WATER UPTAKE INTO STRATUM CORNEUM; PARTITION BETWEEN LIPIDS AND PROTEINS

Water uptake in natural and reaggregated stratum corneum was determined by weight difference after storage in an atmosphere of controlled relative humidity.

Interlayer spacing in separated lipids as a function of their water content was determined by low-angle X-ray diffractometry. These values were used as a calibration curve to determine the water content of the lipid bilayers in reaggregated stratum corneum.

The results revealed different behavior of the lipid models compared to natural lipids of the stratum corneum. The additional water taken up after reaggregation of equilibrated lipids and proteins, was equally partitioned between the protein and the natural lipid fraction, while the models gave a proportionally higher water uptake into the lipids at high relative humidity. It is obvious that the models, so far, do not mimic all the properties of the natural stratum corneum lipids.     

Explanation of Ionic Sequences in Various Phenomena. III. Salting-in and -out of Amino Acids

Abstract

Previous developed theories were applied in explaining the mechanism for the salting-in and -out of various amino acids. Glycine is salted-in according to the cationic sequences Li⁺ > Na⁺ > K⁺ > Rb⁺ and Ca²⁺ > Ba²⁺ > Sr²⁺. The ability of a cation to increase the solubility of an amino acid therefore corresponds to the destruction of the ion-ion bond between the - CO-₂and the -NH_{+ 2}group of the amino acid by forming an insoluble ion-ion bond between the added cation and the - CO?² group. This insolubilizing effect produces a positive charge on the amino acid. If, however, the anion of the added salt forms a relatively insoluble ion-ion bond with the -NH₊₂ group of the amino acid, then the effect is minimized because now both charges on the amino acid are reduced. Consequently, the more insoluble the cation amino acid salt and the more soluble the anion amino acid salt (or vice versa), the greater will be the salting-in effect. Titration of either charged group on the amino acid zwitterion has the same effect, since now the ion-ion bond of the amino acid is again destroyed. Aliphatic and carboxylic acid groups also effect the salting-in sequence, since these groups are salted-out by addition of salt when D^± < D_H2o. These mechanisms explain how leucine is first salted-out, then salted-in (at 4 M) and finally salted-out again (at 9 M) in LiCl solutions. Urea salts-in hydrophobic amino acids by increasing the dielectric constant and salts-out polar amino acids by increasing the interaction between the two charge groups on the amino acid. Glycine reverses the salting-in effect of NaCl on asparagine by competing for the Na⁺ ion.     

Equilibrium Topology and Partial Inversion of Janus Drops: A Numerical Analysis

The equilibrium topology of an aqueous Janus emulsion of two oils, O1 and O2, with water, W, [(O1+O2)/W], is numerically evaluated with the following realistic interfacial tensions (γ): γ_O2/W=5 mN m⁻¹, γ_O1/O2=1 mN m⁻¹, and γ_O1/W varies within the range 4–5 mN m⁻¹, which is the limiting range for stable Janus drop topology. The relative significance of the two independently pivotal factors for the topology is evaluated, that is, the local equilibrium at the line of contact between the three liquids and the volume fraction of the two dispersed liquids within the drop. The results reveal a dominant effect of the local equilibrium on the fraction of the O2 drop surface that is covered by O1. In contrast, for a constant volume of O2, the impact of the interfacial tension balance on the limit of the coverage is modest for an infinite volume of O1. Interestingly, when the O1 volume exceeds this value, an emulsion inversion occurs, and the O1 portion of the (O1+O2)/W topology becomes a continuous phase, generating a (W+O2)/O1 Janus configuration.     

Evaporation/Precipitation from a W/O Microemulsion Base

The evaporation path in a microemulsion base of water, sodium dodecylsulfate, and pentanol was extended to include the subsequent precipitation stage caused by the restriction of the surfactant solubility. The results revealed the surfactant to be the only compound to precipitate during the evaporation/precipitation stage; the relative content of the two volatile compounds in the liquid phase was adjusted to the required level by the evaporation.     

Evaporation from Three Single‐Compound Phases in Equilibrium

The evaporation paths in a system of three single‐compound phases in equilibrium were calculated using a phase diagram approach assuming the evaporation rate proportional to the compound's vapor pressure and molecular weight. The variation with time of the weight of the individual phases was linear, while the weight fraction was not. The approach allowed a simple calculation of the fraction of remaining compounds after one of them was exhausted during the evaporation and a convenient graphical illustration of the importance of the relative vapor pressures.     

The Effects of Alcohol with Different Chain Length on the Phase Equilibria of Nonionic Surfactant System

Addition of alcohol with longer chain length (C₆H₁₃OH, C₈H₁₇OH, and C₁₂H₂₁OH) caused a reduction the cloud point of a commercial nonionic surfactant, Tesgitol (T_15-s-9). The formation of lamellar liquid crystal (LLC) was favored so that isotropic liquid (L₁)-LLC two-phase region became wider with increasing temperature at an appropriate weight ratio of surfactant to alcohol. The isotropic liquid phase/liquid two phase transformation was replaced by a two-phase transformation to isotropic liquid/lamellar liquid crystal at the cloud point for the system without alcohol.     

Influence of Charge Density of Anionic Polyelectrolytes on Structure Formation in Liquid Crystalline Systems

The influence of different amounts of anionic copolymers of N-Methyl N-vinyl acetamide (NMVA) and acrylic acid with various charge densities on the formation of the lamellar liquid crystal formed by sodium dodecyl sulfate (SDS)/decanol/water was investigated by means of polarization microscopy, small angle x-ray scattering (SAXS), transmission electron microscopy and rheology. On the contrary to the incorporation of poly(acrylic acid)     

HYDROTROPES

The structure and action of hydrotropes are discussed using surfactant association structures as a model.

The main action of a hydrotrope is to prevent phase separation from an aqueous solution; either by disordering of a lamellar liquid crystal or by preventing the formation of well defined normal and inverse micelles.     

Some experiments on complex spontaneous emulsification in the system water–benzene–ethanol

The system water–benzene–ethanol was used to illustrate the complexity of spontaneous emulsification, when water-poor emulsions are brought in contact with water. In the first case, an O/W emulsion located close to the plait point in the system was used. The aqueous phase in the emulsion was incompatible with water, and a strong spontaneous emulsification to an O/W between the two liquids took place in the water layer close to the interface between layers. In the second case, a W/O emulsion, also close to the plait point, was brought in contact with water. Now, the spontaneous emulsification between the water and the oil phase of the original emulsion to an O/W emulsion also took place in the water layer forming a distinct emulsion layer beneath the interface.