2.1.2. SEM–EDX

SEM–EDX of THEOS–chitosan and MeTHEOS–chitosan films are presented in Figure 1. The 10,000× amplification illustrates the films' characteristics, showing that they are flexible, thin, and transparent and have no evident imperfections. The elements observed according to EDX analysis are carbon, nitrogen, oxygen, and silicon, in accordance with the hybrid composition.

#### 2.1.3. Thermal Stability of the Hybrids

The thermal stability of the hybrid films was studied. The films were exposed to different temperatures, from room temperature to 700 ◦C, and FTIR–ATR spectra were collected to determine any structural changes. Comparative spectra obtained at 25 ◦C and 350 ◦C are presented (Figure 2). As can be observed, the films are thermally stable until 350 ◦C. In the case of MeTHEOS–chitosan, the fragment –SiCH3 is removed around 500 ◦C.

#### 2.1.4. Structural Characterization by Solid State NMR

A more detailed structural characterization of the hybrid THEOS–chitosan and MeTHEOS– chitosan films was conducted by solid state 13C-NMR (CPMAS) and 29Si-NMR (MAS) and reported recently [32]. The structural analysis of chitosan by 13C-NMR was taken as a reference to point out that the C6, bonded to a terminal hydroxy group, is suggested to be the condensation site of THEOS and MeTHEOS, once the C6 is the most sterically favored for this purpose. The most evident change in the chemical environment corresponds to the region of the chemical shift of C6 (60.71 ppm), exhibiting 2.24 ppm of difference with respect to the C6 of the chitosan film (58.47 ppm). The 29Si MAS and CPMAS spectra of the THEOS and MeTHEOS–chitosan films were collected for complementary structural analysis (Figures S2–S5).

#### *2.2. THEOS–Chitosan and MeTHEOS–Chitosan Formulations Applied to Siliceous and Calcareous Historical Building Materials*

In consideration of the expected compatibility with both siliceous and calcareous materials, the application of the hybrid silane–chitosan in the field of the conservation of historical building stones is suggested. A key aspect is the water base application, which forgoes the use of organic solvents. In order to obtain data regarding the performance of the consolidant and hydrophobic formulations, different characterization methods were used, including FTIR, SEM–EDX, hardness determination, water absorption, and measurement of the contact angle (dynamic and static) and surface free energy. Different formulations in terms of the concentration of silane–chitosan were applied in the three selected materials (caliche, Sostenes, and Compañía). The different percentages of chitosan deacetylation were considered a variable to take into consideration in different determinations (i.e., hardness and contact angle measurements).

The FTIR–ATR spectra of the three studied materials were obtained. Figure 3 illustrates the caliche without treatment (Figure 3a) and after the consolidation (Figure 3b) and hydrophobic treatment (Figure 3c). The IR spectrum shown is typical of a calcite (1550–1350 and 872 cm<sup>−</sup><sup>1</sup> corresponding to stretching and bending vibrations, respectively). The bands at 1052 cm<sup>−</sup><sup>1</sup> and 715 cm<sup>−</sup><sup>1</sup> can be assigned to Si–O stretching due to the presence of a small concentration of silicates. After the consolidation treatment, the most important modifications in the spectrum are an increase in the intensity and broadness of the band

at 1096 (Si–O–Si) and 722 cm<sup>−</sup>1, associated with the siloxane network; in the case of the sample treated with the hydrophobic formulation, a new small band at 1270 cm<sup>−</sup><sup>1</sup> appears, corresponding to the Si–CH3 fragment.

**Figure 1.** Characterization of the hybrid films. (**a**) MeTHEOS–chitosan film and Scanning electron microscopy (SEM) of the (**b**) tris(2-hydroxyethyl)methyl silane (MeTHEOS)–chitosan film; (**c**) EDX of the MeTHEOS–chitosan film; (**d**) THEOS– chitosan film and SEM of the THEOS–chitosan film (**e**); and (**f**) EDX of the THEOS–chitosan film.

**Figure 3.** The FTIR–ATR spectrum of the calcite (**a**) without treatment, (**b**) consolidated, and (**c**) hydrofugated.

In the FTIR–ATR spectra of the Compañía stone, in accordance with its mineralogical composition (see Materials and Methods), bands are displayed at 1000, 1096, and 790 cm<sup>−</sup>1, characteristic of cristobalite, while those at 1000, 790, and 742 cm<sup>−</sup><sup>1</sup> correspond to feldspars and quartz. The most significant modifications after the consolidation and hydrophobic treatment, as in the previous caliche sample, occurred in the region associated with the Si–O–Si network, where the band is more intense and broader (1000 cm<sup>−</sup>1) and the small band at 1272 cm<sup>−</sup><sup>1</sup> (–SiCH3 fragment) appears. Due the siliceous composition, Sostenes and Compañía stones present similar spectra as a result of the application of the formulations (Figures S6 and S7).

