6.2 Surfactant

Surface-active agents or surfactants are substances that，when present at low concentration in a system，have the property of adsorbing onto the surfaces or interfaces of the system and of altering to a marked degree the surface or interfacial free energies of those surfaces （or interfaces）. They have a characteristic molecular structure consisting of a structural group that has very little attraction for the solvent，called the lyophobic group，together with a group that has strong attraction for the solvent，called the lyophilic group. This is known as an amphipathic structure.

When a surface-active agent is dissolved in a solvent，the presence of the lyophobic group in the interior of the solvent may cause distortion of the solvent liquid structure，increasing the free energy of the system. In an aqueous solution of a surfactant this distortion of the water by lyophobic （hydrophobic） group of the surfactant increase the free energy of the system when it is dissolved，means that less work is needed to bring a surfactant molecule than a water molecule to the surface. The surfactant therefore concentrates at the surface. Since less work is now needed to bring molecules to the surface，the presence of the surfactant decreases the work needed to create unit area of surface （the surface free energy per unit area，or surface tension）. On the other hand，the presence of the lyophilic （hydrophilic） group prevents the surfactant from being expelled completely from the solvent as a separate phase，since that would require dehydration of the hydrophilic group. The amphipathic structure of the surfactant therefore causes not only concentration of the surfactant at the surface and reduction of the surface tension of the water，but also orientation of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away from it.^②

The chemical structures of groupings suitable as the lyophobic and lyophilic portions of the surfactant molecule vary with the nature of the solvent and the conditions of use. In a highly polar solvent such as water，the lyophobic group may be a hydrocarbon or fluorocarbon or siloxane chain of proper length，whereas in a less polar solvent only some of these may be suitable （e.g.，fluorocarbon or siloxane chains in polypropylene glycol）. In a polar solvent such as water，ionic or highly polar groups may act as lyophilic groups，whereas in a nonpolar solvent such as heptane they may act as lyophobic groups. As the temperature and use conditions （e.g.，presence of electrolyte or organic additives） vary，modifications in the structure of the lyophobic and lyophilic groups may become necessary to maintain surface activity at a suitable level. Thus，for surface activity in a particular system the surfactant molecule must have a chemical structure that is amphipathic in that solvent under the conditions of use.

The hydrophobic group is usually a long-chain hydrocarbon residue，and less often a halogenated or oxygenated hydrocarbon or siloxane chain; the hydrophilic group is an ionic or highly polar group depending on the nature of the hydrophilic group. Surfactants are classified as:

（1）Anionic. The surface-active portion of the molecule bears a negative charge，for example，RCOO^-Na⁺（soap），RC₆H₄SO₃^-Na⁺ （alkylbenzene sulfonate）.

（2）Cationic. The surface-active portion bears a positive charge，for example，RNH₃⁺Cl^- （salt of a long-chain amine），RN（CH₃）₃⁺Cl^- （quaternary ammonium chloride）.

（3）Zwitterionic or amphoteric. Both positive and negative charges may be present in the surface-active portion，for example，RN⁺H₂CH₂COO^-（long-chain amino acid），RN⁺（CH₃）₂CH₂CH₂SO₃^-（sulfobetaine）.

（4）Nonionic. The surface-active portion bears no apparent ionic charge，for example，RCOOCH₂CHOHCH₂OH （monoglyceride of long-chain fatty acid），RC₆H₄（OC₂H₄）_xOH （polyoxyethylene alkylphenol）.

Most natural surfaces are negatively charged. Therefore，if the surface is to be made hydrophobic （water-repellent） by use of a surfactant，then the best type of surfactant to use is a cationic. This type of surfactant will adsorb onto the surface with its positively charged hydrophilic head group oriented toward the negatively charged surface （because of electrostatic attraction） and its hydrophobic group oriented away from the surface，making the surface waterrepellent. On the other hand，if the surface is to be made hydrophilic （water-wettable），then cationic surfactant should be avoided. If the surface should happen to be positively charged，however，then anionics will make it hydrophobic and should be avoided if the surface is to be made hydrophilic.

Nonionics adsorb onto surfaces with either the hydrophilic or the hydrophobic group oriented toward the surface，depending upon the nature of the surface. If polar groups capable of H bonding with the hydrophilic group of the surfactant are present on the surface，then the surfactant will probably be adsorbed with its hydrophilic group oriented toward the surface，making the surface more hydrophobic; if such groups are absent from the surface，then the surfactant will probably be oriented with its hydrophobic group toward the surface，making it more hydrophilic.

Zwitterionics，since they carry both positive and negative charges，can adsorb onto both negatively charged and positively charged surfaces without changing the charge of the surface significantly. On the other hand，the adsorption of a cationic onto a negatively charged surface reduces the charge on the surface and may even reverse it to a positive charge （if sufficient cationic is adsorbed）. In similar fashion，the adsorption of an anionic surfactant onto a positively charged surface reduces its charge and may reverse it to a negative charge. The adsorption of a nonionic onto a surface generally does not affect its charge significantly，although the effective charge density may be reduced if the adsorbed layer is thick.

Differences in the nature of the hydrophobic groups are usually less pronounced than in the nature of the hydrophilic groups.^③ Generally，they are long-chain hydrocarbon residues. However，they include such different structures as:

①straight-chain，long alkyl groups （C₈-C₂₀）;

②branched-chain，long alkyl groups （C₈-C₂₀）;

③long-chain （C₈-C₁₅） alkylbenzene residues;

④alkylnaphthalene residues （C₃ and greater-length alkyl groups）;

⑤high-molecular-weight propylene oxide polymers （polyoxypropylene glycol derivatives）;

⑥long-chain perfluoroalkyl groups;

⑦polysiloxane groups;

⑧lignin derivatives.