Morphological development of multilamellar phospholipid film depending on drying kinetics

The mechanism for the formation of solid-supported phospholipid membranes during a drying process was investigated. Terracelike multilamellar structures were found to develop from a micellar solution with either spinodal decompositionlike process or nucleation growth, depending on the evaporation rate of an organic solvent. In contrast to the well-known kinetics of phase separation, fast drying induces nucleation while slow drying induces spinodal decompositionlike lipid-film formation. The existing models for the interpretation of phase separation are not sufficient to understand this unexpected kinetics. We suggest a schematic model with which this kinetic feature can be interpreted in terms of a self-assembly pathway in a three-component phase diagram for a phospholipid, organic solvent, and water.

Figure 1

(a) Time-series images of lipid-film formation under the evaporation of chloroform as observed by phase-contrast microscopy. At about 59–60 s after the deposition of solution, bulk solvent evaporates and the interface moves upward from the bottom (white arrow). The nuclei of terracelike lamellar structures grow from the disordered lipid film. (b) Upper images show typical Ostwald ripening in the framed rectangle at 67 s in (a). Time variations in the sizes of small (A) and large (B) terraces are depicted in the lower graph. A relatively small terrace structure shrinks as it is absorbed into a larger one.

Figure 2

(a) Lipid-film formation under the slow evaporation of chloroform by shielding the solution. The formation kinetics of a terracelike lamellar structure proceeded as in spinodal decompositionlike behavior. (b) 1D power spectrum $J\left(k\right)$ obtained by fast Fourier transform analysis of the image at 260 s. A peak at $0.5\mu {\text{m}}^{-1}$ corresponds to the characteristic size of the terrace morphology $D=2\pi /k$. (c) The characteristic size $D$ of the terracelike morphology (open circle) and the peak intensity (closed circle) of the power spectrum at each time point.

Figure 3

Lipid-film formation under the evaporation of $n$-octane. The spinodal decompositionlike formation of a terracelike morphology of lamellae is exhibited.

Figure 4

TR-SAXS profiles of DOPC/$n$-octane under drying. (a) The change in two-dimensional SAXS profiles. (b) One-dimensional SAXS profiles during 40–400 s after the deposition of solution. The solid lines indicate the results of functional fitting using Eq. (1). (c) Time dependence of the effective volume fraction of lipid micelles $\eta$ obtained by the functional fitting with Eq. (1). (d) One-dimensional SAXS profiles during 400–4900 s. The dashed line is the guide of the peak position at 400 s (about $0.125{\text{Å}}^{-1}$).

Figure 5

Upper-half plane of two-dimensional SAXS profile of DOPC dry film formed with drying $n$-octane under a humidity of less than 50%. The dashed lines are the guide at $q_z/q_{z0}=0,1,2,3,6,9$ and $q_r/q_{r0}=0,1,\sqrt{3},2\left(q_r=\sqrt{q_x^2+q_y^2}\right)$. The observed scattering peaks (white circles) are coincident with the reciprocal lattice of the rhombohedral symmetry (Ref. [24]).

Figure 6

Schematic view of the self-assembly of lipid in the drying process with a typical phase diagram of an amphiphile (lipid), oil (organic solvent), and water system ( Ref. [10]). Depending on the evaporation rate, the pathway of self-assembly from micelles to a lamellar phase divides into path A or B. When the water content is low, a sponge or an intermediate mesh structure appears under drying (path C).

Figure 7

(a) The growth rate of the power intensity $\Omega$ with respect to wave number $k$, which is obtained from the microscopic image of drying DOPC/$n$-octane (Fig. 3) at 600 s by using Eq. (9). (b) The Cahn-Hilliard plot, $\Omega /k^2$ with $k^2$.