Alkane/Alcohol mixed monolayers at the solid/liquid interface

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.     

Thermodynamic investigation of the adsorption of amides on graphite from their liquids and binary mixtures

We report a thermodynamic investigation of the adsorption of saturated and unsaturated (cis- and trans-) alkyl amides onto the surface of graphite from their pure liquids and from binary mixtures. We identify the formation of solid monolayers of the amides at temperatures when the bulk materials are liquid. The extent of this presolidification is much more extensive than other related materials, indicating that these amide layers are significantly more stable. The monolayer stability is found to be greatest for saturated amides. In addition, the stability of unsaturated amides is extremely sensitive to the location of the double bonds in the alkyl chain of the molecules, and trans isomers are found to be more stable than cis. We also address the preferential adsorption and mixing behavior of amide mixtures and amides mixed with other species coadsorbed onto graphite from binary solution. The results indicate that the amide molecules appear to be adsorbed with their principal axis parallel to the graphite surface and that amides are found to be strongly preferentially adsorbed with respect to alkanes. Interestingly the amides appear to mix rather better than might have been expected. There is also evidence of a number of other transitions in the adsorbates.     

Use of alkane monolayer templates to modify the structure of alkyl ether monolayers on highly ordered pyrolytic graphite

Scanning tunneling microscopy (STM) has been used to investigate the structure of pure and mixed monolayers formed by adsorption of long-chain alkanes and/or ethers on highly ordered pyrolytic graphite. Application of a pure phenyloctane solution of simple alkanes, such as tritriacontane, CH3(CH2)31CH3, produced a monolayer within which the individual molecular axes were oriented perpendicular to the lamellar axes. In contrast, a pure solution of symmetrical long-chain ethers, such as di-n-hexadecyl ether, CH3(CH2)15O(CH2)15CH3, produced a monolayer within which the molecular axes were oriented at an angle of approximately 65 degrees relative to the lamellar axes. The compositions of the overlying solutions were then gradually changed either from pure alkanes to nearly pure ethers or from pure ethers to nearly pure alkanes. When ethers replaced alkanes in the monolayer, the ethers conformed to the orientation within the existing alkane layer, rather than adopting the characteristic orientation of pure ether monolayers. However, when alkanes were incorporated into monolayers that had been formed from pure ether solutions, the orientation of the molecules within the monolayer converted to that characteristic of pure alkanes. Alkane monolayers thus acted as templates for subsequent ether layers, but ether monolayers did not act as templates for alkane layers.     

Mixing in adsorbed monolayers: perfluorinated alkanes

The mixing behavior of binary combinations of perfluoroalkanes in the bulk and in solid monolayers adsorbed at the graphite/liquid interface, determined by calorimetry and powder diffraction, is reported. The perfluoroalkanes are found to generally have a smaller excess enthalpy of mixing on the surface than in the bulk, and their relative size ratio is a good parameter to predict the mixing behavior. The excess enthalpy of mixing for perfluoroalkanes is found to be significantly smaller than that of the closely related hydrocarbons. The preferential adsorption of longer homologues over shorter ones is observed. Interestingly, the extent of preferential adsorption with relative size ratio is very similar to that of the hydrocarbons. These results can be understood in terms of the increased compressibility and lower polarizability of the perfluoroalkanes compared to hydrocarbons.     

Scanning tunneling microscopy studies of monolayer templates: alkylthioethers and alkylethers

Scanning tunneling microscopy has been used to determine the molecular ordering in stable, ordered monolayers formed from long-chain normal and substituted alkanes in solution on highly oriented pyrolytic graphite surfaces. Monolayers were initially formed using an overlying solution of either a symmetrical dialkylthioether or a symmetrical dialkylether. Initially pure thioether solutions were then changed to nearly pure solutions of the identical chain-length ether, and vice versa. The direct application of a pure solution of long-chain symmetrical ethers onto graphite produced a lamellate monolayer within which the individual molecular axes were oriented at an angle of approximately 65 degrees to the lamellar axes. In contrast, a pure solution of long-chain symmetrical thioethers on graphite produced a monolayer within which the molecular axes were oriented perpendicular to the lamellar axes. When ethers were gradually added to solutions overlying pure thioether monolayers, the ethers substituted into the existing monolayer structure. Thus, the ether molecules could be forced to orient in the perpendicular thioether-like manner through the use of a thioether template monolayer. Continued addition of ethers to the solution ultimately produced a nearly pure ether monolayer that retained the orientation of the thioether monolayer template. However, a monolayer of thioether molecules formed by gradual substitution into an ether monolayer did not retain the 65 degrees orientation typical of dialkylethers, but exhibited the 90 degrees orientation typical of dialkylthioether monolayers. The thioethers and ethers were easily distinguished in images of mixed monolayers, allowing both an analysis of the distribution of the molecules within the mixed monolayers and a comparison of the monolayer compositions with those of the overlying solutions. Substitution of molecules into the template monolayer did not proceed randomly; instead, a molecule within a monolayer was more likely to be replaced by a molecule in the overlying solution if it was located next to a molecule that had already been replaced.     

