Infrared spectra provided an overview of the chemical composition of the investigated silica and of the changes effected by the CVD treatment (Fig. 1A). The prominent peak at 1059 cm−1 is indicative of Si–O bonds. Interestingly, an overlapping peak was observed at 1025 cm−1 for RP-CVD plates that could be explained by the increase of the proportion of Si–O–Si bonds at the surface of silica particles [16]. Furthermore, the peaks present at 1273 cm−1 and 777 cm−1 in the spectra of RP-CVD are evidence of the Si-CH3 bonds. The latter peak is typically observed upon sol–gel synthesis of MTMS [17]. Additionally, the analysis of the region between 2700 and 3800 cm−1 highlighted the proportion of adsorbed water (3000‒3700 cm−1) and C–H bonds (2900 cm−1) (Fig. 1B).

ATR-FTIR spectra of NP-TLC, RP-Paraffin, RP-18 and RP-CVD silica gel plates (A from 600 cm−1 to 4000 and B zoomed range 2700–4000 cm−1)

For paraffin-coated silica plates (RP-Paraffin), the strong C–H stretching bands at 2925 cm−1 and 2855 cm−1 correspond to the CH2 and CH3 groups of the long aliphatic chains in paraffin oil. A broad O–H band is still present, confirming that the paraffin oil is only coating the silica particles without chemically converting the OH groups. For RP-18, sharper peaks are obtained for C–H stretching bands which correspond to the defined aliphatic chains that derivatized the silica. As a consequence of the grafting of C18 chains onto silica that covered approximately 35% of silanol groups [18], the proportion of free hydroxyl groups decreased, which was visible by a flattened O–H peak. Finally, for RP-CVD, a small but sharp peak at 2970 cm−1 is characteristic of the introduced CH3 moieties. The band of hydroxyl groups was practically undetectable, which mirrored the high hydrophobicity of the material.

In all measured spectra, a weak peak at 1735 cm−1 was present that did not correspond to any chemical bond found in siloxanes or alkanes. We assume that this peak can be assigned to the binder that is used in commercially available silica gel plates. This binder, typically poly(methyl methacrylate) (PMMA), contains ester moieties of which the C=O stretching vibration would agree with the observed band.

Sorption isotherms provide information about the capability of the material to retain and release water. This is useful to assess the accessibility and number of hydroxyl groups in materials sensitive towards moisture [19,20,21,22]. The dynamic vapor sorption (DVS) isotherms of hydrophobized silica (RP-Paraffin, RP-18 and RP-CVD) and untreated silica are shown in Fig. 2. Obviously, the more hydrophobic plates—RP-CVD and RP-18—adsorbed very little water even at 100% RH, namely 3.3 wt% and 8.1 wt%, respectively. This indicated a low number of accessible hydroxyl groups. In contrast, RP-Paraffin exhibited a surprisingly large increase from 7 wt% to 50 wt% when relative humidity increased from 60% RH to 100% RH. Thus, a considerable number of hydroxyl groups remained accessible after the simple physical impregnation with paraffin. The highest amount of water was adsorbed by the untreated silica particles from NP-TLC, which retained more than 60 wt% water even at a relative humidity of 80%. Also, the hysteresis was more pronounced for the untreated plates. Considering the ready adsorption of water to normal phase silica, it is evident that humidity can have a considerable effect on a planar separation system. It should be validated and adjusted as a parameter during optimization and needs to be controlled for reproducible results.

Dynamic water vapor sorption (S) and desorption (DS) of aliquots scratched from NP-TLC, RP-Paraffin, RP-18, and RP-CVD silica gel plates starting from dried state (0% RH). All points were measured after equilibration between atmosphere and the aliquot to a constant weight

Determination of the water contact angle provides a direct measure of the hydrophobicity of a surface towards liquid water. Interestingly, a very short CVD treatment time, about 1 h, already resulted in a very hydrophobic material that showed strong water repellence (Fig. 3). In contrast, water did not even form a droplet on RP-Paraffin, RP-18, and NP-TLC but was absorbed instantaneously; it was therefore not possible to measure a contact angle on these surfaces. While this is in agreement with the intuitive understanding of the “hydrophilic” NP-TLC and incompletely covered RP-Paraffin material, such a clear affinity to liquid water was not obvious for “hydrophobic” RP-18 plates. This demonstrates the decisive effect of a comparatively small number of residual hydroxyl groups.

Water contact angles versus duration of the CVD treatment

Capillary action is the driving force of thin-layer chromatography. Its magnitude is described by Jurin’s law (Eq. 1), which relates the interaction between the liquid and a tube wall to the cosine of the contact angle. For the plates that could not sustain a water droplet, the contact angle is equivalent to zero, and the resulting cosine is 1. For contact angles above 90°, a negative cosine is obtained: thus, capillary flow will not happen spontaneously. Pure water can therefore not be used as eluent of the hydrophobized RP-CVD plates. By analogy to the other plates in the sample set, compatibility with pure water was achieved by reducing the extent of the treatment to retain non-reacted hydroxyl groups. The current, simple treatment setup had the disadvantage that it is not capable of precisely dosing the amount of reagent. It also gives little control over equilibration times during the initial saturation of the treatment chamber. The use of pure water was therefore not further investigated; instead, compatible eluents were prepared by mixing water with appropriate organic solvents.

