recently 
 discovered two - dimensional ( 2d ) materials such as graphene , 
 fluorographene , graphene oxide , transition metal dichalcogenides , 
 hexagonal boron nitride , and phosphorene all have very high surface / mass 
 ratios , and many of their potential practical applications rely on 
 their large surface areas . consequently , there is a need to better 
 understand their surface properties and the way in which their surfaces 
 interact with their surroundings . in particular , there is great interest 
 in the adsorption of molecules on 2d materials because of its technological 
 importance . small molecule adsorption can be used to tune the electrical 
 properties of 2d materials and is important 
 in processes that can be exploited in mass , gas , and electrochemical sensing . all kinds of sensors require a contact between 
 an analyte and an active material to generate a readout , so it is 
 essential to have a good understanding of the strength and nature 
 of the interactions between adsorbed molecules and the sensing surface . fluorographene ( i.e. , a fluorographite 
 monolayer ) , fluorinated graphenes , and fluorographite are all active 
 in electrochemical sensing and have sensing properties that depend 
 on their c / f ratio and topology . as such , 
 they could potentially be used to create selective sensors in which 
 specificity is achieved through the interaction of the analyte with 
 an active zone consisting of a suitable fluorinated graphene . in addition 
 to sensing applications , these materials can be used in gas separation 
 and storage . it has been demonstrated that 
 adsorption to graphene is primarily controlled by london dispersive 
 forces , but little is known about adsorption 
 to fluorographene and fluorographite . the few theoretical studies 
 that have explored the adsorption of small molecules to fluorographene 
 have suggested that it may have useful applications in hydrogen storage . adsorption enthalpies on surfaces are usually studied using 
 adsorption 
 calorimetry , temperature - programmed desorption , or equilibrium adsorption 
 isotherms . recently , we witnessed a renaissance 
 of inverse - gas chromatography ( igc ) to study the process of adsorption . 
 this technique measures retention characteristics of gas probes injected 
 to a column loaded by analyzed material . its main advantage is that it provides results that represent averages 
 over the sample s surface . in addition , adsorption enthalpies 
 and entropies , and the dependence of these thermodynamic quantities 
 on the surface coverage , can be obtained directly from igc data . it was shown that igc provides 
 adsorption enthalpies consistent with other experimental techniques . quantum chemistry and solid - state physics 
 calculations can be used 
 to characterize intermolecular interactions and predict their strengths . 
 however , deciphering the nature and strength of molecule surface 
 binding by computational means is frequently rather challenging because 
 the binding energies are usually low and involve physical phenomena 
 such as london dispersion forces that are difficult to model reliably ; physisorption forces are significantly weaker 
 than chemisorption ones . in finite molecular 
 systems , the electron electron correlation effects responsible 
 for these noncovalent interactions can be described using the coupled 
 cluster method , with single and double electron excitations being 
 modeled iteratively and triple excitations perturbatively ( ccsd(t ) ) , 
 or using the perturbative mller plesset mp2.5 method 
 ( in which energies are calculated as the arithmetic mean of the mp2 
 and mp3 energies ) . unfortunately , these 
 methods are not available for periodic systems , which are frequently 
 superior to finite models when studying the adsorption of molecules 
 on surfaces . consequently , methods based 
 on density functional theory ( dft ) are widely used in such applications . classical general gradient approximation dft methods are of semilocal 
 character and thus can not describe the long - range component of the 
 london forces , which result from nonlocal electron  electron 
 correlation . the performance 
 of individual dft methods is usually benchmarked against ccsd(t ) or 
 mp2.5 calculations on finite systems ( molecular clusters ) , in order 
 to identify approaches that accurately describe the system of interest . both ccsd(t ) as well as mp2.5 methods provide highly accurate interaction 
 energies for various types of molecular clusters with errors of less 
 than 2 and 4 relative percent , respectively . ccsd(t ) can be applied to complexes having up to around 35 heavy 
 atoms , while mp2.5 can handle systems up to twice the size . unfortunately , 
 however , no reference method of comparable quality is currently available 
 for use with periodic models , with the exception of the stochastic 
 quantum monte carlo method , that embody exceeding computational demands . in this work , 
 we determined isosteric adsorption enthalpies ( h ) and isosteric adsorption entropies ( s ) 
 for five organic molecules ( acetone , acetonitrile , dichloromethane , 
 ethanol , and ethyl acetate ) on fluorographite by igc . we also performed 
 extensive calculations on finite models of fluorographene to benchmark 
 various dft methods against ccsd(t ) and mp2.5 . the application of 
 symmetry - adapted perturbation theory ( sapt ) to finite model systems allowed us to decipher the nature of the 
 molecular interactions occurring on the fluorographene / fluorographite 
