Alkane/Alcohol Mixed Monolayers at the Solid/Liquid Interface

Abstract

In this work, we present the behavior of solid monolayers of binary mixtures of alkanes and alcohols adsorbed on the surface of graphite from their liquid mixtures. We demonstrate that solid monolayers form for all the combinations investigated here. Differential scanning calorimetry (DSC) is used to identify the surface phase behavior of these mixtures, and elastic neutron incoherent scattering has been used to determine the composition of the mixed monolayers inferred by the calorimetry. The mixing behavior of the alcohol/alkane monolayer mixtures is compared quantitatively with alkane/alkane and alcohol/alcohol mixtures using a regular solution approach to model the incomplete mixing in the solid monolayer with preferential adsorption determining the surface composition. This analysis indicates the preferential adsorption of alcohols over alkanes of comparable alkyl chain length and even preferential adsorption of shorter alcohols over longer alkanes, which contrasts strongly with mixtures of alkane/alkane and alcohol/alcohol of different alkyl chain lengths where the longer homologue is always found to preferentially adsorb over the shorter. The alcohol/alkane mixtures are all found to phase separate to a significant extent in the adsorbed layer mixtures even when molecules are of a similar size. Again, this contrasts strongly with alkane/alkane and alcohol/alcohol mixtures where, although phase separation is found for molecules of significantly different size, good mixing is found for similar size species.

Preferential adsorption of solid monolayers of hydrocarbons over fluorocarbons at the solid/liquid interface

“…The formation of solid monolayers has been identified for many simple molecules including alkanes, alcohols, carboxylic acids and fluoroalkanes [1][2][3][4].…”

The structures of 1-bromoheptane and 1-bromononane monolayers adsorbed on the surface of graphite

“…The monolayer melting point is considerably above the bulk melting point and the melting point at submonolayer coverages. This pre-solidification is not uncommon in physisorbed monolayers of alkyl species such as alcohols [15][16][17][18][19][20] and carboxylic acids [21][22][23].…”

“…The apparatus and procedures for the synchrotron X-ray [24,28], Neutron [9,36] and DSC [19,37] have been described elsewhere. The instrument used for the synchrotron experiment was I11 [38] at the Diamond Light Source, Didcot, UK.…”

“…Even relatively simple species such as alkanes can form solid physisorbed monolayers [11][12][13][14]. In addition alkyl species with functional groups such as alcohols [15][16][17][18][19][20] and fatty acids [21][22][23] are also reported to form similar stable solid monolayers. Recently it has been found that the formation of very stable solid monolayers can be enhanced by a variety of non-covalent interactions within the monolayer, such as hydrogen bonding [24][25][26][27], halogen-halogen bonding [28,29], halogen bonding [28] even at temperatures significantly above the bulk melting point where the bulk adsorbate is liquid.…”

Adsorption of an alkane mixture on carbon nanotubes: Selectivity and kinetics

“…Preferred adsorptions of long chained molecule over short ones have been observed in various systems in both experimental studies, 3 including alkane molecules on graphite 14,15 and simulation studies, such as alkane mixture on Au surface 16 and inside a nanoporous zeolite. 17 A recent experimental study on PE CNT composites also suggested a favorable adsorption of higher molar mass PE component on the tubes.…”