Dissolution and aggregation of zinc oxide nanoparticles at circumneutral pH; a study of size effects in the presence and absence of citric acid

Abstract

I would like to thank my advisor, Professor Vicki H. Grassian, for supporting me during last one and half years in several ways. Upon everything thing I am grateful to her for helping me to wake up the "strong woman" inside me. Your "words and decisions" made me became stronger in my academic life. Many thanks also to my committee members, Professors Sarah Larsen, Ed Gillan, for all of your support. A special thank you goes to all the members of Grassian research group specially Dr. Larissa Stebanovna, Dr. Shaowei-bian, Chia-ming Wu, Gayan Rubasinghege and Imali Mudunkotuwa for their valuable support. I would also like to mention all my teachers since my high school time for showing me the correct path in my life. Next, I would like to thank all my friends for their valuable support. All of you were there for me in my "ups and downs" and your words encouraged me a lot to achieve my goal. A special thank goes to my family. Their support was so precious. I should specially mention my mother; you were my first ever chemistry teacher. Thank you very much for showing me the "beauty" of chemistry when I was a small kid. Finally, thanks to my loving Lahiru for his kind, valuable support. You were with me days and nights when I was working in the lab. Honestly, I don't have words to express my thanks to you for everything you have done for me.

Influence of Organic Ligands on the Colloidal Stability and Removal of ZnO Nanoparticles from Synthetic Waters by Coagulation

“…At pHzpc, NPs have negligible surface charge; due to weak repulsive interaction between NPs, each collision causes particle adherence and increases the HDD of particles [12,30]. These results are also consistent with previous reports [13,16], which affirm that ζ-potential is positive at pH values below the isoelectric point (IEP), and becomes negative at pH values above IEP. In our study, the ζ-potential remains positive (2 ± 0.3 to 15 ± 2.0 mV) for the pH values (4.5-8.5) prior to pHzpc, and HDD ranges between 450 ± 20 to ~700 ± 55 nm.…”

“…The point of zero charge pHzpc of ZnO NPs was at 9.2, where the surface charge −2.7 ± 2.8 mV and HDD (above 800 ± 70 nm) were found to be higher than other pH values. The pHzpc determined in the current study coincides with previous studies which report that the pHzpc of ZnO NPs was between 8.7 to 9.4 [11,12,16]. At pHzpc, NPs have negligible surface charge; due to weak repulsive interaction between NPs, each collision causes particle adherence and increases the HDD of particles [12,30].…”

“…This might be attributed to the charge The ζ-potential of suspension in the pH range (9.5-12) was determined (−15 ± 1.2 to −32 ± 2.4 mV), which demonstrates highly stable colloids with smaller HDDs (190 ± 50 to 250 ± 35 nm). Such smaller sized particles at higher pH might be attributed to the deprotonation effect of pH, which increases the repulsive interaction between the highly-charged NPs [16]. Therefore, to minimize the dissolution effect, further experiments were carried out at circumneutral pH (pH 7.0).…”

“…In contrast, naturally ubiquitous organic ligands like humic acid (HA), salicylic acid (SA) and L-cysteine (L-cys) can adsorb onto the surface of NPs unselectively via ligand exchange, hydrophobic/hydrophilic interaction, and H-bonding, which promotes the stability of NP suspension and bioaccumulation [13,14]. Some recent studies have focused on the effect of physicochemical properties of organic ligands, i.e., molecular weight (MW) distribution and hydrophobic/hydrophilic characteristics on NPs dissolution and aggregation [15,16]. The elevated concentration of hydrophobic, high molecular weight (HMW) ligands like HA consists of abundant hydroxyl, carboxyl, and phenolic groups, which enhance the dissolution of NPs [15].…”

Comparative Characterization of Different Samples Containing Nano-ZnO Particles with Applicability in Topical Therapies

“…Many scientists have reported significant increases in solubility of metal oxides with decreasing sizes of particles [3][4][5][6]. Nano-metal oxides are probably the most widely used particles having applications in cosmetics, medicine, and textile [7].…”