Comment to the paper: Effect of grinding time on fabricating a stable methylene blue/palygorskite hybrid nanocomposite, by Yuan Zhang, Wenbo Wang, Bin Mu, Qin Wang, Aiqin Wang, Powder Technology 280 (2015), 173–179

Comment to the paper: Effect of grinding time on fabricating a stable methylene

blue/palygorskite hybrid nanocomposite, by Yuan Zhang, Wenbo Wang, Bin Mu, Qin Wang,

Aiqin Wang, Powder Technology 280 (2015), 173-179.

Roberto Giustetto*

1_{Department of Earth Sciences, University of Turin, via Valperga Caluso 35, 10125 Torino (Italy)} 2_{NIS – Nanostructured Interfaces and Surfaces Centre, via Quarello 11, 10135 Torino (Italy)}

*corresponding author e-mail: roberto.giustetto@unito.it; tel. +39-011-6705122

Keywords: hybrid nanocomposite; palygorskite; grinding; heating; host/guest interaction.

In this paper, the Authors state that the control of grinding time should be considered a

key-point to promote formation of a stable hybrid composite resulting from fixation of methylene blue

(MB hereafter) on the surface and/or internal channels of palygorskite fibres (PAL hereafter). In

particular, as reported in the abstract, grinding would enhance the composite stability promoting

water removal and structural rearrangement of the host (variation of d110 spacing), thus favouring

formation of specific host/guest interactions responsible for stabilization.

Grinding is a fundamental step in the synthesis of hybrid ‘Maya Blue-inspired’

nano-composites, as it promotes the dissociation of the clay bundles into more or less isolated nanorods

[1]. However, though a certain interaction between the host and the guest is reputed to occur even

while grinding [2-4], all literature points to the fact that the real turning-point allowing formation of

a stable composite is heating. Such a step is fundamental to achieve stability: e.g., an unheated

PAL/indigo mixture is likely to be discolored when attacked with strong acids or alkali [5-8].

The Authors claim that grinding “… greatly influenced the removal of the water molecules”.

Such a statement is arguable: gradual water loss in PAL is caused by temperature rise or vacuum

conditions [9-13]. In particular, loss of zeolitic H2O from the tunnels and/or superficial grooves

activates the PAL framework and its capability of incorporating guest molecules, allowing

formation of specific bonds that stabilize the resulting composite. Recent studies [14-17] proved

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

that the magnitude of heating can even affect the nature and strength of the host/guest interactions,

depending on the kind and amount of water loss. In the commented paper, it should be noted that

the removal of zeolitic H2O from PAL – albeit incomplete – was indeed triggered by heating at 105

°C (a procedure adopted by the Authors). Previous grinding only contributed to increase the host

exposed surface, bringing the guest molecules closer to the PAL structure.

Furthermore, some evidences upon which the Authors base their considerations about dye

fixation in the host and formation of host/guest interactions tend to be misleading.

When dealing with X-ray diffraction (XRD), the Authors remark that the value of the d110

interplanar distances “… decrease sharply with increasing the grinding time from 10 to 20 min, and

then maintain at 10.53 Å after being ground for 20–30 min. To increase the grinding time from 45

to 60 min, the d

values of MB/PAL nanocomposites slightly increase to 10.56 Å.” In the Authors’

opinion, these variations should be ascribed to the grinding action affecting ‘…the micro-structure

around the tetrahedron and octahedron layers, but does not obviously impact the skeleton structure

of PAL crystal”. In particular, these micro-structural modifications should consist in the “…

removal of water molecules…” and formation of “… the intensified host/guest interaction between

PAL and MB molecules … as reported by the other authors [15,19]”.

These considerations are possibly overestimated. While loss of superficially adsorbed and

zeolitic H2O is likely to occur – but due to heating rather than grinding, as aptly evidenced by

Sanchez del Rìo et al. (2009 [18]; i.e., citation number ‘19’ of the commented paper) – XRD data

can provide no information about formation of host/guest interactions. The current literature on

PAL and related hybrid composites [14,17-19] shows that the basal (110) reflection at low angles is

mainly an indicator of the channel content. The same is valid also for zeolites [20-25] – which,

despite what the Authors claim, are not clay minerals. Possible variations in the PAL tunnels are

evidenced by fluctuations in the position and intensity of the d110 values – an occurrence correctly

signaled by the Authors. However, the assumption according to which these variations should be

related to MB insertion in the tunnels and strengthening of host/guest interactions seems rather

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

speculative. XRD analyses may provide a sharp and average picture of the composite crystal

structure, but render no finer details such as the existence of specific bonds between MB and PAL.

In addition, changes in the crystal structure should not be inferred by the behavior of a single

interplanar spacing at low 2θ angles. A more reliable – although preliminary – approach should

involve the computation of the unit cell of PAL and its comparison with those of MB/PAL hybrid

composites [17,18]. As well, the citation numbered ‘15’ seems inaccurate there: Giustetto et al.

(2005 [12]) indeed claimed that stable host/guest interactions exist in Maya Blue – but basing their

evidences on vibrational spectroscopies (FTIR and Raman), rather than XRD. Spectroscopic

techniques may indeed reveal the existence of specific bonds, by checking perturbations of

functional groups in the clay/dye composite with respect to its pristine counterparts.

In the commented paper, however, apart from hinting that the supposed interactions between

MB (in cationic form) and PAL should be triggered by loss of adsorbed and zeolitic H2O, the nature

of such bonds remains unclear. In FTIR spectra some modes are attributed to adsorbed and/or

zeolitic water (which, despite what the Authors claim, is not “…weakly attached to the Mg, Al-OH

by H-bonding”), but no reference is made to structural/Mg-coordinated OH

H2O is H-bound – which is barely mentioned in the TG analyses. No FTIR spectrum of pure MB is

reported, so that no changes can be evaluated before and after dye fixation. Variations of the 1654

cm

_{PAL mode [(OH)], important to check water loss [27-30] as well as interaction with the guest}

[12-13], are difficult to assess. In fact this band is superposed to an intense MB signal at 1601 cm

and the contributions of adsorbed and zeolitic H2O predominate over structural OH2. According to

the Authors, however, this latter kind of water is not lost until temperatures ≥ 259 °C and could thus

represent one of the most feasible candidates in interacting with the guest [12-17,31,32].

Alternatively, bonding between MB and octahedral cations [33-36] or edge silanol units of PAL [2]

may be considered.

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