Dynamic phase diagram and onion formation in the system C₁₀E₃/D₂O

Abstract

The influence of the shear on the lamellar phase, L_α, of the system C₁₀E₃/D₂O was studied along an isoplethal path (40 wt.% C₁₀E₃) in the temperature range 25–42 °C. A dynamic phase diagram was determined by steady-state rheometry, where by shear action the lamellar phase was transformed into multilamellar vesicles (MLVs) (“onions”). The location of “onions” in the dynamic phase diagram, depends only on the temperature and the applied shear rate, and not on the shear history. The classical lamellar phase structure is stable at rest and at low shear rates. When exposed to higher shear rates, the lamellar structure is transformed into onions. The transition from lamellae to onions is shifted to higher shear rates with increasing temperature. In a range of shear rates in between the stable lamellae and stable onion structure, the transition is incomplete. The transformation of lamellae into onions appears to be governed by the imposed strain, in agreement with earlier studies. The effect of temperature can be understood from the general property of nonionic surfactants where the monolayer spontaneous curvature decreases with increasing temperature.

Introduction

In recent years there has been considerable interest in the study of lamellar systems, both the classical lamellar phase of planar sheets (L_α), and systems where the bilayer sheets fold to form a closed structures such as multilamellar vesicles (MLVs). In fact, surfactant lamellar phases are often complicated by the formation of MLV under shear, which can originate simply from shaking the sample [1], [2], [3], [4], [5], [6], [7]. The mechanism that governs this transformation is not yet fully understood, but it was observed that lyotropic lamellar phases under steady shear present modifications and consequently form new phases. These phases can be located on a so called “dynamic phase diagram”. Such a diagram represents the steady states the system adopts under shear flow, as a function of the shear rate.

Zilman and Granek [8] proposed a mechanism for MLV formation based on an undulation instability. They suggested that the flow generates an effective force that acts on the excess area and thereby exerts an effective lateral pressure. Above a critical shear rate the lamellar phase buckles into a harmonic shape modulation and then break up into multi lamellar vesicles. Roux and coworkers [9], [10] presented a pioneering orientation rheogram (shear rate vs. volume fraction) describing the shear effects on the lyotropic lamellar phases and they found three different states of orientation [11]. At very low shear rate (typically below $\text{γ}\text{̇}$=1 s⁻¹) the layers are mainly parallel to the flow, the so-called c-orientation, with defects (dislocations) in the two other directions. At very high shear rates, the orientation is similar but the defects in the flow direction are completely suppressed. For intermediate shear rates, the lamellar phase organizes itself into MLV, which are close packed and fill up space. In the following sections they will be called “onions”, a name they have been given because of their morphological similarity with the vegetable (e.g. the Tropea onion, Calabria, Italy). The characteristic features of this phase can be summarized as follows:

• Electron microscopy and scattering observations have revealed that the onions have a high packing fraction, close to one, and their shape is not spherical but polyhedral [12], [13];

• MLVs formed under shear flow often have a narrow size distribution and the size can be tuned from a few micrometers to a tenth of this depending on the applied shear rate (the size varies as the inverse of the square root of the shear rate) [14];

• The onion structure can be retained when stopping the shear. The relaxation back to the lamellar structure is very slow. It requires days to months, depending on the system and its composition [12];

• Another important characteristic is that the formation rate of the onion structure depends on the value of the applied shear rate.

This paper reports for the first time a systematic study of the dynamic phase diagram of the C₁₀E₃/D₂O system along an isoplethal line (40 wt.% surfactant). We used heavy water (D₂O) rather than ordinary water here in order to facilitate a direct comparison with rheo-SANS studies of the same system [15], [16], [17], [18], [19], where heavy water was used for scattering contrast reasons. The shear induced structures are reported in a phase diagram, where the temperature ranged between 25 and 42 °C and the shear rate ($\text{γ}\text{̇}$) was varied in the range 0.1–100 s⁻¹. The results of this study were interpreted using experimental data present in the literature and structural techniques based on different length scales [16], [17]. The phase diagram without shear of C₁₀E₃/D₂O system was described in the past [18]. A lamellar phase (L_α) is stable at lower temperatures and a narrow sponge phase (L₃) at higher temperatures, both laying within an accessible experimental temperature range between 20 and 50 °C and covers a broad range of concentrations. These characteristics make the system suitable for temperature scan experiments. In [16] it was observed that for a moderate shear rate, e.g. $\text{γ}\text{̇}$=10 s⁻¹, MLV were formed at lower temperatures while planar bilayers (classical lamellar structure) were stable at higher temperatures.