Figure 4 (Tables S1–S3), Figure 5 (Tables S4–S6), and Figure 6 (Tables S7–S9) present the results of SEM–EDX analysis of the stones before and after treatment. The order is caliche, Compañía, and then Sostenes.

In terms of the consolidation process, the aggregation of particles is evident because of the effect of the consolidant treatment. Nevertheless, the most important morphological changes are shown in caliche. In Compañía's sample, which is the least compact stone according to SEM and the one with a higher percentage of water absorption, the consolidation effect is not so evident likely due to the low quantity of added consolidant. However, as is discussed later on, the increment in hardness indicated a positive consolidation effect. On the other hand, in the Sostenes stone, the morphological change is evident; regarding the hydrophobic treatment, a coating is observed in the three stones, with an important reduction in the porosity compared with the untreated materials, though leaving the stone with enough porosity to "breathe", which is the final purpose of hydrophobic treatments in the stone conservation field.

Regarding EDX analysis, Sostenes and caliche stones display an increment in the carbon and nitrogen atomic percentage following treatment. A plausible interpretation is that the chitosan chains are exposed to the surface, not just in the case of the consolidant, but in the hydrophobic treatment (the methyl groups are surface oriented). Additionally, it is interesting to observe that the nitrogen atomic percentage is higher in consolidated Sostenes stone than in caliche, suggesting that the interaction between the consolidant and caliche possibly occurs via the free amine group. On the other hand, in Compañía stone, silicon is the element with a major atomic concentration on surface, probably suggesting, in accordance with SEM, that not enough consolidant formulation was added.

#### 2.2.1. Hardness Determination

The effectiveness of treatment in terms of the mechanical properties was determined by hardness measurements in stones consolidated using the THEOS–chitosan formulation and was performed by indentation with a Shore D durometer. Three variables that influence the hardness increase were considered: the applied formulation (as a function of the silane/chitosan ratio); the nature of the stone; and the percentage of deacetylation of the chitosan used in the formulation (%DDA). A statistical analysis was conducted to evaluate the effect of each variable (not included here). In a next step, the Shore D hardness values were transformed to the most common hardness scale, such as Vickers, Brinell, and finally Mohs, in order to compare the hardness data obtained with respect to reference values of well-studied materials based on the Mohs scale.

The formulations named 1, 2, and 3 (see Materials and Methods) were used in hardness determination. The hardness was measured at four points of the samples before and after treatment to characterize the hardness percentage increase. Interesting results were obtained for every formulation; however, the treatment that remarkably increased the mechanical properties of the stones was formulation 2, which contains chitosan with 66% DDA (Table 1).

**Figure 4.** SEM–EDX analysis for the (**a**) caliche sample without treatment, (**b**) consolidated caliche sample, and (**c**) hydrophobic treated caliche sample.

**Figure 5.** SEM–EDX analysis for the (**a**) Compañía sample without treatment, (**b**) consolidated Compañía sample, and (**c**) hydrophobic treated Compañía sample.

**Figure 6.** SEM–EDX analysis for the (**a**) Sostenes sample without treatment, (**b**) consolidated Sostenes sample, and (**c**) hydrophobic treated Sostenes sample.


**Table 1.** Shore D hardness and hardness increase (%) and the transformation to Vickers and Brinell scales (percentage of deacetylation of chitosan used in the formulation (%DDA) of 66).

> Tables 1 and 2 present illustrative data on the Shore D hardness determination and transformation, first to Vickers and Brinell scales, and then (Table 2) from the Brinell to Mohs scale. In any case, the hardness increase is evident, with some important variations, where the influence of the formulation (silane/chitosan ratio) seems to have a certain effect. However, it is also important to bear in mind the different stone compositions. In the case of caliche, the increase in hardness is quite similar, having a major effect on the siliceous materials.

**Table 2.** Hardness transformation from the Brinell to Mohs scale and % of hardness increase (66% DDA).


f,1 = hardness measured after the consolidation treatment and increase percentage.