Mixing behavior of binary insoluble phospholipid monolayers. Analysis of the mixing properties of binary lecithin and cephalin systems by application of several surface and spreading techniques

The detailed miscibility analysis of binary phospholipid monolayers requires the application of a variety of spreading and surface techniques which often yield complementary results. Testing the equilibrium state of the binary monolayer by long-time experiments is also of great importance. Studies of the compression and spreading behavior of binary monomolecular systems form a basis for the determination of binary monomolecular phase diagrams. Within these plots different phase regions occur which permit clear statements regarding the miscibility state. Additional knowledge of the miscibility properties (phase diagrams) of the binary bulk systems is required. From the analogy of the properties of the bulk systems, the miscibility state of the monolayers is also determined by the temperature, and we can classify the monolayers of binary lecithin and cephalin systems into systems of complete miscibility, partial miscibility and complete immiscibility. In addition to the differences in the chemical structure of the mixing components, the film states in the monolayer and the miscibility behavior of the bulk systems are also influencing factors. If one of the components does not produce a spreading pressure, miscibility gaps occur in the phase diagram of the phospholipid monolayer. The miscibility gap, expressed by a constant spreading pressure, indicates complete immiscibility within this concentration range. If both components produce spreading pressures, and condensed and liquid-expanded film states within the considered temperature range, partial miscibility of the components becomes probable. The most effective parameter is then the difference in the chemical structure of the components. When both components produce spreading pressures and condensed films, the chemical structure of the mixing phospholipid compounds within their hydrophilic and hydrophobic parts is of essential importance. Depending on the differences in the chemical structures of their chains and their head groups in the case of binary phospholipid monolayers, the following possibilities result: complete miscibility, partial miscibility and complete immiscibility of the lecithins and the cephalins. Complete miscibility within the binary phospholipid monolayer takes place in the case of identical head-group structure and where there are only small differences in the chain length of the fatty acid groups. With increasing hydrocarbon chain length differences, partial miscibility or even complete immiscibility can occur within the monolayer. Chemical differences in the head-group structure of the mixing components have a similar influence. In the case of binary lecithin/cephalin mixtures, the differences in the head-group structure affect the miscibility behavior more than the chain length differences do in the case of lecithin/lecithin and cephalin/cephalin mixtures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of concentration and temperature on SDS monolayers at the air-solution interface studied by infrared external reflection spectroscopy

Infrared external reflection (IER) spectra of sodium dodecyl sulfate (SDS) monolayers at the air-solution interface and infrared transmission spectra of the corresponding aqueous solutions were measured at various SDS concentrations and temperatures. A comparison between the spectra of adsorbed monolayers and bulk solutions revealed that the conformational order of the SDS alkyl-chain at the air-solution interface improved with increasing the SDS concentrations, up until the saturation adsorption, and that the conformational order of the adsorbed SDS monolayer was higher than those of monomers and micelles. In addition, below the Krafft point temperature, the adsorbed SDS was maintained in the liquid crystal state, while SDS in the bulk solution was in the crystalline state. Furthermore, the SDS adsorption density was evaluated based on the IER band intensities of the insoluble monolayer of tridecanoic acid with an identical alkyl chain length to SDS.     

烷烃混合物在Cu-BTC中的吸附与分离

用巨正则系综Monte Carlo (GCMC)和构型导向Monte Carlo (CBMC)相结合的方法模拟了298 K下甲烷-乙烷-丙烷体系以及正丁烷-异丁烷体系在1,3,5-苯三甲酸铜(II) (Cu-BTC)中的吸附行为. 结果表明, Cu-BTC对丙烷以及异丁烷的吸附分离都有较好的选择性. 通过我们发展的“材料剖面成像”方法研究了烷烃混合物在Cu-BTC中不同压力下的吸附位点, 从而进一步分析了烷烃混合物在Cu-BTC中的分离性能. 结果发现, 在吸附过程中主要存在着两种效应, 即能量效应和尺寸效应的竞争. 在甲烷-乙烷-丙烷体系中, 较高压力下, 由于尺寸效应的影响, 丙烷主要吸附在主孔道中, 而对甲烷和乙烷组分, 能量效应占主导地位, 从而导致乙烷主要吸附在四面体孔内, 甲烷则主要吸附在三角形孔窗外. 在正丁烷-异丁烷体系中, 能量效应起主导作用, 从而使异丁烷主要吸附在四面体孔内, 而正丁烷主要吸附在主孔道中.     

