Synthesis And Aggregation Behavior Of PAMAM-Based Dendrimers In Aqueous Solution And The Effect On Oil/Water Interface

Polyamidoamine (PAMAM), one of the families of novel dendrimers, has been developed the fastest and researched the most intensively for the remarkable characteristics, such as hyperbranched structure, exact molecular weight and size, well-controlled molecular shape and branch length/density ratio, unique monodispersity, cavity inside of molecule, functionality of molecule surface. On that basis, a variety of study fields focus on PAMAM-based dendrimers with particular molecular structure and physical chemistry properties. This kind of molecules has excellent activity and ability of capacity, introducing many reactive and functional groups in the centre or extremity, and therefore, it can be served as a template of nano particle and drug molecule, or the synthesized simulacrum of protein, enzyme and virus. It also can be led the active centre of catalyst into the cavity, or have the terminal groups modified with gene or antibody. Hence dendrimers become macromolecular materials with special functions.Various examples of PAMAM-based dendrimers functionalized with rigid hydrophobic periphery or chromophoric units, or binding metal ions have been emerged in large numbers. Especially, the amphiphilic PAMAMA-based dendrimers have been attracted on potential and enormous application fields, including biology and medicine, assembly template, catalyst, etc. Little attention, however, has been paid to surface active characteristics, aggregation behavior, pH and salt effects on the aggregagtion properties of PAMAM molecules branched with amphiphilic segments, as well as the molecular interactions with micromolecular surfactants. With this in mind, we synthesized a series of dendrimers with PAMAM as the core and PO_mEO_n units (PO and EO are the abbreviations of propylene oxide and ethylene oxide, respectively; m and n are the numbers of PO and EO in a branch, separately) as the branches, and investigated the influence of dendritic structure on the physicochemical properties and the ability in separating oil/water mixture, as well as the effects of pH and salts on the aggregates conformation, through the measurements of surface tension, steady fluorescence, turbidity analysis, isothermal titration microcalorimetry, dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM).The influences of initiator (namely PAMAM) generation and EO/PO ratio of the branch on the turbidity properties were investigated in this paper, respectively. Dendrimers with different generations of PAMAM have different turbidity properties and cloud point (CP) temperatures. As the mass ratio of EO/PO increases, CP first increases, and then decreases gradually, which indicates the ratio of EO/PO could condition the hydrophilic and hydrophobic properties, and the capability of solubility.The gain of entropy is the main driving force for the spontaneous aggregation of the dendrimer series. In the case of longer PO_mEO_n chains, more EO oxygen atoms contribute to hydrogen bonding, and consequently lead aggregates to form at higher concentration, resulting in higher critical aggregation concentration (CAC) and larger Gibbs free energy for aggregation (ΔGo). In the process, the amphiphilic chain length has impact on the size of aggregates to some extent.pH value has remarkable effect on the aggregation behavior of dendrimers in aqueous solution. When the acidity is below pKa, the dendrimer system is transparent mostly because of the thorough protonation of amide groups of PAMAM; as pH is in the range of pKa 7.5, the protonation process becomes incomplete, which leads dendrimer molecules to form bigger aggregates; with the basic environment coming up, the protonation can hardly take place and the oil phase separates out. With the pH vale increases from 2 to 10, the microenvironment of dendrimer colloid becomes higher polar before aggregates forming and reaches CAC earlier, mainly due to the incomplete protonation process under neutral and basic environment.Salt has an impact on the dendrimer aggregation process in the research. Salt effects on the aggregation behavior in the decreasing order: C₆H₅COONa > Na₂SO₄ > NaCl, resulting in the increase of CP temperature. The influence of inorganic salts is mainly attributed to the hydration process of the preferential movement of water molecules from coordination shells of dendrimer molecules to the hydration layer of SO42- or Cl- ion; and the benzene ring of C₆H₅COONa affects on the size of aggregates significantly, owning to its penetration into the hydrophobic core of aggregates.There is strong interaction between dendrimer and surfactant molecules. When the concentration of added sodium dodecyl sulfate (SDS) is very low such as far from critical micelle concentration (CMC), the system exhibits higher turbidity which results in bigger aggregates, mainly due to the insert of SDS molecules into amine groups and hydrophobic microdomain. Compared with the multilamellar vesicle system of didodecyldimethylammonium bromide (DDAB), the mixed system of dendrimer and DDAB exhibits different size and morphology of aggregates, and rheological properties.The demulsification and dehydration properties of dendrimers were discussed, including the ability to decrease interfacial tension and the parameters of dilational viscosity, half life cycle, and rupture speed constant of droplet. The results showed that the dehydration performance of dendrimer can be improved with the generation of initiator increasing, and it comes to the best dehydration result when the ratio of EO/PO in the branch equals to 1:3.