low supersaturation. It has a very strange shape. The crystal has started to
grow at the apex i.e., at the top in the picture, and the (010) face does not
form. Also the (011) and (011) faces do not grow, but they emerge due to the
growth of other faces. The main explanation is competition byb-lactose;
presumably, its adsorption on the faces mentioned almost completely
prevents incorporation ofa-lactose. The concentration ofb-lactose can be
varied and then kept roughly constant for a time that is long enough to
allow determination of growth rates. Some of such results are given in
Figure 15.11, and it is seen thatb-lactose can strongly reduce growth. This is
part of the explanation of the very slow growth of lactose crystals as
compared to sucrose crystals: sucrose has no anomers.
The extent of growth reduction varies widely with the crystal face,
thereby strongly affecting crystal shape (habit). In the example given,
especially both (010) and bothð 011 Þfaces grow fast in the absence ofb-FIGURE15.11 Growth rateLCof two faces (Miller indices indicated) of a crystal
ofa-lactose monohydrate as a function of the concentration ofb-lactose (in g per kg
water). Supersaturation ofa-lactose: lnb& 1 :0. (After results by A. van Kreveld.
Neth. Milk Dairy J. 23 (1969) 258.)