Swelling and shrinking kinetics of a lamellar gel phase

Abstract

We investigate the swelling and shrinking of L ␤ lamellar gel phases composed of surfactant and fatty alcohol after contact with aqueous poly͑ethyleneglycol͒ solutions. The height change ⌬h͑t͒ is diffusionlike with a swelling coefficient S: ⌬h = S ͱ t. On increasing polymer concentration, we observe sequentially slower swelling, absence of swelling, and finally shrinking of the lamellar phase. This behavior is summarized in a nonequilibrium diagram and the composition dependence of S quantitatively described by a generic model. We find a diffusion coefficient, the only free parameter, consistent with previous measurements. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2913762͔In everyday life and many industrial processes, materials swell by absorption of solvent, e.g., washing powder, 1 foodstuff, 2 diapers, 3 eyeballs, 4 and clay. 5 Conversely, if the solvent flow is reversed materials shrink, as for a hypertonic cell with a lower solute concentration than its environment. Model systems are often preferred for study. Swelling rates of L ␣ surfactant lamellar phases are observed to change when the chemical potential difference between lamellar phase and contacting solution is varied through polymer addition. 6 Artificial liposomes can be swollen or shrunk using glycerol solutions 7 and hard sphere colloidal suspensions shrink when contacted with high concentration polymer solutions. 8 Here, we quantitatively investigate the swelling and shrinking behavior of a complex surfactant system, namely an L ␤ lamellar gel phase, as used in cosmetics and pharmaceuticals. The volume change is initiated by contact with aqueous polymer solution. Our observations are in quantitative agreement with a generic model, which we expect to be applicable to a large variety of situations.The lamellar phase is prepared following an industrial procedure. 9,10 It consists of a cationic quaternary surfactant ͓behenyl trimethyl ammonium chloride ͑BTAC͔͒ and a fatty alcohol ͓1-octadecanol͔ at a molar ratio of 1:3 in water with different total surfactant concentrations c s . Electron and light microscopy reveal a disordered system. Numerous stacks of bilayers form an open structure with small pockets of water and excess fatty alcohol. 10 While the sample is prepared at elevated temperature T, the experiments are performed at T = 25°C, which is below the chain melting temperature ͑about 78°C͒. Approximately 0.5 g of this L ␤ lamellar gel phase is pipetted into a 2 cm 3 cylindrical glass cell and centrifuged for 1 min at 2500 rpm to ensure that the entire highly viscous sample is at the bottom of the cell with a smooth upper surface. Within a range of lamellar masses ͑0.45-1.25 g͒ no systematic change in behavior was observed within the ϳ12% experimental uncertainty. The experiment is started by adding 1.5 g of water or polymer solution on top of the lamellar phase. The use of polymer ͓poly͑ethyleneglycol͒, PEG-10000 with molar mass from 8500 to 11500 g / mol and an average radius of gyration of about 3 nm͔ allows us to vary the diffe...

Three-Dimensional Structure and Growth of Myelins

“…The dependence of S on ϕ is consistent with the osmotic pressure, Π, being the driving force for the swelling of lamellar phase and hence myelin growth. − , With increasing ϕ and thus decreasing water content of the surfactant droplet, the concentration difference, and hence osmotic pressure difference between the surfactant phase and bulk water, increases. An increase in osmotic pressure results in a larger driving force for swelling.…”

“…The initial surfactant mass fraction ϕ was found to affect the swelling parameter S . Using the interdiffusion coefficient, we related S to the mobility of water in the surfactant phase and the osmotic pressure difference between the lamellar phase and water . Since the dependence of the mobility and the osmotic pressure on the properties of the lamellar phase are known, , the dependence of S on ϕ can be calculated; S (ϕ) ∼ (ϕ − ϕ c ) 2 .…”

“…Myelins can form when water is added to a lamellar phase (L α ) which has a large coexistence region of the lamellar phase with water (W). , Myelins are multibilayer tubes. − The bilayer spacing within the myelins corresponds to the maximum stable swelling of the lamellar phase, which occurs at the boundary with the coexistence region. , Many individual tubes are closely packed resulting in a packing fraction of the tubes of ϕ cyl = √3π/6 ≈ 90.7% . For myelins to form, the bilayers need to be fluid and of low permeability. − The addition of molecules which increase the permeability of water or hinder molecular movement within the bilayers , suppresses myelin formation as does solidifying the bilayers, e.g., by lowering the temperature. , …”

“…These experiments suggest that myelin growth is driven by the osmotic pressure difference between the lamellar phase and the contacting water and limited by the water mobility in the lamellar phase. Further models to describe swelling of lamellar phase and myelin growth have been proposed. − …”

Dissolution of anionic surfactant mesophases

“…For a charged surfactant like SLE 3 S, the Na + counterion concentration inside the aqueous part of any mesophase will be relatively high. Hence, there will be a high osmotic pressure gradient perpendicular to the interface that will act as a driving force for the penetration of neat water into the surfactant phase 16,44 . We can estimate the osmotic pressure by assuming that we have a gas of counterions that are free to move only inside the aqueous part of the mesophase.…”