The effect of ethylene oxide number (EON) polydispersity on the phase behavior of alkyl ethoxylates has been well documented in the surfactant literature. These previous studies show that polydisperse alkyl ethoxylates appear more hydrophilic as the surfactant concentration decreases or as the oil-to-water ratio increases. This becomes a troubling issue considering that most surfactant formulations undergo dilution during use, and they experience a wide range of water-to-oil volume ratios. Within the hydrophilic-lipophilic difference framework, the surfactant hydrophobicity is assessed via the sigma (σ) term (also known as the characteristic curvature or Cc). In this work, the effect of surfactant concentration on the apparent value of sigma (σ app ) is evaluated as a function of surfactant concentration. The experimental observations are then explained using a bifunctional model for alkyl ethoxylates that consider the dual nature of polar oils (free alcohol and low EON ethoxymers) as surfactants and as oil components. A segregation-based model and a partition-based model are implemented to account for the distribution of the ethoxymers in the surfactant pseudophase and the oil phase. Combining these distribution models with the bifunctional model and a group contribution model for sigma, one can predict the σ term versus surfactant concentration for a given water/oil ratio, starting from the EON distribution of the surfactant. The practical applications of the model are discussed.

Molecular dynamics of a series of poly(ethylene glycol)-poly(amidoamine) (PEG-PAMAM) dendrimer blends and PEG conjugated PAMAM dendrimers were investigated and contrasted using broadband dielectric relaxation spectroscopy (DRS). The study was carried with 2000 Da PEG and generation 3 PAMAM dendrimers. The results were generated over a wide range of frequencies and temperatures. A number of relaxation processes were detected and their origin and characteristics established. PEG-dendrimer blends show three local processes and a segmental process contributed by the amorphous PEG. The time scale of segmental relaxation decreases with increasing concentration of dendrimers due to the H-bonding that forms between the PEG oxygen and the amino groups on the surface of dendrimers. PEGylated dendrimers show two local processes and a segmental process, also contributed by the amorphous PEG. But unlike the blends, the amorphous PEG in PEGylated dendrimers is constrained not only by the crystalline lamellae but also by the covalent bonds with the dendrimer. This results in a different morphology which, in turn, gives rise to a different time scale of segmental relaxation in PEG-dendrimer blends and PEGylated dendrimers. A comprehensive analysis of the effect of morphology, concentration of dendrimers, number of attached PEG chains, and temperature on the relaxation time, dielectric relaxation strength, and spectral characteristics of various relaxation processes is offered. The results reported here are expected to provide an added dimension to our understanding of the principles that guide drug delivery concepts.

An investigation of the dynamics of aqueous solutions of PEG, PEGylated dendrimers, methotrexate, and PEGylated and drug (methotrexate)-loaded dendrimers was carried out with broadband dielectric relaxation spectroscopy (DRS). Two dielectric dispersions were observed in all systems, and the explanation of their molecular origin was offered. The lower frequency process, characterized by an activation energy of 52.2 kJ/mol, was attributed to motions in ice. The higher frequency process is of the Cole−Cole type and is assigned to bound water around PEG and methotrexate molecules. The effect of the average number of attached PEG chains and the average number of encapsulated methotrexate molecules was probed. Our results indicate that the dynamics are independent of these numbers but are a strong function of the composition of the complex. Spectral characteristics and temperature dependence of all relaxation processes are described.