The migration time for a given distance was recorded on RP-CVD-plates with several pure solvents to determine their elution constants, κ (Eq. 2; Fig. 4). In general, the constants followed the same order and were typically about 1/3 of the elution constants on regular normal-phase silica [15]. In addition, it was tested if mixtures of water with a miscible solvent can achieve capillary flow. Indeed, mixtures of water and methanol showed a flow on the RP-CVD plates, and the determined κ-values increased linearly with the percentage of methanol. By extrapolation, at least 59.5% methanol is required to achieve flow; this agrees with the minimal elution constant of 0.049 cm2/min observed for 60% methanol.

Elution constants κ on RP-CVD plates of several solvents (MeOH‒methanol, THF‒tetrahydrofuran) and various mixtures of water and methanol. The elution duration was measured for a fixed elution distance of 2.5 cm (except H2O‒MeOH [4:6, V/V], which was stopped at 1.4 cm after 40 min of elution)

As established with methanol, water can be used as eluent on RP-CVD plates as a component of a binary solvent system. We selected acetone as eluent in the test systems with parabens, as it had the highest elution constant of the tested solvents and is miscible with water in any ratio. Wettability remained a major issue since it did not allow the use of eluent acetone‒water mixtures with proportions of water above 45% (V/V).

Parabens (esters of para-hydroxybenzoic acid) are convenient test analytes, since they can be detected directly by fluorescence quenching or UV densitometry, and they are available as esters of various alcohols. We used a set with increasing lipophilicity, i.e., methyl-, ethyl-, propyl-, isobutyl-, and butylparaben, that comprised two butyl isomers to test for regioselectivity in the separation. The separation of parabens on RP-18 plates is well described, as mentioned in the introduction. We used this test set to compare the performances of RP-Paraffin, RP-18 and RP-CVD plates.

The best separations of parabens in terms of resolution were obtained for solvent mixtures containing 35‒55% acetone, depending on the plate. It was not possible to place all the peaks in the RF range that theoretically gives the highest resolution (0.2–0.5, [9]), or to achieve more comparable separations on these different stationary phases. All plates were developed to a solvent–front distance of 70 mm. Separations took a long time on all plates (see inserts in Fig. 5). For RP-CVD, this was expected; for the other two types of plates, a contributing factor might lie in the viscosity of the eluent mixture, which passes through its maximum in this compositional range. Also, acetone‒water mixtures might provoke a strong swelling of the PMMA-based binder [11]. A possible yet untested remedy to reduce these elution times could be the addition of 3% NaCl (w/V) in the mobile phase [11].

Left part: chromatograms on RP-Paraffin, RP-18 and RP-CVD plates visualized by fluorescence quenching at 254 nm. The development times were determined in triplicate. From left to right, single parabens and their mixture were applied. Right parts: densitograms of the paraben mixture acquired at 254 nm

Plate height (Fig. 6; Eqs. 3 and 4) and resolution (Fig. 7; Eq. 5) are two parameters that define the efficiency of the separation. They allow a direct comparison between the RP plates (see Table 1 in the Supplementary Information for the determined values). Small plate heights mean more efficient chromatographic systems, sharper peaks and stationary phases with the potential for a good separation. Resolution, in addition, takes the distance between peaks into account. Here, higher values indicate a better separation of the compounds.

Experimental (hollow dots) and mean (filled dots) values of plate heights (H) of methyl- (MP), ethyl- (EP), propyl- (PP), isobutyl- (IBP), and butylparaben (BP), comparing the performances of RP-Paraffin, RP-18, and RP-CVD plates

Experimental (hollow dots) and mean (filled dots) values of resolution (Rs) of adjacent peaks between methyl- and ethylparaben (MP-EP), ethyl- and propylparaben (EP-PP), propyl- and iso-butylparaben (PP-IBP), and iso-butyl- and n-butylparaben (IBP-BP), comparing the performances of RP-Paraffin, RP-18, and RP-CVD plates

The plate heights found for RP-CVD plates were quite consistent for all analytes, with averages from 60 to 80 µm, which are in agreement with the values reported for unmodified TLC plates [9]. Practically, these consistent plate numbers mean that the equilibration between stationary and mobile phase is of similar speed for all fives analytes. The prepared stationary phase is suitable for chromatography and has the potential to yield narrower peaks than the other modified stationary phases. Also, the paraffin-coated plates showed consistent plate heights, albeit somewhat higher than the RP-CVD plates, with averages from 70 to 85 µm. RP-CVD plates should therefore give typically narrower peaks than paraffin coated ones. RP-18 plates gave very good plate heights for all parabens with propyl and butyl substituents—as low as those found for the RP-CVD plates—but showed distinctively higher plates (broader peaks) with higher variability for ethyl and methyl paraben. That the plate heights of RP-18 and RP-CVD plates are similar indicates that the structure and sizes of the particles and pores of the stationary phase were not deteriorated by the CVD treatment.

Surprisingly, the best values for resolution were observed for paraffin coated plates with values around 1. RP-18 achieved values around 0.75, and RP-CVD gave the lowest values at about 0.55. All systems struggled to separate iso-butyl and n-butyl paraben. For RP-Paraffin and RP-CVD, a slight separation is visible for the single standards but not in the mixture. For this regioselective separation to occur, the stationary phase must comprise rigidly positioned points of retentive interaction. This was apparently not the case for the RP-18 material while the other two modified plates still allowed some spatially characteristic interaction—most likely residual OH for RP-Paraffin and methyl for RP-CVD.

RP-CVD plates thus showed the potential to be a useful stationary phase for planar chromatography with a separation efficiency (plate height) equivalent to commercially prepared hydrophobic plates. The highest obstacle to make full use of the plates’ potential is in the choice of solvent to achieve reasonably short separation times.