Influence of temperature on microdomain organization of mixed cationic-zwitterionic lipidic monolayers at the air-water interface

The thermodynamic behavior of mixed DOTAP-DPPC monolayers at the air-water interface has been investigated in the temperature range from 15 to 45 degrees C, covering the temperature interval where the thermotropic phase transition of DPPC, from solid-like to liquid-like, takes place. Based on the regular solution theory, the miscibility of the two lipids in the mixed monolayer was evaluated in terms of the excess Gibbs free energy of mixing DeltaG(ex), activity coefficients f(1) and f(2) and interaction parameter omega between the two lipids. The mixed DOTAP-DPPC film was found to have positive deviations from ideality at low DOTAP mole fractions, indicating a phase-separated binary mixture. This effect depends on the temperature and is largely conditioned by the structural chain conformation of the DPPC lipid monolayer. The thermodynamic parameters associated to the stability and the miscibility of these two lipids in a monolayer structure have been discussed in the light of the phase diagram of the DOTAP-DPPC aqueous mixtures obtained from differential scanning calorimetry measurements. The correlation between the temperature behavior of DOTAP-DPPC monolayers and their bulk aqueous mixtures has been briefly discussed.     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.     

Surface and solution behavior of the mixed dialkyl chain cationic and nonionic surfactants

The surface and solution behavior of the mixed dialkyl chain cationic and nonionic surfactant mixture of dihexadecyldimethylammonium bromide, DHDAB, and hexaethylene monododecyl ether, C12E6, has been investigated, using primarily the scattering techniques of small-angle neutron scattering and neutron reflectivity. Within the time scale of the measurements, the adsorption of the pure component C12E6 at the air-solution interface shows no time dependence. In contrast, the adsorption of the DHDAB/C12E6 mixture and pure DHDAB has a pronounced time dependence. The characteristic time for adsorption varies with surfactant concentration, composition, and temperature. It is approximately 2-3 h for the DHDAB/C12E6 mixture, dependent upon concentration and composition, and approximately 50 min for DHDAB. At the air-solution interface, the equilibrium composition of the adsorbed layer shows a marked departure from ideal mixing, which is dependent upon both the solution concentration and the concentration of added electrolyte. In contrast, the composition of the aggregates in the bulk solution that are in equilibrium with the surface is close to ideal mixing, as expected for solution concentrations well in excess of the critical micellar concentration. The structure of the mixed adsorbed layer has been measured and compared with the structure of the equivalent pure surfactant monolayer, and no substantial changes in structure or conformation are observed. The extreme departure from ideal mixing in the adsorption behavior of the DHDAB/C12E6 mixture is discussed in the context of the structure of the adsorbed layer, changes in the underlying solution structures, and the failure of regular solution theory to predict such behavior.     

Bulk and adsorbed monolayer phase behavior of binary mixtures of undecanoic acid and undecylamine: catanionic monolayers

Differential scanning calorimetry (DSC) and X-ray powder diffraction (PXRD) have been used to determine the phase behavior of the binary mixtures of undecanoic acid (A) and undecylamine (B) in the bulk. In addition, we report DSC data that indicates very similar behavior for the solid monolayers of these materials adsorbed on the surface of graphite. The two species are found to form a series of stoichiometric complexes of the type AB, A(2)B, and A(3)B on the acid rich side of the phase diagram. Interestingly, no similar series of complexes is evident on the amine rich side. As a result of this complexation, the solid monolayers of the binary mixtures exhibit a very pronounced enhancement in stability relative to the pure adsorbates.     

