Evaluation of the surface properties of 4-(Decyloxy) benzoic acid liquid crystal and its use in structural isomer separation

The selectivity of 4-(Decyloxy) benzoic acid (DBA) liquid crystal in surface adsorption region (303.2–328.2 K) and thermodynamic region (423.2 – 433.2 K) was investigated by inverse gas chromatography at infinite dilution (IGC-ID). The selectivity parameters of the structural isomer series named butyl acetate, butyl alcohol, and amyl alcohol series were calculated for the DBA using IGC-ID technique. Additionally, the surface properties including dispersive surface energy (gS D), free energy (DGA S), enthalpy (DHA S), and acidity-basicity constants were calculated with net retention volumes obtained from IGC-ID experiment results. When the DHA S and DGA S are constants, DBA surface was found to be an acidic character (KD/KA @ 0.89).

and tert-butyl alcohol (tBAl)) and amyl alcohol series (n-amyl alcohol (nAAl), iso-amyl alcohol (iAAl) and tert-amyl alcohol (tAAl)), and surface properties were investigated by IGC-ID technique. The selectivity of DBA was investigated in surface adsorption (303.2-328.2 K) and thermodynamic region (423.2-433.2 K). Additionally, the IGC-ID experiments were carried out to investigate the surface properties of DBA in relation to polar and nonpolar probes in surface adsorption region (303.2-328.2 K). Using the retention data obtained from IGC-ID experiments, the parameters used to determine the selectivity parameters and the surface properties were calculated.

The selectivity coefficient
To determine selectivity of materials, the net retention volumes (V N ) in surface adsorption region and thermodynamic region should be calculated as main data. For volatile polar and nonpolar solvents used in the analysis, the V N is closely related to the interaction of these solvents with the materials [32][33][34][35][36]. V N is calculated as follows: .T/T f (1) Here, t R and t A are retention times of volatile probes and air, respectively; Q is the volumetric flow rate; T and T f are the column and ambient temperature, respectively; J is James-Martin pressure correction factor.
The selectivity of the stationary phase contained in the chromatographic column can be calculated from the proportioning of the numerical difference between the retention times obtained from the IGC-ID experiments. Besides, selectivity coefficient can also be calculated from the ratio of V N calculated according to Eq. (1). The selectivity of stationary phase is determined depending on the size of the selectivity coefficient (a). This value is calculated as follows [37,38]: Here, t R1 and t R2 are the retention time of the first and second isomer from the isomer pairs, respectively; t A is the retention time of air; V N1 and V N2 are the net retention volume of the first and second isomer, respectively.

