Measurement of droplet size distributionin emulsions using benchtop NMR

Application Note

In many industries, there is a requirement to reliably andsimply determine the droplet size distribution in the dispersed
phase of an emulsion without disrupting the sample structure.
The MQR time-domain Nuclear Magnetic Resonance (NMR)
spectrometer fulfils this requirement with technology that provides
a non-destructive method for measuring the distribution. Sample
preparation is simple and results are fast and accurate. The useof the MQR allows more definitive and accurate formulation ofnew products, increased consistency in manufacturing operations,better process control and ultimately reduced costs.
Summary
MQR is a time-domain NMR spectrometer which
addresses a range of industry specific quality control
and research and development requirements using
proven and reliable technology.

The droplet size distribution (DSD) is an important characteristicof emulsions. Droplet size analysis by NMR is used as aneffective technique for the quality control of raw materials,
characterisation of products at different stages of manufacturing
and for monitoring properties of new products in research anddevelopment. Droplet size measurements help product developers

Advantages of the NMR technique for evaluation ofand manufacturers achieve the required properties of emulsions

droplet size distribution in emulsions include:

and optimise production costs.

• One of the easiest and most reliable techniques

Advantages of NMR

available – suitable for unskilled personnel•	No hazardous solvents required; no hazardous waste
produced• Simple sample preparation minimising damage to thesample structure

The NMR method is applicable to a wide range of products
and materials and in contrast to other techniques (e.g. optical
microscopy and electric sensing) is much less destructive and canbe quickly performed on a sample “as is”. This means that sample
structure distortion is minimised and the use of hazardous solvents,• Measurements are made directly on the sample• Uses standard approaches for data analysis• Provides information about the dimensions of droplets,

contrast agents or any other testing fluids is avoided, allowing• Non-destructive, so the same sample may be

step-by-step instructions.

not droplet clusters

measured several times• Applicable to a wide variety of emulsions

multiple repeats of NMR measurements and further analysis of thesame sample by other techniques. From a practical point of view,
NMR measurements can be routinely performed by non-NMR
experts using intuitive and user-friendly software which provides

MethodThe time-domain NMR method for analysing droplet size
distributions is based on the phenomenon of restricted diffusion,

Application
An emulsion is a mixture of two or more immiscible liquids
which form a system of dispersed droplets (dispersed phase)
separated by the continuous phase matrix. Spreads, margarine,
mayonnaise, salad dressing, crude oil and paints are examplesof emulsion products and materials. Emulsions remain stableand keep their structure (the droplets do not coalesce) for aperiod of time under certain conditions.

where the effective mobility of the dispersed phase moleculesmoving within the droplets is significantly lower comparedwith that of the equivalent bulk liquid due to interactions withthe droplet walls. This means that information about droplet size
can be evaluated from
NMR diffusometry
measurements.

Measurement of droplet size distributionin emulsions using benchtop NMR
The raw data are acquired in the form of NMR signal
diffusion decays. For an emulsion sample, where continuousand dispersed phases are present, both these components
produce an NMR signal. In order to collect NMR data only for

Application Note

The measurement protocol consists of four steps:
Step 1: Adjusting the parameters of the T1 filter to remove thecontinuous phase NMR signal.

the dispersed phase, the signal from the continuous phase isStep 2: Collecting diffusion data for pure water to adjust theremoved by applying a suitable T1 relaxation filter, followed byeffective strength of the PFG.

a pulsed-field gradient (PFG) diffusometry protocol. To improve

Step 3: Collecting diffusion data for the pure dispersed phase to

the signal selection further, the sample temperature can be

define the initial parameters for processing data.

maintained at a certain value by using a variable temperature
NMR probe (VT probe). Under such conditions, the effective T1
relaxation time of the continuous phase is much shorter than
the relaxation time of the dispersed phase, and the efficiency of

Step 4: Collecting diffusion data for an emulsion sample(product). The data acquired during this step are used to obtaina droplet size distribution for the sample.

the T1 filter becomes even better. This approach is used for theSteps 1-3 are the calibration steps and step 4 is the analysis

DSD analysis of dairy products (water-in-oil emulsions) wherestep. The calibration steps may be skipped if the requirement

the samples are measured at a temperature of +5°C.

is to measure several emulsion samples containing the same
continuous and dispersed phases.

