Theoretical Description of Mixed Film Formation at the Air/Water Interface: Carboxylic Acids–Alcohols

Abstract

The thermodynamic parameters of formation and clusterization of aliphatic alcohols C n H 2 n +1 OH and carboxylic acids C n H 2 n +1 COOH ( n = 6–16) are calculated using the quantum-chemical semiempirical PM3 method. Four types of dimers are constructed in two directions of the spread monolayer comprising the most energetically advantageous monomer structures. The hydrophobic chains of alcohol and carboxylic acid molecules in the regarded dimers are found to be tilted within 12° to the normal of the spread monolayer. The structures of the mixed and pure surfactant dimers are the basis for the mixed alcohol–carboxylic acid monolayers of the following types: two dimensional (2D) film 1 with single distribution of the individual component in the other one, when the molecules of the first component do not interact with each other but are completely surrounded by the molecules of the second component; 2D film 2 with domain structure, when the film consists of “islands” of the individual components. The dependences of the clusterization Gibbs’ energy per one monolayer molecule on the molar fraction of the components for the mixed 2D films 1 formed by surfactants with equal alkyl chain length are found to be limited from top to bottom by the corresponding dependences for pure components. This indicates the absence of synergetic interaction between the hydrophilic head groups of carboxylic acids and alcohols and conforms to the available experimental data. The formation of the described types of mixed films is competitive. The preferential formation of 2D films 1 with single distribution of the first component among the molecules of the second one is possible when the length of the carboxylic acid hydrocarbon chain is longer by Δ n = 1–2 methylene units than that of the corresponding alcohol alkyl chain. According to the fractionally linear law, the highest possible content of the carboxylic acids in such 2D films 1 depends on the Δ n value and does not exceed 33.3%.

Theoretical Description of Mixed Film Formation at the Air/Water Interface: Carboxylic Acids–Fatty Amines

“…Figure shows that the dependences for clusterization Gbbs energy of mixed films are lower than the corresponding dependences for the individual components (except for the cases with X COOH = 80 and 90 mol %). This indicates a synergetic interaction between amine and acid hydrophilic parts unlike in the case considered in our previous study devoted to mixtures of aliphatic alcohols and carboxylic acids . In the last case, dependences of Δ G 298 Cl,∞ / m for mixed alcohol–acid films are between the lines of the corresponding dependences for pure monolayers.…”

“…This indicates a synergetic interaction between amine and acid hydrophilic parts unlike in the case considered in our previous study devoted to mixtures of aliphatic alcohols and carboxylic acids. 18 In the last case, dependences of ΔG 298 Cl,∞ /m for mixed alcohol−acid films are between the lines of the corresponding dependences for pure monolayers. This indicates the absence of synergetic action between alcohol and acid molecules.…”

“…In addition, this approach has been used to estimate the behavior of carboxylic acids and aliphatic alcohols in the mixed monolayers. 18 Good agreement between the obtained calculated and experimental results encouraged us to spread the used scheme to the case of carboxylic acids and amines. This study aims at (1) calculation of thermodynamic parameters of mixed clusterization of aliphatic amines C n H 2n+1 NH 2 and carboxylic acids C n H 2n+1 COOH with n = 6−16 methylene units in the chain at the air/water interface at standard conditions; and (2) defining the conditions for miscibility of the components on the molecular level or domain formation of the individual components.…”

Zinc–Carboxylate Binding in Mixed Octadecanoic Acid and Octadecanol Monolayers on Proxy Seawater Solution Surfaces

“…Due to the immense diversity in sources from which atmospheric and oceanic organic molecules originate, mixed monolayer films are more suitable for the purpose of representing sea spray aerosol and the SSML. Experimental techniques and molecular dynamics simulations have both been used to study the interfacial properties of mixed monolayers at the air–water interface. ,, Surface properties of binary mixtures of fatty acids with other organic molecules such as amines, esters, cholesterols, and alcohols have received significant attention during the past two decades. ,, These studies have revealed substantial differences between the physical and chemical properties of a pure fatty acid monolayer and a mixed monolayer that contains fatty acid. The inclusion of a secondary surfactant in a fatty acid monolayer has been shown to alter film packing structure, thermodynamic behavior, and monolayer stability .…”

“…Experimental techniques and molecular dynamics simulations have both been used to study the interfacial properties of mixed monolayers at the air−water interface. 22,46,47 Surface properties of binary mixtures of fatty acids with other organic molecules such as amines, esters, cholesterols, and alcohols have received significant attention during the past two decades. 46,48,49 These studies have revealed substantial differences between the physical and chemical properties of a pure fatty acid monolayer and a mixed monolayer that contains fatty acid.…”