Controlling water evaporation through self-assembly

Abstract

Water evaporation concerns all land-living organisms, as ambient air is dryer than their corresponding equilibrium humidity. Contrarily to plants, mammals are covered with a skin that not only hinders evaporation but also maintains its rate at a nearly constant value, independently of air humidity. Here, we show that simple amphiphiles/water systems reproduce this behavior, which suggests a common underlying mechanism originating from responding self-assembly structures. The composition and structure gradients arising from the evaporation process were characterized using optical microscopy, infrared microscopy, and smallangle X-ray scattering. We observed a thin and dry outer phase that responds to changes in air humidity by increasing its thickness as the air becomes dryer, which decreases its permeability to water, thus counterbalancing the increase in the evaporation driving force. This thin and dry outer phase therefore shields the systems from humidity variations. Such a feedback loop achieves a homeostatic regulation of water evaporation.T he evaporation of water from an aqueous medium to a dry gas phase is a ubiquitous phenomenon in nature. Evaporation can occur freely, as from oceans into the air, or be hindered by membranes or barrier films. Land-living organisms face the challenge of adjusting to the relative humidity (RH) of ambient air, which varies from a few percent to saturation at 100%, whereas the living-cell water chemical potential corresponds to a RH of above 99%. This difference drives water transport from the cells to the ambient air, exposing life to a drying-out threat. Different strategies have emerged to counter this threat. Plant leaves are covered with a waxy cuticle layer composed of polymers and associated lipids (1), whereas animals like mammals are protected by a skin composed of dead cells embedded in a lipid matrix (2), and a lipid film on the tear liquid of their eyes (3). Water transport across an inert diffusional barrier is proportional to the difference in water chemical potential between the inside and the outside. Total water loss through an inert membrane would thus vary in response to changes in the environmental humidity, with the risk of massive water loss in dry conditions. This phenomenon is typically observed in the plant cuticular film that coats the leaves (1, 4), as displayed in Fig. 1. On the contrary, several studies show that, for healthy human stratum corneum, the outermost layer of skin, the evaporation rate increases with lowering RH at high humidities, whereas it is virtually constant and independent of the outside humidity for RH < 85% (5-7) (Fig. 1). The stratum corneum is a thin and dry layer composed of dead keratin-filled cells embedded in a lipid multilamellar matrix, which realizes the barrier function of the skin (2). This membrane responds to drier conditions by decreasing its water permeability and can thus not be described as an inert barrier membrane. Sparr and Wennerström (8) previously pointed out a mechanism for this responsive behavior of ...

Humidity-insensitive water evaporation from molecular complex fluids

“…as also reported by Roger et al for water/surfactant solutions, in an experimental configuration close to the one studied here [14]. α is extremely insensitive to the variations of a e (variations below < 0.1% over the range a e = 0-0.9) suggesting again that humidity does not play any role (at long time scales) on the evaporation kinetics, even in this timedependent configuration.…”

“…One recovers strictly Eqs. (11)(12)(13)(14)(15)) from the above model describing the drying of a quiescent thick film by using the change of variable:…”

Buoyancy-driven dispersion in confined drying of liquid binary mixtures

“…(8-10) with boundary conditions Eqs. (11)(12)(13)(14)(15) has been performed with the commercial software Comsol Multiphysics (finite elements, Galerkin method). The boundary condition Eq.…”

Tracking solvents in the skin through atomically resolved measurements of molecular mobility in intact stratum corneum

“…When the skin is exposed to a formulation the SC molecular and macroscopic properties may be altered due to interactions between SC components and solvents, active substances, humectants, penetration enhancers, and other excipients (12,13,20). The effects of the formulation also depend on skin hydration, the state of the skin, and the occurrence of skin diseases (13,(57)(58)(59)(60). The exposure time also varies between different applications in, for example, dermal patches, creams, salves, and washing formulations.…”