 surfaces . moreover , dft calculations on periodic models helped us 
 to clarify the roles of various adsorption sites and surface defects 
 on adsorption to fluorographene , as well as the influence of molecular 
 clustering . we conclude that the enthalpies of adsorption to fluorographene / fluorographite 
 are slightly lower than those for graphene / graphite , i.e. , small molecules 
 generally bind more strongly to fluorographene and fluorographite 
 than to their nonfluorinated counterparts . using 
 igc we determined the isosteric adsorption enthalpies of five molecular 
 probes ( table 1 ) to 
 fluorographite for coverage values ranging from 2 to 20% of the adsorbate 
 monolayer . the isosteric adsorption enthalpies of acetone , acetonitrile , 
 and ethanol decreased as the surface coverage increased , with saturation 
 occurring at a coverage level slightly above 10% ( figure 1 ) . the isosteric adsorption 
 enthalpies of ethyl acetate and dichloromethane were rather coverage - independent . the saturated adsorption enthalpies h  h reported in table 1 ranged from 6.9 kcal / mol ( dichloromethane ) 
 to 12.8 kcal / mol ( ethanol ) . isosteric adsorption 
 enthalpies h ( top ) 
 and entropies s ( bottom ) for five organic 
 molecules on fluorographite obtained from inverse gas chromatography 
 as a function of surface coverage . the dotted lines are eye - guides . averaged over coverage values greater 
 than 10% . averaged over 
 coverage values over 
 220% . the temperature 
 interval tmin - tmax ( in k ) 
 was used for data fitting ( see the supporting information ) . standard 
 enthalpies of condensation 
 hcond ( negative standard enthalpies 
 of vaporization in kcal / mol ) for tmin and tmax were adopted from the literature . adsorption 
 enthalpies ( in kcal / mol ) 
 of the same molecules on graphene hgr . were taken from previous works . when explaining 
 coverage dependence of the adsorption enthalpies , 
 one should take into account the fact that the real material surface 
 is really complex containing various structural and chemical features 
 and defects , e.g. , edges , steps , cavities , pores , vacancies , or adatoms . such features may represent sites , where the adsorbate preferentially 
 binds ( the high - energy sites ) . in addition , 
 some adsorbates may tend to form clusters over the surface . as the igc provides averaged adsorption enthalpies 
 over the surface all these effects are involved . fortunately , the 
 complexity of the process can be typically deciphered from the plot 
 of adsorption enthalpy vs coverage in combination with atomistic simulations . the decreasing adsorption enthalpies of acetone , acetonitrile , and 
 ethanol with increasing coverage ( figure 1 ) can be explained by the clustering of these 
 molecules over the adsorbent surface , because the same behavior was 
 observed for ethanol adsorption to graphene and was attributed to 
 ethanol clustering over the graphene surface . this behavior might occur when the adsorption enthalpy of a single 
 molecule to the surface is greater than the enthalpy of condensation ; 
 however , we should note that a tendency of clustering is given by 
 a delicate balance among adsorption enthalpy , enthalpy , and entropy 
 of clustering . the rather constant adsorption 
 enthalpies of dichloromethane to the surface ( at very low coverage 
 levels ; see figure s1 in the supporting information ) may indicate that the material used in this work had few high - energy 
 sites or that the enthalpies of adsorption to structural features 
 that typically correspond to high energy sites ( e.g. , steps , edges , 
 cavities , and defects ) are comparable to those for adsorption to a 
 fluorographene / fluorographite surface lacking such features . computational 
 methods ( see below ) were used to determine which of these potential 
 explanations was most plausible . we measured the adsorption 
 enthalpies of the same probe molecules 
 to graphene powder in a previous investigation . on comparing the adsorption enthalpies for 
 fluorographene / fluorographite and graphene / graphite , we found that 
 the enthalpies of adsorption to fluorographite are generally slightly 
 lower than those for graphene / graphite ( by 1.2 kcal / mol on average , 
 corresponding to 14% of the h for graphene ; 
 see table 1 ) . this 
 indicates that small organic molecules bind more strongly to fluorographite 
 than to graphite and hence that fluorographene / fluorographite more 
 readily adsorbs guest molecules from its environment . we used finite 
 systems to benchmark the accuracy of selected dft 
 methods when applied to the systems of interest and used the best - performing 
 methods in these benchmarking studies to perform further calculations 
 on periodic - boundary models . two small models of fluorographene / fluorographite 
 surfaces were used in the benchmarking calculations . the smallest 
 model , perfluorohexamethylcyclohexane ( c12f24 ) , was small enough to permit the use of the ccsd(t ) method , which 
 provides very accurate interaction energies ei for a wide range of complexes . however , because of the 
 small size of this model , it may not constitute an adequate representation 
 of the theoretically infinite fluorographene / fluorographite surface . we therefore also considered a larger model , hexatriacontafluorotetracosahydrocoronene 
 ( c24f36 ) , which is more representative and was 
 also used to obtain geometries and enthalpy corrections ( see methods ) . because it was computationally unfeasible 
