In the differential scanning calorimetry measurements, we use an empty pan as a reference to track heat flow in our sample. Both solid samples and liquid samples are treated in the same way. In the case of solid samples, it is quite obvious that there is no other impurity in the sample, but only air (and maybe moist). Therefore it is OK to use an empty pan as a reference. However, the liquid sample is generally composed of a solid material suspended in an aqueous phase with the help of at least one excipient which is usually a surface active agent (a surfactant). The dispersed material can be a pure drug or material such as solid lipids or polymers used for nanoparticle formulation. In solution state, surfactants show different self assemly patterns and micellar structures under elevated temperatures and contribute to changes in the heat flow. In this case I wonder if it wouldn't be better to use the solution, or dispersion of these surfactants in an aqueous phase (which can be referred to as the blank formulation) as reference in order to study the in situ crystal structure of the dispersed materialby eliminating the signal coming from free syrfactant molecules.
I would be grateful if anybody shares their experience, or relevant literature describing the usage of a blank formulation as a reference in DSC measurements.
Complex systems are always a challenge.
You always can use a liquid reference. In fact that is the case in microcalorimetry where you measure heats of unfolding proteins. In that case, the reference cell is full of buffer solution while sample cell contains buffer solution and protein.
However, what you are looking for in DSC is a reference material whose Cp does not present any processes during your heating program and varies in a constant and predictable way so you can measure differences in Cp due to your sample only. Are you sure this is the case with your reference?
In the case you are referring to, using a reference with the surfactant could be of help in order to isolate the thermogram of the substance of interest but you should be sure that this surfactant is not interacting with your substance of interest (or your substance is disturbing those assembly patterns you are talking about). If this is the case, you will not be able to isolate heat flow due to your substance and would be having some heat flow coming form interactions.
Simply, it balances both pans in two cells (reference and sample) and it traces the heating scan of the desired sample.
Thanks Ion.
Actually the surfactant interacts with the substance, by hytrophobic attraction. It surrounds nanoparticles composed of solid lipids. I wonder if the lipids in these nanoparticles, which have Tm higher than 50oC remain in supercooled liquid state. Therefore, instead of treating the sample as a material composed of a single component I wanted to know if it is possible to treat the sample as a mixture of separate components and to retract the signal coming from the "buffer" refernece.
Indeed. I believe you can do so. The best would be to use exactly the same amount of surfactant/solvent in the reference cell as you have in your sample cell. That is not always easy.
The reference PAN is to balance the heat and fix a baseline to identify the transitions. if you have your material in a solvent or any other solution you could use them in ref but the weight should be exactly the same. But one caution, you should operate below the boiling point of the solvent otherwise your DSC measurements will be messed up with the interactions due to vaporization. Too volatile solvents can also give serious trouble in DSC measurements
hope this helps
Thank you Surianarayanan. Actually we do not exceed the boiling point of the dispersant (we perform heating to a maximum which is approx 10oC below the boiling temperature). In addition we use hermetically sealed pans designed for experiments with liquid samples. The notice that the weight of both samples should be exactly the same might be of help.
Unfortunately, the evaporation pressure may be too high at that temperature, check which pressure the hermetic closed pan tolerates! Good luck.
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