Enhanced stability of short- and long-chain diselenide self-assembled monolayers on gold probed by electrochemistry, spectroscopy, and microscopy

Well-ordered, compact, self-assembled monolayers (SAMs) of hexyl and dodecyl diselenides have been formed on oriented (111) gold surfaces. Monolayer formation has been effected by adsorption from neat diselenides as well as millimolar solutions of diselenides in alcohol. The monolayer formation is confirmed using electrochemical quartz crystal microbalance studies. The stability and permeability of the monolayers at various temperatures have been probed using reflection absorption infrared spectroscopy (RAIRS) and electrochemistry. The RAIRS studies in the dry state show the formation of highly ordered, compact structures when adsorbed from neat compounds compared to the monolayers adsorbed in the presence of alcohol. The monolayers adsorbed from neat diselenide are quite stable as a function of temperature irrespective of the chain length. The electrochemical studies based on the blocking behavior of the monolayers toward electron transfer between a diffusing species and the electrode surface reflect the stability and the compactness of the structure. The results point out that the presence of solvent molecules during the SAM formation hinders the organization of the monolayer structure, especially in the case of short-chain diselenide monolayers.     

Enhanced thermal stability and structural ordering in short chain <Emphasis Type="Italic">n</Emphasis>-alkanethiol monolayers on gold probed by vibrational spectroscopy and EQCM

Monolayers of alkanethiols with varied chain lengths, CH₃(CH₂)_nSH where n = 3, 5, and 7, on gold substrates have been prepared by adsorption from (1) neat thiol, (2) millimolar thiol solution in alcohol (conventional method), and (3) potential-controlled adsorption. Reflection absorption infrared spectroscopy (RAIRS) and electrochemical quartz crystal microbalance (EQCM) have been used to characterize the integrity of the monolayers. Methylene and methyl stretching modes along with C–S stretching modes have been used as benchmarks to follow the order–disorder transitions in the monolayer structure, in the temperature range from 25 to 175 °C. Monolayers adsorbed from neat thiol show superior quality in terms of stability and structural arrangement. Short chain thiols with n = 3, 5, and 7 do show substantial stability. The possibility of multilayer formation is ruled out by EQCM studies comparing the frequency and mass change associated with the monolayer desorption. Self-assembled monolayers (SAMs) adsorbed under potential control behave very similarly to the monolayers adsorbed from neat thiol as far as stability and structural orientation are concerned, irrespective of the chain length. The adsorption from neat thiol gets rid of the solvent-induced defects and arrests the propagation of defects under temperature constraints.     

Miscibility and segregated structures in mixed Langmuir monolayers

The phase behavior and morphology of segregated structures are considered for mixed Langmuir monolayers, which comprise a type of supramolecular polymer having a complex internal structure mixed with a long chain fatty acid. We fabricated two different series of mixed monolayers from a polyglutamate (PG) copolymer having 30% octadecyl ester side chains and 70% methyl ester side chains and fatty acids. These mixed monolayers deposited on a solid substrate were studied by pressure-area diagram measurements, X-ray analysis, and atomic force microscopy. Stearic acid (STA) and hexacosanoic acid (HCA) with alkyl chain lengths of 17 and 25 carbon atoms, respectively, were used as low molecular weight components. For the mixture PG:STA, where the length of the STA molecules is comparable to the length of the PG side chains, we observed the formation of partially miscible monolayers. These mixtures exhibit a nanometer scale domain morphology formed by the STA molecules dissolved in the outer shell of the PG monolayer. In contrast, for the PG:HCA mixture we observed a strong tendency for microphase separation and the formation of well-defined submicron segregated structures in the monolayers. Lateral compression of the mixed monolayers to a point close to the collapse pressure promotes microphase separation in both types of mixed monolayers with the formation of anisotropic surface morphology and oriented domains.     

Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. I. Structure

The structure of a monolayer film of the branched alkane squalane (C30H62) adsorbed on graphite has been studied by neutron diffraction and molecular dynamics (MD) simulations and compared with a similar study of the n-alkane tetracosane (n-C24H52). Both molecules have 24 carbon atoms along their backbone and squalane has, in addition, six methyl side groups. Upon adsorption, there are significant differences as well as similarities in the behavior of these molecular films. Both molecules form ordered structures at low temperatures; however, while the melting point of the two-dimensional (2D) tetracosane film is roughly the same as the bulk melting point, the surface strongly stabilizes the 2D squalane film such that its melting point is 91 K above its value in bulk. Therefore, squalane, like tetracosane, will be a poor lubricant in those nanoscale devices that require a fluid lubricant at room temperature. The neutron diffraction data show that the translational order in the squalane monolayer is significantly less than in the tetracosane monolayer. The authors' MD simulations suggest that this is caused by a distortion of the squalane molecules upon adsorption on the graphite surface. When the molecules are allowed to relax on the surface, they distort such that all six methyl groups point away from the surface. This results in a reduction in the monolayer's translational order characterized by a decrease in its coherence length and hence a broadening of the diffraction peaks. The MD simulations also show that the melting mechanism in the squalane monolayer is the same footprint reduction mechanism found in the tetracosane monolayer, where a chain melting drives the lattice melting.     

