Diar Nizar:

When heated oleic acid decarboxylated. RCOOH = CO2 +R-H. Silicone-oil surfactant containing a functional group which forms a chemical bond with the surface of the particles and a tail group which is soluble in the silicone-oil carrier, to provide a stable ferrofluid composition at high temperature. See, for example,http://www.google.com/patents/US4356098. Mechanisms of degradation of surfactants see in books of Organic Chemistry.

What is the temperature you are refering to? Oxidation, oxydative degeneration, molecular transformation, disociation, association etc. etc. a variety of phenomena can cause a surfactant to fail or degrade.  

as written privately already, SDBS should not degrade markedly below 100°C in water. E.g. it is used to wash at 90°C. Alumina itself dissolves in water slowly, likely faster at elevated temperature. Adding (poly)phosphate may help to passivate the surface.

Yuri, at which temperature we have to consider decarboxylation of oleic acid? Would further consider transesterification when oil is vegetable base. Epoxy formation and consecutive reaction may also take place starting from cis double bond. Copper may also act as catalyst.

Dear Titus:

You are right there are other possible reactions. Oleic acid is decarboxylated at 330 degrees. Carboxylic acid salts decarboxylate at lower temperatures.

Thank you for your answers

when I spoke about high temperatures: it was around 90 ° C for the case of water using alumina and SDBS as surfactant

and around 200 ° C using oil and copper as nanoparticle and oleic acid as a surfactant

can you give me an idea for the mechanisms of degradation at such temperature thank you

After knowing your sytem, it seems, most probably it is due to the photocatalytic degradation of SDBS in the presence of Cu/CuO being exposed to visible light. There are many literature reference. Please check them.

After knowing your sytem, it seems, most probably it is due to the photocatalytic degradation of SDBS in the presence of Cu/CuO being exposed to visible light. There are many literature reference. Please check them.

After knowing your sytem, it seems, most probably it is due to the photocatalytic degradation of SDBS in the presence of Cu/CuO being exposed to visible light. There are many literature reference. Please check them.

After knowing your sytem, it seems, most probably it is due to the photocatalytic degradation of SDBS in the presence of Cu/CuO being exposed to visible light. There are many literature reference. Please check them.

After knowing your sytem, it seems, most probably it is due to the photocatalytic degradation of SDBS in the presence of Cu/CuO being exposed to visible light. There are many literature reference. Please check them.

The answer depends also on the time scale you consider. A radical mechanism of oxidation is likely. The main question you need to answer is what is initiating the radical chain. O2 dissolved in the system is a common initiator, but it is not very reactive and few days may pass before appreciable oxidation is observed. Light makes the reaction faster. Cu is a good catalyst for radical chain oxidation. Oil content can be an essential factor. The reaction at 90oC should go initially like that:

RCH2ФSO3Na + i  ->  RCH.ФSO3Na + iH   (i can be O2 or another)

RCH.Ф... + O2  ->  RCH(O2.)Ф...

RCH(O2.)Ф... + RH  ->  RCH(O2H)Ф... + R.   (R. may be RCH.Ф...)

What happens next with the peroxide depends on the conditions, esp. temperature. A possibility is formation of diperoxide RC(O2H)2ФSO3Na, which is unstable and ultimately degrades to HOOCФSO3Na.

Basic literature: 

Comprehensive Chemical Kinetics. Liquid-Phase Oxidation; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, Netherlands, 1980

ET Denisov, IB Afanas’ev, Oxidation and antioxidants in organic chemistry and biology (CRC Taylor & Francis, 2005)

Thank you for your answers