表面活性劑是指在較低濃度時，即能(neng)顯著(zhu)降低液體(ti)表面(mian)張力，改變體(ti)系(xi)界面(mian)狀(zhuang)態，從而產生(sheng)潤濕、乳(ru)化、起泡、增溶等作(zuo)用(yong)的物質。

分類方法：

1）按照(zhao)離(li)子類型(xing)(xing)分(fen)類：分(fen)為非離(li)子型(xing)(xing)表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)、離(li)子型(xing)(xing)表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)，其(qi)中(zhong)離(li)子型(xing)(xing)表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)又(you)可以分(fen)為陰離(li)子表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)、陽(yang)離(li)子表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)、兩性(xing)表(biao)面(mian)活(huo)性(xing)劑(ji)(ji)。

2）按親水基的結構分類：羧酸鹽(yan)(yan)型(xing)(xing)(xing)(xing)、磺酸鹽(yan)(yan)型(xing)(xing)(xing)(xing)、硫酸酯(zhi)鹽(yan)(yan)型(xing)(xing)(xing)(xing)、磷(lin)酸酯(zhi)鹽(yan)(yan)型(xing)(xing)(xing)(xing)、胺鹽(yan)(yan)型(xing)(xing)(xing)(xing)、季銨鹽(yan)(yan)、鎓鹽(yan)(yan)型(xing)(xing)(xing)(xing)（鏻、鉮、硫、碘鎓化合物(wu)）、多羥基型(xing)(xing)(xing)(xing)、聚氧乙烯型(xing)(xing)(xing)(xing)。

3）按疏水基種類(lei)分(fen)類(lei)：直鏈、支鏈烷(wan)基(碳原子數為8～20)、烷(wan)基苯基、萘基(烷(wan)基碳原子數為8～16)、松(song)香衍(yan)生物、高分(fen)子量聚(ju)環(huan)(huan)氧丙(bing)烯基、長鏈全(quan)氟(fu)(或高氟(fu)代)烷(wan)基、聚(ju)硅氧烷(wan)基、全(quan)氟(fu)聚(ju)環(huan)(huan)氧丙(bing)烷(wan)基(低(di)分(fen)子量)。

4）按表(biao)(biao)面(mian)活(huo)性劑(ji)的特殊性分類：碳氟表(biao)(biao)面(mian)活(huo)性劑(ji)、含硅(gui)表(biao)(biao)面(mian)活(huo)性劑(ji)、高分子表(biao)(biao)面(mian)活(huo)性劑(ji)、生物表(biao)(biao)面(mian)活(huo)性劑(ji)、冠醚型(xing)表(biao)(biao)面(mian)活(huo)性劑(ji)。

在(zai)兩相(xiang)(特別是氣-液(ye))界面(mian)上(shang)，處處存在(zai)著一(yi)種(zhong)張力(li)(li)(li)(li)，這種(zhong)力(li)(li)(li)(li)垂直與表(biao)面(mian)的(de)邊界，指向液(ye)體方向并(bing)與表(biao)面(mian)相(xiang)切。把作用于單(dan)位邊界線(xian)上(shang)的(de)這種(zhong)力(li)(li)(li)(li)稱為表(biao)面(mian)張力(li)(li)(li)(li)，用 g 或 s 表(biao)示(shi)。表(biao)面(mian)張力(li)(li)(li)(li)的(de)單(dan)位通常為N/m. 表(biao)面(mian)活(huo)性：因溶質在(zai)表(biao)面(mian)發生吸(xi)附（正吸(xi)附）而使溶液(ye)表(biao)面(mian)張力(li)(li)(li)(li)降低的(de)性質被稱為表(biao)面(mian)活(huo)性。

界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)是指兩相接觸的約幾個分(fen)子(zi)厚度的過渡區，若其中(zhong)一(yi)相為氣(qi)體(ti)(ti)，這種界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)通(tong)常稱(cheng)為表(biao)(biao)面(mian)(mian)(mian)(mian)(mian)。界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)包括氣(qi)-液(ye)(ye)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)，氣(qi)-固(gu)(gu)(gu)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)，液(ye)(ye)-液(ye)(ye)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)，液(ye)(ye)-固(gu)(gu)(gu)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)，固(gu)(gu)(gu)-固(gu)(gu)(gu)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)。界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)包括表(biao)(biao)面(mian)(mian)(mian)(mian)(mian)，表(biao)(biao)面(mian)(mian)(mian)(mian)(mian)屬于界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)，如氣(qi)-液(ye)(ye)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)成為液(ye)(ye)體(ti)(ti)表(biao)(biao)面(mian)(mian)(mian)(mian)(mian)，氣(qi)-固(gu)(gu)(gu)界(jie)(jie)面(mian)(mian)(mian)(mian)(mian)常成為固(gu)(gu)(gu)體(ti)(ti)表(biao)(biao)面(mian)(mian)(mian)(mian)(mian)。