Scanning tunneling microscopy of prochiral anthracene derivatives on graphite: chain length effects on monolayer morphology

The morphology of monolayers formed upon adsorption of prochiral 1,5-substituted anthracene derivatives on highly oriented pyrolytic graphite is investigated using scanning tunneling microscopy at the liquid-solid interface. The adsorption orientation of these prochiral anthracene derivatives positions one of their enantiotopic faces in contact with the graphite. The molecules adsorb in rows with contact between adjacent anthracenes. The anthracene side chains extend perpendicular to the direction of the row repeat. All molecules within a single row adsorb via the same enantiotopic face. Anthracenes with side chains containing an even number of non-hydrogenic atoms (C, S) form monolayers in which molecules in adjacent rows adsorb via opposite enantiotopic faces. Anthracenes with side chains that contain an odd number of non-hydrogenic atoms form two-dimensional chiral domains in which all rows contain molecules adsorbed via the same enantiotopic face. This chain length effect on monolayer morphology represents a generalized example of structural effects previously observed in alkanoic acid monolayers formed on HOPG. The variation of the STM current with position in the vicinity of the anthracenes indicates that the highest occupied molecular orbital is the predominant mediator of tunneling for the aromatic group.     

Adsorption and micellar properties of a mixed system of nonionic-nonionic surfactants

We study the surface adsorption and bulk micellization of a mixed system of two nonionic surfactants, namely, ethylene glycol mono-n-dodecyl ether (C12E1) and tetraethylene glycol mono-n-tetradecyl ether (C14E4), at different mixing ratios at 15 degrees C. The pure C14E4 monolayer cannot show any indicative features of phase transition because of both hydration-induced and dipolar repulsive interactions between the bulky head groups. On the other hand, the monolayers of pure C12E1 and its mixture with C14E4 undergo a first-order phase transition, showing a variety of surface patterns in the coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases under the same experimental conditions. For pure C12E1, the domains are of a fingering pattern while those for the C12E1/C14E4 mixed system are found to be compact circular and small irregular structures at 2:1 and 1:1 molar ratios, respectively. The critical micelle concentration (cmc) values of both the pure and the mixed systems were measured to understand the micellar behavior of the surfactants in the mixture. The cmc values of the mixed system were also calculated assuming ideal behavior of the surfactants in the mixture. The experimental and calculated values are found to be very close to each other, suggesting an almost ideal nature of mixing. The interaction parameters for mixed monolayer and micelle formation were calculated to understand the mutual behavior of the surfactants in the mixture. It is observed that the interaction parameters for mixed monolayer formation are more negative than those of micelle formation, indicating a stronger interaction between the surfactants during monolayer formation. It is concluded that since both the surfactants bear EO units in their head groups, structural parity and hydrogen bonding between the surfactants allow them to be closely packed during monolayer and micelle formation.     

Interplay between the surface adsorption and solution-phase behavior in dialkyl chain cationic-nonionic surfactant mixtures

Neutron reflectivity, NR, and surface tension have been used to study the adsorption at the air-solution interface of mixtures of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) and the nonionic surfactants monododecyl triethylene glycol (C12E3), monododecyl hexaethylene glycol (C12E6), and monododecyl dodecaethylene glycol (C12E12). The adsorption behavior of the surfactant mixtures with solution composition shows a marked departure from ideal mixing that is not consistent with current theories of nonideal mixing. For all three binary surfactant mixtures there is a critical composition below which the surface is totally dominated by the cationic surfactant. The onset of nonionic surfactant adsorption (expressed as a mole fraction of the nonionic surfactant) increases in composition as the ethylene oxide chain length of the nonionic cosurfactant increases from E3 to E12. Furthermore, the variation in the adsorption is strongly correlated with the variation in the phase behavior of the solution that is in equilibrium with the surface. The adsorbed amounts of DHDAB and the nonionic cosurfactants have been used to estimate the monomer concentration that is in equilibrium with the surface and are shown to be in reasonable qualitative agreement with the variation in the mixed critical aggregation concentration (cac).     

Structural evolution of hexa-peri-hexabenzocoronene adlayers in heteroepitaxy on n-pentacontane template monolayers

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha-->beta-->gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.