2.2.1. Particle Size

Particle size distribution was determined by a Malvern Mastersizer 2000 LF granulometer (Malvern InstrumentsTM). Measurements were performed in purified water after dispersing the solids by midst sonication for 30 s. The amount of sample added in each experiment was determined by the real-time laser obscuration degree indicated by "Mastersizer 2000" software. The optimal laser obscuration was delimited between 10–20%. Three replicates were performed for each sample, and statistical particle diameters (d10, d50, d90) were calculated together with the SPAN factor as an index of the amplitude of particle size distribution calculated according to Equation (1).

$$\text{SPAN} = \frac{\mathbf{d} \circ \mathbf{0} - \mathbf{d}\_{10}}{\mathbf{d}\_{50}} \tag{1}$$

#### 2.2.2. Cation Exchange Capacity

Cation exchange capacity (CEC) of PS9 and G30 was determined by dispersing 1 g of dry powder in 25 mL of tetramethyl ammonium bromide (TMAB) aqueous solution (1 M). The resultant dispersions were shaken in a Roller Mixer for 24 h and subsequently centrifuged (8000 rpm, 30 min) and filtered (0.45 μm pore size, HAWP-Millipore filter). Three replicates were performed for each sample together with two blanks of pure TMAB (1 M). The CEC determination is based on the exchange between bromine (in the TMAB solution) and cations present in the clay. The main exchangeable cations released from the clay mineral, once the Br exchanged, are K+, Na<sup>+</sup>, Mg2<sup>+</sup> and Ca2+. Cations released, present in the resultant solution, were determined by ICP-OES (Optima 8300 ICP-OES Spectrometer, Perkin Elmer, Waltham, MA, USA), and CEC was calculated as the sum of exchangeable cations, expressed in mEq/100 g of clay mineral.

## 2.2.3. Zeta Potential

Zeta-potential (ζ-potential) measurements determine the electrical potential difference between the stationary layer of fluid surrounding the solid particles and the bulk (electric double layer). Zeta potentials of different concentrations of PS9 and G30, suspended in Dulbecco's modified Eagle medium, supplemented with 10% fetal bovine serum, 200 IU/mL penicillin and 0.2 mg/mL streptomycin, were also measured. Afterwards, ζ-potentials of the aforementioned suspensions were determined by using an electrophoretic light scattering (ELS) Zetasizer Naso-ZTM (Malvern Instruments, Worcestershire, UK). Samples were placed inside a folded capillary zeta potential cell (DTS1061, Malvern Instruments). Three replicates were obtained for each sample, and analyses were done at 25 ± 0.5 ◦C. During the analyses, 20 points were collected for each replicate and results expressed in mV.
