The in situ surface modification of inorganic nanoparticles (NPs) and its influence on the size, morphology, and particle surface properties is increasingly receiving attention. Control of the size and morphology and perfect dispersion of inorganic NPs in polymer matrices fabricates soft materials with unique optical, electrical, magnetic, gas barrier, self-healing, and thermal and mechanical properties. This study explores the strategy of the in situ modification of inorganic NPs (CaCO3) with cationic and anionic surfactants and the role of in situ modification on the dispersion of these NPs in thermoplastic polymers (poly ε-caprolactone, PCL). The surfactants having an appropriate polar head with a high charge density bind onto the crystal’s nuclei, protect them against extensive aggregation, and consequently control the size, morphology, and surface properties of the produced NPs. This permits formulation of hybrid materials with enhanced thermal stability and tensile modulus and with a marked increase of the crystallization rate.