Zeta potential measurement

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Zeta potential 측정

장비 모델 : Zetasizer Nano Series

Introduction

콜로이드 입자의 분산도와 안정성을 판단할 수 있는 기준으로 입자의 표면 전위인 제타포텐셜이 적용된다. 제타 포텐셜이란 콜로이드 상에서 전하를 띄고있는 입자들이 외부에서 가해 준 전기장에 반응하여 일정한 속도로 움직일 때 이 속도를 전기영동속도(electrophoretic velocity)라고 부른다. 외부에서 전기장을 가할 때 콜로이드 입자는 그표면전위의 부호와 반대방향으로 이동하게 되는데, 이때 입자의 이동속도를 가해준 전기장의 세기와 유체역학적인 효과(용매의 점도, 유전율 등)를 고려하여 계산된 것이 제타포텐셜이다. 액체에 부유하는 콜로이드 입자의 표면 전기적 일반적으로 제타포텐셜의 크기 값은 수 mV이다. 콜로이드 입자의 표면 전하에 의하여 입자의 분산도와 안정성이 결정된다. 액체 내에 있는 입자의 분산도와 입자의 안정성은 이러한 입자 표면 전하의 크기로서 판단할 수 있는데 표면 전위가 높으면 입자들 간의 반발력으로 인해 용액내의 입자들은 안정하게 된다. 반대로 표면 전위가 낮으면 입자의 자유 브라운 운동으로 인해 입자들이 충돌하고 뭉쳐서 불안정한 상태가 된다. 일반적으로 입자가 전기적으로 안정되어 있고 제타포센셜이 -30mV이하, +30mV이상이면 콜로이드 입자는 안정되어 있다고 볼 수 있다.

시료 준비

제타포텐셜을 측정하는 시료의 농도는 다양한 조건의 농도를 측정함으로써, 실험적으로 적절한 농도를 찾아야한다. 왜냐하면, 제타포텐셜은 입자의 Optical properties, size, polydispersity of the particle size distribution 과 같은 파라미터에 영향을 받는데 이를 종합하여 측정되는 결과(Data reports)를 바탕으로 적정한 농도를 결정할 수 있다. 일반적으로, 처음 측정 기준 농도는 샘플의 비율과 용매의 비율이 1:9 또는 2:8 비율로 제조하여 측정한다.


Figure 2 샘플의 적절한 농도 nanoComposix guidelines for zetapotential analysis of nanoparticles 매뉴얼

측정방법

측정하기에 앞서 사용하는 장비의 모습은 이러하다. (Zetasizer Nano Series)


1) 기기의 안정화를 위해 장비를 켜고 30분 동안 예열시켜준 후 software를 실행시킨다.

2) 준비한 시료 1 mL를 주사기를 이용하여 cell에 넣는다.

주의: 시료 주입 시 cell 안에 기포가 생기지 않도록 천천히 주입해야 한다.

3) 기기 holder에 cell의 앞, 뒷면을 확인하여 넣는다. 이때, 전극이 기기에 부착되어있는 전극과 잘 맞닿았는지 확인한다.

4) 프로그램 클릭 후, 측정모드를 Zeta로 지정한다.

5) 위의 메뉴에서 File → New → Measurement File을 눌러 측정 및 저장할 파일을 만든다.

6) Measure 메뉴에서 → Manual을 클릭한다.

7) 아래 각각의 항목들을 입력한다.

a. Material : 측정하고자 하는 시료를 지정하고, 그 물질의 Refractive index (RI)값을 선택한다.

b. Dispersant : 시료의 solvent를 선택한다.

c. Temperature : 측정온도를 입력한다.

d. Cell type : 사용할 셀을 지정한다. (DTS1070)

8) 항목들에 대한 정보를 입력하고, START 버튼을 눌러주면 그림과 같은 데이터가 얻어지고 자동 저장된다.


★주의사항

1) cell 전극이 손상된 경우 (안쪽 전극이 검은색으로 변함) 재현성 있는 결과가 측정되지 않음으로 분석 전,후에 전극의 상태를 확인 해야한다.

