Reversible size of shear-induced multi-lamellar vesicles

Abstract

IntroductionThe behavior of lamellar phases under shear is a subject that has been receiving a lot of attention in the recent years [1][2][3][4]. In fact, many different shear effects have been reported: transformation from lamellar phases to Multi-lamellar vesicles, MLVs (or ''onions''), in different amphiphilic systems and followed by different techniques [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]; formation of MLVs in presence of submicron-size particles (''stuffed onions'') [21]; changes in lamellar orientation [17,22,23]; formation of intermediates with cylindrical structure between a lamellar and MLV phases [24][25][26]; reduction in lamellar spacing [27]; transitions from MLVs to unilamellar vesicles [28] and ''layering'' effects on onions [8,29]. As a practical application, these MLVs can be used, for instance, to encapsulate chemicals leading to a new kind of controlled micro-reactor [30] or as carriers for oligonucleotide delivery [31].The size distribution of shear-induced multi-lamellar surfactant vesicles depends on the applied shear rate. In general, the MLV systems are shear thinning and the average MLV radius, R, decreases as the shear rate increases [5,26,32]. For higher surfactant concentrations, the inter-bilayer spacing is independent of the MLV size [33]. Hence, a decrease in the MLV size is accompanied by an increase in the MLV number density, N/V, where N is the number of vesicles and V the total volume. The MLVs at higher surfactant concentrations are polyhedral. They fill space with a total volume fraction which is essentially unity [8,34] and N/V is approximately proportional to R )3 . Abstract We have investigated the reversibility in the shear-induced multi-lamellar vesicle (MLV) size during stepwise cycling of the shear rate by employing common rheometry, polarized light microscopy and rheo-optic techniques. We thus address the question whether there is a true MLV steady state, irrespective of history. The system studied, was the nonionic surfactant triethylene glycol decyl ether (C 10 E 3 ) with a concentration of 40 wt.% in D 2 O and a constant temperature of 25°C. It was found that the MLV size varies reversibly with varying shear rate, and hence there exists a true steady state in the presence of shear flow. The experimental observations of reversibility are however restricted to higher shear rates. Because the transformation of the size results from the shear strain, the process is very slow at lower shear rates, where the steady state cannot be reached within a reasonable experimental time.

Multi-lamellar vesicle formation in a long-chain nonionic surfactant: C16E4/D2O system

“…When the shear rate is increased, some layers are stripped off and this causes a decreasing of the size of MLVs which may vary from hundreds of nanometers to tens of micrometers. Obviously the vesicle number density increases and the total bilayer area can be considered constant [12]. Shear-induced MLVs can be stable for a long time, but they do not correspond to the thermodynamic equilibrium structure of the lamellar system [15][16][17][18].…”

“…An example is the stability of multi-lamellar vesicles (MLVs) upon shearing and temperature variations [11,12]. Knowledge about the MLV formation mechanism is important for various applications, which often depend on the viscosity and rely on the vesicle structures.…”

“…Shear-induced MLVs can be stable for a long time, but they do not correspond to the thermodynamic equilibrium structure of the lamellar system [15][16][17][18]. Combining different structural techniques such as X-ray, neutron and light scattering, and NMR (all of these rheo tools equipped), make it possible to investigate fluid structures under flow like MLVs [1][2][3] and to determine their characteristic dimensions [10][11][12]18,20,[22][23][24][25][26][27][28][29][30].…”

Transient and Steady-State Shear Banding in a Lamellar Phase as Studied by Rheo-NMR

“…% surfactant [16,27,[29][30][31][32][33][34][35][36][37][37][38][39][40][41][42][43], although shear-induced MLVs can be found in all surfactants of the C n E m type studied so far [26,28,33,[44][45][46][47][48][49][50][51][52][53][54][55]. The shear diagram of C 10 E 3 /water (40 wt.…”

Phase Coexistence in a Dynamic Phase Diagram

“…Mixtures of c and a orientationsh ave been observed under shear flow. [9][10][11] Moreover,c losed structures identified as multilamellar vesicles (MLVs) can be formed under shear flow. The mechanism of MLVf ormation was described in detail [30] by using the 1-2 shear cell [33,34] in time-resolved smallangle neutrons cattering experiments (SANS).…”