 to perform ccsd(t ) calculations on this larger system , we instead 
 performed reference calculations using the mp2.5 method , which is 
 known to approach the quality of ccsd(t ) for many noncovalent complexes . the use of mp2.5 in this case was validated 
 by comparing the energies calculated with this method for the smaller 
 c12f24 system to those obtained with ccsd(t ) . both ccsd(t ) and mp2.5 explicitly model the dispersion energy , whereas 
 most dft methods model it implicitly using some kind of correction . this is one of the reasons why the performance 
 of dispersion - corrected dft techniques must be carefully tested . the ccsd(t ) and mp2.5 interaction energies for the dichloromethane 
 and ethanol complexes of c12f24 were in reasonably 
 good agreement ( table 2 ) , although the ccsd(t ) interaction energies are systematically more 
 attractive ( by 10% on average ) than the mp2.5 energies . this justified 
 the use of the less expensive mp2.5 method as a source of reference 
 data for calculations on the large models . dft functionals with london 
 dispersion corrections generally provided reasonably accurate energies 
 for the smaller complexes ( table 2 , figure 2 , figure s2 ) , but optb86b - vdw and vdw - df 
 overestimated the interaction energy by over 30% for this model . however , 
 it should be noted that the optb88-vdw functional provided the best 
 agreement with experimental data in a study on the adsorption of small 
 molecules to graphene . this may indicate 
 that the individual dispersion - corrected dft methods do not provide 
 a consistent treatment of dispersion interactions for finite size 
 and periodic systems . together with the limited amount of available 
 experimental data on such systems , this complicates the assessment 
 of theoretical methods for adsorption studies . models of fluorographene 
 ( top ) and their interaction energies ei with dichloromethane , showing the dependence 
 of the interaction energy on the size of the model system and the 
 computational method used ( bottom ) . table 3 summarizes 
 the interaction energies calculated for complexes of the larger c24f36 system with the five organic molecules using 
 mp2.5 and various dft functionals . the mp2.5 results suggest that 
 the complexes with the largest and smallest interaction energies are 
 those of ethyl acetate and dichloromethane , respectively , and all 
 of the tested functionals replicated this trend . the optb86b - vdw and 
 vdw - df functionals again strongly overestimated the absolute interaction 
 energies ( by more than 56% ) , whereas the b97-d3 functional underestimated 
 the interaction energies by more than 25% . other dft functionals provided 
 interaction energies that agreed reasonably well with the reference 
 mp2.5 values . the b3lyp - d3 and pbe - d3 functionals gave the most accurate 
 interaction energies with respect to mp2.5 ( with deviations below 
 10% ) ; however , pbe - d2 , pbe - ts+scs , and vdw - df2 functionals performed 
 also well because the ccsd(t ) energies were more negative than the 
 mp2.5 values for the smaller model . we obtained near - identical pbe - d3 
 interaction energies by two different approaches : using localized 
 gaussian orbitals ( the tzvpp basis set ) as implemented in turbomole 
 and using plane waves ( pw ) as implemented in vasp ( see also the methods section ) . the relative deviation from the 
 mp2.5 energy , ( ei-ei)/ei , is given in parentheses . to determine how the interaction 
 energy ei depends on the model 
 size , we performed calculations on a larger finite model system - 
 c54f72 . we also compared all of the results 
 obtained using finite size models to results for an infinite ( periodic ) 
 model based on a 5  5 fluorographene supercell . this comparison 
 was justified by the fact that the calculated adsorption configurations 
 on fluorographene were similar to those for the finite models ( figure 3 ) . interaction energies 
 for small molecules on all four models of fluorographene ( c12f24 , c24f36 , c54f72 , and periodic c50f50 ; see figure 2 ) could be computed 
 using dft methods implemented in the vasp package ( see the methods section ) , namely the empirically corrected 
 density functionals pbe - d2 , pbe - d3 , pbe - ts , and pbe - ts - scs , as well 
 as the nonlocal correlation functionals vdw - df , vdw - df2 , and optb86b - vdw . we also performed less expensive wave function - based calculations 
 using the mp2/aug - cc - pvdz method to compare the interaction energies 
 computed in this way for the three finite systems ( c12f24 , c24f36 , and c54f72 ) to those obtained by dft . adsorption geometries of acetone , acetonitrile , 
 dichloromethane , 
 ethanol , and ethyl acetate ( from the top to the bottom , respectively ) 
 on perfluorinated tetracosahydrocoronene ( left column ) and fluorographene 
 ( right column ) . structures shown in the left and right columns were 
 obtained by optimization with the b97d and optb86b - vdw density functionals , 
 respectively . dark gray , green , red , blue , yellow , and white represent 
 c , f , o , n , cl , and h atoms , respectively . figures 2 and s2 present the results of the calculations performed 
 for the adsorption of dichloromethane ( and ethanol ) on all four fluorographene 
 model systems using various methods . in all cases , the calculated 
 interaction energy decreased as the model size increased , i.e. the 