2) 시료의 농도가 측정값의 정확도를 좌우하는데 큰 역할을 하므로 충분한 신호가 검출기로 전달되는 count rate의 농도를 조절하여 측정해야 신뢰성 있는 데이터를 얻을 수 있다.

Result processing

1) 필요한 값인 ZP (zeta potential) 값만을 가지고 그래프를 그린다. (excel, origin, sigmaplot 등) sigma plot을 이용하여 그래프를 그린경우를 보면,

1-1) Sigma plot을 실행시킨다.

1-2) 결과값을 입력한 후, 행과 열에 각각에 맞게 라벨링 힌다. (샘플조건, 측정횟수, 샘플이름 등)

1-3) 왼쪽 다양한 graph 종류 중에서 vertical bar chart를 선택한다.

1-4) Many Y를 선택한 후, 각각의 컬럼을 선택해준 후, 마침을 누르면 zeta potential에 대한 그래프가 그려진다.

Zeta potential measurement

Introduction

Most colloidal dispersions in aqueous media carry an electric charge. There are many origins of this surface charge depending upon the nature of the particle and its surrounding medium but some of the most important mechanisms are: ionisation of surface groups, differential loss of ions from the crystal lattice and adsorption of charged species.

The development of a net charge at the particle surface affects the distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counter ions, ions of opposite charge to that of the particle, close to the surface. Thus an electrical double layer exists round each particle.

The liquid layer surrounding the particle exists as two parts; an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move it. Those ions beyond the boundary stay with the bulk dispersant. The potential at this boundary (surface of hydrodynamic shear) is the zeta potential.

The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system. If all the particles in suspension have a large negative or positive zeta potential then they will tend to repel each other and there will be no tendency for the particles to come together. However, if the particles have low zeta potential values then there will be no force to prevent the particles coming together and aggregating.

The general dividing line between stable and unstable suspensions is generally taken at either +30 or -30 mV. Particles with zeta potentials more positive than +30 mV or more negative than -30 mV are normally considered stable.

Zeta potential is a measure of the magnitude of the electrostatic or charge repulsion/attraction between particles, and is one of the fundamental parameters that affect stability. Its measurement clarifies the causes of dispersion, aggregation or flocculation, and can be applied to improve the formulation of dispersions, emulsions and suspensions.

The measurement of zeta potential has important applications in a wide range of industries including ceramics, pharmaceuticals, medicine, mineral processing, electronics and water treatment.

Zeta potential analysis is a technique for determining the surface charge of nanoparticles in solution (colloids). It is an important tool for understanding the state of the nanoparticle surface and predicting the long-term stability of the nanoparticle.

Preparing the sample

For meaningful measurements, the solvent is crucially important. A measurement result given with no reference to the solvent in which the material is dispersed is meaningless. The zeta potential is as dependent on the composition of the disperse phase as it is on the nature of the particle surface.

Solvent

The solvent of most samples can be put into one of two categories:

  • Polar dispersants are defined as those with a dielectric constant greater than 20 e.g. ethanol and water.
  • Non-polar or low polarity dispersants are defined as those with a dielectric constant less than 20, e.g. hydrocarbons, higher alcohols.

Aqueous/Polar Systems

The aim of sample preparation is to preserve the existing state of the surface during the process of dilution. There is only one way to ensure this is the case. This is by filtering or centrifuging some clear liquid from the original sample and using this to dilute the original concentrated sample. In this way the equilibrium between surface and liquid is perfectly maintained.

If extraction of a supernatant is not possible, then just letting a sample naturally sediment and using the fine particles left in the supernatant is a good method.

Another method is to imitate the original medium as closely as possible. This should be done with regard to:

  • pH
  • Total ionic concentration of the system
  • Concentration of any surfactants or polymers present

Non-Polar Systems

Sample preparation for such systems will follow the same general rules as for polar systems. As there will be generally fewer ions in a non-polar dispersant to suppress the zeta potential, the actual values measured can seem very high, as much as 200 or 250 mV. In such non-polar systems, equilibration of the sample after dilution is the time dependent step, equilibration can take in excess of 24 hours.

Samples are prepared in solution or suspension state. If the sample is in solid state, it should be dissolved or suspended in a suitable solvent.

