reported for the C10E3/D2O system at 40 wt%, which required a slightly lower strain [12], [13]. The SALS data confirm the MLV formation permitting furthermore the estimation of the MLV radii. The rheo-SALS data also show a characteristic pattern at 10 s−1 that can be attributed to a multi-lamellar cylinders, although the MLCs and stripe buckling of the lamellae have the same spatial refractive index variation. Nevertheless, in order to clarify the MLCs presence, rheo-Small Angle Neutron Scattering (SANS) investigations are required. Moreover, rheo-Nuclear Magnetic Resonance (NMR) experiments are planned.

The MLV size is related directly to the original lamellar phase and indirectly to the CnEm molecular structure. By comparing the MLV sizes, previously determined by using SALS for C10E3 and C12E4 water systems [9], [10], [11], two observations can be evidenced: (i) the MLVs formed from a lamellar phase of CnEm/water systems have sizes comprised between 0.1 and 3 μm; (ii) there might be a correlation between the different MLV sizes and the CnEm molecular structures. Further investigations are needed to understand this correlation. In fact Suganuma et al. [15], in the case of C12E4, have reported a jump of the MLV size with temperature.

Section snippets

Acknowledgment

We thank Knut and Alice Wallenberg Foundation for supporting the purchase of rheo-SALS instrument.

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