Role of Tail−Tail Interactions versus Head-Group/Subphase Interactions in the Pressure−Area Isotherms of Fatty Amines at the Air−Water Interface. 1. Influence of Subphase Acid Counterions

Abstract

The pressure−area isotherms obtained in the initial stages after the spreading of fatty primary amines on the aqueous subphase has been investigated as a function of pH, length of hydrocarbon chain, basicity of the amine, nature of the acid counterion, etc. The main emphasis is on the high-area liquidlike features observed prominently at low pH. The dependence on the specific nature of the acid counteranion or the nature of the amine (aliphatic or aromatic) highlights the importance of the interactions at the interface in deciding the nature of the pressure−area isotherm in addition to the well-known tail−tail interactions between the hydrocarbon chains. The marked influence of small amounts (∼2 × 10-4 M) of alcohol such as n-butanol in influencing the stability as well as the nature of the pressure−area isotherms of fatty acids and amines is demonstrated. The relation between the lift-off areas and length scales, such as the size of the solvated subphase ionic species, repulsive limits between charged −NH3 + head groups, and ion-pair interaction distances, as well as the valence of the subphase acid species is examined. The role of the area-expanding head-group/subphase interactions vis-a-vis the area-condensing tail−tail interactions is discussed in terms of recent two-state models for liquid monolayers.

Phase behavior of Langmuir monolayers with ionic molecular heads: Molecular simulations

“…More importantly, the shift of the LE-LC phase transition to smaller temperatures when the molecular heads carry an ion is in qualitative agreement with the experimental observations of fatty acid monolayer with ionic head groups [45]. We also calculated two isotherms at T = 1.5 and T = 0.5 for two system sizes N = 144 and N = 256 and found that their behavior looks very similar to the experimental isotherms [45,64,65]. From these isotherms we calculated the variation ranges of the monolayer's area compression modulus and encountered that they are consistent with the experimental values reported in Refs.…”

“…These isotherms correspond to a monolayer with ionic molecular heads and were calculated for two system sizes, one with 144 (•) and the other with 256 (•) molecules. Their behavior looks very similar to the experimental isotherms: as pressure decreases the area per molecule increases and the higher the temperature the larger the area per molecule [45,64,65]. Note that at these two temperatures there appears to be no signature of a Surface-pressure-versus-area-per-molecule isotherms for a monolayer with ionic molecular heads for systems with 144 and 256 molecules at T * = 0.5 and T * = 1.5.…”

Adsorption of DNA to octadecylamine monolayers at the air–water interface

“…For pure ODA monolayer, the collapse surface pressure (60 mN m À1 ) and the limiting area (about 17 Å 2 molecule À1 ) are consistent with most values found in the literature [43][44][45] while other authors provide higher values under the same conditions [40,47,49]. This size is smaller than the corresponding value of a carbohydrate amphiphilic chain ($20 Å 2 molecule À1 ) and this difference is usually attributed to mass transfer to the aqueous subphase [39].…”

“…Moreover, anchoring of DNA in a Langmuir-Blodgett film has made possible its immobilization [34][35][36] and the construction of DNA chips [37]. Most of these studies were developed at a lipidic interface, while surface behavior of fatty amines [38][39][40][41][42] has received few research efforts compared with other lipidic matrices. However, fatty amines provide a positive charge below its pK value (around 11 for most soluble primary amines) that is suitable for many specific molecular architectures such us Langmuir-Blodgett preparation of anionic compounds as adsorption layer [43].…”

Chemical instability of octadecylammonium monolayers

“…Primary alkylammoniums, such as octadecylammonium (ODA), and alkylamines are compounds frequently used for monolayer studies on water [1][2][3][4][5][6][7] and for the preparation of hydrophobic clay minerals. [7][8][9][10] It is implicitly assumed that these compounds remain chemically stable during the experiment.…”