The sample preparation process can be summarized as below:

  1. Pipet a suitable volume of stock nanoparticles into a microtube.
  2. Dilute the stock nanoparticles with an appropriate solvent (DI water or cell culture media) to a desired concentration. The sample volume should be at least 1 mL.
  3. Transfer the sample into a syringe.
  4. Place the sample syringe into one of the sample ports.
  5. Invert the cell and slowly inject the sample from its syringe into the cell, filling the U tube to just over the half way.
  6. Turn the cell upright and continue injecting the sample slowly into the cellŒ.
  7. Once sample starts to emerge from the second sample port, insert a stopper. Remove the syringe and replace with a second stopperŽ. The stoppers must be fitted before a measurement is performed.
  8. Check that no bubbles are in the cell. Tap the cell slightly to dislodge any present bubbles. Check that the cell electrodes are completely covered.
  9. Remove any liquid that may have spilt onto the electrodes.

Inserting the cell

  1. Turn on the instrument and wait 30 minutes for the laser to stabilize. The power button is on the back of the instrument.
  2. Start the Zetasizer software.
  3. Open the cell area lid by pushing the button in front of the lid.
  4. When inserting the cell, make sure that the Malvern logo faces towards the front of the instrument. Press down until the cell clicks into place.
  5. Close the cell area lid.

Making a manual measurement

Creating a new measurement file

  1. Select File - New - Measurement File.
  2. A dialogue will appear allowing the new measurement file to be named and specify where it will be saved. The file name should clearly indicate user name, measurement type and measurement date.
  3. All new measurement files automatically have the .DTS extension.

Starting a manual measurement

  1. Select Zeta, Manual measurement and click on the green triangular button to start a measurement. This will open the Manual measurement dialogue allowing some settings to be configured.
    • Sample name should indicate the material, dispersant and other important conditions of the sample. For example: 12h_Fe3O4_DIW_50ppm means that the sample is Fe3O4 nanoparticles dispersed in DI water with the concentration 50 ppm and sedimentation time 12 hours.
    • Sample materials are chosen from the list in the software. If the material is not on the list, users have to add it manually. Sample materials are defined by refractive index and absorption value.
    • Sample dispersants are chosen from the list in the software. If the dispersant is not on the list, users also have to add it manually. Sample dispersants are defined by temperature, viscosity, refractive index and dielectric constant.
    • In the temperature tab, there is also equilibration time option. Equilibration time is the amount of time required for the current to stabilize before measurement. If it is set high, the current is more stable but the measurement duration is longer. If it is set low, the measurement duration is shorter but the current is less stable. Therefore, users have to balance between current stability and measurement duration.
    • DTS1060 and DTS1070 cells look very similar, so users have to be careful to choose the correct cell that is used. In fact, DTS1070 cells are the replacement for DTS1060 cells. DTS1060 cells can be inserted either way round, but DTS1070 cells have to be inserted with the Malvern logo facing towards the front of the instrument.
    • In automatic mode, users set minimum and maximum number of runs, and the measurement will be repeated until the results meet quality criteria. In manual mode, the measurement will only be repeated for the number of runs that are set by users.
    • Users can also set number of measurements and delay time between measurements. Number of measurements is the number of times that one sample is measured. Delay time between measurements is the number of seconds that the current is allowed to stabilize between each replicate measurement.
  2. Once all settings have been made, click OK to close the dialogue and return to the measurement display.
  3. Click the Start button to start a measurement.
    • If the sample meets all quality criteria, the software will notify users of good result. This means that the data is reliable and the sample does not have to be adjusted.
    • If the result is not good, the software will also notify users of some possible adjustments that can be made to improve the data quality. Usually, the sample concentration needs to be increased or decreased, or the sample cell needs to be checked for bubbles.

Data processing

Copy the data table to Excel and calculate mean and standard deviation values of replicate measurements.

Use SigmaPlot software to visualize data by bar chart.

  • In the data set, the first column is sample name, the next pairs of columns are mean and standard deviation values.
  • In the 2D Graph Toolbar, select Vertical Bar Chart, and then Grouped Vertical Bar – Error Bars.
  • In the dialogue box, select X Many Y, and then select the appropriate columns for X and Y.
  • Click Finish to plot the chart.