 small molecules were most strongly bound on periodic fluorographene . the ratio of mp2.5/cbs interaction energies of dichloromethane and 
 ethanol molecules with c24f36 and c12f24 equals to 1.2 and 1.3 , respectively . the same ratio 
 of mp2/aug - cc - pvdz for dichloromethane complexes equals to 1.4 , whereas 
 it becomes 1.2 when we consider c54f72 and c24f36 models . it should be noted here that mp2.5 
 and mp2 describe the dispersion energy explicitly , which means that 
 they model well both pairwise and many - body energy terms ( however , strictly speaking , mp2 does not provide 
 reliable values for the many - body dispersion term ) . the ratios of the calculated energies of the dichloromethane and 
 ethanol complexes of c24f36 and c12f24 obtained with different dft methods were generally 
 similar to the mp2.5 values ( in the cases of pbe - d2 , pbe - d3 , vdw - df2 , 
 and optb86b - vdw ) or the mp2 values ( for pbe - ts , pbe - ts - scs , and vdw - df ) . moreover , the dft interaction energy ratios for the c54f72 and c24f36 models were similar 
 to those obtained with mp2 . the tested dft methods thus predict the 
 effects of increasing surface size similarly to mp2.5 and mp2 , both 
 of which correctly model many - body terms . the interaction energies 
 calculated for the largest finite model 
 system , c54f72 , were only slightly ( 0.20.8 
 kcal / mol ) higher than those for the infinite periodic surface ( figures 2 and s2 ) . this suggests that calculations on c54f72 provide quite good estimates of interaction 
 energies for fluorographene . in addition , the relative ordering of 
 the interaction energies for the studied small molecules on c54f72 matched that for the periodic system . the calculated interaction energies obtained for the periodic model 
 system with different density functionals decreased in the following 
 order : ei > ei > 
 ei > 
 ei , ei > ei , ei ( see the interaction curves 
 in figure s3 ) . the interaction energies 
 provided by the dft - d3 and optb86b - vdw / vdw - df functionals thus corresponded 
 to the upper and lower limits of the dft energy range , which spanned 
 approximately 3 kcal / mol on average ( 5 kcal / mol for 
 ethyl acetate ) . the dft energies calculated for the finite c24f36 model were less variable ( spanning a range of 2 
 kcal / mol for all small molecules other than ethyl acetate ; see table 3 ) , and those for the 
 smallest c12f24 model varied still less , with 
 a range of 1 kcal / mol ( table 2 ) . the calculations performed using the dft functionals 
 with empirical dispersion corrections indicated that dispersion interactions 
 are the most important component of the interaction energy resulting 
 from the binding of small molecules to the fluorographene surface . the pure pbe functional yielded a very shallow potential well for 
 molecular adsorption to fluorographene ( < 1 kcal / mol , figure s3 ) , but substantially more negative adsorption 
 energies were obtained using functionals with dispersion corrections . 
 based on the adsorption energies computed using the many - body d3 dispersion 
 correction , dispersion accounted for 92% of the total binding energy 
 ( e/e ) in the case of acetone , 64% for acetonitrile , 
 69% for dichloromethane , 73% for ethanol , and 83% for ethyl acetate . this trend was confirmed by more rigorous dft based symmetry adapted 
 perturbation theory ( dft - sapt ) calculations 
 on the intermediate finite model c24f36 ( see figure 4 ) . the dispersion 
 contribution ( calculated as ei/(ei + 
 ei + ei ) dominated , accounting for 
 72% , 70% , 73% , 70% , and 76% of the total attractive interaction energies 
 for acetone , acetonitrile , dichloromethane , ethanol , and ethyl acetate , 
 respectively . the electrostatic term ( 1923% ) represented the 
 second largest attractive contribution , followed by the induction 
 or polarization term ( 49% ) ; see figure 4 . this trend is similar to that observed 
 for the interaction of small molecules with a finite model of graphene 
 ( coronene , see table s1 for a comparison ) . 
 in the graphene case , the interactions were similarly dominated by 
 dispersion ( 6266% ) , with lesser contributions from electrostatics 
 ( 2629% ) and induction ( 812% ) . the relative contribution of electrostatics to binding in the case 
 of c24f36 was lower than that for graphene , 
 which is somewhat surprising given that c semi - ionic  ) , making the distribution of electron density 
 across the fluorographene plane rather inhomogeneous . decomposition of the total attractive energy into dispersion , induction , 
 and electrostatic contributions calculated by dft - sapt for the small 
 model system c24f36 . data for the hexatriacontafluorotetracosahydrocoronene model system 
 ( c24f36 ; see figure 3 ) were used to estimate the different contributions 
 to the adsorption enthalpies of small molecules on fluorographene ( table 4 ) by applying standard expressions from statistical 
 mechanics under the ideal gas , rigid rotor , and harmonic oscillator 
 approximations . the derived enthalpy / energy differences ( h  e ) ranged from 1.2 to 
 1.5 kcal / mol and were used as corrections to derive adsorption enthalpies 
 from the adsorption energies calculated for the periodic model . these 
 adsorption energies ranged from 7.8 to 13.8 kcal / mol 
 ( optb86b - vdw functional , table 5 ) and dominated the calculated adsorption enthalpies because 
 the correction terms were equal to at most 19% of the calculated 
 interaction energies . e and e0 with and without zpe , respectively , internal 
