*3.1. Limits of Solid Solutions*

Relatively similar sizes and shapes of the alanine and serine molecules (Figure 2a,b) would suggest isomorphic miscibility or, in other words, the formation of solid solutions in their mixtures.

**Figure 2.** Molecules of alanine (**a**) and serine (**b**).

This hypothesis was totally valid in the case of the L-threonine–L-*allo*-threonine diastereomers with the same chirality (L referring to Cα), where the authors observed the phenomenon of the continuous isomorphic miscibility of the components, which is only rarely found in chiral systems [10]. On the other hand, in the system of the L-threonine–D-threonine enantiomers [33] and the system of the D-threonine–L-*allo*-threonine diastereomers [34], both containing components with different chiralities, despite the identical molecular compositions and very close sizes and shapes of the molecules, the regions of solid solutions appeared to be rather limited. Three other investigated systems formed by the L-enantiomers of different amino acids—i.e., L-valine–L-isoleucine [15,16], L-valine–L-leucine [17], and L-isoleucine–L-leucine [18]—are characterized by the presence of non-equimolar discrete heterocompounds (V2I (valine/isoleucine = 2:1), V3L (valine/leucine = 3:1), and I3L (isoleucine/leucine = 3:1), respectively) and limited solid solutions in the vicinities of the heterocompounds and pure components.

As seen in Figure 3 from the X-ray patterns of the co-precipitated mixtures of L-ala and L-ser, in the L-alanine–L-serine system neither discrete heterocompounds nor solid solutions were found within the studied composition range. The X-ray patterns shown contain two sets of peaks, one corresponding to L-ala and the other to L-ser. The relative intensities of each peak set vary with the changes in the mixture composition, which proves that the mixtures studied are conglomerates or just mechanical mixtures.

**Figure 3.** X-ray patterns (2θ CuKα) of co-precipitated mixtures (mol. %): L-ser/L-ala = 0/100, 15/85, 25/75, 35/65, 50/50, 65/35, 75/25, 85/15, and 100/0.

In order to more precisely delineate the potential limits of the solid solutions in the L-alanine–L-serine system, several co-precipitated mixtures with compositions close to the pure components were prepared and investigated. The obtained X-ray patterns are shown in Figure 4. These experiments succeeded in revealing a limited isomorphic miscibility in the vicinity of the components. This is proved by the absence of the admixture phase peaks and the altered peak positions in the samples with L-ser/L-ala ratios of 7/93 and 93/7 compared to the corresponding positions of the L-ser and L-ala peaks. This is also reflected in the nature of changes in the respective calculated parameters and volume of the orthorhombic cells given in Table 1. A monocrystal obtained at the L-ser/L-ala ratio of 93/7 was studied by means of Single Crystal X-ray Diffractometry (SCXRD), but revealed only the presence of the L-ser phase. This fact can be explained by the low sensibility of the SCXRD method in the diagnostics of solid solutions with low contents of light molecules. The samples with L-ser/L-ala compositions of 10/90 and 90/10 (Figure 4) show the additional presence of the other solid solution and thus biphasic behavior.

**Figure 4.** X-ray patterns (2θ CoKα) of the co-precipitated mixtures (mol. %): L-ser/L-ala = 0/100, 7/93, 10/90, 90/10, 93/7, and 100/0.


**Table 1.** Parameters and volume of the orthorhombic cell for L-ser, L-ala, and their solid solutions with L-ser/L-ala ratios of 7/93 and 93/7.
