A new type of colloidal mesoporous silica nanoparticles (MSNs) is synthesized in liquid-liquid biphasic systems consisting of tetraethoxysilane (TEOS) and water in the presence of primary amines and cationic surfactants (cetyltrimethylammonium chloride, CTAC) under controlled pH conditions (pH 11.1-11.5). The obtained MSNs are characterized by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and nitrogen adsorption-desorption. The results show that the colloidal MSNs with an average diameter in the range of 28-54 nm and a size polydispersity below ca. 15% have been obtained. Importantly, each MSN is composed of a number of tiny primary silica nanoparticles (PSNPs) forming 3D connected pore structure. The pore size of the MSNs can be tuned from 2.5 to 3.0 nm by changing the pH of catalyst stock solution, and larger pore sizes (3.1-4.5 nm) can be achieved by using pore swelling agent. The Brunauer-Emmett-Teller (BET) surface areas and total pore volumes vary from 550 to 750 m(2) g(-1) and from 1.2 to 1.7 cm(3) g(-1), respectively. Compared with conventional MCM-41-type MSNs, our new MSNs show outstanding colloidal and hydrothermal stabilities. They are colloidally stable at room temperature over 1 year, and their mesostructure was retained even after hydrothermal treatment at 120 degrees C for 24 h. Finally, based on the analysis of the morphology and structure of MSNs, a formation scheme based on the cooperative self-assembly of PSNPs and surfactant molecules is proposed. (c) 2012 Elsevier Inc. All rights reserved.