P.E. Vickers et al./ Carbon 38(2000)675-689 interactions, it is assumed that AG has additive dispersive RT In(N) values" for molecular probes adsorbed onto untreated (d)and acid-base(AB)contributions [21, 281 Degree of fibre treatment (% △G=G4+△GAB re a nu 4.42 tion of AG to AG of a material [29-35 8.13 7.24 approach uses the boiling points of the probe 13.8 1.1 [29]. If the AG, values of the n-alkanes are plotte = 5.0 14.1 their boiling points a straight line should be attained TMP 9.22 pecific acid-base interactions between a probe and the CHCI 3.79 04 5.09 3.85 stationary phase cause the retention volume of the probe to 4.19° deviate from the line due to the n-alkanes AG can be t-BuOH 18.7 EtAc 16.2 14.9 15.8 13.7 (△G-△Ga)=RTln(VN/ RT In(N) values calculated with RT in k mol and Nin where IN and VN.ref are the net retention volumes of the Determined using a flow-rate of 15 ml min polar probe and a hypothetical reference n-alkane having the same the same boiling point, respectively 3.2.1.I. Free energy of adsorption of n-alkanes. Fig 4 Fig 5 shows graphs of RT In(n) values plotted against shows a plot of RT In(N) values vs the number of carbon the boiling point of the probe molecules. Again the n- atoms per n-alkane for the homologous series of n-alkanes alkanes yield a straight line. The distance between the adsorbed onto the untreated and the 200% treated fibres. It specific probe markers and the reference line is a measure can be seen that a straight line is obtained when the of AG a the acid-base contribution to the free energy of n-alkane series are used as IGC probe molecules. The adsorption. For the untreated fibre, the markers corre- gradient of each plot is a measure of AG, the free sponding to the specific probes are lying close to the energy of adsorption per methylene dispersive interactions reference line, an indication of proportional to the square root of y he slight weak specific interactions with the solid surface except for decrease in the slope as a result of surface treatment is thus BuoH(see Table I for abbreviations) which undergoes an indication of a decrease of y- as a result of surface substantial acid-base interactions. Chloroform has its treatment marker lying on the reference line and its interaction is thus mainly via dispersive forces. In the case of the 200% 3.2. 1.2. Free energy of adsorption of polar probes and the DFT, the basic probes EtAc and THF and the amphoteric branched alkane. If the material under test and a molecu- l-Buoh exhibit significant deviations from the reference, lar probe interact via both dispersive and acid-base which indicates that the oxidised fibres are showing acidic 25 200% 2E Fig. 4. RT In(VN)vS number of carbons, ne.682 P.E. Vickers et al. / Carbon 38 (2000) 675 –689 Table 5 interactions, it is assumed that DG has additive dispersive a a RT ln(VN ) values for molecular probes adsorbed onto untreated (d) and acid–base (AB) contributions [21,28]: and treated carbon fibres at 508C d AB DG 5 DG 1 DG (3) Degree of fibre treatment (%) aaa Probes 0 25 50 100 200 There are a number of methods in calculating the contribu- AB tion of DG to DG of a material [29–35]. A simple C6 5.21 4.42 a a approach uses the boiling points of the probe molecules C7 9.63 8.13 7.24 8.89 6.65 C8 13.8 12.0 11.1 12.5 10.3 [29]. If the DG values of the n-alkanes are plotted against a C9 16.1 15.0 16.2 14.1 their boiling points a straight line should be attained. TMP 9.22 7.97 7.35 8.51 6.69 Specific acid–base interactions between a probe and the b CHCl 3.79 3.54 4.04 5.09 3.85 3 stationary phase cause the retention volume of the probe to CCl 4.30 2.84 2.60 4.19 2.40 b AB 4 deviate from the line due to the n-alkanes. DG can be a t-BuOH 11.1 16.8 18.7 16.9 computed by: EtAc 7.94 14.0 15.5 16.2 14.5 AB d THF 8.09 13.8 14.9 15.8 13.7 2 DG 5 2 (DG 2 DG ) 5 RT ln(V /V ) (4) a a a N N,ref a 21 RT ln(VN ) values calculated with RT in kJ mol and VN in 3 where V and V are the net retention volumes of the cm . N N,ref b Determined using a flow-rate of 15 ml min . 21 polar probe and a hypothetical reference n-alkane having the same the same boiling point, respectively. 3.2.1.1. Free energy of adsorption of n-alkanes. Fig. 4 Fig. 5 shows graphs of RT ln(V ) values plotted against N shows a plot of RT ln(V ) values vs. the number of carbon the boiling point of the probe molecules. Again the n- N atoms per n-alkane for the homologous series of n-alkanes alkanes yield a straight line. The distance between the adsorbed onto the untreated and the 200% treated fibres. It specific probe markers and the reference line is a measure AB can be seen that a straight line is obtained when the of DG , the acid–base contribution to the free energy of a n-alkane series are used as IGC probe molecules. The adsorption. For the untreated fibre, the markers corre- CH2 gradient of each plot is a measure of DG , the free sponding to the specific probes are lying close to the a energy of adsorption per methylene group, which is dispersive interactions reference line, an indication of d [27] proportional to the square root of g . The slight weak specific interactions with the solid surface except for s decrease in the slope as a result of surface treatment is thus t-BuOH (see Table 1 for abbreviations) which undergoes d an indication of a decrease of g as a result of surface substantial acid–base interactions. Chloroform has its s treatment. marker lying on the reference line and its interaction is thus mainly via dispersive forces. In the case of the 200% 3.2.1.2. Free energy of adsorption of polar probes and the DFT, the basic probes EtAc and THF and the amphoteric branched alkane. If the material under test and a molecu- t-BuOH exhibit significant deviations from the reference, lar probe interact via both dispersive and acid–base which indicates that the oxidised fibres are showing acidic Fig. 4. RT ln(V ) vs. number of carbons, n . N c