Soap monolayers

Effect of pH and Nature of Monovalent Cations on Surface Isotherms of Saturated C16 to C22 Soap Monolayers... 

The authors of the present paper (7, 10, 21) used the planar soap monolayers as a simple model for the complex surface of a porous ion exchanging resin. Thus, a study of the effect of the separation between the ionized sites of a model exchanger on its selectivity between two competing counterions can be attempted. 

Figure 3 shows a plot of this vs. the quantity (1/ V T which is proportional to the separation between the positive charges of the adsorbed soap monolayer. 

The first theory for film fluctuations resembles Cahn s theory of spinodal decomposition of unstable bulk systems. A very simple mechanism was adopted for the liquid flow. It was assumed that because of the presence of the soap monolayers, the film surfaces were stagnant (see Section 11) and that the film liquid was pumped back and forth through a slab with thickness h according to Reynolds s law ... 

The films are thinner at the top than at the bottom. After a while, the upper part often ends up becoming extremely thin, reducing to two soap monolayers sandwiching just a few molecules of water. Such films no longer produce optical interference effects instead, they look quite black. Newton studied these black films. Long before, the Assyrians had observed them and even used their unpredictable shapes (in horizontal geometries) to divine the future. 

It would therefore seem Ukely that these antifoam effects are largely associated with the formation of calcium soap precipitates. Curiously, Peper [195] ignores this possibility and interprets his resnlts in terms of formation of rigid islands of calcium soap monolayer interspersed with gaseous film of adsorbed surfactant. Peper [195] asserts that these islands wiU make the film unstable because of their inflexible brittle nature. No theoretical arguments are given for why these should be unstable. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Soap is assumed to form a continuous monolayer over the polymer particles at the point of exhaustion of micellar soap. If we let represent the area thus occupied by a gram of soap, the total area of the particles in one cc. at the time h when this occurs will be CsUs, where Cs is the soap concentration in grams per cc. Equating at t = t to this quantity, we have from Eq. (32)... 

The Gibbs adsorption theory (Birdi, 1989,1999, 2002, 2008 Defay et al., 1966 Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The surface tension of water decreases appreciably on the addition of very small quantities of soaps and detergents. The Gibbs adsorption theory relates the change in surface tension to the change in soap concentration. The experiments that analyze the spread monolayers are also based on one molecular layer. The latter data indeed conclusively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and other similar kind of molecules are found to exhibit self-assembly characteristics. The subject related to self-assembly monolayer (SAM) structures will be treated extensively (Birdi, 1999). However, no procedure exists that can provide information by direct measurement. The composition of the surface of a solution with two components or more would require additional comments. 

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

Figure 12. Fraction of soap molecules present in monolayer vs. pH of substrate at 30°C. and 0.5N total cation concentration (KOH + KCl). Stearic and behenic acids at 20 and 30 sq. A. per molecule...

Kinetics of Adsorption (8) (Results), (a) The Ions PoC162. The curves obtained by plotting the intensity of the -rays emitted by 210Po adsorbed by the soap film vs. time conform to the law represented by the Figure 1. The slope k of the line shown on Figure 1 is the constant of the rate of exchange of these ions by the pre-existing film of the neutral adsorbed soap. No such adsorption can be found for an anionic soap or oleic acid monolayers. 

The kinetics and the equilibrium of adsorption of anions PoCl62" by ionized monolayers of cationic soaps have been studied and both phenomena are analogous to those observed with the polymer ion exchangers... 

---