Interactions of PAMAM dendrimers with SDS at the solid-liquid interface

Area of Science:

• Surface Science
• Colloid and Interface Science
• Materials Chemistry

Background:

• Investigating the behavior of poly(amidoamine) (PAMAM) dendrimers and sodium dodecyl sulfate (SDS) at interfaces is crucial for understanding complex fluid behavior.
• The interplay between cationic dendrimers and anionic surfactants at hydrophilic surfaces like silica presents unique structural and dynamic challenges.

Purpose of the Study:

• To elucidate the structural and nonequilibrium effects governing the interactions between PAMAM dendrimers (generations 4 and 8) and SDS at a hydrophilic silica-water interface.
• To compare adsorption behaviors resulting from sequential surfactant addition versus premixed dendrimer/surfactant solutions.

Main Methods:

• Utilized neutron reflectometry to probe the structure of adsorbed layers.
• Employed quartz crystal microbalance with dissipation monitoring to quantify adsorption and viscoelastic properties.
• Investigated adsorption from both sequential addition and premixed solution pathways.

Main Results:

• PAMAM dendrimers form compact, irreversible monolayers on silica.
• SDS adsorption causes dendrimer layer expansion at low concentrations and forms aggregates/bilayers at higher concentrations.
• Adsorption from premixed solutions near charge neutrality forms multilayers with lower surface excess for negatively charged complexes; nonequilibrium effects are significant.
• Sequential addition shows reversible surfactant adsorption with minor monolayer changes; nonequilibrium effects are less critical.

Conclusions:

• The pathway of adsorption significantly impacts the resulting interfacial structure.
• Nonequilibrium effects are critical for adsorption from premixed PAMAM/SDS solutions, rendering thermodynamic models inappropriate.
• Understanding these complex interfacial dynamics is key for designing advanced materials and processes.