The commercial alkyl polyglucosides with mixed-chain C8/10G1.31 and C12/14G1.43 used in this work were kindly provided by Research Institute of Daily Chemical Industry, China, which are supplied as 50 wt% solutions in water with high pH value to avoid microbial attack. Their compositions were determined by chromatograms to be C8/10G1.31 (mono- glucoside 77.18%, diglucoside 16.61%, triglucoside 4.30%, tetraglucoside 1.91%) and

C12/14G1.43 (monoglucoside 71.56%, diglucoside 17.52%, triglucoside 6.77%, tetraglu-

coside 4.15%), respectively.

Samples are prepared where 1-butanol varies monotonically, while the APG concentrations are fixed at different values. A water- to- octane weight ratio of unity is preferred. All samples are allowed to equilibrate at (40±0.1)℃ in a water bath for one or two weeks. Phase equilibrium was determined by visual observations of a large number of samples. At last the volumes of the middle-phase microemulsions were recorded and the modified fishlike phase diagrams were plotted.

The methodology used here is established by ourselves. For a quaternary system of water(W)-oil(O)-alcohol(A)-surfactant(S), the following composition variables are most suitable for our purposes and are defined as the mass ratio of oil to water plus oil in the system, α= O/(W＋O); the mass fraction of the surfactant in the system, b = S/(W+O+A+S); and the mass fraction of alcohol in the system, e = D/(W+O+A+S). If

αwas held constant (0.50), the b is plotted horizontally and the e is plotted vertically, a two-dimensional phase diagram (b ～e ) can be obtained.

Results and Discussion

The phase diagram for water/APG/1-butanol/n-octane system is shown in Figure 1.

Figure 1 The modified fishlike phase diagram of the quaternary system C8/10G1.31 or C12/14G1.43 / 1-butanol / n-octane / water system at 40 ℃ andα=0.5

It can be seen that increasing b at constant e causes a series of phase inversions

Winsor I→III→II. It is known that the whole phase behavior of a quaternary system at

constant temperature and pressure can be done in a phase tetrahedron, a section through such a phase tetrahedron atα=0.5 is a particular three-phase tie triangle including the microemulsion in the midst of the Winsor III region (Figure 1). This section is called a hydrophile-lipophile balanced plane (HLB plane)8.

The hydrophile-lipophile property of the mixed amphiphilic film in the quaternary system is just balanced in the midst line of the middle-phase region (see the broken line in Figure 1). The composition of the microemulsion in the midst line of the Winsor III region obeys the HLB plane equation

Where So and Ao are solubilities of APG monomers and 1-butanol in oil, and SS, AS denote the mass fractions of APG and 1-butanol in the interfacial film which is composed of surfactant and 1-butanol, respectively. A plot of e vs. b is a straight line (the broken line in Figure 1). If the slope and intercept of Eq. (1) are K and I, respectively, then AS can be obtained by

The volume fractionφof the middle phase at the midst line (Figure 1) was measured for a series of b values and plotted as a function of b (the Fig. was omitted). Extrapolation of this linear function to φ=0 and φ=1 yields the abscissas of the start point B (b B) and the end point E (b E). The ordinates of these two points（e B and e E）can be obtained by using the HLB plane equation (Eq. 1). The values are listed in Table 1. From b B and e B values, the solubilities of APG and 1-butanol in n-octane can be obtained (See Table 1), respectively. b E and e E reveal the minimum concentration of APG and 1-butanol, respectively, for getting a single microemulsion system while the ratio of water to oil is equal 1.

Acknowledgment

This project is supported by the National Natural Science Foundation of China (No:20243005).