 energies u , enthalpies h , gibbs energies g , and entropies s ( in cal / molk ) , and the contributions of the zero - point 
 energy ( e0 ) , thermal ( et ) , enthalpy ( eh ) , and gibbs energy corrections ( eg ) . the adsorption process c24f36 + x  c24f36x 
 was modeled at 313.15 k and 101.325 kpa using the b97d functional . the correction 
 to the adsorption 
 enthalpy was obtained from calculations on perfluorotetracosahydrocoronene 
 ( table 4 ) . we investigated the potential contributions of 
 surface irregularities , defects , and molecular configurations to the 
 adsorption process by studying the roles of ( i ) multilayers , ( ii ) 
 surface steps and edges , and ( iii ) surface defects ( figure 5 ) . specifically , we compared 
 the adsorption of ethanol on monolayer and bilayer fluorographene , 
 because studies on graphene had previously shown that adsorption to 
 multilayer graphene was slightly stronger than that to a graphene 
 monolayer . conversely , the energy of adsorption 
 for small molecules on bilayered fluorographene was 1.4 kcal / mol higher 
 than that for a fluorographene monolayer ( table s2 , supporting information ) . the addition of a third layer of 
 fluorographene changed the adsorption energy by less than 0.1 kcal / mol 
 relative to that for the bilayer . adsorption geometries of an ethanol molecule 
 on multilayer fluorographene 
 and a fluorographene step / edge ( top ) . adsorption geometries of an 
 ethanol molecule on fluorographene with vacancy defects and a stone all adsorption 
 energies were obtained with the optb86b - vdw density functional . for 
 molecular clusters , steps are regarded as high - energy 
 sites in multilayered graphene 
 and graphite because the energy change upon adsorption to steps is 
 up to 2.5 times greater than that for adsorption to a stepless surface . the calculated adsorption energies 
 on steps were only 1020% lower than those for the stepless 
 surface ( table 6 ) . taking into account boltzmann distribution of probes between the high - energy 
 sites and surface , such differences are 
 barely experimentally detectable , as demonstrated by the corresponding 
 igc data ( cf . in addition , the calculated adsorption 
 energies for ethanol on fluorographene edge sites were less favorable ( 4.0 and 3.1 kcal / mol ) than those for 
 the surface ( 8.7 kcal / mol , figure 5 ) . calculated with the optb86b - vdw 
 density functional ( results obtained with pbe - d3 in parentheses ) . we also considered four types 
 of defect sites : ( i ) f vacancies , 
 ( ii ) c vacancies , ( iii ) c  f vacancies , and ( iv ) stone wales 
 ( sw ) defects , the latter corresponding to lattice reconstructions 
 in which four hexagons were transformed into two pentagons and two 
 heptagons [ an sw(5577 ) defect ] ( figure 5 ) . ethanol bound preferentially to the defect - free 
 surface : its adsorption energies on the defect sites ( 6.7 
 to 7.2 kcal / mol ) were less negative than those for the perfect 
 surface ( 8.7 kcal / mol ) . the experiments 
 indicated that clustering played a significant role in the adsorption 
 of ethanol , acetonitrile , and acetone to fluorographene / fluorographite 
 ( cf . our calculations revealed the formation 
 of cyclical planar ethanol clusters lying flat on the fluorographene 
 surface ( figure 5 ) . the adsorption energies of ethanol dimers , tetramers , and hexamers 
 on fluorographene were 10.7 kcal / mol ,  11.2 kcal / mol , 
 and 13.1 kcal / mol per molecule , respectively , and were lower 
 than the adsorption energy of single molecules ( 8.7 kcal / mol ) . this strongly suggests that ethanol forms clusters on fluorographene 
 surfaces and that the measured adsorption enthalpies corresponded 
 to the binding of ethanol clusters . the adsorption energy of a single ethanol molecule on fluorographene 
 ( 8.7 kcal / mol ) was lower than on graphene ( 7.7 kcal / mol ) . 
 on the other hand , the adsorption energies of ethanol clusters on 
 fluorographene were higher than on graphene ( 11.2 kcal / mol 
 on fluorographene compared to 15.6 kcal / mol for ( etoh)4 on graphene ) . this different 
 adsorption behavior of molecules and clusters is probably due to competition 
 between h - bonding in the cyclic ethanol clusters ( as occurs on the 
 graphene surface ) and possible h - bonding between the oh group of an 
 isolated ethanol molecule and the f atoms of fluorographene : the calculated 
 o hf h - bond was 
 3.2  for a single ethanol molecule positioned on a fluorographene 
 surface such that its oh moiety was situated in the middle 
 of an f - triangle ( figure 5 ) . the calculations indicate that acetonitrile also 
 formed clusters on the fluorographene surface : the calculated adsorption 
 energy for a single acetonitrile molecule ( 7.8 kcal / mol ) was 
 substantially less negative than those for acetonitrile dimers ( 10.6 
 kcal / mol per molecule ) or trimers ( 10.9 kcal / mol per molecule ) . the flat antiparallel adsorption geometries predicted for acetonitrile 
 clusters on the fluorographene surface ( see figure 6 for an image of the trimer ) were very similar 
 to those identified for free clusters . hn hydrogen bonding within the acetonitrile 
 clusters appeared to be weak given the 
 calculated d(c f ) distances 
 for the putative c  hf bonds to the fluorinated 
 surface ranged between 3.2 and 3.5  ( figure 6 ) . top ( top ) and side ( bottom ) views of the adsorption 
 geometries 
 of acetonitrile ( left ) and acetone ( right ) trimers on fluorographene . 