Figure 1: The droplet size distribution results obtained after the fitting of NMR diffusometry data for a butter sample. The experimental
diffusion decay ( , the red solid circles) and the log-normal fitting curve (—, the solid black line) are shown in the left-hand graph window. The
volume-weighed droplet size distribution (—, the blue curve) and the mean size distribution (—, the red curve) are plotted in the right-handgraph. The numerical parameters of the distribution are listed in the “Fit Summary” and “Droplet Size Summary” tables below the data chart.

•Measurement of droplet size distributionin emulsions using benchtop NMR

Application Note

The data analysis is carried out using a log-normal distribution

NB:of diffusion coefficients in a separate piece of provided•	Samples (1), (3) and (4) are required for each different

software. The fitting results are reported as a graph of thedistribution function and as a set of the distribution function
parameters (see example in Figure 1): the mean radius ofdroplets (R00), the volume-mean radius (R33), the medianradius R0, the distribution width (σ) and the percentage offormulation of emulsion.
•	Information about the recommended sample sizes andconditioning time is provided in the respective Method
Sheets for different probe sizes.

droplets with the size falling in the predefined radius ranges.

Tables 1 and 2 show that NMR has good repeatability for theThe radius ranges can be specified by users.

evaluation of the droplet size distributions.

Sample Preparation
The NMR method for measuring droplet size distributionfunction requires four samples:
1) A sample of the pure continuous phase (for Step 1)2) A sample of pure deionised water (for Step 2)3) A sample of the pure dispersed phase (for Step 3)4)	An emulsion sample or a set of emulsion samples madeof the continuous and dispersed phases mentioned in (1)and (3)Samples (1) to (4) are transferred into suitable NMR tubes tothe required sample height. The samples must be conditioned
at the required temperature in a conditioning block or in theVT probe of the MQR analyser for a suitable period of time.
The DSD software allows a time delay to be set to temper thesample prior to measurement.

Conclusion
The NMR technique is an accurate and repeatable methodfor defining droplet size distributions in a wide variety ofemulsions.
Sample preparation for the NMR measurements is simple,
does not distort the structure of emulsions and does not
require hazardous solvents or contrast (testing) additives
meaning that the emulsion is analysed “as is”.
The NMR technique is non-destructive, so the same
sample can be measured several times before being
analysed by other techniques.

Table 1. Repeatability of NMR measurements for droplet size distribution
parameters: Butter sample

Repeat1

Repeat2Repeat3Averagevalue forthreerepeats(μm)

Mean radius, μm

0.76

0.70

0.75

0.74

0.03

Volume weighed mean, μm

2.32

2.41

2.32

2.350.05

Median radius, μm

0.63

0.57

0.62

0.61

0.04

Distribution width, μm

0.61

0.64

0.61

0.62

0.02

Droplet size
distribution
parameters

Standard
deviation forthree repeat
measurements(μm)

Table 2. Repeatability of NMR measurements for content of droplets withspecific radius: Butter sample

Droplet radius
ranges*Percentage of dropletswith radius within therespective range (%)
Repeat Repeat Repeat
123Averagevalue forthreerepeats (%)

Standard
deviation forthree repeat
measurements (%)

smaller than 2 μm

51.75

50.74

51.96

51.48

0.65

from 2 to 5 μm

42.21

41.87

41.92

42.00

0.18

from 5 to 10 μm

5.68

6.81

5.74

6.08

0.64

from 10 to 15 μm

0.32

0.50

0.34

0.39

0.10

from 15 to 30 μm

0.04

0.08

0.04

0.05

0.02

larger than 30 μm

0.00

0.00

0.00

0.00

0.00

* The number of the radius
ranges and their numericalborders can be defined by
the uservisit www.oxinst.com/mqr for more information or email magres@oxinst.com

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