Palmitic Acid Monolayers Research Articles

Proton transport at the monolayer-water interface

It is shown that when monolayers of stearic acid, palmitic acid, DPPC, or DPPS are compressed above some critical area A c a lateral conduction mechanism is initiated at the monolayer/water interface. The interfacial conductance increases on further increasing the molecular packing density in the monolayer. All compounds also show major changes in surface potential at A c the potential becoming more positive in all cases. It is argued that this is a consequence of structural reorganisation at the headgroup/water interface causing a significant reduction in the local permittivity. The critical area, A c, is approximately double the molecular areas estimated from the pressure-area isotherm, and experiments with stearic acid monolayers show that A c decreases significantly when the chaotropic ion SCN −, which is known to disrupt the molecular structure of water, is added to the subphase. It is likely, therefore, that the structural changes occurring at A c involve the formation of a hydrogen bonded network between monolayer headgroups and adjacent water molecules at the monolayer/water interface. It is suggested that the conduction mechanism initiated at A c arises from proton hopping along this hydrogen-bond network.

Surface pressure and wave-damping effects of surface-active organic compounds on seawater

The surface pressure vs concentration and area per molecule curves for monolayers of stearic acid, palmitic acid, oleic acid, palmitoleic acid, triolein, and cholesterol were determined on a clean artificial seawater surface using a modified Langmuir trough. The effectiveness of each compound at wave damping was then estimated mathematically using equations derived by Dorrestein [(1951) General linearized theory of the effect of surface films on water ripples. Proc. Amster. Aca. Sci. 54, 260–350]. We conclude from these analyses that compounds forming highly expanded monolayers on seawater are more effective at damping capillary and short gravity waves (<6 cm wavelength) and generally require lower surface concentrations for this effect than materials forming highly condensed monolayers. Maximal variations in wave amplitude attenuation by different materials are predicted for wavelengths between 2 and 6 cm. All six materials are predicted to cause some wave damping at wavelengths less than 20 cm. However, between 6 and 20 cm waves, a rapid transition occurs which makes the condensed monolayers more effective at damping the longer wavelengths.

The temperature dependence of mixed monolayers of poly-γ-methyl- l-glutamate and palmitic acid at the air/water interface

There is a flat region (plateau region) ascribed to the transition from monolayer to bilayer in the surface pressure ( F)-area ( A) isotherm of poly-γ-methyl- l-glutamate (PMLG) at the air/water interface. The change in entropy from initial to final state of this transition is very small and negative. This indicates that the order in the monolayer and bilayer states is almost the same but that the former is a little more random. At or below 20°C, the monolayer of palmitic acid (PA) forms a solid condensed film and no difference in the F- A isotherm was detected regardless of temperature. However, there is a break at about 16.8°C in the curve of temperature versus surface pressure of the flat region in the F- A isotherm corresponding to the collapse of the PA monolayer. This suggests that the monolayer states of PA above and below 16.8°C are different. PA at or above 20°C is more miscible with PMLG in the monolayer state than PA at or below 15°C. The difference between miscibility of PMLG in the monolayer state and PA at or above 20°C and at or below 15°C can perhaps be ascribed to the difference between the monolayer states of PA above and below ca. 16.8°C.

The Dissolving-Rates of Spread Monolayer of Palmitic Acid into Aqueous Sodium Tetradecyl Sulfate Solution at Constant Surface Area

Rates and process of dissolution of tritium labelled palmitic acid monolayer into a solution of sodium tetradecyl sulfate (STS) were studied by means of a radiotracer method. The plots of the logarithm of the surface concentration of the monolayer against the square root of the time elapsed showed a linear relationship. The rate constant of dissolution at the concentration below the critical micellar concentration (cmc) was nearly zero. These constants began to increase suddenly at the cmc and continued to rise linearly above at 4.0×10-3mol/dm3. The mechanism of dissolution was explained on the basis of the solubilization of palmitic acid into STS micelles. The diffusion constants of the micelles were calculated from the rates of dissolution and the equilibrium distributions of palmitic acid between the monolayer and the micelles.

Autoxidation of fatty acid monolayers adsorbed on silica gel: III. Effects of saturated fatty acids and cholesterol.

Autoxidation of fatty acid monolayers on silica consisting of multiple components to simulate biomembranes has been studied by the rate of fatty acid disappearance and the products formed. When palmitic acid was incorporated into linoleic acid monolayers, the decrease in rate was proportional to the amounts of plamitic acid present. The protective effect of the saturated fatty acid diminished rapidly as the chain length of the saturated fatty acid decreased below C12. With acids of medium chain length, C12 was more effective than C16. In pure linoleic acid monolayers, when the surface coverage was reduced to only 5% of the available adsorption sites, and in the case of palmitic acid-linoleic acid monolayers, the rate dropped drastically and the major identified product formed was hydroxyepoxyoctadecenoic acid. On the contrary, the major product formed in the case of saturated monolayers of pure linoleic acid was a mixture of unsubstituted epoxy acids. The inclusion of cholesterol in linoleic acid monolayers increased the rate of disappearance of linoleic acid slightly, whereas cholesteryl acetate decreased the rate. The protective effect exerted by cholesteryl acetate appeared to be similar to that of palmitic acid.

