Reasonable values of the thermodynamic characteristics for the cluster formation of odd alcohols at the air/water interface are obtained by the quantum chemical PM3 approximation. The calculated values of enthalpy Δ , entropy Δ and Gibbs energy Δ for the formation of clusters of a given structure depend linearly on the number of CH2 groups in the odd alcohol molecule. The additive approach, proposed in a recent paper, was further developed to extend the results of the calculations of the thermodynamic properties of small associates (2 to 7 alcohol molecules) to infinite clusters of odd and even homologues. Hydrogen O···H bonds, O···O interactions between the oxygen lone pairs, and four types of H···H interaction are taken into account, and the thermodynamic characteristics of the formation of linear, rhombic, rectangular, etc. clusters are calculated. Depending on the cluster type, the dependence of the Gibbs free energy on the alkyl chain length can be either the same for all homologous alcohols or can be different for the even and odd homologues. The most stable associates are clusters possessing rhombic or dodecahedral structures and linear clusters possessing one H···H bond per each methylene group. The former type exhibits a monotonic dependence of the thermodynamic parameters on the number of methylene groups, whereas for the latter type this dependence is stepwise. The results of the quantum chemical calculations agree well with the results obtained on the basis of a thermodynamic model that assumes equilibrium between oligomers and clusters within the monolayer. The experimental Π−A isotherms, indicating the existence of a first-order phase transition, and the microscopic morphology of the condensed-phase domains of tridecanol monolayers support the results of the quantum chemical and thermodynamic calculations.

Within the framework of PM3 molecular orbital approximation the thermodynamic function characteristics for the formation and geometrical structure of monomers, dimers, trimers, and tetramers of nondissociated n-carboxylic acids C(n)H(2n+1)COOH with n = 5-15 are calculated. It is shown that spontaneous aggregation of homologous fatty acids for the homologues with carbon atoms numbers n > or = 13 at the air/water interface can take place, leading to the formation of infinite plane rectangular clusters, whereas for the homologues with n < 11 spontaneous decomposition of large aggregates is energetically preferable. At the same time, the formation of trimers is more probable for the lower homologues (8 < n < 13). These results agree well both with the experimental data reported by various authors and with thermodynamic models developed earlier for soluble and insoluble monolayers. The slopes of the regressions calculated for the dependencies of the thermodynamic parameters on the alkyl chain length for all the clusters considered are all equal to each other. This fact indicates that the contributions of the CH2 groups to the thermodynamic characteristics of alcohols and acids are the same, and the differences in the formation of clusters by these substances should be attributed only to the differences in the structure and interactions of relevant functional groups. Therefore, it enables one to describe both acids and alcohols within the framework of the developed method, and it makes it possible to extend the proposed approach onto other classes of amphiphilic compounds.

The thermodynamics of the two-dimensional cluster formation of normal fatty alcohols (n ) 8-16) andthat of 2-methylhexadecanolat in the air/water interface is quantum chemically analyzed. The calculation provides reasonable values for the thermodynamic characteristics for the formation of alcohol clusters (m ) 2-7) with various structures at the air/water interface. The calculated values of enthalpy ∆H m cl , entropy ∆S m cl , and Gibbs energy ∆G m cl for the formation of a definite cluster structure can be satisfactory represented by a linear dependence on the number of CH 2 groups in the alcohol molecule. The absolute terms and coefficients of these equations can be characterized in form of the dependencies on the number of bonds formed between the alkyl groups (one to four bonds) and the contributions to the interactions from hydrogen bonds and lone pairs of oxygen atoms. The enthalpy and entropy of the cluster formation can be estimated from the molecular geometry of the clusters (relative positions of methylene and hydroxyl groups), the number of carbon atoms in the monomer and the number of molecules in the cluster. Reliable estimates predict plane clusters with tetragonal or hexagonal structure, and linear clusters with an arbitrary number of CH 2 groups and an arbitrary (up to infinite) number of monomers in the cluster. The calculations show that stable tetramers are formed by n-decanol, whereas n-dodecanol and the higher homologues cannot only form tetramers but also infinite clusters. These results are in agreement with the existence of a first-order phase transition in the experimental surface pressure-area isotherms of n-dodecanol, n-tetradecanol, n-hexadecanol, and 2-methylhexadecanol monolayers that does not occur in n-decanol monolayers. The thermodynamic model which assumes equilibrium between the oligomers and clusters within the monolayer agrees well with the experimental results, and suggests that in the fluid state the monolayers are comprised of monomers and oligomers (dimers to tetramers), the aggregation degree of which increases with the increase of the alkyl chain length. The data obtained by the thermodynamic model agree qualitatively, and also quantitatively (especially for Gibbs energy) with the quantum chemical calculations.

