Bowers James, Zarbakhsh Ali, Querol Aranzazu, Christenson Hugo K, McLure Ian A, Cubitt Robert
Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom.
J Chem Phys. 2004 Nov 8;121(18):9058-65. doi: 10.1063/1.1805501.
Using neutron reflectometry, adsorption from an equimolar mixture of hexane + perfluorohexane to a fluorophobic, octadecyl-coated, silicon substrate has been investigated as a function of temperature in the one-phase region upon approach to liquid-liquid coexistence. The composition of the investigated mixture, x(F) = 0.50, is well removed from the critical composition of x(F) = 0.36, where x(F) is the perfluorohexane mole fraction. To aid the modeling, mixtures with three different neutron refractive index contrasts have been used: namely, mixtures of C(6)H(14) + C(6)F(14) (H-F), C(6)D(14) + C(6)F(14) (D-F), and a mixture of C(6)H(14) + C(6)D(14) + C(6)F(14) which has been adjusted to have the same refractive index as silicon (CMSi). For all three contrasts, the principal features of the composition profile normal to the interface follow similar trends as the temperature T is reduced towards T(0), the coexistence temperature. These features consist of: (i) a hexane-rich primary adsorption layer appended to the octadecyl coupled layer. This primary layer is 22 +/- 5 A thick and becomes increasingly enriched in hexane as T(0) is approached. (ii) A tail that decays exponentially towards the bulk composition with a characteristic decay length zeta. As T(0) is approached, zeta increases. The scattering length density profiles have been converted to volume fraction profiles and the surface excess of hexane Gamma has been determined as a function of temperature for all three contrasts. As T(0) is approached Gamma increases, and its behavior can be represented using the scaling law Gamma approximately |T - T(0)|(-m). The resulting values of m are 0.71 +/- 0.09, 0.68 +/- 0.04, and 0.68 +/- 0.06 for the D-F, H-F, and CMSi contrasts, respectively. The behavior of Gamma with temperature does not adhere to the Gamma approximately |T - T(0)|(-1/3) law expected for complete wetting in systems with van der Waals interactions nor does it correspond to Gamma approximately |T - T(c)|(-0.305) expected for critical adsorption. The magnitude of the exponent m indicates that the adsorption resides in the crossover region between critical adsorption and complete wetting.
利用中子反射法，研究了在接近液 - 液共存的单相区域中，己烷 + 全氟己烷等摩尔混合物在憎氟的十八烷基涂层硅衬底上的吸附情况随温度的变化。所研究混合物的组成，x(F) = 0.50，与临界组成x(F) = 0.36相差较大，其中x(F)是全氟己烷的摩尔分数。为了辅助建模，使用了具有三种不同中子折射率对比度的混合物：即C(6)H(14) + C(6)F(14)（H - F）、C(6)D(14) + C(6)F(14)（D - F）以及已调整为与硅具有相同折射率的C(6)H(14) + C(6)D(14) + C(6)F(14)混合物（CMSi）。对于所有这三种对比度，当温度T朝着共存温度T(0)降低时，垂直于界面的组成分布的主要特征遵循相似的趋势。这些特征包括：（i）附着在十八烷基耦合层上的富含己烷的初级吸附层。该初级层厚度为22±5 Å，并且随着接近T(0)，在己烷中变得越来越富集。（ii）一个朝着本体组成呈指数衰减的尾部，其特征衰减长度为ζ。随着接近T(0)，ζ增加。散射长度密度分布已转换为体积分数分布，并且对于所有三种对比度，已确定己烷的表面超额Γ随温度的变化。随着接近T(0)，Γ增加，并且其行为可以用标度律Γ≈|T - T(0)|^(-m)来表示。对于D - F、H - F和CMSi对比度，所得的m值分别为0.71±0.09、0.68±0.04和0.68±0.06。Γ随温度的行为既不遵循具有范德华相互作用的系统中完全润湿所预期的Γ≈|T - T(0)|^(-1/3)定律，也不对应于临界吸附所预期的Γ≈|T - T(c)|^(-0.305)。指数m的大小表明吸附处于临界吸附和完全润湿之间的交叉区域。
