Exploring surface sensitive technologies, you often encounter the real-time method of QCM. This three-letter abbreviation is short for Quartz Crystal Microbalance, which essentially means that this is a balance for very small masses. If you have browsed the instrumentation area a bit more, you may have noticed that there are several versions of QCMs, such as QCM-D. So, what is the difference between QCM and the four-letter version where a D is added at the end?
Unlike traditional QCM, the QCM with an added D measures an additional parameter, namely the dissipation factor. The dissipation factor provides real time information about the softness and rigidity, also called the viscoelasticity, of the molecular layers on the sensor surface. This means that QCM-D provides more information about the system under study than traditional QCM.
What is dissipation?
So, QCM-D measures dissipation, but what is this parameter, what does it consist of and what does it mean? Sometimes dissipation is also referred to as damping of the oscillating sensor. The strict definition of dissipation is the sum of all energy losses in the system per oscillation cycle. Using an equation to describe it, it can be written as 1/Q, where Q is the so-called quality factor of the oscillator used. Another way to define dissipation is the energy dissipated per oscillation, divided by the total energy stored in the system.
So much for the definition, but how is this related to an actual experiment? As already mentioned, the dissipation correlates to the softness of the layers on the sensor surface. A rigid material deposited on the sensor surface will follow the sensor oscillation without being deformed during the oscillatory motion. This means that a rigid material will not dampen the oscillation and in this situation, the measured dissipation will be low. On the other hand, a soft film will not be able to follow the sensor oscillations entirely. This means that it will be deformed during the measurement. The soft film will dampen the oscillation and result in high dissipation. Consequently, the dissipation parameter offers a real-time characterization of the softness or viscoelasticity of films deposited on the sensor surface, and it makes it possible to monitor conformational changes.
Why is it important to measure the energy dissipation?
The possibility to collect more information about the system under study is of course positive, but is the dissipation information really needed? There are three important aspects of the dissipation parameter:
The dissipation reveals how the properties of the system under study vary qualitatively with time.
It provides key information, if quantification of the mass, thickness and viscoelastic properties is desired.
Not only will the dissipation reveal whether a model for soft or rigid film should be used, but it also represents key information, should it turn out that soft film modeling is needed. Whereas the mass of a rigid film can be calculated using the Sauerbrey equation, which only needs the resonance frequency response of one single harmonic as input, the multiparameter fitting involved in the viscoelastic modeling of a soft film will need f and D input from at least two harmonics. Generally, the more harmonics available as input to the model, the better the fitting result.
Going into more detail about the first important aspect, the qualitative insight of the time resolved behavior of the system under study, the dissipation parameter will complement the information from the collected resonance frequency. Together the two captured parameters f and D will paint a picture of how molecules arrange themselves on the surface and how the arrangement varies with time; the frequency response of a QCM sensor reflects the amount of mass coupled to the surface, including the mass of solvent which is trapped in the molecular layer.
The dissipation reflects the softness of the film. Monitoring these two parameters as a function of time makes it possible to detect for example if molecules are lying flat on the surface (rigid film and low level of hydration) or if they are in a stretched configuration (soft film and high level of hydration), as well as if there are rearrangements such as for example swelling (from flat to stretched) or collapse (from stretched to flat), see fig 1.
Figure 1. To the left, molecules in a stretched configuration, high level of hydration. To the right, molecules in a collapsed configuration, low level of hydration.
Three benefits of QCM-D over QCM
To summarize, the dissipation parameter offers three important benefits over traditional QCM.
It offers relevant qualitative information on how soft the film is and how the softness varies with time
It offers key information on what model to use for quantification
It provides essential input for performing soft film modeling
To get more out of your experiments, you can use the information that the dissipation parameter provides.
Download the white paper to read more about the differences between QCM-D and other QCMs.
This post was originally published in Nov 23, ’17.
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