Interfacial water structure at polymer gel/quartz interfaces investigated by sum frequency generation spectroscopy

Area of Science:

• Surface Science
• Polymer Science
• Physical Chemistry

Background:

• Understanding water's interfacial structure is crucial for tribology and material interfaces.
• Polymer gels like polyvinyl alcohol (PVA) and poly(2-acrylamido-2-methypropane) sulfonic acid sodium salt (PNaAMPS) are used in various applications.
• Friction at polymer gel/solid interfaces is influenced by interfacial water properties.

Purpose of the Study:

• To investigate the interfacial water structures at PVA and PNaAMPS/quartz interfaces.
• To correlate water structure with friction at polymer gel/solid interfaces.
• To elucidate the role of electrostatic interactions in modifying interfacial water structure and friction.

Main Methods:

• Sum Frequency Generation (SFG) spectroscopy was employed to probe interfacial water structures.
• Experiments were conducted on quartz surfaces modified with aminopropyltrimethoxysilane (APS).
• PVA and PNaAMPS gels were brought into contact with modified quartz surfaces under varying pH conditions.

Main Results:

• Two distinct water structures, 'ice-like' (strongly hydrogen-bonded) and 'liquid-like' (weakly hydrogen-bonded), were identified at both interfaces.
• 'Liquid-like' water became dominant when PVA gel was pressed against quartz.
• At PNaAMPS/APS/quartz interfaces, increased 'liquid-like' water and reduced friction were observed at pH 12 (negative surface charge, repulsive interaction), while friction increased at pH 2 (positive surface charge, attractive interaction).

Conclusions:

• Interfacial water structure plays a significant role in determining friction at polymer gel/solid interfaces.
• Electrostatic interactions can modulate interfacial water structure and consequently influence friction.
• The findings suggest that promoting 'liquid-like' water structures can lead to reduced friction.