Surface properties
In recent years, IGC-ID is commonly used for examining the surface properties of the materials. The standard free energy (DG A º) value for the adsorption of volatile probes on the stationary phase is calculated with the help of the V N resulting from the interaction between probe and stationary phase [39][40][41]. DG A º is calculated as follows: Surface energy is an extremely important parameter in explaining the interaction between stationary phase and volatile probes. The greater the surface energy, the more interactions between molecules. On the contrary, when this energy is low, the interaction decreases. Surface energy of the stationary phase (g S ) can be calculated as a sum of dispersive energy (g S D ) generated by weak interactions on surface and specific energy (g S S ) generated by strong interactions on surface [42][43][44]: g S = g S S + g S D (4) g S D of stationary phase is determined when non-polar probes are injected at Henry's law region. This energy is due to dispersive interactions between molecules on the surface of the material and non-polar probe molecules [45]. g S D values can be calculated in the surface adsorption region according to the method proposed by Dorris-Gray [46] as follows: (5) Here, g S D is the dispersive energy of the surface (mj/m 2 ), DG [CH2] is the adsorption free energy of a methylene group, which is determined the slope of the plot between the number of alkanes versus RTlnV N values, N A is the Avogadro's number, a [CH2] is the molecular area of a methylene group (0.06 nm 2 ) and g [CH2] is the surface energy of a methylene group. g [CH2] values are calculated at any temperature (t o C) as follows [47]: Additionally, the method proposed by Schultz is widely used to calculate dispersive energy of surface [48]. This energy is calculated as follows: Here, a is the cross-sectional area of the probes, N A is the Avogadro's number, g L D is the dispersive energy of the probes. The a and g L D values were taken from the literature, and were listed in Table 1. g S D values of stationary phase can be calculated from the slope of plot between RTlnV N versus a(g L D ) 0.5 of non-polar probes. DG [CH2] values are calculated as follows [49]: Here, R is the universal gas constant; V N,n and V N,n+1 are the net retention volumes of two n-alkanes having n and n+1 carbon atoms, respectively. DG A S for the polar probes are calculated as follows: When the studies are carried out at different temperatures, DH A S and DS A S values can be calculated as follows [50]: The value of DH A S is linked with K A (donor or acidity group) and K D (acceptor or basicity group) parameters. This situation is due to the interactions that occur between probes and surfaces that do not have dispersive and entropic interactions. These values are calculated as follows [51,52]: Here, DN is an electron donor or acidity number and AN* is an electron acceptor or basicity number determined by Gutmann [53]. By calculating the value of DH A S for polar probes, a linear plot is drawn between -DH A S /AN * and DN/AN*. The values of K A and K D of solid materials can be obtained from the slope and intercept of the line, respectively. If K D /K A > 1, the surface is considered to be a basic; whereas, if K D /K A < 1, the surface is considered to be an acidic.

Materials and methods
All the properties of the chemicals used in this study are given in Table 2.
All measurements in IGC-ID studies were carried out using an Agilent Technologies HP-6890N device combined with thermal conductivity detector (TCD) (Hewlett-Packard, Palo Alto, CA, USA). The stainless-steel column (1/8" o.d., 2.10 mm i.d. 10 m) was purchased from Alltech Associates, Inc. (Chicago, IL, USA). Chromosorb W (AW-DMCS-treated, 80/100 mesh) was used as the support material and obtained from Sigma Aldrich. The DBA liquid crystal was dissolved in the Chloroform, and Chromosorb W was added slowly. A homogeneous mixture was obtained by continuous stirring in heating controlling water bath, and the LC was coated on support. Silane-treated glass wool used to plug the ends of the column was obtained from Alltech Associates Inc (Deerfield, IL, USA). The ends of the column were loosely plugged with silanized glass wool. After the column was cut to a size of 1 m and cleaned thoroughly, approximately 1.21 g of the prepared column interior material was filled. The total loading of DBA liquid crystal on the support was determined as 10.38% by weighing. Helium (He), which kept at a constant flow rate of 3.6 mL/min, was used as the mobile phase during the experiments. Probes and air were injected into the column with 1 mL and 10 mL Hamilton syringes, respectively. For infinite dilution, the probe (0.1 mL) was taken into the syringe and flushed into the air. Then, the retention times for probe and air were determined. At least four consecutive injections were made for each probe and air at each set of measurements.

