測(cè)Zeta電位為什么不能稀釋？

什么是Zeta電位？

Zeta potential is an electrostatic potential that exists very near the surface of particles suspended in liquids1. Zeta potential (ζ) is responsible for particle-particle repulsion forces in colloidal suspensions and thus can be used to predict colloid stability against particle aggregation. Figure 1 illustrates a particle suspended in a liquid along with various notional regions around it.  The“slipping plane” or “shear plane” is where Zeta potential is located versus the potential in the bulk solution. Within this slipping plane, the liquid is bound to the particle while it moves freely outside this boundary. The net potential far from the particle (in the bulk of the liquid) is zero.

Zeta電位是液體中懸浮的粒子很接近表面位置的靜電勢(shì)1。Zeta電位(ζ)是由膠體中粒子與粒子間的相互作用造成的，因此它可以用來(lái)預(yù)測(cè)膠體體系里粒子聚集的穩(wěn)定性。圖1顯示了懸浮在液體中的粒子及其周圍的各種概念區(qū)域。Zeta電位指的是液體中滑動(dòng)面或者剪切面的電位。在這個(gè)滑動(dòng)平面內(nèi)，當(dāng)液體在這個(gè)邊界外自由運(yùn)動(dòng)時(shí)，它與粒子結(jié)合在一起。遠(yuǎn)離粒子的凈電勢(shì)(在液體中)為零。

Figure 1. A negatively charged particle suspended in a liquid. Notional boundaries are shown.

圖1懸浮在液體中的帶負(fù)電的粒子及其周圍的各種概念區(qū)域

PS：這個(gè)是另外的一種說(shuō)法，但是要描述的內(nèi)容是一樣的；

由于分散粒子表面帶有電荷而吸引周圍的反號(hào)離子，這些反號(hào)離子在兩相界面呈擴(kuò)散狀態(tài)分布而形成擴(kuò)散雙電層。根據(jù)Stern雙電層理論可將雙電層分為兩部分，即Stern層和擴(kuò)散層。Stern層定義為吸附在電極表面的一層離子（IHP or OHP）電荷中心組成的一個(gè)平面層，此平面層相對(duì)遠(yuǎn)離界面的流體中的某點(diǎn)的電位稱為Stern電位。穩(wěn)定層(Stationary layer) (包括Stern層和滑動(dòng)面slipping plane以內(nèi)的部分?jǐn)U散層) 與擴(kuò)散層內(nèi)分散介質(zhì)(dispersion medium)發(fā)生相對(duì)移動(dòng)時(shí)的界面是滑動(dòng)面(slipping plane)，該處對(duì)遠(yuǎn)離界面的流體中的某點(diǎn)的電位稱為Zeta電位或電動(dòng)電位（ζ-電位）。

測(cè)Zeta電位為什么不能稀釋？

In aqueous media, Zeta potential is typically generated as the ions on the particle surface dissociate, leaving a net electric charge near the surface surrounded by a cloud of counter-ions. Various types of ions can diffuse in and out through the slipping plane which allows Zeta potential to vary depending on the ion composition in the liquid such as pH. Ions may also participate in chemical reactions within the slipping plane which can affect the Zeta potential. Sample dilution can significantly shift the Zeta potential as ions may adsorb or desorb from the particle. Thus, Zeta potential can be positive or negative, or zero (Iso-Electric Point, IEP) depending on the liquid (solvent) pH or ion type and concentration.

在水相介質(zhì)中，Zeta電位通常是由于粒子表面的離子離解而產(chǎn)生的，在表面附近留下一個(gè)被反離子云包圍的凈電荷。各種類型的離子可以通過(guò)滑動(dòng)面擴(kuò)散進(jìn)來(lái)和出去，滑動(dòng)面允許Zeta電位根據(jù)液體中的離子組成而變化，例如pH值。離子也可以通過(guò)參與滑動(dòng)面內(nèi)的化學(xué)反應(yīng)，從而影響Zeta電位。樣品稀釋可以顯著地改變Zeta電位，因?yàn)殡x子可以吸附或者解析顆粒。因此，Zeta電位可以是正的或負(fù)的，也可以是零(等電點(diǎn)，IEP)，這取決于液體(溶劑)的pH值或離子的類型和濃度。

測(cè)量Zeta電位的方法

Particle-filtration systems may benefit from low Zeta potential levels as aggregated particles are easier to remove. Most other colloidal systems require higher Zeta potentials, e.g. over +/- 20 millivolts in order to maximize shell life. Coatings tend to be more efficient when the particles and coated surface have opposite polarities. Zeta potential normally cannot be directly measured. For example, one cannot place a voltmeter probe against a particle surface in order to measure its surface potential. Instead, Zeta potential is calculated from electrophoretic measurements which measure particle velocity under an applied electric field, i.e. make the particles move and measure their particle mobility (see www.matec。。ｃｏｍ/mas). Thus, the calculated Zeta potential depends on the theory used in these computations to relate particle mobility to Zeta potential. An alternative measurement for large particles or surfaces is to move the liquid against stationary particles, fibers, or surfaces and measure the resulting streaming potential。

顆粒過(guò)濾系統(tǒng)可能受益于較低的Zeta電位水平，因?yàn)榫奂w粒更容易去除。大多數(shù)其他膠體系統(tǒng)需要較高的Zeta電位，例如超過(guò)+/- 20毫伏，以大限度地提高殼體壽命。當(dāng)顆粒和涂層表面具有相反的極性時(shí)，涂層往往更有效。Zeta電位通常不能直接測(cè)量。例如，不能將伏特計(jì)探頭靠在粒子表面上以測(cè)量其表面電位。相反，Zeta電位是通過(guò)電泳測(cè)量來(lái)計(jì)算的，電泳測(cè)量是在外加電場(chǎng)下測(cè)量粒子速度，也就是通過(guò)粒子移動(dòng)并測(cè)量其粒子遷移率（見www.yh-tek。。ｃｏｍ/mas）。因此，計(jì)算出的Zeta電位取決于這些計(jì)算中使用的理論，即粒子遷移率與Zeta電位的關(guān)系。另一種測(cè)量大顆?；虮砻骐娢坏姆椒ㄊ菍⒁后w移到靜止的顆粒、纖維或表面上，然后測(cè)量產(chǎn)生的流動(dòng)電位。