 selected weak bonds between molecules in clusters ( top ) and between 
 molecules and surfaces ( bottom ) are highlighted by red dotted lines . the clustering of acetone 
 on fluorographene appeared to be less 
 favorable than ethanol and acetonitrile because the adsorption energies 
 of the acetone dimer ( 10.9 kcal / mol ) and trimer ( 11.5 
 kcal / mol ) were comparable to that for a single acetone molecule ( 10.7 
 kcal / mol ) . the planar acetone clusters that were predicted to form 
 on the fluorographene surface ( figure 6 ) do not adopt the typical cyclical structures of free 
 acetone clusters . ho = c hydrogen bonding ( figure 6 , d(o  c ) = 3.3  ) observed on the fluorographene has also 
 been observed in free acetone clusters . the 
 difference in behavior of the ethanol , acetonitrile , and acetone 
 molecules and other two adsorbates ( as indicated by experiment ) motivated 
 us to perform additional calculations with ethyl acetate . we performed 
 periodic calculations and evaluated the overall energy balance for 
 the creation and adsorption of selected ethanol and ethyl acetate 
 clusters . the thermodynamic cycle shown in the scheme 1 explained the different behavior of ethanol 
 and ethyl acetate on the surface , because the energy of clustering 
 e of ethanol molecules over the surface is 
 negative ( 2.5 kcal / mol ) favoring formation of clusters , while 
 the energy of clustering of ethyl acetate over the surface is close 
 to zero ( 0.3 kcal / mol ) . e for the process of 
 clustering is of order of 1 kcal / mol per molecule ( e.g. , h  e corrections for ethanol 
 clusters up to pentamer ranged between 0.91.4 kcal / mol per 
 molecule ) ; therefore , enthalpy of clustering 
 of ethyl acetate dimer from monomers on the surface will be positive , 
 i.e. , disfavoring formation of surface clusters . all energies ( in kcal / mol ) 
 are normalized to one ethanol ( ethyl acetate ) molecule . the clustering on the fluorographene surface complicates 
 direct comparisons of the measured adsorption enthalpies for the five 
 molecular probes ( table 1 ) to those obtained from the calculations ( table 5 ) . in fact , direct comparisons are only really 
 justifiable for dichloromethane and ethyl acetate . we therefore corrected 
 the adsorption enthalpies calculated for acetate , acetonitrile , and 
 ethanol to account for the effects of clustering , as discussed in 
 the preceding section . we also corrected the calculated h values for adsorption to fluorographene ( table 5 ) using the correction terms 
 calculated for fluorographite surface adsorption ( table s2 ) to enable meaningful comparison of the experimental 
 and calculated quantities . a comparison of the experimental and modified 
 computational results is presented in figure 7 . experimental ( table 1 , including error bar ) and calculated adsorption 
 enthalpies ( h ) . calculated values corresponding 
 to molecular adsorption 
 ( squares ) were corrected for the effect of clustering ( triangles ) . the calculated enthalpies are based on energies obtained 
 with the optb86b - vdw density functional . all of the dft methods systematically underestimated the 
 strength 
 of molecule / cluster binding to the fluorographite surface , i.e. , the 
 calculated h values were always higher than 
 the experimental values . the best accuracy was achieved with the optb86-vdw 
 functional because its h values were closer 
 to the experimental results than those obtained with any other method . 
 for clarity , only results obtained with this functional are shown 
 in figure 7 . the inclusion 
 of corrections for clustering always shifted the calculated adsorption 
 enthalpies toward the experimental values . for dichloromethane and 
 ethyl acetate , dft methods that include dispersion corrections based 
 on nonlocal correlation functional ( i.e. , optb86b - vdw , vdw - df , and 
 vdw - df2 ) gave adsorption enthalpies that were closer to experiment 
 than those that use corrections based on atom - centered empirical functions 
 ( dft - dx methods ) . it has many possible 
 causes , ranging from the precise implementations of the empirically 
 corrected dft methods to the correct inclusion of many - body effects , and warrants further investigation . the 
 langmuir adsorption model enabled enumeration of both the adsorption 
 enthalpies and entropies . the entropies ranged from 19 cal / molk 
 for dichloromethane to 36 cal / molk for ethanol . the overall 
 trends in the adsorption entropies mirrored those for the adsorption 
 enthalpies ( figure 1 ) . it has previously been shown that the adsorption entropy of physisorbed 
 molecules is surface - independent , being governed by the temperature 
 and the gas phase entropy of the adsorbate . this might explain the rather good agreement between the measured 
 adsorption energies and those calculated for the finite fluorographene 
 models ( table 1 and table 4 ) , although it is 
 important to bear the effects of the experimentally observed clustering . the entropy loss upon adsorption corresponded to around 40% of the 
 total gas phase entropy on average ( table s3 in the supporting information ) , which is consistent with previously 
 published data . a restriction of translational 
 and rotational degrees of freedom of the adsorbed molecule was responsible 
 for the entropy loss ( table s3 ) . it should 
 be noted that the discussed mirroring of entropies might indicate 
 that stronger binding leads to larger entropy loss due to larger restriction 
 of the probe conformational freedom on the surface . we measured adsorption enthalpies of acetone , 
 acetonitrile , dichloromethane , 