Results and discussion
The main data (V N ) obtained from IGC-ID studies were calculated for all probes injected surface adsorption region (303.2-328.2 K) and thermodynamic region (423.2-433.2 K) according to Eq. (1). The retention diagrams of DBA used as separator stationary phase in two regions were given in Figure 1 and 2, respectively. The "a" values for nBAc/iBAc, nBAc/tBAc, nBAl/iBAl, nBAl/tBAl, nAAl/iAAl, and nAAl/tAAl were obtained using their V N in two regions. "a" values calculated according to Eq. (2) determined the separation ability of DBA. The higher the values of the separation factor calculated according to Eq. (2), the better the selectivity for isomers. Table 3 and 4 shows the calculated the values for the isomer pairs in two regions. Considering these values, it is seen that isomers are separated. Besides, it was observed that structural isomers were better separated in the surface adsorption region than in the thermodynamic region. The main data (V N ) obtained from IGC-ID studies was calculated for all probes between 303.2 and 328.2 K according to Eq. (1). Retention diagrams of non-polar and polar probes were given in Figure 3 and 4, respectively.
The surface energy of a solid materials depends on the chemical structure, physical properties, and composition. Interactions between molecules on a solid surface and polar or nonpolar probe molecules are due to long-and shortrange interactions known as weak interactions (London dispersive forces) and strong interactions (acid-base interactions). Dispersive surface energy occurs as a result of nonspecific interactions caused by the London dispersive forces known as weak or long-range interactions [54]. g S D can be calculated using IGC-ID technique based on well-known approaches for data analysis, such as Dorris-Gray (Eq. (5)) and Schultz (Eq. (7)) methods. In these a S D calculations, homologous alkane vapor series are used in infinite dilution, resulting in a single numerical a S D value. DG A for the all probes were calculated from the Schultz method using Eq. (7) in the surface adsorption region (303.2-328.2 K). A plot of RTlnV N versus a(g L D ) 0.5 for all probes was plotted at 303.2 K in Figure 5. From Eqs. (5) and (7), g S D of DBA was calculated using Schultz and Dorris-Gray methods. The results obtained from studies were listed in Table 5.
It is showed that the value determined for g S D of DBA have different ranges from 47.51-44.06 (Schultz method) to 47.74-46.19 mj/m 2 (Dorris-Gray method). Besides, it is observed that the g S D values calculated by Dorris-Gray method are higher than those obtained from the Schultz method. The g S D values obtained from the Schultz method decrease faster than the g S D values obtained by the Dorris-Gray method with increasing temperature. The results obtained for the Schultz and Dorris-Gray method at surface adsorption region are close to each other, showing that these two methods are compatible and feasible. There is no study on DBA in the literature. A rough comparison can be made with reported LCs. In the literature, g S D values for LCs were ranged from 30 to 42 mj/m 2 in agreement with this study [50,55]. The values of -DG A S were calculated by the numerical difference between the calculated value of RTlnV N and that which was obtained from Eq. (7) of the linear plot of the nonpolar reference line. The variation of DG A S between DBA and the polar probes for the studied temperatures is given in Table 6. Regarding the Table 6, it was seen that the temperature did not change the DG A S values much. DH A S values were calculated for polar probes and the results were given in Table  7. The DH A S values were calculated as the degree of interaction between the DBA molecule surface and the polar probe        [56,57]. Considering the values of DH A S and DG A S for each polar probe, adsorption occurs exothermically and spontaneously for all studied temperatures. The specific intermolecular interactions are derived from the interaction between the polar probe and the Lewis acidic-basic sites on surface [58][59][60].  A plot of -DH A S /AN* versus DN/AN* was plotted by K A as the slope and K D as the intercept using Eq. (11), and it is shown in Figure 6. The character of DBA surface was determined by the ratio of K D /K A . The obtained K A and K D values were listed in Table 8. Due to the K D /K A value is lower than 1, DBA surface is an acidic character.

Conclusion
IGC-ID technique was used to investigate the separation of isomer series in surface adsorption (303.2-328.2 K) and thermodynamic region (423.2-433.2 K) and the surface properties of DBA in surface adsorption region. Considering the separation factors, it was determined that the DBA in IGC-ID technique can be used to separate the isomer series in surface adsorption and thermodynamic region. The values of g S D for DBA were determined to be 47.51-44.06 mj/m 2 using the Schultz method and 47.74-46.19 mj/m 2 using the Dorris-Gray method. g S D values from both calculation methods decrease linearly with the increase in temperature in the range from 303.2 to 328.2 K. The values of K A and K D were found to be 0.2134 and 0.1907, respectively. As shown that, the K D value is lower than the K A . In this case, it can be said that the DBA surface is an acidic character. The IGC-ID technique is very important in improving the quality of products for industrial fields, since isomers can be separated effectively and the surface energy of samples can be easily determined.

Acknowledgment
This research has been supported by Yildiz Technical University Scientific Research Projects Coordination Department. Project Number: FDK-2020-4071.