Microfluidic SAXS Study of Lamellar and Multilamellar Vesicle Phases of Linear Sodium Alkylbenzenesulfonate Surfactant with Intrinsic Isomeric Distribution

Abstract

The structure and flow behavior of a concentrated aqueous solution (45 wt %) of the ubiquitous linear sodium alkylbenzenesulfonate (NaLAS) surfactant is investigated by microfluidic small-angle X-ray scattering (SAXS) at 70 °C. NaLAS is an intrinsically complex mixture of over 20 surfactant molecules, presenting coexisting micellar (L1) and lamellar (Lα) phases. Novel microfluidic devices were fabricated to ensure pressure and thermal resistance, ability to handle viscous fluids, and low SAXS background. Polarized light optical microscopy showed that the NaLAS solution exhibits wall slip in microchannels, with velocity profiles approaching plug flow. Microfluidic SAXS demonstrated the structural spatial heterogeneity of the system with a characteristic length scale of 50 nL. Using a statistical flow-SAXS analysis, we identified the micellar phase and multiple coexisting lamellar phases with a continuous distribution of d spacings between 37.5 and 39.5 Å. Additionally, we showed that the orientation of NaLAS lamellar phases is strongly affected by a single microfluidic constriction. The bilayers align parallel to the velocity field upon entering a constriction and perpendicular to it upon exiting. On the other hand, multilamellar vesicle phases are not affected under the same flow conditions. Our results demonstrate that despite the compositional complexity inherent to NaLAS, microfluidic SAXS can rigorously elucidate its structure and flow response.

Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS)

“…The symmetric Maltese cross patterns found within the droplets ( for MLVs formed upon shearing and addition of salts to solutions of NaLAS at higher concentrations (> 45 wt%). [15] Similarly, continuous cooling of the solutions at lower rates (α c ≈ 0.1 • C/min) shows simultaneous growth of micro-scale liquid droplets and formation of lamellar structures below the phase transition temperature T c , for a fixed concentration of NaLAS (Fig. 4a).…”

“…The NaLAS sample used here is a mixture of alkyl chains of C 11 to C 14 and different phenyl isomers (2-to 7-phenyl), as described in an earlier work. [15] 2.1. Quiescent measurements Optical microscopy.…”

“…[6,7,8,9,10,11,12] Previous investigations of the phase behaviour of LAS were mainly dedicated to higher concentrations (> 30 wt%), for which a densely packed planar lamellar phase L α coexists with a micellar phase L 1 at room temperature. [13,14,15] For intermediate LAS concentrations (40 to 60 wt%), a nearly compositionindependent line in the phase diagram, around 50 • C, defines the boundary between an opaque planar lamellar phase (L α in Fig. 1) with a uniform d-spacing (32−34Å), and a semi-transparent lamellar phase (L ′ α in Fig.…”

“…[14] Applying shear stress or addition of salt to the planar lamellar phase of anionic surfactants, such as LAS, is known to promote the bending and wrapping of the bilayers, and thus transform them into multilamellar vesicles (MLVs). [16,17,18,15] These onion-like liquid crystalline droplets typically have diam-eters ranging from tens of nanometers to a few micrometers. [19,20,21,22,23] The MLVs formed under shear are not, however, thermodynamically stable and revert to the original equilibrium planar lamellar phase in the absence of shear.…”

Rapid contrast matching by microfluidic SANS

“…Microfluidics is routinely coupled with an array of characterisation approaches, including dynamic light scattering 6,7 and small angle X-rays scattering (SAXS). [8][9][10][11][12][13][14][15][16][17][18][19] Microdevices accurately formulate mixtures, requiring minute sample volumes [20][21][22] and manipulate flow fields with unprecedented precision 23 . Automated sample preparation microfluidic platforms for SAXS of biological systems have been previously demonstrated, based on either sequential mixing, stopped-flow SAXS and sample wash, 11,12 or by multi-sample, parallel formulation and measurement, 19 greatly improving measurement throughput.…”