 ethanol , and ethyl acetate on fluorographite by inverse gas chromatography 
 at surface coverage levels ranging from 2 ( 0.2 for dichloromethane ) 
 to 20% . plots of the resulting isosteric adsorption enthalpies revealed 
 that acetone , acetonitrile , and ethanol cluster on the fluorographite / fluorographene 
 surface . the calculated saturated adsorption enthalpies 
 on fluorographene ranged from 6.9 kcal / mol for dichloromethane 
 to 12.8 kcal / mol for ethanol and were 12 kcal / mol 
 lower than those previously determined for graphene . computational 
 investigations provided deeper insights into the strength and nature 
 of adsorbate - fluorographene / fluorographite binding . finite size models 
 amenable to study using reference theoretical methods were a bit too 
 small for reliable estimation of interaction energies but were useful 
 in evaluating the accuracy of the adsorption energies calculated with 
 various dft methods because they permitted benchmarking against ccsd(t ) 
 and mp2.5 results . these benchmarking studies showed that dispersion 
 corrected dft functionals performed well and can be safely used for 
 relative comparisons of adsorption energies . the finite size models 
 also provided information on enthalpy corrections , which were rather 
 modest , ranging from 1.3 to 1.5 kcal / mol . despite the good performance 
 of the dispersion - corrected functionals in the benchmarking study , 
 nonlocal vdw dft functionals ( particularly optb86b - vdw ) achieved the 
 best agreement with experimental data when using a periodic model 
 of fluorographene . computational investigations using these functionals 
 revealed that we did not detect binding to high - energy sites in the 
 igc experiments because binding to these structural features , which 
 is typically very energetically favorable in layered materials , was 
 either only slightly ( by 1020% for steps ) more favorable or 
 less favorable ( in the case of edges ) than binding to the surface . the adsorption enthalpies were largely controlled by the interaction 
 energies , which were dominated by london dispersive forces . the clustering 
 of ethanol , acetonitrile , and acetone on the fluorographene / fluorographite 
 surface was explained by a delicate interplay between intracluster 
 and cluster - surface bonding . finally , we estimated adsorption entropies 
 for the different adsorbates , which ranged from 19 cal / molk 
 for dichloromethane to 36 cal / molk for ethanol and corresponded 
 to a loss of 40% of the gas phase entropy upon adsorption . these results indicate that calculations on finite size models are 
 adequate for estimating adsorption entropies on fluorographene surfaces . all measurements were conducted using 
 an sms igc - sea 2000 instrument 
 ( surface measurement systems ltd . , uk ) in a silanized column ( 3 mm 
 diameter and 30 cm long ) filled with a 23.9 mg sample of graphite 
 fluoride ( sigma  aldrich ) . before each measurement , the sample 
 was washed at 80 c using he as the carrier gas at a flow rate 
 of 10 sccm . the used graphite 
 fluoride crystals have a laminar morphology , and their surfaces are 
 dominated by exposed fluorographene planes with a small proportion 
 of edges and steps . measurements were carried out with acetone ( merck , 
 lichrosolv , for hplc , 99.8% ) , acetonitrile ( lach : ner , hplc supergradient , 
 min . 99.9% ) , dichloromethane ( merck , for lc lichrosolv , 99.9% ) , 
 ethanol ( merck , gradient grade for lc lichrosolv , 99.9% ) , 
 and ethyl acetate ( lach : ner , hplc , min . primary chromatograms 
 were recorded at temperatures from 303 to 363 k using he as the carrier 
 gas at the flow rate of 10 sccm . the column temperature was controlled 
 by the instrument oven with declared stability of 0.1 c . partial pressures of adsorbates were calculated from the primary chromatograms , 
 i.e. , peak maxima , using instrument calibration and cirrus plus software 
 advanced version 1.2.1.2 ( surface measurement systems ltd . , uk ) . the 
 partial pressures of individual adsorbates were measured at the given 
 targeted surface coverage i as 
 a function of temperature . the measurements were repeated for various 
 target surface coverage i , which 
 ranged from 2% to 20% of monolayer . the saturated ( 40 c ) adsorbate 
 vapors were injected into the column , and the injection time was set 
 up to reach the targeted surface coverage . the required injection 
 time was calculated from the targeted surface coverage , known surface 
 area ( 236.9 m / g ) of the material , adsorbate vapor tension at 40 c , and adsorbate cross sectional 
 area using cirrus control software advanced version 1.3.3 ( surface 
 measurement systems ltd . , uk ) . the low surface coverage 
 values considered in the experiments permitted application of the 
 langmuir adsorption model . this assumption leads to the equation , which we used directly to fit the isosteric 
 enthalpy of adsorption hi  hi and the entropy of adsorption si  si ( figure s3 ) for 
 each considered target surface coverage i . the partial pressure of the adsorbate , the standard pressure 
 of 760 torr , the universal gas constant , and the column temperature 
 are denoted as p , p , r , and t , respectively . the saturated 
 adsorption enthalpy h  h was obtained as the mean value   of n particular enthalpies 
 obtained by fitting . the error in the saturated enthalpy , h , was determined as the maximum individual hi value . the 
 adsorption energy , 
 e , was calculated as the difference between 
 the energy of the most favorable configuration of the complex ( comprising 
 one molecule / cluster on fluorographene ) and the sum of the energies 
