On estimation of the area per a surfactant molecule in a 2D monolayer during the liquid-expanded-liquid-condensed phase transition at the air/water interface
L.M. Litvinenko Institute of Physical Organic and Coal Chemistry
Abstract: A method is proposed to estimate the area per molecule of a surfactant monolayer Ac at the transition onset of the liquid-expanded to a liquid-condensed phase based on two theoretical models. A thermodynamic model with account for nonideality of the mixing entropy makes it possible to estimate the Gibbs energy of surfactant clusterization using the Π-A isotherms obtained at different temperatures. On the other hand, the quantum-chemical approach also makes it possible to calculate this thermodynamic parameter and assess the structural features of the obtained monolayers. The values of the Gibbs clusterization energies of small surfactant associates and the geometric parameters of the monolayer unit cells were previously calculated using the quantum-chemical semiempirical method PM3 for eight classes of amphiphilic compounds: saturated and ethoxylated alcohols, saturated and cis-monoenic carboxylic acids, α -hydroxylic and α -amino acids, N -acyl-substituted alanines and dialkyl-substituted melamine. These parameters are used in the thermodynamic model with account for nonideality of the mixing entropy to calculate Ac . The estimated values Ac adequately reflect the experimental temperature dependence for the considered phase transition: with an increase in temperature the area per surfactant molecule of a fixed chain length decreases, and vice versa, with an increase in the surfactant chain length at a fixed temperature, the value Ac increases. This makes it possible to use the proposed approach for prognostic purposes.
Keywords: amphiphilic monolayer, clusterization Gibbs’ energy, unit cell, phase transition, thermodynamic model
- Elena S. Kartashynska – Dr. Sc., Researcher, Supramolecular Chemistry Department, L.M. Litvinenko Institute of Physical Organic and Coal Chemistry
Kartashynska, E.S. On estimation of the area per a surfactant molecule in a 2D monolayer during the liquid-expanded-liquid-condensed phase transition at the air/water interface / E.S. Kartashynska // Physical and chemical aspects of the study of clusters, nanostructures and nanomaterials. – Tver: TSU, 2021. — I. 13. — P. 196-208. DOI: 10.26456/pcascnn/2021.13.196. (In Russian).
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