Surface activity of urea

The effects of high concentrations of urea to increase surface pressure and alter surface potentials of stearic and palmitic acid monolayers were examined at temperatures between 15° and 40°C. Results of the investigation show that penetration of urea into an interfacial region depends upon the presence of molecules at the interface, where dipole-dipole interactions with the urea can occur. Such interactions allow the concentration of urea to increase to equal the bulk concentration. In the absence of such molecules urea is desorbed from the interface.

Calcium action on fatty acid and phospholipid monolayers and its relation to the cell membrane

Calcium ions interact with stearic and palmitic acid monolayers producing a solid-condensed film which is rigid, has no measurable surface viscosity, and has a high solid-to-liquid transition temperature measured by the talc test. This effect can be explained by charge destruction, formation of di-soap molecules, or formation of a copolymeric soap lattice. Surface viscosity experiments indicate that charge destruction has a measurable but minor effect on film properties. Di-soap analogs have increased surface viscosities. However, these analogs form plastic films with moderate transition temperatures. The data suggest that the unusual properties of calcium stearate and palmitate films are most readily explained by a copolymeric lattice structure. Calcium has no effect on the surface viscosity of cis-and trans-monoethenoic acid and phospholipid monolayers. Saturated fatty acid soaps thus have unique surface properties which depend on their packing mode and crosssectional area and limit their value as model systems for calcium interactions with the cell membrane.

Double layer capacity studies of adsorption at mercury-solution interfaces effects of expanding and receding surfaces stability of oriented langmuir monolayers

Double layer capacity measurements were performed on aqueous solutions of n-octyl alcohol, n-nonanoic acid, and n-butyric acid with a pendant mercury drop electrode capable of expansion or recession. No appreciable differences of double layer capacity were observed whether the mercury drop was formed by expansion or by recession. Temporary differences are probably due to different rates of approach to equilibrium rather than to structural differences. A Langmuir isotherm is applicable for n-nonanoic acid at concentrations below two-thirds of saturation. Beyond that concentration there is evidence of multilayer adsorption. Compressed, oriented Langmuir monolayers of palmitic acid on mercury-coated platinum are found to reach the same equilibrium state as bare mercury-coated platinum. The double layer capacity of an oriented monolayer of palmitic acid is 4.3 μf. /cm 2. at the potential of maximum adsorption, as measured against 0.1 M NaClO 4.

Stability of condensed monolayers of palmitic acid on gold

Stability of condensed monolayers of palmitic acid on gold

Adsorption of Surface-Active Substances at Mercury-Solution Interfaces

A review is given of adsorption of surface-active substances at mercury-solution interfaces, especially with regard to the structure of adsorbed layers. The difficulty in studying adsorption equilibrium at a mercury electrode surface exists in the fact that the amount adsorbed depends on the electrode potential as well as the bulk concentration of organic substances. It is generally accepted that both the Langmuir isotherm and the Gibbs isotherm are valid for adsorption of organic substances. The dependence of the amount adsorbed on the electrode potential has been explained quantitatively by Frumkin and Butler independently. The physical picture of the latter theory, regarding adsorbed molecules as dielectrics with a permanent dipole and a given polarizabillty, may be easier for chemists to follow, but no less important is the fundamental expression for charge density of the former theory. A linear relationship which is usually assumed to hold between the surface coverage and the double layer capacity should be tested by further experiments. Some conclusions on adsorption rate and mechanism of adsorption can be derived from the frequency dependence behavior of the heights of adsorption-desorption peaks, which appear on a differential capacity-potential curve. The dependence of peak potential on concen-tration has been successfully explained. The rate constant of the adsorption process has also been estimated by Lorenz for aliphatic alcohols and acids of low molecular weight, while the formation of a condensed adsorbed layer has been pointed out for higher homologues of those. The frequency effect observed for those substances has been interpreted semiquantitatively by assuming that a relatively slow association process may take place between adsorbed molecules after a rapid adsorption process. 4.3μF/cm2, recently obtained by Laitinen and his co-worker for a monolayer of palmitic acid on a mercury-coated platinum electrode, would be reasonable for the differential capacity value of a monolayer film of higher fatty acids. In comparison to this value, 1.8 μF/cm2 observed with nearly saturated solutions of higher aliphatic alcohols and acids, might indicate the formation of a bimolecular adsorbed layer. There seems to remain some ambiguity for the orientation of neutral molecules at an electrode surface, but this problem may be solved in the case of long chain surfactant anions, which have been lately studied in detail by Eda. With solutions of sodium dodecyl sulfate in sodium sulfate, four peaks are observed on the differential capacity-potential curve. The two peaks which appear at the most positive or negative potential may correspond to the adsorption-desorption reaction in an ordinary sense. The relatively small peak which appears on the positive branch near the electrocapillary maximum may be attributed to the reorientation of the adsorbed anions ; and therefore a bimolecular adsorbed layer may be formed on the more positively charged mercury surface than this peak potential. The other small peak on the negative branch may correspond to the phase separation, which means the transformation of a gaseous phaseinto a condensed phase.