 of the optimized isolated species ( fluorographene and the molecule 
 in vacuum ) . in contrast , the interaction energy , ei , was calculated on the basis of each separated fragment 
 in the geometry of the complex . the enthalpy of adsorption , h , was calculated by adding the zero - point energy ( e0 ) and the thermal ( et ) and enthalpic ( eh ) corrections to the adsorption energy , i.e. , h = e + e0 + et + eh . the corrections e0 , et , and eh were evaluated for the molecule on a finite 
 model of fluorographene , fully fluorinated tetracosahydrocoronene 
 ( c24f36 ) , i.e. , h  e + ( h  e ) . for this purpose , geometry optimizations and frequency calculations 
 were performed , and partition functions and thermochemical data ( at 
 313.15 k and 101.325 kpa ) were obtained . we used the b97d functional and cc - pvtz basis sets for all elements and 
 the gaussian09 package for calculations 
 on c24f36 . reference wave function - based 
 calculations of interaction energies ei were performed with a small finite model of fluorographene , 
 perfluorohexamethylcyclohexane ( c12f24 ) . accurate 
 and computationally demanding ccsd(t)/complete basis set limit ( cbs ) 
 interaction energies were determined using the formulahere , the ccsd(t ) correction term was determined 
 as the difference between the ccsd(t ) and mp2 interaction energies 
 with the 6 - 31g**(0.25,0.15 ) basis set . the basis set 6 - 31g**(0.25,0.15 ) 
 is a modified version of the 6 - 31 g * * basis set in which the exponential 
 parameters of the polarization functions were altered from their original 
 values to 0.25 ( c , n , f , o atoms ) and 0.15 ( h atom ) . the mp2/cbs interaction 
 energies were determined by summing the hartree  fock and mp2 
 correlation energy components , ei = ei + ei , both of which were extrapolated 
 from the aug - cc - pvdz ( adz ) and aug - cc - pvtz ( atz ) basis sets . alternative calculations 
 were performed using the mp2-f12 method 
 and the cc - pvdz - f12 basis set to evaluate interaction energies . in 
 these calculations , the core electrons were frozen , and interaction 
 energies were corrected for the basis set superposition error ( bsse ) . the mp2.5/cbs interaction energies obtained 
 for both the previously 
 mentioned finite models were calculated using the following extrapolation 
 equation the mp2.5 correction term was computed as a difference between 
 the mp3 and mp2 interaction energies with the 6 - 31g**(0.25,0.15 ) basis 
 set . as an alternative to the cbs limit , explicitly correlated mp2-f12 
 was used without any extrapolation . all 
 reference calculations were performed in turbomole 6.6 under the cuby framework with the exception of the mp2.5 correction terms , which 
 were calculated in molcas 8.0 sapt 
 decomposition allows the interaction energy to be partitioned 
 into physically meaningful components . for this purpose , we used dft - sapt as implemented by hesselmann et al . in the molpro program package , and we collected sapt components into four terms corresponding 
 to electrostatics , exchange repulsion , induction , and dispersion : we used the lpbe0ac exchange - correlation 
 potential for monomer 
 calculations and the cc - pvtz basis set . dft calculations on periodic fluorographene and its finite models 
 were performed using the projector - augmented wave ( paw ) method in 
 the vienna ab initio simulation package ( vasp , version 5.3.5 ) . empirically corrected density functionals ( pbe - d2 , pbe - d3 , pbe - ts , pbe - ts - scs ) and 
 nonlocal correlation functionals ( vdw - df , vdw - df2 , optb86b - vdw ) that approximately account for nonlocal dispersion interactions 
 were employed . the fluorographene sheet was modeled using a 5  
 5 supercell ( c50f50molecule ) 
 or 7  7 supercell ( c98f98cluster ) 
 with a chair structure , which has been shown to be the most stable 
 fluorographene conformer . input geometrical 
 parameters for fluorographene were obtained by pbe optimization from 
 our previous works and were as follows : lattice 
 constant of a = 2.61  , c  c bond length 
 of 1.58  , c  f bond length of 1.38  , and c  c  c 
 bond angle of 110.9 deg . multilayer fluorographene structures were 
 modeled by fluorographene sheets with aa stacking ; an interlayer lattice constant of c = 6  
 was used as an input for optimization . for instance , the bilayer step 
 of fluorographene consists of two sheets in a 5  10 supercell 
 with the second sheet reduced to a 5  5 supercell and terminated 
 by 5 + 5 fluorine atoms ( 150 carbon and 160 fluorine atoms in total ) . as periodic boundary conditions were applied in all three dimensions , 
 a periodicity of at least 20  in the out - of - plane direction 
 was imposed to minimize ( spurious ) interactions between adjacent layers . 
 the energy cutoff for the plane - wave ( pw ) expansion was set to 400 
 ev , a convergence criterion of 10 ev was used 
 in the scf cycle , and -point calculations were performed . the 
 positions of the atoms were relaxed using the conjugate gradient method 
 until the forces on each atom were below 1 mev /  . for the final 
 geometries of the 5  5 supercells , we recalculated all total 
 energies with the 3  3 k - point mesh and an 
 energy cutoff of 500 ev ( see table s4 for 
 the convergence test ) . finally , the interaction energies for 
 the c24f36 complexes were also calculated by 
 dft using gaussian orbitals . several 
 functionals ( b97d , b3lyp , and pbe ) and tzvpp basis sets were used in conjunction with the empirical 
 d3 dispersion correction ; the hybrid 
 meta m06 - 2x functional was used with 
 the cc - pvtz basis set .