The hardness values transformed to the Mohs scale and reported in Table 2 indicate a hardness increment of one unit in siliceous materials, and in the case of caliche (formulations 1 and 2), even 2 units. In general, the most important increase in hardness occurred in caliche. In terms of the Mohs scale, the hardness values from 5 to 7 obtained for the samples range between apatite to orthoclase and quartz. The hardness studies indicate that the samples treated with THEOS–chitosan displayed an important increase in the mechanical properties of the three materials.

#### 2.2.2. Water Absorption

Water absorption was tested using the Karsten tube technique, and measurements were taken before and after the application of the hydrophobic treatment (MeTHEOS– chitosan) on the stones (Table 3). The stone samples subjected to treatment present different mineral composition and water absorption values.

The penetration of water in the Compañía stone was quite high (51%) and was reduced to 7% with the hydrophobic treatment; such behavior makes sense due to its high pore diameter (macropores) in comparison with the other stones. Sostenes samples, that also possess a siliceous composition, had a water absorption value of 29% before treatment with a reduction to 10% as a result of the hydrophobic treatment. The calcareous stone

(caliche) from the archeological site with an initial water absorption value of 46% exhibited remarkable reduction to 23%.


**Table 3.** Water absorption percentage on untreated and hydrophobic formulation (MeTHEOS)-treated stones.

#### 2.2.3. Contact Angle Measurements

The evaluation of the hydrophobic formulation MeTHEOS–chitosan was studied by static and dynamic contact angle measurements using the same formulations 1, 2, and 3 and the % of DDA of 66. Because of the natural existence of defects on certain materials, as is the case of the stones studied in the current investigation, it has been suggested that a static water contact angle does not necessarily characterize the intrinsic water wettability [41]. Dynamic contact angle determination in the three stones is presented. The dynamic contact angle was obtained by the degree of hydrophobicity calculated by the hysteresis, representing the difference as θR (receding angle) − θA (advancing angle). The hysteresis values and the average of three measurements in different surface sections of the three stones are reported in Table 4. The dynamic angle measurements indicate that the surfaces of the three stones studied display water repellency.

**Table 4.** Dynamic contact angle. Three contact angle measurements were performed at different points of the stones.


1 θA = advancing angle. 2 θR = receding angle.

The static contact angle was measured in three mediums (water, diiodomethane, and formamide) to take into consideration the different contributions of polar and non-polar mediums. The information obtained in the three mediums was useful for calculating the surface free energy or free energy of hydrophobicity by using the Owens and Van Oss (acid–base) methods [42,43].

The results are presented in Table 5 (1, 2, and 3 correspond to the formulation applied).


**Table 5.** Static angle determinations for the treated stones.

The value obtained for static contact angle showed that hydrophobic properties were achieved after the application of the MeTHEOS–chitosan formulation. In terms of the static contact angle in water, all values are over 90◦, with caliche as an exception (formulation 1,89.1◦). Some authors consider that 90◦ demarcation can generally be applied to classify hydrophilic and hydrophobic behaviors; however, they consider contact angles closer to 90◦ to be relatively hydrophobic and lower contact angles to be relatively hydrophilic [41].

#### 2.2.4. Determination of the Surface Free Energy

In general terms, a sample with a low surface energy will cause poor wetting (a high contact angle). The reason for this is that the surface is not capable of forming strong bonds, so there is little energetic reward for the liquid to break bulk bonding in favor of interacting with the surface. On the contrary, a high surface energy will generally cause good wetting with a low contact angle. A surface will always try to minimize its energy. This can be done by adsorbing a material with a lower energy onto its surface [42,43]. The energy surface data are presented in Table 6.


**Table 6.** Surface free energy of the stones treated with MeTHEOS–chitosan using the Owens and Van Oss (acid–base) methods 1.

1 Surface energy units, mN/m.

The data interpretation reported in Table 6 is based on the criterium of a low surface energy value of 40 mN/m as a reference to consider a hydrophobic surface, although it is dependent on the model used; in the Owens model, the interval ranges from 49 to 3.16 mN/m, while in the Van Oss (acid–base), it ranges from 47 to 0 mN/m, so a lower value means a more hydrophobic surface [42,43]. According to the 40 mN/m reference

value, or either the Owens or Van Oss model, caliche (formulation 1) is the only sample considered not to be hydrophobic. Some stones have a very low surface energy value, in agreemen<sup>t</sup> with the static contact angle obtained; Compañía is the most hydrophobic, followed by Sostenes and, finally, caliche. Moreover, the energy surface data indicate that formulation 2, in some way, is the most appropriate in terms of the silane/chitosan ratio (1 g of THEOS and 10 mL of a 0.5% aqueous solution of chitosan, and 66% DDA).

#### **3. Materials and Methods**