The thermodynamic parameters of the formation of monomers, dimers, trimers, tetramers, and one hexamer of alkylamines with the alkyl chain length in the range of 6 to 16 carbon atoms are calculated using the semiempirical PM3 method. The dependencies of potential energy surfaces of monomers and dimers on corresponding torsion angles are analyzed to determine the most stable conformations (local and global minima) of these entities. The thermodynamic parameters of cluster formation (enthalpies, entropies, and Gibbs' energies) are calculated for the dimers, trimers, tetramers, and the hexamer. The additive approach was further developed to extend the results of the calculations of the thermodynamic properties of small associates (2-6 amine molecules) to infinite clusters. It is shown that these parameter values are stepwise-dependent on the alkyl chain length, and spontaneous cluster formation takes place for this class of compounds when the alkyl chain length becomes 18-19 carbon atoms and higher.

The semiempirical PM3 method is used to calculate the thermodynamic parameters of the formation of monomers, dimers, trimers, tetramers, and one of the possible hexamers of saturated thioalcohols with normal structure and alkyl chain lengths of 6-16 carbon atoms. The dependencies of the potential energy surfaces on the torsion angles for monomers and dimers of normal thioalcohols are calculated. The most stable conformations of monomers and dimers are determined and used to construct the structures of trimers, tetramers, and larger clusters. For various conformations of dimers, trimers, tetramers, and for the hexamer, the thermodynamic parameters of clusterization (enthalpy, entropy, and Gibbs energy) are calculated. It is found that these parameters are stepwise-dependent on the alkyl chain length. It is shown that a two-dimensional square-symmetric infinite cluster and clusters constructed on the basis of cyclic trimers are formed for alkyl chain lengths of 14-15 carbon atoms and above.

The semiempirical quantum-chemical PM3 method is used to calculate the thermodynamic parameters of clusterization of the S-form of alpha-amino acids with the general composition C(n)H(2n+1)CHNH(2)COOH (n = 5-15) at 278 and 298 K. It is shown that six stable conformations of monomers exist, for which the thermodynamic parameters (enthalpy and Gibbs' energy) of the formation and absolute entropy are calculated. The correlation dependencies of the calculated parameters on the alkyl chain length are found to be linear. The structures of the monomers are used to build larger clusters (dimers, tetramers, hexamers). For all small clusters (comprised of two to six molecules), the thermodynamic parameters of formation and clusterization are calculated. It is shown that for tetramers and hexamers the enthalpy, entropy, and Gibbs' energy of clusterization are linearly dependent on the alkyl chain length, whereas for the dimers these dependencies are stepwise. The thermodynamic characteristics of clusterization of associates tilted by angles of 9 and 30 degrees with respect to the normal to the interface are calculated. It is shown that the 30 degrees angle orientation is more energetically advantageous for this class of compounds. The geometric parameters of the unit cell characteristic for the infinite 2D film which corresponds to the most advantageous conformation of the monomer were calculated using the PM3 parametrization to be a = 4.57-4.71 A and b = 5.67-5.75 A, with the angle between the axes theta = 100-103 degrees . These values agree well with the available experimental data. Spontaneous clusterization of alpha-amino acids at the air/water interface at 278 K takes place if the alkyl chain length exceeds 11-12 carbon atoms, whereas for 298 K this clusterization threshold corresponds to 13-14 carbon atoms in the alkyl chain, also in agreement with the experimental data.

No abstract

In the framework of the semiempirical PM3 method, the thermodynamic parameters of cis isomers of unsaturated carboxylic acids at the air/water interface are studied. The model systems used are unsaturated cis fatty acid of the composition Delta = 12-15 and omega = 6-11, where Delta and omega refer to the number of carbon atoms between the functional group and double bond, and that between the double bond and methyl group, respectively. For dimers, trimers, and tetramers of the four acid series, the thermodynamic parameters of clusterization are calculated. It is shown that the position of the double bond does not significantly affect the values of thermodynamic parameters of formation and clusterization of carboxylic acids for equal chain lengths (n = Delta + omega). The calculated results show that for cis unsaturated fatty acid with odd Delta values the spontaneous clusterization threshold corresponds to n = 17-18 carbon atoms in the alkyl chain, while for monounsaturated acids with even Delta values this threshold corresponds to n = 18-19 carbon atoms in the alkyl chain. These differences in the clusterization threshold between the acids with even and odd Delta values are attributed to the formation of additional intermolecular hydrogen bonds between the ketonic oxygen atom of one monomer and the hydrogen atom linked to the alpha-carbon atom of the second monomer for the acids with odd Delta values or between the hydroxyl oxygen atom of one monomer and hydrogen atom linked to the alpha-carbon atom of the second monomer for the acids with even Delta values. The results obtained in the study agree satisfactorily with our experimental data for cis unsaturated nervonic (Delta15, omega9) and erucic acids (Delta13, omega9), and published data for some fatty acids, namely cis-16-heptadecenoic (Delta16, omega1), cis-9-hexadecenoic (Delta7, omega9), cis-11-eicosenoic (Delta11, omega9) and cis-9-octadecenoic acid (Delta9, omega9).