I'm afraid it can be answered only by comparing the conditions. You should cmpare the chemical structure, crosslonk density etc. of your gel and the particles size dsitribution of your MgO particles with those described in the literature where an opposites effect is observed. Swelling is a delicate property influenced by several factors.

The more swelling capacity of the MgO hydrogels may be attributed to the presence of MgO nanoparticles with different size, morphology and surface charges. Charged MgO nanoparticles results in the penetration of more water molecules to balance the build-up ion osmotic pressure, which causes the hydrogel to swell.

Dr.Gyorgy and Dr.Yadollahi,

Would you please send me a reference regarding to your reasons? Because I must use a reference for my answers to this topic.

Regards

Addition of nanoparticles may change the water-uptake in hydrogels by: 1. direct water adsoprtion (at the filler/water interface), 2. by changing the netowrk density (by adding physical crosslink points to the existing chemical crosslinks). If you add the MgO nanoparticles to the monomer system before preparing the hydrogel, even the chemical crosslink density itself may change due to the selectivee adsorption of monomers (if the hydrogel is a copolymer) or by the selective adorption of initiators, crosslinkert etc. It requires a separate study.

Some references which I found and whic may be relevant to your topic:

http://eprints.utm.my/32550/

http://www.wseas.us/e-library/conferences/2013/Brasov/BIOMED/BIOMED-10.pdf

http://www.naun.org/main/NAUN/bio/c082010-117.pdf

http://pubs.rsc.org/en/Content/ArticleLanding/2010/JM/b927314j#!divAbstract

http://faculty.mu.edu.sa/public/uploads/1369709096.8631Synthesis%20and%20characterization%20of%20novel%20hydrogels.pdf

https://www.researchgate.net/publication/224116668_Impact_of_magnetic_nanofillers_in_the_swelling_and_release_properties_of_kappa-carrageenan_hydrogel_nanocomposites

http://onlinelibrary.wiley.com/doi/10.1002/app.39544/abstract

http://connection.ebscohost.com/c/articles/74299748/hes-hema-nanocomposite-polymer-hydrogels-swelling-behavior-characterization

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8499755

http://www.hfmi.stevens.edu/Soft%20Matter%20Vural_Dikovics_Kalyon_2010.pdf

Article Impact of magnetic nanofillers in the swelling and release p...

please see these references:

http://onlinelibrary.wiley.com/doi/10.1002/app.22164/abstract;jsessionid=F1267521321B309FA1E1C10E29B77D44.f02t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false

http://www.sciencedirect.com/science/article/pii/S0141813010000061

http://www.sciencedirect.com/science/article/pii/S0144861708003494

http://www.sciencedirect.com/science/article/pii/S0014305707004624

Thanks alot dear Dr.Gyorgy , Dr.Yadollahi and Dr.Haraszti. Of course all your answers is helpful for my research. Also Thank you to give me such an important references.

Regards

Farzaneh